Lithium-ion rechargeable battery

Abstract

This invention provides a secondary battery with good interfacial contact between the active material and the electrolyte. [Solution] A secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive and negative electrode layers, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, and the electrolyte layer has a third solid electrolyte and an ionic liquid, the ionic liquid being impregnated into the voids of the third solid electrolyte. The secondary battery is foldable.

Description

[Technical Field]

[0001] One aspect of the present invention relates to a secondary battery, electronic equipment, and an aircraft.

[0002] Furthermore, one aspect of the present invention relates to a product, method, or method of manufacture. Alternatively, the present invention relates to a process, machine, manufacture, or composition of matter. One aspect of the present invention relates to a semiconductor device, display device, light-emitting device, energy storage device, lighting device, electronic device, or a method of manufacturing the same.

[0003] In this specification, "electronic devices" refers to all devices having a secondary battery, and includes electro-optical devices having a secondary battery, information terminal devices having a secondary battery, and so on. [Background technology]

[0004] In recent years, there has been a great deal of activity in the development of various energy storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. In particular, the demand for lithium-ion secondary batteries, which offer high output and high capacity, has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable as a source of rechargeable energy in today's information society.

[0005] Most lithium-ion batteries currently in use employ an electrolyte (also called an organic electrolyte) in which lithium salts are dissolved in a polar organic solvent. However, since this organic solvent is flammable, secondary batteries using it pose a risk of ignition or fire.

[0006] Large rechargeable batteries used in automobiles and other applications require high reliability, especially safety. Therefore, solid-state batteries, which use a solid electrolyte instead of a liquid electrolyte between the positive and negative electrodes, are being considered. Solid electrolytes are broadly classified into organic and inorganic types.

[0007] As an inorganic solid electrolyte, for example, Patent Document 1 discloses a secondary battery having a sulfide-based solid electrolyte or an oxide-based solid electrolyte. Non-Patent Documents 1 to 3 describe changes in the crystal structure of lithium cobalt oxide. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2012-14892 [Non-patent literature]

[0009] [Non-Patent Document 1] Motohashi, T. et al, “Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-Patent Document 2] Zhaohui Chen et al, “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609 [Non-Patent Document 3] Belsky, A. et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58 364-369. [Overview of the project] [Problems that the invention aims to solve]

[0010] Patent Document 1 proposes a non-aqueous electrolyte battery that includes an intervening layer between the negative electrode active material layer and the solid electrolyte layer, considering that the volume change (expansion and contraction) of the negative electrode active material layer is large, which reduces the bonding between the negative electrode active material layer and the solid electrolyte layer and increases the lithium ion transfer resistance at the interface between the two layers. Patent Document 1 states that the intervening layer consists of a polymer containing a lithium salt or an ionic liquid.

[0011] However, when an intervening layer is included, interfacial contact between the negative electrode active material layer and the intervening layer, or between the solid electrolyte layer and the intervening layer, becomes a problem. Interfacial contact is sometimes referred to as interfacial resistance.

[0012] Therefore, one aspect of the present invention aims to improve the contact between interfaces in a secondary battery, for example, between the active material and the electrolyte. Alternatively, one aspect of the present invention aims to provide a secondary battery with improved safety. Alternatively, one aspect of the present invention aims to provide a flexible secondary battery.

[0013] Furthermore, the description of these problems does not preclude the existence of other problems. These problems are considered independent of each other, and one aspect of the present invention does not need to solve all of them. Moreover, it is possible to extract other problems from the description, drawings, and claims of this specification, etc. [Means for solving the problem]

[0014] To solve the above problems, one aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, and the electrolyte layer has a third solid electrolyte and an ionic liquid, the ionic liquid being impregnated into the voids of the electrolyte layer, specifically the voids of the third solid electrolyte.

[0015] Another aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, and the electrolyte layer has a third solid electrolyte, and the positive electrode layer, the negative electrode layer and the electrolyte layer have an ionic liquid, the ionic liquid being impregnated into the voids of the electrolyte layer, specifically the voids of the third solid electrolyte.

[0016] Another aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a first electrolyte layer to a third electrolyte layer, the first electrolyte layer to the third electrolyte layer has an ionic liquid, and the ionic liquid is impregnated into the voids of the second electrolyte layer, specifically into the voids of the third solid electrolyte in the second electrolyte layer.

[0017] Another aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a first electrolyte layer and a second electrolyte layer, the first electrolyte layer and the second electrolyte layer have an ionic liquid, and the ionic liquid is impregnated into the voids of the second electrolyte layer, specifically into the voids of the third solid electrolyte in the second electrolyte layer.

[0018] In any one aspect of the present invention, the positive electrode active material preferably has a composite oxide having a layered rock salt type crystal structure, a spinel type crystal structure, or an olivine type crystal structure.

[0019] In any one aspect of the present invention, the positive electrode active material having a layered rock salt type crystalline structure is preferably lithium cobaltate or lithium nickel-manganese-cobaltate.

[0020] In any one aspect of the present invention, the negative electrode active material preferably comprises silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, or indium.

[0021] In any one aspect of the present invention, the negative electrode active material preferably has a carbon material.

[0022] An electronic device, a wristwatch-type electronic device, or an aircraft having a secondary battery according to one aspect of the present invention. [Effects of the Invention]

[0023] According to one aspect of the present invention, a secondary battery with good interfacial resistance can be provided. Alternatively, according to one aspect of the present invention, a secondary battery with improved safety can be provided. Alternatively, according to one aspect of the present invention, a flexible secondary battery can be provided.

[0024] Furthermore, the description of these effects does not preclude the existence of other effects. These effects are considered independent of each other, and one embodiment of the present invention does not need to exhibit all of these effects. Moreover, it is possible to extract other effects from the description in this specification. [Brief explanation of the drawing]

[0025] [Figure 1] Figures 1A and 1B illustrate a secondary battery according to one embodiment of the present invention. [Figure 2] Figures 2A and 2B illustrate a secondary battery according to one embodiment of the present invention. [Figure 3] Figure 3 illustrates a secondary battery according to one embodiment of the present invention. [Figure 4] Figures 4A and 4B illustrate a secondary battery according to one embodiment of the present invention. [Figure 5] Figures 5A to 5C illustrate a method for manufacturing a secondary battery according to one embodiment of the present invention. [Figure 6] Figures 6A to 6D illustrate a method for manufacturing a secondary battery according to one embodiment of the present invention. [Figure 7] Figures 7A to 7D illustrate a method for manufacturing a secondary battery according to one embodiment of the present invention. [Figure 8] Figures 8A and 8B illustrate a method for manufacturing a secondary battery according to one embodiment of the present invention. [Figure 9] Figures 9A and 9B illustrate a method for manufacturing a secondary battery according to one embodiment of the present invention. [Figure 10] Figure 10 illustrates a manufacturing apparatus for a secondary battery according to one embodiment of the present invention. [Figure 11] Figure 11 is a flowchart illustrating a method for fabricating the electrolyte layer of a secondary battery according to one embodiment of the present invention. [Figure 12] Figures 12A and 12B illustrate the heating process of the electrolyte layer of a secondary battery according to one embodiment of the present invention. [Figure 13] Figures 13A and 13B are cross-sectional views of the positive electrode active material, and Figures 13C to 13F are partial cross-sectional views of the positive electrode active material. [Figure 14] Figure 14 shows an example of a TEM image where the crystal orientation is roughly consistent. [Figure 15] Figure 15A is an example of a STEM image where the crystal orientation is roughly consistent. Figure 15B is the FFT pattern of the region with the rock salt type crystal structure RS. Figure 15C is the FFT pattern of the region with the layered rock salt type crystal structure LRS. [Figure 16] Figure 16 illustrates the crystal structure of the positive electrode active material. [Figure 17] Figure 17 illustrates the crystal structure of a conventional positive electrode active material. [Figure 18] Figures 18A and 18B are cross-sectional views of the positive electrode active material, and Figures 18C1 and 18C2 are partial cross-sectional views of the positive electrode active material. [Figure 19] Figure 19 shows the XRD pattern calculated from the crystal structure. [Figure 20] Figure 20 shows the XRD pattern calculated from the crystal structure. [Figure 21] Figure 21 is a cross-sectional view of the positive electrode active material. [Figure 22]Figures 22A to 22C illustrate the method for preparing the positive electrode active material. [Figure 23] Figures 23A and 23B illustrate a laminate-type secondary battery according to one embodiment of the present invention. [Figure 24] Figures 24A to 24C illustrate a method for manufacturing a laminate-type secondary battery according to one embodiment of the present invention. [Figure 25] Figures 25A and 25B illustrate a bent secondary battery according to one embodiment of the present invention. [Figure 26] Figures 26A and 26B illustrate a secondary battery according to one embodiment of the present invention. [Figure 27] Figures 27A and 27B illustrate a bent secondary battery according to one embodiment of the present invention. [Figure 28] Figures 28A to 28C illustrate a wristwatch-type electronic device according to one embodiment of the present invention. [Figure 29] Figures 29A to 29G illustrate a wristwatch-type electronic device according to one embodiment of the present invention. [Figure 30] Figures 30A to 30C illustrate a wristwatch-type electronic device according to one embodiment of the present invention. [Figure 31] Figure 31 illustrates a wristwatch-type electronic device according to one embodiment of the present invention. [Figure 32] Figures 32A and 32B are perspective views showing an example of an aircraft according to one embodiment of the present invention. Figure 32C is a cross-sectional view showing an example of an aircraft according to one embodiment of the present invention. [Figure 33] Figures 33A and 33B are perspective views showing an example of an aircraft according to one embodiment of the present invention. [Figure 34] Figures 34A to 34C illustrate a coin-type secondary battery according to one embodiment of the present invention. [Figure 35] Figures 35A to 35D illustrate a cylindrical secondary battery according to one embodiment of the present invention. [Figure 36] Figures 36A to 36C show the external appearance of a secondary battery pack according to one embodiment of the present invention. [Figure 37]Figures 37A to 37C show the external appearance of a secondary battery pack according to one embodiment of the present invention. [Figure 38] Figures 38A to 38C illustrate an example of application to electric vehicles (EVs). [Figure 39] Figures 39A to 39D illustrate an example of a vehicle. [Figure 40] Figures 40A to 40C illustrate an example of a vehicle. [Figure 41] Figures 41A to 41E illustrate an example of an electronic device. [Figure 42] Figure 42 illustrates an example of an electronic device. [Figure 43] Figures 43A and 43B are planar SEM images of the first sheet-like electrolyte layer. [Figure 44] Figures 44A and 44B are planar SEM images of the second sheet-like electrolyte layer. [Figure 45] Figures 45A and 45B are planar SEM images showing the state in which the ionic liquid has impregnated the voids in the second sheet-like electrolyte layer. [Modes for carrying out the invention]

[0026] Embodiments will be described in detail with reference to the drawings. However, it will be readily apparent to those skilled in the art that the present invention is not limited to the following description, and that its form and details can be modified in various ways without departing from the spirit and scope of the present invention. Accordingly, the present invention is not to be interpreted as being limited to the contents of the embodiments shown below. In addition, in the configuration of the invention described below, the same reference numerals are used in common between different drawings for the same parts or parts having similar functions, and the repetition of their descriptions may be omitted.

[0027] Furthermore, the position, size, and scope of each component shown in the drawings, etc., may not represent the actual position, size, and scope in order to facilitate understanding of the invention. For this reason, the invention disclosed herein is not necessarily limited to the position, size, and scope disclosed in the drawings, etc.

[0028] In this specification, the terms "above" or "below" do not limit the positional relationship of the components to being directly above or directly below. Nor do the terms "above" or "below" limit the positional relationship to being in contact with each other. For example, in the expression "active material layer B on current collector A," it is not necessary for the active material layer B to be formed in contact with the current collector A, and other components may be included between the current collector A and the active material B.

[0029] In this specification, ordinal numbers such as "first," "second," etc., are used to avoid confusion of constituent elements and do not indicate order or rank, such as process order or layering order. Furthermore, even if an ordinal number is not used for a term in this specification, it may be used in the claims to avoid confusion of constituent elements. Also, even if an ordinal number is used for a term in this specification, a different ordinal number may be used in the claims. Furthermore, even if an ordinal number is used for a term in this specification, the ordinal number may be omitted in the claims, etc.

[0030] In this specification, examples are sometimes shown in which lithium metal is used as the negative electrode in a secondary battery using a positive electrode and positive electrode active material, but the secondary battery of one aspect of the present invention is not limited to this. Other materials, such as graphite or lithium titanate, may be used for the negative electrode. As long as one aspect of the present invention consists of a positive electrode and a positive electrode active material, the material of the negative electrode is not limited in any way.

[0031] In this specification, the electrolyte layer refers to a region that electrically insulates the positive electrode and the negative electrode and is conductive to lithium ions. The electrolyte layer sandwiched between the positive electrode and the negative electrode may appear as a layer.

[0032] In this specification, a semi-solid battery is defined as a battery having a semi-solid material in at least one of its components: the electrolyte layer, positive electrode, and negative electrode. It is particularly preferable that the electrolyte layer be made of a semi-solid material. The term "semi-solid" means a material that possesses solid properties, such as small volume change, while also exhibiting properties similar to a liquid, such as fluidity. It does not mean that the solid material content is 50%. The term "semi-solid material" applies whether these properties are exhibited by a single material or multiple materials. For example, a gel-like material can exhibit these properties as a single material and is therefore considered a semi-solid material. Furthermore, if a material is formed by impregnating (or infiltrating) a porous solid material with a liquid material, and exhibits these properties, it may also be called a semi-solid material.

[0033] In this specification and other documents, the positive electrode and the negative electrode together may be referred to as electrodes.

[0034] In this specification, space groups are expressed using international notation (or Hermann-Mauguin notation) in short notation. Crystal planes and crystal directions are expressed using Miller indices. Individual planes are indicated using ( ). In crystallography, space groups, crystal planes, and crystal directions are indicated by a bar above the number, but in this specification, due to formatting constraints, a minus sign (-) may be placed before the number instead of a bar above it. Individual orientations within a crystal are indicated by [ ], collective orientations indicating all equivalent directions are indicated by < >, individual planes indicating crystal planes are indicated by ( ), and collective planes with equivalent symmetry are indicated by {}. Furthermore, for ease of understanding the structure, a trigonal crystal represented by the space group R-3m is generally represented as a composite hexagonal lattice of a hexagonal crystal, and (hkil) may be used as Miller indices in addition to (hkl). Here, i is -(h+k).

[0035] In this specification, the term "particle" is not limited to a spherical shape with a circular cross-section, but includes particles with elliptical, rectangular, trapezoidal, triangular, rounded quadrilateral, asymmetrical shapes, and other cross-sectional shapes. Furthermore, the shapes of multiple particles do not have to be uniform, and individual particles may have irregular shapes.

[0036] In this specification and the like, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the insertable and extractable lithium has been extracted from the positive electrode active material. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 275 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0037] In this specification and the like, the degree to which insertable and extractable lithium remains in the positive electrode active material is represented by x in the composition formula of the positive electrode active material, for example, Li in CoO2, or Li x in MO2. The value of x indicates the lithium occupancy in Li x CoO2, or Li x MO2. In this specification and the like, Co in Li x CoO2 is an example of a transition metal, and can be appropriately read as Li x MO2 (M represents a transition metal). In the case of the positive electrode active material in a secondary battery, x = (theoretical capacity - charge capacity) / theoretical capacity can be used. For example, when a secondary battery using LiCoO2 as the positive electrode active material is charged to 219.2 mAh / g, it can be said that Li 0.2 CoO2 or x = 0.2. When x in Li x CoO2 is small, for example, it means 0.1 < x ≤ 0.24.

[0038] When the synthesized lithium cobaltate approximately satisfies the stoichiometric ratio, it is LiCoO2 and x = 1. Also, when the discharge of a secondary battery using LiCoO2 as the positive electrode ends, it can be said that LiCoO2 or x = 1. Here, the end of discharge means a state where the voltage becomes 3.0 V or 2.5 V or less at a current of 100 mA / g or less, for example.

[0039] Li x The charge capacity and / or discharge capacity used for calculating x in Li

[0040] In this specification, the space group of a crystal structure is identified by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. Therefore, in this specification, "belonging to a certain space group," "being part of a certain space group," or "being a certain space group" can be rephrased as "being identified to a certain space group."

[0041] In this specification, an anion structure in which three layers are stacked with a slight offset from each other, such as ABCABC, is referred to as a cubic close-packed structure. Therefore, the anion does not have to be a strictly cubic lattice. At the same time, since real crystals always have defects, the analytical results do not necessarily conform to theory. For example, in fast Fourier transforms (FFTs) of electron diffraction or transmission electron microscope (TEM) images, spots may appear at positions slightly different from the theoretical positions.

[0042] In this specification, homogeneity refers to the phenomenon in which, in a solid composed of multiple elements (e.g., A, B, C), a certain element (e.g., A) is distributed in a specific region with similar characteristics. Furthermore, if the concentrations of element (e.g., A) in specific regions are substantially the same, the material can be considered homogeneous. For example, if the difference in concentration of element (e.g., A) between specific regions is within 10%, the material can be considered homogeneous. In active materials, specific regions include, for example, the surface, protrusions, depressions, and interior.

[0043] In this specification, the positive electrode active material may be expressed as a composite oxide, positive electrode material, positive electrode material, positive electrode material for secondary batteries, etc. In this specification, the positive electrode active material of one embodiment of the present invention preferably has additive elements, and the positive electrode active material having additive elements may be expressed as a compound, composition, or composite.

[0044] As the charging voltage of a secondary battery increases, the voltage at the positive electrode generally rises. The positive electrode active material according to one aspect of the present invention has a stable crystal structure even at high voltages. The stability of the crystal structure of the positive electrode active material in the charged state suppresses the decrease in discharge capacity that occurs with repeated charging and discharging.

[0045] Short circuits in secondary batteries not only cause malfunctions in the charging and / or discharging operations of the secondary battery, but can also lead to overheating and ignition. To realize a safe secondary battery, it is desirable that short circuits be suppressed even at high charging voltages. The positive electrode active material of one aspect of the present invention suppresses short circuit current even at high charging voltages. Therefore, it is possible to create a secondary battery that achieves both high discharge capacity and safety.

[0046] (Embodiment 1) In one embodiment of the present invention, the electrolyte layer comprises a solid material and a liquid material. This embodiment describes an electrolyte layer according to one aspect of the present invention and an example of a secondary battery having the same.

[0047] Figure 1A is a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. The secondary battery 100 has a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107. The positive electrode layer 106 has a positive electrode current collector 101 and a positive electrode active material layer 102. The negative electrode layer 107 has a negative electrode current collector 105 and a negative electrode active material layer 104.

[0048] Figure 1B is a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention, and unlike Figure 1A, it shows a configuration in which the negative electrode active material layer 104 is unnecessary. For example, when a metal foil containing lithium is used for the negative electrode current collector 105, the negative electrode active material layer 104 can be omitted. The electrolyte layer 103 may be placed at a certain distance from the negative electrode current collector 105. This is to ensure a region on the negative electrode current collector 105 where lithium is deposited.

[0049] In Figures 1A and 1B, the electrolyte layer 103 contains a solid material and a liquid material.

[0050] The electrolyte layer 103 has the function of moving carrier ions. Lithium ions, sodium ions, etc., can be used as carrier ions. The electrolyte layer 103 is conductive to carrier ions and exhibits the function of moving carrier ions. Specifically, it is preferable to use a solid material with high carrier ion conductivity for the electrolyte layer 103. For example, the lithium ion conductivity of the solid material used in the electrolyte layer 103 should be between 0.1 mS / cm and 20 mS / cm at room temperature. In addition, it is preferable for the electrolyte layer 103 to contain a liquid material with high carrier ion conductivity in addition to the solid material. For example, the lithium ion conductivity of the liquid material used in the electrolyte layer 103 should be between 0.1 mS / cm and 20 mS / cm at room temperature. If the lithium ion conductivity of the solid material is higher than that of the liquid material, the electrolyte layer 103 should contain more solid material than liquid material.

[0051] To satisfy the above lithium-ion conductivity requirements, it is preferable to use a solid electrolyte as the solid material. Specific examples of solid electrolytes will be described later.

[0052] To satisfy the lithium-ion conductivity requirements mentioned above, it is preferable to use an ionic liquid as the liquid material. Specific examples of ionic liquids will be discussed later.

[0053] Furthermore, the electrolyte layer 103 may contain a lithium salt. For example, if the electrolyte layer 103 contains an ionic liquid as the solvent, it is preferable to contain a lithium salt as the solute.

[0054] In an electrolyte layer 103 having a solid material and a liquid material, it is preferable that the solid material can hold the liquid material. For example, voids may occur in a solid material after firing, but the liquid material is easily impregnated into these voids and can be easily held in them.

[0055] Furthermore, controlling the viscosity of the liquid material makes it easier for it to impregnate and retain in the solid material. For example, when impregnating a solid material, it is preferable for the liquid material to have low viscosity. After being retained in the solid material, it is preferable for the liquid material to have high viscosity. A highly viscous state is sometimes described as a gel state. A gel state can be described as an intermediate material form between a solid and a liquid.

[0056] By having the above-mentioned solid material in a configuration that holds the liquid material, that is, a configuration in which the solid material is impregnated with the liquid material, it is possible to suppress the liquid material from seeping out of the electrolyte layer 103 compared to a configuration in which the solid material and the liquid material are simply mixed.

[0057] This configuration makes it possible to provide a secondary battery in which liquid materials in the electrolyte layer and other parts are less likely to leak, thereby improving safety.

[0058] As long as the above-mentioned retention configuration is satisfied, the ratio of solid material to liquid material in the electrolyte layer 103 is not particularly limited, but it is preferable for the proportion of solid material to be greater than the proportion of liquid material for easier retention. The ratio of solid material to liquid material can be determined using volume percent. For example, it is preferable for the solid material in the electrolyte layer 103 to be 70% to 95% by volume, preferably 80% to 93% by volume. Since the liquid material in the electrolyte layer 103 occupies the remainder, it is preferable for it to be 5% to 30% by volume, preferably 7% to 20% by volume. Note that lithium salt may be dissolved in the liquid material, but it is sufficient to satisfy the above ratio while the lithium salt is dissolved in the liquid material.

[0059] An electrolyte layer 103 that satisfies the above ratio can be said to have a semi-solid material. An electrolyte layer 103 having a semi-solid material is sometimes referred to as a semi-solid electrolyte layer. A secondary battery having such a semi-solid electrolyte layer is preferable because it is easily bendable.

[0060] When a solid electrolyte is used as the solid material and an ionic liquid as the liquid material, the transport fraction of lithium ions in the solid electrolyte is often higher than that of the ionic liquid. Therefore, it is preferable for the proportion of the solid electrolyte to be higher than that of the ionic liquid, and satisfying this condition suppresses stagnation of lithium ion movement in the electrolyte layer 103. Lithium ion transport fraction is an indicator similar to lithium ion conductivity and indicates the ease of lithium ion movement.

[0061] In a configuration in which a solid material is impregnated with a liquid material, carrier ions, such as lithium ions, can move between the solid and liquid materials. Furthermore, in a configuration in which a solid material is impregnated with a liquid material, lithium ions can also move only within the solid material. Additionally, in a configuration in which a solid material is impregnated with a liquid material, lithium ions can also move only within the liquid material.

[0062] While the case where solid and liquid materials are present in the electrolyte layer has been described, one or both of the solid and liquid materials may also be present in the positive electrode layer. The presence of one or both of the solid and liquid materials in both the electrolyte and positive electrode layers can lower the interfacial resistance between the electrolyte and positive electrode layers compared to the case where an intervening layer is present. Furthermore, one or both of the solid and liquid materials may also be present in the negative electrode layer. The presence of one or both of the solid and liquid materials in both the electrolyte and negative electrode layers can lower the interfacial resistance between the electrolyte and negative electrode layers compared to the case where an intervening layer is present. When one or both of the solid and liquid materials are present in the positive or negative electrode layer, one or both of the solid and liquid materials may be mixed into the positive or negative electrode slurry. If the active material can be held by one or both of the solid and liquid materials, the binder in the positive or negative electrode layer can be eliminated or reduced. Furthermore, if conductivity can be ensured by the solid material, the conductive additive in the positive or negative electrode layer can be eliminated or reduced.

[0063] The solid material of the positive electrode layer or negative electrode layer may have a different shape from the solid material of the electrolyte layer. While the electrolyte layer is preferably made of a solid material with voids, the positive electrode layer or negative electrode layer may be made of a particulate solid material. If the positive electrode layer or negative electrode layer does not contain a liquid material, it is particularly preferable that it be made of a particulate solid material. The solid material of the positive electrode layer or negative electrode layer may be different from the solid material of the electrolyte layer, but it is preferable that they be the same material to suppress interfacial resistance.

[0064] The liquid material in the positive electrode layer or negative electrode layer may be in a different state from the liquid material in the electrolyte layer. While the electrolyte layer is preferably in a gel state, the positive electrode layer or negative electrode layer may be in a liquid state. The liquid material in the positive electrode layer or negative electrode layer may be different from the liquid material in the electrolyte layer, but from the perspective of suppressing interfacial resistance, it is preferable to use the same material, with only the state being different.

[0065] If the liquid material in the positive electrode layer or the negative electrode layer is the same material as the liquid material in the electrolyte layer and is in a liquid state, the secondary battery can be completed by injecting the liquid material after the secondary battery has been assembled.

[0066] Furthermore, due to processes such as the pressing process used to assemble the secondary battery, the solid and liquid materials present in the electrolyte layer may move to the positive or negative electrode layer. In such cases, the solid and liquid materials may seep out from the electrolyte layer, and the solid and liquid materials in the positive or negative electrode layer will be the same as those in the electrolyte layer. However, if the solid and liquid materials used in the electrolyte layer are those described in one aspect of the present invention, the solid material will hold the liquid material, thus preventing the solid and liquid materials from leaking out of the secondary battery.

[0067] Furthermore, the condition that the proportion of solid electrolyte is higher than that of ionic liquid is limited to the electrolyte layer; if one or both of the solid electrolyte and ionic liquid are present in the positive electrode layer or negative electrode layer, the above condition does not need to be met. For example, in the electrolyte layer, the proportion of solid electrolyte may be higher than that of ionic liquid, but in the positive electrode layer or negative electrode layer, the proportion of ionic liquid may be higher than that of solid electrolyte. Also, in the positive electrode layer or negative electrode layer, ionic liquid may be present but solid electrolyte may not be present. Also, in the positive electrode layer or negative electrode layer, solid electrolyte may be present but ionic liquid may not be present.

[0068] When a positive electrode layer or negative electrode layer contains both a solid and a liquid material, the liquid material may be held in place by the solid material; this state is sometimes referred to as a semi-solid state.

[0069] In the electrolyte layer, positive electrode layer, and negative electrode layer, it is preferable that the solid material be an inorganic material in order to hold the liquid material with a solid material, but organic materials can also be used as the solid material. If a non-moving gel-like material is used as the organic material, it becomes possible to hold the liquid material and exhibit a semi-solid state.

[0070] Furthermore, the liquid material should preferably have a viscosity sufficient to be retained by the solid material; for example, a highly viscous gel-like material can be used. When an ionic liquid is used as the liquid material, a gel-like ionic liquid can be applied.

[0071] Furthermore, the viscosity of the liquid material is preferably such that it does not seep out of the electrolyte layer 103 if it is filled at least when the electrolyte layer 103 is completed, and it is not necessary to fill it at the time of the starting material. In other words, the viscosity of the liquid material may be changed. For example, if a liquid material with low viscosity is used at the time of the starting material, it will be easier to impregnate the voids. Subsequently, in order to maintain the state in which it is held by the solid material, it is preferable to increase the viscosity of the liquid material at least when the electrolyte layer 103 is formed or when the secondary battery is completed. Specifically, a gelation treatment can be performed on the liquid material using a heating process, which is a manufacturing process for the electrolyte layer, and the viscosity of the liquid material should increase after the heating process. Alternatively, the viscosity of the liquid material may be reduced by using a heating process when mixing the solid material and the liquid material.

[0072] Furthermore, other materials may be added to adjust the viscosity of the liquid material when using starting materials. For example, the viscosity of the liquid material can be controlled by mixing an organic solvent with an ionic liquid. As organic solvents, one or more selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc. can be used.

[0073] Furthermore, the viscosity of the liquid material can be controlled by controlling the amount of lithium salt added to the ionic liquid. As the lithium salt, one or more selected from LiPF6, LiClO4, LiBF4, Li(C2F5SO2)2N, Li(CF3SO2)2N, Li(SO2F)2N, lithium bis(oxalate) borate (Li(C2O4)2, LiBOB), etc., can be used.

[0074] When a solid electrolyte is used as a solid material, the starting material for the solid electrolyte is often in the form of particles. Particle shape includes shapes that are circular or perfectly round when viewed from a cross-section. However, the shape of the solid electrolyte changes through a firing process, i.e., a heating process, or a mixing process, applied to the starting material. That is, the solid electrolyte may take on a shape different from its particle shape. A different shape includes shapes with irregularities or elliptical shapes when viewed from a cross-section. In other words, the solid material in the electrolyte layer 103 of the secondary battery 100 is not limited to a particle shape; the effects of this invention can be achieved even with a variety of shapes.

[0075] The shape changes described above can also occur in active material. For example, the starting material for active material is often in granular form. However, the shape changes through a firing process, i.e., a heating process, or a mixing process. In other words, the active material may have a shape different from the granular form. That is, the active material in the positive electrode active material layer 102 is not limited to granular form. Similarly, the active material in the negative electrode active material layer 104 is not limited to granular form. The effects of this invention can be achieved even if the active material has a variety of shapes.

[0076] Figure 2A shows a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. Figure 2A shows a configuration having a negative electrode active material layer 104, as shown in Figure 1A. Of course, in Figure 2A, the negative electrode active material layer 104 can be omitted, as shown in Figure 1B.

[0077] In Figure 2A, the positive electrode active material layer 102 comprises at least a positive electrode active material 111 and a solid electrolyte 113. In Figure 2A, the positive electrode active material 111 and the solid electrolyte 113 are shown in particle shape, but are not limited to particle shape. Because the positive electrode active material layer 102 has a solid electrolyte 113, it can operate as a secondary battery 100 even if the positive electrode active material layer 102 does not contain an ionic liquid. Furthermore, as shown in Figure 2A, since the solid electrolyte 113 is continuously present from the electrolyte layer 103 to the positive electrode active material layer 102, interfacial resistance between the layers can be suppressed. It is preferable that the solid electrolyte 113 in the positive electrode active material layer 102 be made of the same material as the solid electrolyte 113 in the electrolyte layer 103, and that the shape be different, with one being in particle shape and the other a sintered body. A sintered body includes a state in which particles are bonded together, and voids may occur between these particles.

[0078] The positive electrode active material layer 102 may contain a conductive additive, but in Figure 2A, the conductive additive is omitted. Since the positive electrode active material layer 102 contains a solid electrolyte 113, it is also possible to eliminate the need for a conductive additive. The positive electrode active material layer 102 may also contain a binder, but in Figure 2A, the binder is omitted. Since the positive electrode active material layer 102 contains a solid electrolyte 113, it is also possible to eliminate the need for a binder.

[0079] Furthermore, the positive electrode active material layer 102 may contain an ionic liquid instead of the solid electrolyte 113, or it may contain an ionic liquid in addition to the solid electrolyte 113. Preferably, the ionic liquid in the positive electrode active material layer 102 is the same material as the ionic liquid in the electrolyte layer 103, and the two states may be different, with one in a gel state and the other in a liquid state.

[0080] In Figure 2A, the negative electrode active material layer 104 comprises at least a negative electrode active material 117 and a solid electrolyte 113. In Figure 2A, the negative electrode active material 117 and the solid electrolyte 113 are shown in particle shape, but are not limited to particle shape. Because the negative electrode active material layer 104 has a solid electrolyte 113, it can operate as a secondary battery 100 even if the negative electrode active material layer 104 does not contain an ionic liquid. Furthermore, as shown in Figure 2A, since the solid electrolyte 113 is continuously present from the electrolyte layer 103 to the negative electrode active material layer 104, interfacial resistance between the layers can be suppressed. It is preferable that the solid electrolyte 113 in the negative electrode active material layer 104 be made of the same material as the solid electrolyte 113 in the electrolyte layer 103, and that the shape be different, with one being in particle shape and the other a sintered body. A sintered body includes a state in which particles are bonded together, and voids may occur between these particles.

[0081] The negative electrode active material layer 104 may contain a conductive additive, but in Figure 2A, the conductive additive is omitted. Since the negative electrode active material layer 104 contains a solid electrolyte 113, it is possible to eliminate the need for a conductive additive. The negative electrode active material layer 104 may also contain a binder, but in Figure 2A, the binder is omitted. Since the negative electrode active material layer 104 contains a solid electrolyte 113, it is possible to eliminate the need for a binder.

[0082] Furthermore, the negative electrode active material layer 104 may contain an ionic liquid instead of the solid electrolyte 113, or it may contain an ionic liquid in addition to the solid electrolyte 113. Preferably, the ionic liquid in the negative electrode active material layer 104 is the same material as the ionic liquid in the electrolyte layer 103, and the states may be different, with one in a gel state and the other in a liquid state.

[0083] In Figure 2A, the electrolyte layer 103 has a solid electrolyte 113 as a solid material and an ionic liquid 118 as a liquid material. Figure 2B shows an enlarged schematic diagram of a part of region 114 of the electrolyte layer 103. As shown in Figure 2B, the solid electrolyte 113 is preferably a sintered body. In addition, some of the solid electrolyte 113 in the electrolyte layer 103 may have a particle shape.

[0084] As shown in Figure 2B, the sintered solid electrolyte 113 also has voids. Furthermore, these voids can be formed according to the firing conditions of the solid electrolyte. For example, in order to suppress short circuits between the positive and negative electrodes, firing conditions can be set to reduce the voids in the electrolyte layer 103. However, in this invention, the voids are not necessarily reduced in order to fill them with ionic liquid 118. To suppress short circuits in the voids, for example, the viscosity of the ionic liquid 118 can be controlled.

[0085] Increasing the proportion of ionic liquid 118 in the electrolyte layer 103 will also increase the voids. To increase the voids, the firing process may be carried out with an organic material having a melting point below the sintering temperature mixed in. During the firing process, voids corresponding to the melted and lost organic material can be formed. Thus, increasing the voids while controlling them, rather than reducing them, is also included in one aspect of the present invention.

[0086] In Figure 2B, the region containing the ionic liquid 118 corresponds to a void. It is preferable that the filled ionic liquid 118 is gelled in order to suppress short circuits between the positive and negative electrodes.

[0087] The electrolyte layer 103 may be processed into a sheet shape through processes such as pressing. The electrolyte layer 103 may also have multiple solid electrolyte particles. That is, even if it does not form a sintered body and has multiple solid electrolyte particles, the electrolyte layer 103 can have voids between the particles.

[0088] The film thickness of the sheet-like electrolyte layer 103 is preferably 1 μm to 100 μm, more preferably 1 μm to 50 μm, and more preferably 1 μm to 20 μm.

[0089] In the electrolyte layer 103, the position of the voids may be controlled. If the voids connect from the positive electrode layer to the negative electrode layer, forming holes, the possibility of a short circuit in the secondary battery increases due to dendrites, for example, that form in the negative electrode layer. Therefore, it is preferable to control the position of the voids in the electrolyte layer 103 so that they are offset from each other, preventing the formation of such holes.

[0090] As mentioned above, since the voids are filled with a liquid material, increasing the viscosity of the liquid material can also suppress short circuits in secondary batteries caused by dendrites and the like.

[0091] The proportion of voids in the electrolyte layer 103 may be controlled. For example, the proportion of voids in the center of the electrolyte layer 103 may be made higher than the proportion of voids in the electrolyte layer 103 closer to the positive or negative electrode layer. To suppress dendrite formation in the negative electrode layer, the proportion of voids in the electrolyte layer 103 may be made lower as it approaches the negative electrode layer.

[0092] The electrolyte layer 103 may also be a stacked structure, preferably consisting of two or more, more than three, electrolyte layers. In the case of a three-layer structure, the proportion of voids in the central electrolyte layer can be made different from the proportion of voids in the electrolyte layers above and below it. Such an electrolyte layer 103 can suppress the short circuit of the secondary battery described above. Furthermore, a separator may be placed in place of the central electrolyte layer.

[0093] To reiterate, it is preferable to prepare the electrolyte layer 103 in sheet form. A sheet-like electrolyte layer is also suitable for the above-described laminated structure. To make the electrolyte layer 103 sheet-like, it is preferable to have an ionic liquid in a gel state rather than a liquid material, as this allows the shape to be maintained.

[0094] To suppress short circuits in the secondary battery as described above, a separator may be placed in addition to the electrolyte layer 103.

[0095] As shown in Figures 2A and 2B, in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 can be any solid material, and the ionic liquid 118 can be any liquid material.

[0096] The electrolyte layer 103 may also have a binder. This makes it easier for the solid electrolyte 113 to hold the ionic liquid 118. However, the binder is omitted in Figures 2A and 2B.

[0097] Although Figure 2A shows that there are boundaries between each layer, clear boundaries may not be visible in the secondary battery 100. For example, when bonding the sheet-like electrolyte layer 103 to the positive electrode layer 106, pressing can blur the boundary between the electrolyte layer 103 and the positive electrode layer 106. This is because some of the positive electrode active material 111 enters the electrolyte layer 103, and some of the solid electrolyte 113 enters the positive electrode layer 106. Similarly, some of the negative electrode active material 117 enters the electrolyte layer 103, and some of the solid electrolyte 113 enters the negative electrode layer 107, blurring the boundary between the electrolyte layer 103 and the negative electrode layer 107.

[0098] Figure 3 shows a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. Figure 3 shows a configuration having a negative electrode active material layer 104, as shown in Figure 1A. Of course, in Figure 3, the negative electrode active material layer 104 can be omitted, as shown in Figure 1B.

[0099] Unlike Figure 2A, the secondary battery 100 shown in Figure 3 has the ionic liquid 118 located throughout the entire battery 100. By forming the secondary battery 100 through processes such as stacking the positive electrode layer 106, electrolyte layer 103, and negative electrode layer 107 and then injecting the ionic liquid 118, the ionic liquid 118 can be located throughout the entire battery 100, as shown in Figure 3. In this case, the ionic liquid does not gel, or a gelation treatment is performed after injection.

[0100] The rest of the configuration is the same as in Figures 2A and 2B.

[0101] Figures 2A and 3 both show that, at least in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118 may be any liquid material.

[0102] Note that, as with Figure 2A, a clear boundary line may not be visible in the secondary battery 100 in Figure 3.

[0103] Figure 4A shows a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. Figure 4 shows a configuration having a negative electrode active material layer 104, as shown in Figure 1A. Of course, in Figure 4, the negative electrode active material layer 104 can be omitted, as shown in Figure 1B.

[0104] Unlike Figure 2A and the like, the secondary battery 100 shown in Figure 4A has a solid electrolyte 113 located only in the center of the electrolyte layer 103, with regions on the positive electrode side and the negative electrode side that do not contain the solid electrolyte 113. An electrolyte layer 103 having such a structure can be classified into a first electrolyte layer 103a, a second electrolyte layer 103b, and a third electrolyte layer 103c depending on the content ratio of the solid electrolyte 113. Such a structure is sometimes referred to as a laminated structure, and Figure 4A illustrates the case where the electrolyte layer 103 has a three-layer laminated structure.

[0105] The electrolyte layer 103 can be constructed in a stacked structure of two or more layers. Figure 4B shows a secondary battery 100 having an electrolyte layer 103 with a two-layer stacked structure.

[0106] In Figure 4A, the first electrolyte layer 103a located in the region without the solid electrolyte 113, and the third electrolyte layer 103c located in the region without the solid electrolyte 113, may use a gel-like ionic liquid or the like. In Figure 4B, the first electrolyte layer 103a, which does not have the solid electrolyte 113, may use a gel-like ionic liquid or the like.

[0107] The rest of the configuration is the same as in Figures 2A, 2B, and 3.

[0108] Figures 2A, 3, 4A, and 4B all show that, at least in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118 may be any liquid material.

[0109] Although Figures 4A and 4B show boundaries between each layer, as in Figure 2A, a clear boundary may not be visible in the secondary battery 100.

[0110] Next, we will describe each component of the secondary battery 100 shown in Figures 1 to 4.

[0111] <Current collector> The positive electrode current collector 101 and the negative electrode current collector 105 can be made of highly conductive materials such as stainless steel, gold, platinum, aluminum, copper, titanium, and alloys thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the positive electrode potential. Aluminum alloys with added elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. Alternatively, they may be formed from metallic elements that react with silicon to form silicides. Examples of metallic elements that react with silicon to form silicides include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in various shapes, such as foil, plate, sheet, mesh, perforated metal, or expanded metal. It may also have a layer of carbon black or graphene as an undercoat. The current collector should preferably have a thickness of 5 μm to 30 μm. The term "foil-like" refers to a material with a thickness of 1 μm to 100 μm, preferably 5 μm to 30 μm.

[0112] Furthermore, especially when using LiFSI (FSI is an abbreviation for bis(fluorosulfonyl)imide anion) as the lithium salt, it is preferable that the positive electrode current collector 101 and the negative electrode current collector 105 are made of materials that are resistant to corrosion by LiFSI. For example, titanium and titanium compounds are preferred because they are resistant to corrosion. Similarly, titanium, titanium compounds, or aluminum coated with carbon are also preferred.

[0113] <Active material> As the positive electrode active material 111 in the positive electrode layer 106, for example, a composite oxide having a layered rock salt type crystal structure, a spinel type crystal structure, or an olivine type crystal structure can be used. For example, composite oxides having lithium and a transition metal such as lithium cobalt oxide, lithium nickel oxide, lithium cobalt oxide in which part of the cobalt is substituted with manganese, lithium cobalt oxide in which part of the cobalt is substituted with nickel, nickel-manganese-cobalt oxide, lithium iron phosphate, lithium iron oxide, and lithium manganese oxide can be used. Furthermore, any material that functions as a positive electrode active material does not necessarily have to contain lithium, and materials such as V2O5, Cr2O5, and MnO2 may also be used.

[0114] Other positive electrode active materials will be discussed later.

[0115] As the negative electrode active material 117 in the negative electrode layer 107, elements capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium can be used. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc., can be used. Such elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Compounds containing these elements may also be used. For example, SiO(silicon monoxide) and SiO X It can also be expressed as (where x is preferably between 0.2 and 1.5), and examples include Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying and de-alloying reactions with lithium, and compounds containing such elements, are sometimes called alloying materials.

[0116] Silicon nanoparticles can be used as the negative electrode active material containing silicon. The median diameter (D50) of the silicon nanoparticles is 5 nm or more and less than 1 μm, preferably 10 nm or more and 300 nm or less, and more preferably 10 nm or more and 100 nm or less. The silicon nanoparticles may be crystalline. Furthermore, the silicon nanoparticles may have both crystalline and amorphous regions.

[0117] The silicon-containing negative electrode active material may be in the form of silicon monoxide particles containing one or more silicon crystal grains. The silicon monoxide may be amorphous. The silicon monoxide particles may be carbon-coated. These particles can be mixed with graphite to form the negative electrode active material.

[0118] Suitable carbon materials include graphite, easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon nanotubes, graphene, and carbon black. It is preferable to include fluorine in these carbon materials. Fluorine-containing carbon materials can also be called particulate or fibrous fluorinated carbon materials. When measuring carbon materials by X-ray photoelectron spectroscopy, the fluorine concentration is preferably 1 atomic% (sometimes denoted as at%) or more relative to the sum of the concentrations of fluorine, oxygen, lithium, and carbon.

[0119] Furthermore, while the negative electrode active material may undergo volume changes during charging and discharging, placing fluorine-containing organic compounds, such as fluorinated carbonate esters, between the negative electrode active materials improves their ability to slide smoothly even when volume changes occur during charging and discharging, thereby suppressing cracking and improving cycle characteristics. The presence of fluorine-containing organic compounds between multiple negative electrode active materials is crucial.

[0120] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spheroidal graphite having a spherical shape can be used as artificial graphite. For example, MCMB may have a spherical shape and is therefore preferable. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, which may also be preferable. Examples of natural graphite include flake graphite and spheroidized natural graphite.

[0121] When lithium ions are inserted into graphite (during the formation of lithium-graphite intercalation compounds), graphite exhibits a potential as low as that of lithium metal (0.05V to 0.3V vs. Li / Li). + This allows lithium-ion secondary batteries to exhibit a high operating voltage. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0122] Furthermore, titanium dioxide (TiO2) and lithium titanium oxide (Li4Ti5O2) are used as negative electrode active materials. 12 ), lithium-graphite intercalation compound (Li x Oxides such as C6, niobium pentoxide (Nb2O5), tungsten oxide (WO2), and molybdenum oxide (MoO2) can be used.

[0123] Furthermore, as the negative electrode active material, a Li3N type structure is used, which is a lithium and transition metal binitride. 3-x M x N(M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 The N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm²). 3 ) indicates a preference.

[0124] Using a lithium-transition metal complex nitride is preferable because it contains lithium ions in the negative electrode active material, allowing it to be combined with lithium-ion-free materials such as V2O5 and Cr3O8 as the positive electrode active material. Even when using a lithium-ion-containing material as the positive electrode active material, the lithium-transition metal complex nitride can be used as the negative electrode active material by pre-desorbing the lithium ions contained in the positive electrode active material.

[0125] Furthermore, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. The conversion reaction can also occur with oxides such as Fe2O3, CuO, Cu2O, RuO2, Cr2O3, and CoS 0.89 This can also occur with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorides such as FeF3 and BiF3.

[0126] Furthermore, lithium can be used as the negative electrode active material. When lithium is used as the negative electrode active material, foil-like lithium can be provided on the negative electrode current collector. Alternatively, lithium may be provided on the negative electrode current collector by vapor deposition or by vapor phase methods such as sputtering. In addition, lithium may be deposited on the negative electrode current collector by electrochemical methods in a solution containing lithium ions.

[0127] The conductive additive and binder that the negative electrode active material layer may contain can be the same materials as those used for the conductive additive and binder that the positive electrode active material layer may contain.

[0128] Furthermore, in addition to the same materials as the positive electrode current collector, copper and other materials can also be used as the current collector. It is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0129] Furthermore, as another form of negative electrode, a negative electrode without negative electrode active material can be used. In a secondary battery using a negative electrode without negative electrode active material, lithium is deposited on the negative electrode current collector during charging, and the lithium on the negative electrode current collector can be dissolved during discharge. Therefore, except in a completely discharged state, lithium is present on the negative electrode current collector.

[0130] When using a negative electrode without a negative electrode active material, a film may be provided on the negative electrode current collector to homogenize the deposition of lithium. As a film to homogenize the deposition of lithium, for example, a solid electrolyte having lithium ion conductivity can be used, and an electrolyte layer can be placed on the negative electrode current collector.

[0131] As solid electrolytes, sulfide-based solid electrolytes, oxide-based solid electrolytes, halide-based solid electrolytes, and polymer-based solid electrolytes can be used. Among these, polymer-based solid electrolytes are suitable as films for homogenizing lithium deposition because it is relatively easy to form a uniform film on the negative electrode current collector. Sulfide-based solid electrolytes, oxide-based solid electrolytes, halide-based solid electrolytes, and polymer-based solid electrolytes will be described later.

[0132] Furthermore, when using a negative electrode without negative electrode active material, a negative electrode current collector with irregularities can be used. When using a negative electrode current collector with irregularities, the recesses in the negative electrode current collector become cavities where lithium can easily be deposited, thus suppressing the formation of dendrite-like shapes when lithium is deposited.

[0133] <Solid electrolyte> A solid electrolyte is an example of a solid material used in an electrolyte layer or the like according to one aspect of the present invention. Solid electrolytes include oxide-based, sulfide-based, or halide-based materials, and a solid electrolyte obtained by mixing these may be used in the electrolyte layer or the like.

[0134] As an oxide-based solid electrolyte, a material having a perovskite-type crystal structure (La 2 / 3-x Li 3xMaterials having a NASICON-type crystal structure (Li 1+X Al X Ti 2-X (PO4)3 etc.) Materials having a garnet-type crystal structure (Li7La3Zr2O 12 (LLZO), or Li 6.25 La3Zr2Al 0.25 O 12 Materials having a LISICON-type crystal structure (Li) (e.g., LLZAO), 14 ZnGe4O 16 (etc.), oxide glass (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), or oxide crystallized glass (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 Examples of materials include (PO4)3, etc. Oxide-based solid electrolytes have advantages such as being heat-resistant and being more stable in air than sulfide-based solid electrolytes, which will be discussed later.

[0135] As a sulfide-based solid electrolyte, thiolysicone-based (Li 10 GeP2S 12 Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glass (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), or sulfide crystallized glass (Li7P3S 11 Li 3.25 P 0.95 Examples of materials include S4, etc. Sulfide-based solid electrolytes have advantages such as the availability of materials with high conductivity, the ability to be synthesized at low temperatures, and their relatively soft nature which helps maintain the conductive path even after charging and discharging.

[0136] Examples of halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, and LiI.

[0137] In one embodiment of the present invention, a mixed material in which a solid electrolyte is filled into the pores of porous aluminum oxide or porous silica can also be used as the solid material for the electrolyte layer, etc. That is, a mixture of a solid electrolyte and a ceramic material may be used for the electrolyte layer, etc.

[0138] While the use of a solid electrolyte was used to explain the solid nature of the material, any non-flowing material that can hold an ionic liquid is acceptable, and a polymer material may be used to hold the ionic liquid. A configuration in which a polymer material holds an ionic liquid is also sometimes called a semi-solid. In this case, the electrolyte layer 103 of the secondary battery 100 is sometimes called a semi-solid electrolyte layer.

[0139] Lithium-ion conductive polymers are used as polymer materials in the electrolyte layer and the like according to one embodiment of the present invention. Lithium-ion conductive polymers are sometimes referred to as polymeric solid electrolytes. Examples of lithium-ion conductive polymers that can be used include polyethylene oxide (PEO), derivatives having polyethylene oxide as the main chain, polypropylene oxide, polyacrylic acid esters, polymethacrylate esters, polysiloxanes, and polyphosphazenes.

[0140] In one embodiment of the present invention, a graphene compound or graphene may be mixed with the solid electrolyte described above. Graphene compounds have excellent physical properties, including high flexibility and high mechanical strength, and can therefore impart high flexibility and high mechanical strength to the solid electrolyte.

[0141] Graphene compounds include multilayer graphene, graphene oxide, multilayer graphene oxide, and multilayer graphene oxide. A graphene compound is defined as a material containing carbon atoms, having a plate-like or sheet-like shape, and possessing a two-dimensional structure formed by six-membered carbon rings. This two-dimensional structure formed by six-membered carbon rings is sometimes referred to as a carbon sheet. Graphene compounds may also contain functional groups. Furthermore, graphene compounds preferably have a bent shape. Additionally, graphene compounds may be rolled up to resemble carbon nanofibers.

[0142] Graphene oxide refers to a material that contains carbon and oxygen, has a sheet-like structure, and possesses functional groups, particularly epoxy groups, carboxyl groups, or hydroxyl groups.

[0143] <Ionic Liquid> An ionic liquid can be used as the liquid material in the electrolyte layer or the like in one embodiment of the present invention. Ionic liquids will be described below.

[0144] Ionic liquids, sometimes referred to as room-temperature molten salts, contain cations and anions. The cations include imidazolium, ammonium, pyrrolidinium, piperidinium, pyridinium, or phosphonium-based basic skeletons. Cations with an imidazolium-based skeleton can provide ionic liquids with lower viscosity compared to cations with an ammonium-based skeleton. Lower viscosity tends to increase the conductivity of carrier ions. Furthermore, the viscosity of the ionic liquid can be controlled by the alkyl group in the side chain of the cation.

[0145] <General formula for cations> A cation of an ionic liquid according to one aspect of the present invention will be described.

[0146] An ionic liquid according to one aspect of the present invention has an imidazolium-based cation represented by the general formula (G1).

[0147] [ka]

[0148] In the above general formula (G1), R 1 R represents an alkyl group with 1 to 10 carbon atoms. 2 ~R 4 Each of these independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and R 5A represents an alkyl group having 1 to 6 carbon atoms, or an ether group, thioether group, or siloxane having a main chain composed of two or more atoms selected from C, O, Si, N, S, and P. In the above general formula (G1), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0149] An ionic liquid according to one aspect of the present invention has a pyridinium-based cation represented by the general formula (G2).

[0150] [ka]

[0151] In the above general formula (G2), R 6 It has a main chain composed of an alkyl group with 1 to 6 carbon atoms, or two or more atoms selected from C, O, Si, N, S, and P. 7 ~R 11 Each of these independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 8 or R 9 A can represent a hydroxyl group. In the above general formula (G2), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0152] An ionic liquid according to one aspect of the present invention may have a quaternary ammonium cation, for example, a quaternary ammonium cation represented by the general formula (G3).

[0153] [ka]

[0154] In the above general formula (G3), R 28 ~R 31 Each of these independently represents an alkyl group having 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethyl group, or a hydrogen atom. In the above general formula (G3), A -This represents an anion, and is preferably FSI or TFSI as described later.

[0155] An ionic liquid according to one aspect of the present invention has a cation represented by the general formula (G4).

[0156] [ka]

[0157] In the above general formula (G4), R 12 and R 17 Each of these independently represents an alkyl group with 1 to 3 carbon atoms. 13 ~R 16 Each of these independently represents either a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. In the above general formula (G4), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0158] An ionic liquid according to one aspect of the present invention has a cation represented by the general formula (G5).

[0159] [ka]

[0160] In the above general formula (G5), R 18 and R 24 Each of these independently represents an alkyl group with 1 to 3 carbon atoms. 19 ~R 23 Each of these independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. In the above general formula (G5), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0161] An ionic liquid according to one aspect of the present invention has a cation represented by the general formula (G6).

[0162] [ka]

[0163] In the above general formula (G6), n and m are 1 to 3, α is 0 to 6, β is 0 to 6, and X or Y represents a linear or side-chain alkyl group having 1 to 4 carbon atoms as a substituent, a linear or side-chain alkoxy group having 1 to 4 carbon atoms, or a linear or side-chain alkoxyalkyl group having 1 to 4 carbon atoms. In the above general formula (G6), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0164] An ionic liquid according to one aspect of the present invention has a tertiary sulfonium cation represented by the general formula (G7).

[0165] [ka]

[0166] In the above general formula (G7), R 25 ~R 27 Each of these independently represents a hydrogen atom, an alkyl group with 1 to 4 carbon atoms, or a phenyl group. 25 ~R 27 Each has a main chain composed of two or more atoms independently selected from C, O, Si, N, S, and P. In general formula (G7), A - This represents an anion, and is preferably FSI or TFSI as described later.

[0167] An ionic liquid according to one aspect of the present invention has a quaternary phosphonium cation represented by the following general formula (G8).

[0168] [ka]

[0169] In the above general formula (G8), R 32 ~R 35Each independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. Also, R 32 to R 35 each independently has a main chain composed of two or more atoms selected from the atoms of C, O, Si, N, S, and P. In general formula (G8), A - represents an anion, and is preferably FSI or TFSI described later.

[0170] <Cation> Specific examples of the cation of the above general formula (G1) include, for example, structural formulas (111) to structural formula (174). Structural formula (111) is 1-ethyl-3-methylimidazolium cation, and the abbreviation is EMI. Structural formula (113) is 1-butyl-3-methylimidazolium cation, and the abbreviation is BMI.

[0171] [Chemical formula]

[0172] [Chemical formula]

[0173] [Chemical formula]

[0174] [[ID=...]] [Chemical formula]

[0175] [Chemical formula]<00..0901>

[0176] [Chemical formula]

[0177] Specific examples of the cation of the general formula (G2) include, for example, Structural Formulas (701) to Structural Formula (719).

[0178]

Chemical Formula

[0179]

Chemical Formula

[0180] Specific examples of the cation of the general formula (G4) include, for example, Structural Formulas (501) to Structural Formula (520).

[0181]

Chemical Formula

[0182] Specific examples of the cation of the general formula (G5) include, for example, Structural Formulas (601) to Structural Formula (630).

[0183]

Chemical Formula

[0184]

Chemical Formula

[0185] Specific examples of the cation of the general formula (G6) include, for example, Structural Formulas (301) to Structural Formula (309), and Structural Formulas (401) to Structural Formula (419).

[0186]

Chemical Formula

[0187]

Chemical Formula

[0188] Furthermore, structural formulas (301) to (309) and structural formulas (401) to (419) show examples where m is 1 in general formula (G6), but in structural formulas (301) to (309) and structural formulas (401) to (419), m may be replaced with 2 or 3.

[0189] Furthermore, specific examples of the cation of the general formula (G7) mentioned above include structural formulas (201) to (215).

[0190] [ka]

[0191] <Anion> An anion in an ionic liquid according to one aspect of the present invention will be described. Examples of anions include halide ions, tetrafluoroborates, hexafluorophosphates, bis(trifluoromethylsulfonyl)amides, or bis(fluorosulfonyl)imides.

[0192] Specific anions that can be used include one or more selected from monovalent amide anions, monovalent methide anions, fluorosulfonic acid anions, perfluoroalkyl sulfonic acid anions, tetrafluoroborate anions, perfluoroalkyl borate anions, hexafluorophosphate anions, perfluoroalkyl phosphate anions, and tetrafluoroborate anions.

[0193] Monovalent amide anions have the general formula (C n F 2n+1 SO2)2N - (where n is between 0 and 3)

[0194] When n is 0, the above general formula is called a bis(fluorosulfonyl)imide anion and is represented by the following structural formula (H11). The abbreviation for bis(fluorosulfonyl)imide anion is FSI or FSA.

[0195] [ka]

[0196] When n is 1, the above general formula is called the bis(trifluoromethanesulfonyl)imide anion and is represented by the following structural formula (H12). The abbreviation for the bis(trifluoromethanesulfonyl)imide anion is TFSI or TFSA.

[0197] [ka]

[0198] Another monovalent cyclic amide anion is called 4,4,5,5-tetrafluoro-1,3,2-dithiazolidinetetraoxide anion, which is represented by the following structural formula (H13).

[0199] [ka]

[0200] Monovalent methide anions have the general formula (C n F 2n+1 SO2)3C - (where n is between 0 and 3)

[0201] One of the monovalent cyclic methide anions is called 4,4,5,5-tetrafluoro-2-[(trifluoromethyl)sulfonyl]-1,3-dithiolanetetraoxide anion, which is represented by the following structural formula (H14).

[0202] [ka]

[0203] Fluoroalkyl sulfonate anions have the general formula (C m F 2m+1 SO3) -It is represented by (m is 0 or more and 4 or less).

[0204] When m is 0, the above general formula is a fluorosulfonic acid anion, and when m is 1, 2, 3, or 4, the above general formula is a perfluoroalkylsulfonic acid anion.

[0205] The fluoroalkyl borate anion is represented by the general formula {BF n (C m H k F 2m+1-k ) 4-n}} - (n is 0 or more and 3 or less, m is 1 or more and 4 or less, k is 0 or more and 2m or less).

[0206] The fluoroalkyl phosphate anion is represented by the general formula {PF n (C m H k F 2m+1-k ) 6-n}} - (n is 0 or more and 5 or less, m is 1 or more and 4 or less, k is 0 or more and 2m or less).

[0207] The ionic liquid of one aspect of the present invention can have one or more selected from the above-mentioned anions.

[0208] Since such an ionic liquid is a liquid composed only of ions, it has strong electrostatic interactions, shows non-volatility and thermal stability, and has high heat resistance. The secondary battery using the ionic liquid does not catch fire within the temperature range of use and is excellent in safety.

[0209] <Organic solvent> An organic solvent can be used as the liquid material used for the electrolyte layer and the like of one aspect of the present invention. It is preferable to use a mixed material of an organic solvent and an ionic liquid as the liquid material used for the electrolyte layer and the like of one aspect of the present invention. The organic solvent will be described.

[0210] In one embodiment of the present invention, an aprotic organic solvent may be used as the organic solvent. For example, one or more selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc. can be used.

[0211] Furthermore, the organic solvent may include fluorinated carbonates or cyclic carbonates. An example of a fluorinated carbonate is a fluorinated cyclic carbonate. Fluorinated cyclic carbonates have a high flash point, which can enhance the safety of secondary batteries.

[0212] As the fluorinated cyclic carbonate, fluorinated ethylene carbonates such as monofluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), or tetrafluoroethylene carbonate (F4EC) can be used. Note that DFEC has isomers such as cis-4,5 and trans-4,5.

[0213] One of the fluorinated cyclic carbonates according to one aspect of the present invention is monofluoroethylene carbonate, abbreviated as FEC.

[0214] One of the fluorinated cyclic carbonates according to one aspect of the present invention is tetrafluoroethylene carbonate, abbreviated as F4EC.

[0215] One of the fluorinated cyclic carbonates according to one aspect of the present invention is difluoroethylene carbonate, abbreviated as F2EC.

[0216] Although fluorinated cyclic carbonates have been described, the organic solvent in one embodiment of the present invention can also be a cyclic carbonate having a cyano group.

[0217] <Gelling agent> The ionic liquid or organic solvent described above may be gelled. Gelation can suppress leaching from the electrolyte layer 103. The gelling agent can be selected according to methods such as chemical gelling treatment or physical gelling treatment. The gelling agent used in chemical gelling treatment preferably contains a polymer and a crosslinking agent.

[0218] The gelling agent is added to the ionic liquid or organic solvent described above and mixed. At this time, the mixture is heated at a temperature of 75°C to 100°C, preferably 85°C to 95°C. This yields a gelled ionic liquid or gelled organic solvent.

[0219] As a specific gelling agent, poly(dimethylaminoethyl methacrylate) can be used as the polymer, and N,N,N',N'-tetra(trifluoromethanesulfonyl)-dodecane-1,12-diamine can be used as the crosslinking agent. The crosslinking agent causes the polymer to have a crosslinked structure, and the gel state is formed when an ionic liquid or organic solvent is retained in this crosslinked structure.

[0220] <Lithium salts> In one embodiment of the present invention, the lithium salt used in the electrolyte layer and the like is preferably a lithium salt containing a halogen. Furthermore, it is preferably a fluoride-containing lithium imide salt. Examples of fluoride-containing lithium imide salts that can be used include Li(CF3SO2)2N (hereinafter sometimes referred to as "LiTFSI" or "LiTFSA"), Li(C2F5SO2)2N (hereinafter sometimes referred to as "LiBETI"), or Li(SO2F)2N (hereinafter sometimes referred to as "LiFSI" or "LiFSA").

[0221] Other lithium salts containing halogens, such as LiPF6, LiBF4, and LiClO4, can also be used.

[0222] Furthermore, LiBOB may be used as a halogen-free lithium salt.

[0223] These lithium salts may be used individually or in combination with those described above.

[0224] <Exterior> The exterior of a secondary battery according to one aspect of the present invention will now be described. As the exterior of the secondary battery, metal materials such as aluminum or resin materials can be used. Resin materials include rubber materials. Rubber includes natural rubber and synthetic rubber. Synthetic rubbers include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer.

[0225] Furthermore, the outer casing of the secondary battery is preferably in the form of a film. Materials that enable a film shape for the outer casing include, for example, polyethylene, polypropylene, polycarbonate, ionomer, and polyamide. Alternatively, a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel may be used for the outer casing.

[0226] Furthermore, the outer casing that enables the film shape may have a laminated structure. The first layer may have a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and the second layer may have a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel.

[0227] Furthermore, the outer surface of the exterior body may be provided with an insulating synthetic resin film such as a polyamide resin or polyester resin. By applying this outer surface configuration to the laminated structure of the first and second layers described above, a three-layer film can be used.

[0228] A secondary battery having an electrolyte layer according to one aspect of the present invention is preferred because it is easily bendable. The outer casing using the insulating synthetic resin film described above is suitable for a bent secondary battery, or a secondary battery that changes between a bent and an extended state. Since the ionic liquid is held in the solid electrolyte, leakage is suppressed even when the secondary battery is bent. Furthermore, even if the ionic liquid does leak out, the outer casing described above, especially an outer casing having a laminated structure, can suppress the leakage of the ionic liquid from the secondary battery.

[0229] <Binder> Although not shown in Figures 1 to 4, the positive electrode layer 106 and the negative electrode layer 107 may have a binder. Furthermore, the electrolyte layer 103 may also have a binder. Preferably, the binder is a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer. Fluororubber can also be used as a binder.

[0230] Furthermore, it is preferable to use a water-soluble polymer as the binder. Examples of water-soluble polymers include polysaccharides. Examples of polysaccharides include cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, diacetylcellulose, and regenerated cellulose, as well as starch. It is even preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0231] Alternatively, it is preferable to use one or more materials selected from polystyrene, polyvinyl butyral (PVB), methyl polyacrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, nitrocellulose, etc., as the binder.

[0232] You may use a combination of several of the binders mentioned above.

[0233] <Conductive materials (conductive additives)> Although not shown in Figures 1 to 4, the positive electrode layer 106 and the negative electrode layer 107 may contain a conductive additive. As the conductive additive, carbon materials such as acetylene black (AB), carbon nanotubes, graphene, and fullerene can be used.

[0234] Graphene is in the form of thin flakes and possesses excellent electrical properties, including high conductivity, as well as excellent physical properties, including mechanical strength. Therefore, by using graphene as a conductive additive, the contact points or contact area between active materials can be increased.

[0235] Graphene includes monolayer graphene or multilayer graphene consisting of 2 to 100 layers. Monolayer graphene refers to a sheet of carbon molecules with one atomic layer containing π bonds.

[0236] <Manufacturing Process 1> An example of the manufacturing process for the secondary battery 100 shown in Figures 1 to 4 will be explained. The ionic liquid used in explaining the manufacturing process example may contain a lithium salt. An ionic liquid containing a lithium salt may be referred to as a lithium liquid electrolyte or lithium ion electrolyte.

[0237] As shown in Figure 5A, a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a slurry containing a dispersion medium, positive electrode active material 111, and solid electrolyte 113 onto the positive electrode current collector 101. When the dispersion medium and other components are removed from the slurry, it becomes the positive electrode active material layer 102. If the firing process is not performed, the solid electrolyte 113 will be in particulate form, and if the firing process is performed, the solid electrolyte 113 may form a sintered body. In Figure 5A, the positive electrode layer 106 has particulate solid electrolyte 113.

[0238] As shown in Figure 5B, an electrolyte layer 103 is prepared. The electrolyte layer 103 contains a solid electrolyte 113 and an ionic liquid 118. It is preferable to sinter the solid electrolyte 113 through a firing process to make it easier to retain the ionic liquid 118. Furthermore, it is preferable to use an electrolyte layer 103 that has been processed into a sheet shape. This is sometimes referred to as a sheet-shaped electrolyte layer. The sheet-shaped electrolyte layer is placed on the positive electrode layer 106 and a pressing process is carried out. The pressing process may also be carried out after the negative electrode layer 107, which will be described later, is placed on the electrolyte layer 103. Heat may be applied during the pressing process. Figure 5B shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0239] As shown in Figure 5C, a negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, and a solid electrolyte 113 onto the negative electrode current collector 105. When the dispersion medium and other components are removed from the slurry, it becomes the negative electrode active material layer 104. In Figure 5C, the negative electrode layer 107 has particulate solid electrolyte 113.

[0240] The negative electrode layer 107 is placed on the electrolyte layer 103, and a pressing process is carried out. Heat may be applied during the pressing process. Figure 5C shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0241] The pressing process described in Figure 5C can be combined with the pressing process described in Figure 5B, therefore the pressing process described in Figure 5B can be omitted.

[0242] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 in the electrolyte layer 103 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118 may be any liquid material.

[0243] <Manufacturing Process 2> This section describes an example of a manufacturing process for a secondary battery 100 that differs from manufacturing process 1.

[0244] As shown in Figure 6A, a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a slurry containing a dispersion medium, positive electrode active material 111, and solid electrolyte 113 onto the positive electrode current collector 101. When the dispersion medium and other components are removed from the slurry, it becomes the positive electrode active material layer 102. If the firing process is not performed, the solid electrolyte 113 will be in particulate form, and if the firing process is performed, the solid electrolyte 113 may form a sintered body. In Figure 6A, the positive electrode layer 106 has particulate solid electrolyte 113.

[0245] As shown in Figure 6B, an electrolyte layer 103 is prepared. At this stage, the electrolyte layer 103 has a solid electrolyte 113 and is processed into a sheet shape. It is preferable to sinter the solid electrolyte 113 through a firing process to make it easier to hold the ionic liquid 118, which will be described later. The sheet-shaped electrolyte layer is placed on the positive electrode layer 106 and a pressing process is carried out. The pressing process may also be carried out after the negative electrode layer 107, which will be described later, is placed on the electrolyte layer 103. Heat may be applied during the pressing process. Figure 6B shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0246] As shown in Figure 6C, a negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, and a solid electrolyte 113 onto the negative electrode current collector 105. When the dispersion medium and other components are removed from the slurry, it becomes the negative electrode active material layer 104. In Figure 6C, the negative electrode layer 107 has particulate solid electrolyte 113.

[0247] The negative electrode layer 107 is placed on the electrolyte layer 103, and a pressing process is carried out. Heat may be applied during the pressing process. Figure 6C shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0248] The pressing process described in Figure 6C can be combined with the pressing process described in Figure 6B, therefore the pressing process described in Figure 6B can be omitted.

[0249] Subsequently, as shown in Figure 6D, the ionic liquid 118 is injected. The injection of the ionic liquid 118 should be performed under a vacuum or reduced pressure atmosphere.

[0250] A gelation treatment may be performed on the injected ionic liquid 118. If heating is used for the gelation treatment, the heating performed in the pressing process described above should be used. That is, the gelation treatment may be carried out while pressing.

[0251] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 in the electrolyte layer 103 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118 may be any liquid material.

[0252] <Manufacturing Process 3> An example of a secondary battery 100 manufacturing process different from manufacturing process 1 and manufacturing process 2 will be described.

[0253] As shown in Figure 7A, a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a slurry containing a dispersion medium, positive electrode active material 111, and solid electrolyte 113 onto the positive electrode current collector 101. When the dispersion medium and other components are removed from the slurry, it becomes the positive electrode active material layer 102. If the firing process is not performed, the solid electrolyte 113 will be in particulate form, and if the firing process is performed, the solid electrolyte 113 may form a sintered body. In Figure 7A, the positive electrode layer 106 has particulate solid electrolyte 113.

[0254] As shown in Figure 7B, a first electrolyte layer 103a is prepared. The first electrolyte layer 103a is a layer having a gelled ionic liquid 118a, and preferably does not have a solid electrolyte. The first electrolyte layer 103a is placed on the positive electrode layer 106. If the first electrolyte layer 103a is a layer having a gelled ionic liquid, it may be tacky, and the pressing process may be unnecessary. Of course, the pressing process may also be performed. The pressing process may be performed after lamination up to the third electrolyte layer 103c, which will be described later, or after the negative electrode layer 107 is placed on the third electrolyte layer 103c. Heat may be applied during the pressing process. Figure 7B shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0255] As shown in Figure 7C, a second electrolyte layer 103b is prepared. The second electrolyte layer 103b has a solid electrolyte 113, and a sheet-like electrolyte layer is preferable. By sintering the solid electrolyte 113 through a firing process, it becomes easier to process it into a sheet. Alternatively, the firing process may be performed when processing into a sheet to create a sintered body. The sheet-like electrolyte layer is placed on the first electrolyte layer 103a. If the first electrolyte layer 103a is a layer having a gelled ionic liquid, it may be tacky, and the pressing process may be unnecessary. Of course, the pressing process may be performed, and heat may be applied during the pressing process. Figure 7C shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0256] Furthermore, as shown in Figure 7C, a third electrolyte layer 103c is prepared. The third electrolyte layer 103c can be the same as the first electrolyte layer 103a, and it is preferable to use a layer containing gelled ionic liquid 118b. The third electrolyte layer 103c is placed on the second electrolyte layer 103b. If the third electrolyte layer 103c is a layer containing gelled ionic liquid, it may be tacky, and the pressing process may be unnecessary. Of course, a pressing process may be performed, and heat may be applied during the pressing process. Figure 7C shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process.

[0257] As shown in Figure 7D, a negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, and a solid electrolyte 113 onto the negative electrode current collector 105. When the dispersion medium and other components are removed from the slurry, it becomes the negative electrode active material layer 104. In Figure 7D, the negative electrode layer 107 has particulate solid electrolyte 113.

[0258] The negative electrode layer 107 is placed on the third electrolyte layer 103c, and a pressing process is carried out. Heat may be applied during the pressing process. Figure 7D shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process. Also, as shown in Figure 7D, a portion of the ionic liquid 118a may be impregnated into the voids of the solid electrolyte 113 in the second electrolyte layer 103b.

[0259] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 in the electrolyte layer 103 holds the ionic liquid 118a or ionic liquid 118b. In other words, at least in the electrolyte layer 103, the ionic liquid 118a or ionic liquid 118b is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a or ionic liquid 118b does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118a or ionic liquid 118b may be any liquid material.

[0260] <Manufacturing Process 4> An example of a secondary battery 100 manufacturing process different from manufacturing processes 1 to 3 will be explained.

[0261] As shown in Figure 8A, following the secondary battery manufacturing process 3 described above, a battery is prepared with the layers stacked up to the second electrolyte layer 103b.

[0262] As shown in Figure 8B, a negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, and a solid electrolyte 113 onto the negative electrode current collector 105. The dispersion medium and other components are removed from the slurry to form the negative electrode active material layer 104. In Figure 8B, the negative electrode layer 107 has particulate solid electrolyte 113.

[0263] The negative electrode layer 107 is placed on the second electrolyte layer 103b, and a pressing process is carried out. Heat may be applied during the pressing process. Figure 8B shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process. Furthermore, by the process shown in Figure 8B, etc., a portion of the ionic liquid 118a can be impregnated into the voids of the solid electrolyte 113 in the second electrolyte layer 103b.

[0264] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 in the second electrolyte layer 103b holds a portion of the ionic liquid 118a. In other words, at least in the electrolyte layer 103, a portion of the ionic liquid 118a is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118 may be any liquid material.

[0265] <Manufacturing Process 5> An example of a secondary battery 100 manufacturing process different from manufacturing processes 1 to 4 will be described.

[0266] As shown in Figure 9A, structure A is prepared by stacking the second electrolyte layer 103b according to the secondary battery manufacturing process 3 described above. Also, as shown in Figure 9A, structure B is prepared by stacking the negative electrode layer 107 and the third electrolyte layer 103c. Structures A and B are then bonded together as indicated by the white arrows.

[0267] As shown in Figure 9B, a pressing process is performed after bonding. Heat may be applied during the pressing process. Figure 9B shows the boundaries of each layer, but clear boundaries may not be visible after the pressing process. Furthermore, through the process shown in Figure 9B, etc., a portion of the ionic liquid 118a or a portion of the ionic liquid 118b can be impregnated into the voids of the solid electrolyte 113 in the first electrolyte layer 103a.

[0268] In the secondary battery obtained through this manufacturing process, the electrolyte layer 103 contains a solid electrolyte 113 that holds a portion of the ionic liquid 118a and a portion of the ionic liquid 118b. In other words, at least in the electrolyte layer 103, a portion of the ionic liquid 118a and a portion of the ionic liquid 118b are impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a or ionic liquid 118b does not seep out. This electrolyte layer 103 is sometimes referred to as a semi-solid electrolyte layer. As mentioned above, the solid electrolyte 113 may be any solid material, and the ionic liquid 118a or ionic liquid 118b may be any liquid material.

[0269] This embodiment can be used in combination with other embodiments.

[0270] (Embodiment 2) The above-described manufacturing process is preferably carried out continuously using a roll-to-roll manufacturing apparatus. The roll-to-roll method can be applied to manufacturing processes 1 to 5 described above, but in this embodiment, the manufacturing process shown in manufacturing process 5 will be explained using the manufacturing apparatus shown in Figure 10.

[0271] Using the manufacturing apparatus shown in Figure 10, the following steps can be performed: a step 310 of applying slurry onto the positive electrode current collector 101; a step 320 of drying the slurry to form a positive electrode active material layer 102; a step 330 of superimposing an electrolyte layer 103 onto the positive electrode active material layer 102; and a step 340 of passing the positive electrode current collector 101 with the positive electrode active material layer 102 formed on it, together with the electrolyte layer 103, between a pair of pressure rolls (a first pressure roll 325 and a second pressure roll 326).

[0272] The above process 310 will now be explained. As shown in Figure 10, the manufacturing apparatus has a feeding mechanism 311 (also called an unwinder), and a first bobbin 312 on which the positive electrode current collector 101 is wound is placed in the feeding mechanism 311. The positive electrode current collector 101 is moved by utilizing the rotation of the roller 313, and the slurry is applied to one surface of the positive electrode current collector 101 by the first slurry application means 314a. The slurry contains at least a dispersant, a positive electrode active material, and a solid electrolyte. The rollers 313 are paired and can also press as the material passes between them.

[0273] As the first slurry coating means 314a, for example, a slot die coater, lip coater, blade coater, reverse coater, gravure coater, etc., can be used. Depending on the type of coater used, the number of rollers for reversing the positive electrode current collector 101 may be increased. In addition, a dip method or a spray method can be used as the first slurry coating means 314a. Furthermore, depending on the material used, the slurry may be applied while heating the first slurry coating means 314a. It is preferable that the slurry be applied while heated.

[0274] In step 320, the slurry applied to the positive electrode current collector 101 is dried in a heating chamber 321a having an air intake port 322, an exhaust port 323, and a drying means 324. By drying the slurry, a positive electrode active material layer 102 can be formed on the positive electrode current collector 101. The air intake port 322 and exhaust port 323 are preferably installed on the ceiling (also referred to as the top surface) of the heating chamber 321a, but they may also be installed on the walls (also referred to as the sides) or floor (also referred to as the bottom surface) of the heating chamber 321a. As the drying means 324, one or more methods selected from hot air heating, lamp heating, induction heating, blowing air, etc., can be used.

[0275] Step 320 shows an example where the slurry is allowed to cool naturally after drying, and no cooling means are provided. However, a cooling means may be provided in or near the heating chamber 321a for forced cooling.

[0276] In step 330, the first electrolyte layer 103a and the second electrolyte layer 103b are formed on the positive electrode active material layer 102 by the second slurry adhesion means 314b. It is preferable to prepare the first electrolyte layer 103a and the second electrolyte layer 103b as a laminate before forming them on the positive electrode active material layer 102. Alternatively, multiple adhesion means corresponding to the second slurry adhesion means 314b may be installed to form the first electrolyte layer 103a and then the second electrolyte layer 103b on the positive electrode active material layer 102 in that order.

[0277] In parallel with the processing of the positive electrode current collector 101, the negative electrode current collector 105 is also processed. The second bobbin 405 on which the negative electrode current collector 105 is wound is placed in the feeding mechanism 315, and the slurry is applied to one side of the negative electrode current collector 105 by the third slurry application means 314c using the rotation of the roller 316. The slurry contains at least a dispersion medium, a negative electrode active material, and a solid electrolyte. The rollers 316 are paired and can also press as the material passes between them.

[0278] As the third slurry coating means 314c, for example, a slot die coater, lip coater, blade coater, reverse coater, or gravure coater can be used. Depending on the type of coater used, the number of rollers for reversing the negative electrode current collector 105 may be increased. Alternatively, a dip method or spray method can be used as the third slurry coating means 314c. Depending on the material used, the slurry may be applied while the third slurry coating means 314c is heated. It is preferable that the slurry be applied while heated.

[0279] Next, the slurry applied to the negative electrode current collector 105 is dried in the heating chamber 321b. The heating chamber 321b may have the same configuration as the heating chamber 321a. By drying the slurry, a negative electrode active material layer 104 can be formed on the negative electrode current collector 105. After drying the slurry, it may be allowed to cool naturally, or it may be forcibly cooled by installing a cooling means in or near the heating chamber 321b.

[0280] Next, the slurry is applied to the negative electrode active material layer 104 by the fourth slurry adhesion means 314d and passed through the heating chamber 321c to form the third electrolyte layer 103c. After passing through the roller 406, the process proceeds to step 340. The heating chamber 321c may have the same configuration as the heating chamber 321a.

[0281] As the fourth slurry coating means 314d, for example, a slot die coater, lip coater, blade coater, reverse coater, or gravure coater can be used. Depending on the type of coater used, the number of rollers for reversing the negative electrode current collector 105 may be increased. Alternatively, a dip method or spray method can be used as the fourth slurry coating means 314d. Depending on the material used, the slurry may be applied while heating the fourth slurry coating means 314d. It is preferable that the slurry be applied while heated.

[0282] In step 340, the positive electrode current collector 101 is pressed against the negative electrode current collector 105 by utilizing the rotation of a pair of pressure rolls (first pressure roll 325, second pressure roll 326). Heating may be performed during pressing. This step may cause the ionic liquid, etc., contained in the electrolyte layer to temporarily melt (become a sol). The melted ionic liquid, etc., can impregnate the adjacent positive electrode layer or negative electrode layer.

[0283] Finally, the laminate is wound onto a second bobbin 328 installed in the winding mechanism 327 (also called a winder). Then, it is cut into the desired shape by a cutting means such as a laser cutter or cutter (not shown).

[0284] Furthermore, although Figure 10 shows an example of winding up the laminate, it may also be cut into the desired shape using a cutting method such as laser cutting or a cutter, which are not shown in the figure, instead of winding it up.

[0285] By following the above steps, a secondary battery can be manufactured.

[0286] This embodiment can be implemented in appropriate combination with other embodiments.

[0287] (Embodiment 3) This embodiment describes a process for processing an electrolyte layer according to one aspect of the present invention into a sheet. The electrolyte layer processed into a sheet is preferable because it is easy to handle and improves productivity.

[0288] As shown in step S50 of Figure 11, prepare the electrolyte source, binder, plasticizer, and solvent. Prepare LLZAO powder as the electrolyte source. Prepare polyvinyl butyral (PVB) as the binder. Prepare dioctyl phthalate (DOP) as the plasticizer. Prepare N-methyl-2-pyrrolidone (NMP) as the solvent.

[0289] In addition to PVB, the aforementioned materials, such as polyvinyl alcohol (PVA), may be used as a binder, and acrylic resin may also be used as a binder. In addition to DOP, phthalate esters may be used as plasticizers, and it is preferable to use one or more selected from, for example, dimethyl phthalate (DMP), diethyl phthalate (DEP), or dibutyl phthalate (DBP). In addition to NMP, one or more selected from, for example, water, dimethylformamide (DMF), may be used as a solvent.

[0290] As shown in step S52 of Figure 11, the materials described above are mixed to obtain a slurry as shown in step S54. Before the mixing shown in step S52, the materials shown in step S50 may be mixed independently. Step S52 can be performed using, for example, a rotary-orbit mixer. The rotation speed can be 1000 rpm or more and 3000 rpm or less. The rotation time can be 1 minute or more and 10 minutes or less. The mixing using the above mixer may be performed two or more times instead of just once.

[0291] As shown in step S54 of Figure 11, the slurry is applied to the coating substrate. For the coating substrate, a material that allows the sheet-like electrolyte layer to peel off easily, such as a silicone substrate, is preferable. Additionally, a release agent may be applied to the surface of the coating substrate to facilitate peeling.

[0292] As shown in step S55 of Figure 11, the slurry is dried using a drying oven or the like. The drying temperature should be between 25°C and 200°C, preferably between 45°C and 85°C. Drying removes the solvent and other substances present in the slurry.

[0293] As shown in step S56 of Figure 11, the electrolyte layer sheet is peeled off the coating substrate. Step S56 can also be called separating the electrolyte layer sheet from the coating substrate. It is preferable to peel off the unpressed electrolyte layer, i.e., the electrolyte layer before pressing, from the coating substrate. The unpressed electrolyte layer is sometimes referred to as the unpressed electrolyte layer.

[0294] As shown in step S58 of Figure 11, pressing is performed after drying. A roll press can be used for pressing. The gap of the roll press is set to, for example, 50% to 70% of the thickness of the unpressed electrolyte layer. For example, if the thickness of the unpressed electrolyte layer is 140 μm, the gap of the roll press is set to 60 μm to 100 μm, preferably 70 μm to 85 μm.

[0295] As shown in step S59 of Figure 11, a first sheet-like electrolyte layer can be obtained. The first sheet-like electrolyte layer should have a film thickness of 100 μm to 150 μm, preferably 120 μm to 140 μm. Voids can be confirmed in the first sheet-like electrolyte layer from SEM (scanning electron microscope) observation images, etc. Also, from SEM observation images, etc., it can be confirmed that the LLZAO powder, which is a solid electrolyte, is connected to each other via a binder in the first sheet-like electrolyte layer.

[0296] As shown in step S60 of Figure 11, the first sheet-like electrolyte layer is heated. The heating temperature is 1000°C to 1300°C, preferably 1100°C to 1250°C. The heating atmosphere is preferably an oxygen-containing atmosphere, but an atmosphere containing oxygen and an inert gas, or an atmosphere containing only an inert gas, may also be used.

[0297] Figures 12A and 12B show the state of the first sheet-like electrolyte layer during heating. The first sheet-like electrolyte layer 125 is heated in a circular punched-out state. Figure 12A is a schematic top view in which the first sheet-like electrolyte layer 125 is placed on an alumina substrate 126. Between the alumina substrate 126 and the first sheet-like electrolyte layer 125 is a region 128 in which LLZAO powder is scattered. It is advisable to scatter LLZAO powder to suppress adhesion between the alumina substrate 126 and the first sheet-like electrolyte layer 125.

[0298] Figure 12B is a schematic cross-sectional view in which a region 128, where LLZAO powder is scattered, can be seen between the alumina substrate 126 and the first sheet-like electrolyte layer 125. A substrate 129 opposite the alumina substrate 126 is placed to cover it using a gap-holding material 130. It is preferable to use an alumina substrate for substrate 129 as well. Furthermore, it is preferable to scatter LLZAO powder on the upper surface of the first sheet-like electrolyte layer 125, and this scattered region is designated as region 128b.

[0299] As shown in step S61 of Figure 11, a second sheet-like electrolyte layer is obtained. Because the second sheet-like electrolyte layer undergoes a heating process, it may have shrunk compared to the first sheet-like electrolyte layer. For example, if the first sheet-like electrolyte layer is punched out in a circular shape with a diameter of 12 mm, the second sheet-like electrolyte layer will shrink to a circular shape with a diameter of 10 mm. The second sheet-like electrolyte layer preferably has a film thickness of 80 μm to 120 μm, more preferably 90 μm to 110 μm, and the film thickness is also reduced compared to the first sheet-like electrolyte layer.

[0300] SEM observations of the second sheet-like electrolyte layer revealed that the solid electrolyte LLZAO had become a sintered body, and voids were observed in the second sheet-like electrolyte layer. Furthermore, SEM observations sometimes showed no binder in the second sheet-like electrolyte layer. For example, it is thought that the binder is removed by heating in step S60.

[0301] The second sheet-like electrolyte layer obtained in this way can be used as the solid material for the electrolyte layer 103 shown in the above embodiment.

[0302] This embodiment can be used in combination with other embodiments.

[0303] (Embodiment 4) This embodiment describes a positive electrode active material that can be used in a secondary battery according to one aspect of the present invention, and a method for producing the same.

[0304] [Cathode active material] Figures 13A and 13B are cross-sectional views of a positive electrode active material 200 that can be used in a secondary battery according to one embodiment of the present invention. Figures 13C and 13D show enlarged views of the area around AB in Figure 13A. Figures 13E and 13F show enlarged views of the area around CD in Figure 13A.

[0305] As shown in Figures 13A to 13F, the positive electrode active material 200 has a surface layer 200a and an interior layer 200b. In these figures, the boundary between the surface layer 200a and the interior layer 200b is indicated by a dashed line. Also, in Figure 13B, an example of a grain boundary 201 is shown by a dashed line.

[0306] In this specification, the surface layer 200a of the positive electrode active material 200 refers to, for example, a region within 50 nm from the surface toward the interior, more preferably within 35 nm from the surface toward the interior, even more preferably within 20 nm from the surface toward the interior, and most preferably within 10 nm from the surface toward the interior. Surfaces formed by cracks and / or fissures may also be considered the surface. The surface layer 200a is synonymous with the vicinity of the surface, the vicinity of the surface region, or the shell.

[0307] Furthermore, the region deeper than the surface layer 200a of the positive electrode active material is called the interior 200b. Interior 200b is synonymous with the interior region or core.

[0308] Furthermore, the surface of the positive electrode active material 200 refers to the surface of the composite oxide including the surface layer 200a, the interior 200b, and the protrusions 203, etc. The positive electrode active material 200 is assumed to be free from carbonates, hydroxyl groups, etc., that have been chemically adsorbed after fabrication. The positive electrode active material 200 is also assumed to be free from electrolytes, binders, conductive materials, or compounds derived therefrom that have adhered to the positive electrode active material 200. Furthermore, the surface of the positive electrode active material 200 in cross-sectional STEM (scanning transmission electron microscope) images, etc., refers to the boundary between the region where the electron beam coupling image is observed and the region where it is not observed, and is the outermost region where bright spots originating from the atomic nuclei of metal elements with atomic numbers greater than lithium are confirmed. The surface in cross-sectional STEM images, etc., may be determined in conjunction with the results of analyses with higher spatial resolution, such as electron energy loss spectroscopy (EELS).

[0309] Furthermore, a grain boundary 201 refers to, for example, a region where positive electrode active material 200 particles are fixed together, a region where the crystal orientation changes within the positive electrode active material 200, that is, a region where the repetition of bright and dark lines in STEM images becomes discontinuous, a region containing many crystal defects, or a region where the crystal structure is disordered. A crystal defect is defined as a defect observable by cross-sectional TEM (transmission electron microscope), cross-sectional STEM images, etc., that is, a structure in which other elements have entered between the lattice, a cavity, etc. A grain boundary 201 can be considered a type of surface defect. Furthermore, the vicinity of a grain boundary 201 refers to the region within 10 nm of the grain boundary 201.

[0310] <Contained elements> The positive electrode active material 200 comprises lithium, a transition metal M, oxygen, and an additive element A. Alternatively, the positive electrode active material 200 may be a composite oxide (LiMO2) containing lithium and a transition metal M, to which the additive element A is added. However, the composition of the composite oxide is not strictly limited to Li:M:O=1:1:2. Furthermore, a positive electrode active material to which the additive element A is added may also be referred to as a composite oxide.

[0311] The positive electrode active material of a lithium-ion secondary battery needs to contain a redox-capable transition metal in order to maintain charge neutrality even when lithium ions are inserted and removed. In one embodiment of the present invention, the positive electrode active material 200 preferably uses cobalt as the transition metal M responsible for the redox reaction. In addition to cobalt, one or more selected from nickel and manganese may also be used. It is preferable that the cobalt content of the transition metal M in the positive electrode active material 200 is 75 atomic% or more, preferably 90 atomic% or more, and more preferably 95 atomic% or more, as this offers many advantages, such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics.

[0312] Furthermore, if the cobalt content of the transition metal M in the positive electrode active material 200 is 75 atomic percent or more, preferably 90 atomic percent or more, and more preferably 95 atomic percent or more, then compared to composite oxides such as lithium nickelate (LiNiO2) in which nickel accounts for the majority of the transition metal M, Li xThe stability of CoO2 is better when x is small. This is thought to be because cobalt is less affected by strain due to the Jahn-Teller effect than nickel. In transition metal compounds, the strength of the Jahn-Teller effect varies depending on the number of electrons in the d orbital of the transition metal. In layered rock salt type composite oxides in which octahedral low-spin nickel(III) ions make up the majority, such as lithium nickelate, the Jahn-Teller effect is significant, and strain is likely to occur in the layers consisting of octahedra of nickel and oxygen. Therefore, there is a growing concern that the crystal structure may collapse during charge-discharge cycles. In addition, nickel ions have a larger ionic radius than cobalt ions and are close in size to lithium ions. Therefore, in layered rock salt type composite oxides in which nickel makes up the majority, such as lithium nickelate, there is a problem that cation mixing of nickel and lithium is likely to occur.

[0313] On the other hand, if nickel is used as the transition metal M in the positive electrode active material 200 in an amount of 33 atomic percent or more, preferably 60 atomic percent or more, and more preferably 80 atomic percent or more, the raw materials may be cheaper compared to the case where cobalt is abundant, and the discharge capacity per unit weight may increase, which is preferable.

[0314] The additive element A in the positive electrode active material 200 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. Furthermore, the amount of additive element A is preferably less than 25 atomic percent relative to the transition metal (or less than 10 atomic percent relative to the sum if there are two or more transition metals), more preferably less than 5 atomic percent.

[0315] In other words, the positive electrode active material 200 can include lithium cobalt oxide with magnesium and fluorine added, lithium cobalt oxide with magnesium, fluorine and titanium added, lithium cobalt oxide with magnesium, fluorine and aluminum added, lithium cobalt oxide with magnesium, fluorine and nickel added, lithium cobalt oxide with magnesium, fluorine, nickel and aluminum added, and so on.

[0316] These additive elements A further stabilize the crystal structure of the positive electrode active material 200, as will be described later. In this specification, although additive elements A are part of the raw materials for the positive electrode active material, they are called additive elements because their concentration is lower than that of the main components.

[0317] Furthermore, the additive element A does not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0318] For example, if the positive electrode active material 200 is substantially free of manganese, the above advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics become even greater. The weight of manganese contained in the positive electrode active material 200 is preferably 600 ppm or less, more preferably 100 ppm or less. The weight of manganese can be analyzed, for example, using GD-MS (glow discharge mass spectrometry).

[0319] <Crystal structure> Using Figures 14 to 20, Li x The change in crystal structure due to x in CoO2 will be explained by comparing a conventional positive electrode active material with a positive electrode active material 200 according to one embodiment of the present invention. The value of x indicates the extent to which insertable and detachable lithium remains in the lithium cobalt oxide. x This can be described as the lithium occupancy rate in CoO2. Note that Co is just one example of a transition metal, and you may substitute cobalt with transition metal M and cobalt site with transition metal M site as appropriate.

[0320] In this specification, the layered rock salt crystal structure belonging to space group R-3m of a composite oxide containing lithium and a transition metal M, such as cobalt, refers to a crystal structure that has a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, thereby enabling two-dimensional diffusion of lithium. Defects such as vacancies in cations or anions may be present. Furthermore, strictly speaking, a layered rock salt crystal structure may have a distorted lattice structure compared to a rock salt crystal structure.

[0321] Furthermore, a rock salt-type crystal structure refers to a cubic crystal structure, including the space group Fm-3m, in which cations and anions are arranged alternately. It is also acceptable for there to be vacancies in either the cations or anions.

[0322] Furthermore, the presence of characteristics of both layered rock salt crystal structures and rock salt crystal structures can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc.

[0323] In rock salt crystal structures, there is no distinction between cation sites, but in layered rock salt crystal structures, there are two types of cation sites: one is mostly occupied by lithium, and the other by the transition metal M. The layered structure, in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately, is the same for both rock salt and layered rock salt crystal structures. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes forming these two-dimensional planes, when the central spot (transmission spot) is taken as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in an ideal rock salt crystal structure, and for example, the (003) plane in a layered rock salt crystal structure. For example, when comparing the electron diffraction patterns of MgO with a rock salt crystal structure and LiCoO2 with a layered rock salt crystal structure, the bright spot on the (003) plane of LiCoO2 is observed at about half the distance of the bright spot on the (111) plane of MgO. Therefore, if the analysis region contains two phases, for example, MgO with a rock salt crystal structure and LiCoO2 with a layered rock salt crystal structure, the electron diffraction pattern will show crystal planes in which bright spots of high and low brightness are alternately arranged. Bright spots common to both the rock salt crystal structure and the layered rock salt crystal structure will have high brightness, while bright spots occurring only in the layered rock salt crystal structure will have low brightness.

[0324] Furthermore, in cross-sectional STEM images, when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers with high brightness and layers with low brightness are observed alternately. This feature is not observed in rock salt crystal structures because there is no distinction in the sites of cations. In the case of a crystal structure that has characteristics of both rock salt and layered rock salt crystal structures, when observed from a specific crystal orientation, layers with high brightness and layers with low brightness are observed alternately in cross-sectional STEM images, and the layers with low brightness correspond to lithium layers, and a metal with an atomic number greater than lithium is present in part of the lithium layer.

[0325] The anions in the layered rock salt crystal structure and the rock salt crystal structure adopt a cubic close-packed structure (face-centered cubic lattice structure). It is also presumed that the anions in the O3' type crystal structure, which will be described later, adopt a cubic close-packed structure. Therefore, when the layered rock salt crystal structure and the rock salt crystal structure are in contact, there exists a crystal plane in which the orientation of the cubic close-packed structure composed of anions is aligned.

[0326] Alternatively, it can be explained as follows: The anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt crystal structure has a space group R-3m and is a rhombohedral structure, but to facilitate understanding of the structure, it is generally represented as a composite hexagonal lattice, and the (0001) plane of the layered rock salt crystal structure has a hexagonal lattice. The triangular lattice of the cubic {111} plane has the same atomic arrangement as the hexagonal lattice of the (0001) plane of the layered rock salt crystal structure. The consistency between the two lattices can be described as the orientation of the cubic close-packed structure being aligned.

[0327] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure described later is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (the space group Fm-3m is the space group of the general rock salt type crystal structure). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different for the layered rock salt type crystal structure and the O3' type crystal structure and for the rock salt type crystal structure. In this specification, it is sometimes said that the crystal orientations are roughly the same when the orientations of the cubic close-packed structure composed of anions are aligned in the layered rock salt type crystal structure, the O3' type crystal structure and the rock salt type crystal structure.

[0328] Furthermore, the general agreement of crystal orientation in two regions can be determined from TEM images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle scattering annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, electron diffraction patterns, FFT patterns of TEM and STEM images, etc. XRD, electron diffraction, neutron diffraction, etc. can also be used as criteria for determination.

[0329] Figure 14 shows an example of a TEM image in which the orientation of the layered rock salt crystal structure LRS (circled) and the rock salt crystal structure RS (circled) are roughly consistent. Such TEM images, as well as STEM images, HAADF-STEM images, and ABF-STEM images, etc., provide images that reflect the crystal structure.

[0330] For example, in high-resolution images obtained by TEM, contrast originating from crystal planes can be obtained. Due to the diffraction and interference of electron beams, for example, when an electron beam is incident perpendicular to the c-axis of a composite hexagonal lattice of a layered rock salt crystal structure, contrast originating from the (0003) plane is obtained as a repetition of bright bands (bright strips or bright lines) and dark bands (dark strips or dark lines). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines interact with each other (for example, L shown in Figure 14). RS and L LRS If the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly coincidental, that is, the crystal orientations are roughly coincidental. Similarly, if the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly coincidental.

[0331] Furthermore, HAADF-STEM images yield contrast proportional to the atomic number, with elements having higher atomic numbers appearing brighter. For example, in the case of layered rock salt lithium cobalt oxide belonging to space group R-3m, cobalt (atomic number 27) has the highest atomic number, so electron beams are strongly scattered at the positions of cobalt atoms, and the arrangement of cobalt atoms is observed as bright lines or a sequence of bright points. Therefore, when lithium cobalt oxide with a layered rock salt crystal structure is observed perpendicular to the c-axis, the arrangement of cobalt atoms is observed perpendicular to the c-axis as bright lines or a sequence of bright points, while the arrangement of lithium atoms and oxygen atoms is observed as dark lines or low-brightness regions. The same applies when lithium cobalt oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0332] Therefore, in HAADF-STEM images, if the repetition of bright and dark lines is observed in two regions with different crystal structures, and the angle between the bright lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the atomic arrangement is roughly consistent, i.e., the crystal orientation is roughly consistent. Similarly, if the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can also be determined that the crystal orientation is roughly consistent.

[0333] In ABF-STEM, elements with smaller atomic numbers appear brighter, but since contrast is obtained according to atomic number, just like with HAADF-STEM, ABF-STEM images can be used to determine crystal orientation, similar to HAADF-STEM images.

[0334] Figure 15A shows an example of a STEM image where the orientation of the layered rock salt crystal structure LRS, marked with a square, and the rock salt crystal structure RS, also marked with a square, are in roughly agreement. Figure 15B shows the FFT of the region of the rock salt crystal structure RS, and Figure 15C shows the FFT of the region of the layered rock salt crystal structure LRS. In Figures 15B and 15C, the composition is shown on the left, the JCPDS card number is shown to the right of the composition, and the d value and angle calculated from it are shown. Measured values ​​are shown on the right. Spots marked with O are zero-order diffraction.

[0335] The spot labeled A in Figure 15B originates from the 11-1 reflection of the cubic crystal. The spot labeled A in Figure 15C originates from the 0003 reflection of the layered rock salt crystal structure. From Figures 15B and 15C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal structure are roughly coincide. That is, the line passing through AO in Figure 15B and the line passing through AO in Figure 15C are roughly parallel. Here, roughly coincidental and roughly parallel means that the angle between each line is 5 degrees or less, or 2.5 degrees or less.

[0336] Thus, in FFT and electron diffraction, if the orientations of the layered rock salt crystal structure and the rock salt crystal structure are roughly the same, the <0003> orientation of the layered rock salt crystal structure and the <11-1> orientation of the rock salt crystal structure may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. The fact that the reciprocal lattice points are spot-like and not continuous with other reciprocal lattice points means that the crystallization is high.

[0337] Furthermore, as mentioned above, if the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal structure roughly coincide, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt crystal structure may be observed in a reciprocal space different from the orientation of the 0003 reflection of the layered rock salt crystal structure. For example, the spot labeled B in Figure 15C originates from the 1014 reflection of the layered rock salt crystal structure. This spot may be observed at an angle of 52° to 56° from the orientation of the reciprocal point (A in Figure 15C) originating from the 0003 reflection of the layered rock salt crystal structure (i.e., ∠AOB is between 52° and 56°), with d between 0.19 nm and 0.21 nm. Note that this index is just an example and does not necessarily have to match. For example, equivalent reciprocal points in each case would also be acceptable.

[0338] Similarly, spots not originating from the cubic 11-1 reflection may be observed in reciprocal space at a different orientation than the orientation where the cubic 11-1 reflection was observed. For example, the spot labeled B in Figure 15B originates from the cubic 200 reflection. This is because diffraction spots may be observed at an angle between 54° and 56° from the orientation of the cubic 11-1 reflection (A in Figure 15B) (i.e., ∠AOB is between 54° and 56°). Note that this index is just an example and does not necessarily have to match. For example, equivalent reciprocal points in each case would also be acceptable.

[0339] It is known that in positive electrode active materials with a layered rock salt-type crystalline structure, such as lithium cobalt oxide, the (0003) plane and its equivalent plane, as well as the (10-14) plane and its equivalent plane, tend to appear as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material with an SEM or the like, it is possible to thin the observation sample using a FIB or the like so that the electron beam is incident at [12-10] in a TEM or the like, in order to make the (0003) plane easier to observe. When it is necessary to determine the consistency of the crystal orientation, it is preferable to thin the sample so that the (0003) plane of the layered rock salt-type crystalline structure is easily observable.

[0340] ≪Li x When x in CoO2 is 1 >> Figure 17 shows the discharge state, i.e., Li x The crystal structure of conventional lithium cobalt oxide when x = 1 in CoO2 is shown. This crystal structure has a layered rock salt type crystal structure belonging to space group R-3m. The conventional positive electrode active material shown in Figure 17 is lithium cobalt oxide (LiCoO2) without additive element A. Changes in the crystal structure of lithium cobalt oxide without additive element A are described in Non-Patent Documents 1 to 3, etc.

[0341] Furthermore, in this crystal structure, lithium occupies the octahedral sites, and there are three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called the O3 type crystal structure. The CoO2 layer is a layer in which octahedral structures, in which oxygen is 6-coordinated to cobalt, form a continuous structure in a plane with shared edges. This is sometimes referred to as a layer consisting of octahedra of cobalt and oxygen. In Figure 17, the crystal structure when x=1 is labeled R-3m(O3).

[0342] Figure 16 shows the discharge state, i.e., Li xThe crystal structure of lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention is shown when x = 1. This crystal structure has a layered rock salt type crystal structure belonging to space group R-3m. In this crystal structure, lithium occupies octahedral sites, and there are three CoO2 layers in the unit cell. In Figure 16 as well, the crystal structure when x = 1 is labeled R-3m(O3).

[0343] Lithium cobalt oxide having a layered rock salt crystal structure has a high discharge capacity, possesses a two-dimensional lithium ion diffusion pathway, is suitable for lithium ion insertion / deinsertion reactions, and is therefore excellent as a positive electrode active material for secondary batteries. For this reason, in one embodiment of the present invention, it is preferable that the interior 200b, which accounts for the majority of the volume, is lithium cobalt oxide having a layered rock salt crystal structure.

[0344] In one embodiment of the present invention, the surface layer 200a of lithium cobalt oxide used in the positive electrode active material 200 preferably has the function of reinforcing the layer structure of the octahedral layer of cobalt and oxygen (e.g., CoO2 layer) in the interior 200b so that it does not break down even if lithium is extracted from the positive electrode active material 200 due to charging. That is, it is preferable that the surface layer 200a functions as a barrier film for the positive electrode active material 200. Alternatively, it is preferable that the surface layer 200a reinforces the positive electrode active material 200. Reinforcing includes suppressing structural changes in the surface layer 200a and interior 200b of the positive electrode active material 200, and / or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 200.

[0345] Therefore, in lithium cobalt oxide used in the positive electrode active material 200 of one aspect of the present invention, it is preferable that the surface layer 200a has a different crystal structure from the interior 200b. Specifically, it is preferable that the surface layer 200a has a composition and crystal structure that is more stable at room temperature (25°C) than the interior 200b. For example, it is preferable that the surface layer 200a has at least a rock salt type crystal structure. It is more preferable, but not limited to, that the entire surface layer 200a has a rock salt type crystal structure. For example, the surface layer 200a may have both a rock salt type crystal structure and a layered rock salt type crystal structure.

[0346] Here, we will explain the surface layer 200a. The surface layer 200a is the region where lithium ions first desorb during charging, and it is a region where the lithium concentration tends to be lower than in the interior 200b. Also, on the surface, which is the surface layer 200a, it can be said that the atoms that make up lithium cobalt oxide (for example, oxygen) exist in a state where their bonds are broken due to the desorption of lithium ions. In other words, the surface layer 200a is more unstable than the interior 200b, and it is a region where degradation of the crystal structure is more likely to begin. Therefore, if at least the surface layer 200a can be made sufficiently stable, Li x Even when x in CoO2 is small (for example, x is 0.24 or less), the layered structure of the octahedrons of cobalt and oxygen in the interior 200b can be made less prone to breaking. Note that the breakdown of the layered structure includes the shifting of the edges of the layered structure of the octahedrons of cobalt and oxygen, and if the surface layer 200a is sufficiently stable, this shifting can be suppressed.

[0347] To stabilize the surface layer 200a, it is sufficient for the surface layer 200a to have a stable composition or a stable crystal structure, and for this purpose, it is preferable that the surface layer 200a contains additive element A. It is more preferable that additive element A comprises two or more elements with different concentration distributions, such as additive element X and additive element Y, which will be described later. Furthermore, it is said that the surface layer 200a contains additive element A, which includes the fact that the concentration of additive element A present in the surface layer 200a is higher than the concentration of additive element A present in the interior 200b. Furthermore, the presence of high and low concentrations of additive elements includes the fact that additive element A has a concentration gradient in the surface layer 200a, or that additive element A has a concentration gradient from the surface layer 200a toward the interior 200b. In the case of additive element X and additive element Y having a concentration gradient, it is preferable that the concentration distributions showing the concentration gradients are different from each other. Even more preferably, it is preferable that the peak position showing the maximum concentration of additive element X and the peak position showing the maximum concentration of additive element Y are different. The maximum value of a concentration is sometimes referred to as the peak top, and the maximum value of a concentration is sometimes referred to as the peak.

[0348] For example, the additive element X selected from additive element A preferably has a concentration distribution that increases from the interior 200b towards the surface, as shown in the gradient in Figure 13C. Specifically, it is one or more elements selected from magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, calcium, etc. The peak top of additive element X is preferably located in the surface layer 200a. For example, it is preferable that the additive element X has a concentration distribution such that the peak top is located in a region of 0.5 nm to 10 nm from the surface towards the interior.

[0349] The additive element Y selected from additive element A preferably has a concentration gradient as shown by the density of the hatches in Figure 13D, and preferably has its peak top in a region deeper than the peak top in Figure 13C. Specifically, it is one or more elements selected from aluminum, manganese, etc. The peak top of additive element Y may be located in the surface layer 200a or deeper than the surface layer 200a. For example, it is preferable that additive element Y has a concentration distribution such that its peak top is located in a region of 5 nm to 30 nm from the surface inward. The position of the peak top of additive element Y may differ from the position of the peak top of additive element X. Furthermore, the concentration distribution of additive element Y may differ from the concentration distribution of additive element X.

[0350] For example, magnesium ions, one of the additive elements X, are divalent, and these magnesium ions are more stable in lithium sites than in cobalt sites in the layered rock salt crystal structure, so they readily enter the lithium sites. In other words, the presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 200a makes it easier to maintain the layered rock salt crystal structure of the interior 200b. This is presumed to be because the magnesium present in the lithium sites of the surface layer 200a functions as pillars supporting the CoO2 layers. Furthermore, the presence of magnesium in lithium cobalt oxide means that Li x Even when x in CoO2 is, for example, 0.24 or less, the desorption of oxygen around magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of lithium cobalt oxide. In addition, if the magnesium concentration in the surface layer 200a is higher than that in the interior layer 200b, it is expected that the corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte will improve.

[0351] At appropriate concentrations, magnesium does not adversely affect lithium insertion and removal during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may adversely affect lithium insertion and removal. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because, at high magnesium concentrations, magnesium enters not only lithium sites but also cobalt sites. In addition, magnesium may segregate on the surface of the positive electrode active material as magnesium compounds (oxides or fluorides, etc.) without substituting for either lithium or cobalt sites, potentially becoming a resistive component of the secondary battery. Moreover, the discharge capacity may decrease as the magnesium concentration of the positive electrode active material increases. This is thought to be because too much magnesium enters the lithium sites, reducing the amount of lithium that contributes to charging and discharging.

[0352] Therefore, it is preferable that the total amount of magnesium in the positive electrode active material 200 is appropriate. For example, the number of magnesium atoms is preferably 0.001 times or more and 0.1 times or less the number of cobalt atoms, more preferably greater than 0.01 times and less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium in the total positive electrode active material 200 referred to here may be a value obtained by performing an elemental analysis of the entire positive electrode active material 200 using, for example, GD-MS or ICP-MS (inductively coupled plasma mass spectrometry), or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 200.

[0353] Furthermore, nickel, one of the additive elements X, can be present in both cobalt sites and lithium sites. When present in cobalt sites, its oxidation-reduction potential is lower compared to cobalt, which leads to an increase in discharge capacity and is therefore preferable.

[0354] Furthermore, when nickel is present at the lithium site, the displacement of the octahedral cobalt and oxygen layers (e.g., the CoO2 layer) during charging and discharging is suppressed. Volume changes during charging and discharging are also suppressed. Additionally, the elastic modulus increases, meaning it becomes harder. This is presumed to be because the nickel present at the lithium site also functions as a pillar supporting the CoO2 layers. Therefore, it is desirable that the crystal structure becomes more stable, especially during charging at high temperatures, such as above 45°C.

[0355] On the other hand, an excess of nickel may exacerbate the distortion caused by the Jahn-Teller effect. Furthermore, an excess of nickel may negatively affect the insertion and removal of lithium.

[0356] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 200 is appropriate. For example, the number of nickel atoms in the positive electrode active material 200 is preferably more than 0% and 7.5% or less of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferable that it is more than 0% and 4% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.57.5% or less. Alternatively, it is preferable that it is more than 0% and 4% or less. The amount of nickel shown here may be a value obtained by performing an elemental analysis of the entire positive electrode active material using, for example, GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the process of manufacturing the positive electrode active material.

[0357] Furthermore, aluminum, one of the additive elements Y, can be present in the cobalt site of a layered rock salt crystal structure. Since aluminum is a trivalent typical element and its valency does not change, lithium around the aluminum does not easily move during charging and discharging. Therefore, the aluminum and the surrounding lithium can function as pillars, suppressing changes in the crystal structure. In addition, aluminum suppresses the dissolution of surrounding transition metals M, improving continuous charging endurance. Moreover, since the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, having aluminum as an additive element Y can improve safety when used in secondary batteries. Furthermore, it is possible to create a positive electrode active material 200 whose crystal structure is less likely to collapse even after repeated charging and discharging.

[0358] On the other hand, an excess of aluminum may negatively affect the insertion and removal of lithium.

[0359] Therefore, it is preferable that the total amount of aluminum in the positive electrode active material 200 is appropriate. For example, the total number of aluminum atoms in the positive electrode active material 200 is preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.3% to 1.5% of the total number of cobalt atoms. Alternatively, 0.05% to 2% is preferred, or 0.1% to 4% is preferred. The total amount of aluminum in the positive electrode active material 200 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 200 using, for example, GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 200.

[0360] Furthermore, fluorine, one of the additive elements X, is a monovalent anion, and if some of the oxygen in the surface layer 200a is replaced by fluorine, the lithium desorption energy decreases. This is because the change in the valence of cobalt ions accompanying lithium desorption is from trivalent to tetravalent when fluorine is absent, and from divalent to trivalent when fluorine is present, resulting in different oxidation-reduction potentials. Therefore, if some of the oxygen in the surface layer 200a is replaced by fluorine, the desorption and insertion of lithium ions near the fluorine can occur more smoothly. As a result, when lithium cobalt oxide containing fluorine is used in secondary batteries, the charge-discharge characteristics, current characteristics, etc. can be improved. In addition, the presence of fluorine in the surface layer 200a, which is the part that comes into contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid. Furthermore, as will be discussed later, if the melting point of fluorides, including lithium fluoride, is lower than the melting point of other additive element A sources, it can function as a flux (also called a fluxing agent) that lowers the melting point of other additive element A sources.

[0361] Furthermore, titanium oxide, one of the additive elements X, is known to be superhydrophilic. Therefore, by using a positive electrode active material 200 having titanium oxide in its surface layer 200a, it is possible that wettability with highly polar solvents will be improved. When used in a secondary battery, good contact at the interface between the positive electrode active material 200 and the highly polar electrolyte may be achieved, potentially suppressing an increase in internal resistance.

[0362] Furthermore, if phosphorus, one of the additive elements X, is present in the surface layer 200a, Li x Maintaining a small value of x in CoO2 can suppress short circuits, which is preferable. For example, it is preferable that the compound containing phosphorus and oxygen exists in the surface layer 200a.

[0363] If the positive electrode active material 200 contains phosphorus, it is preferable that the hydrogen fluoride generated by the decomposition of the electrolyte reacts with the phosphorus, potentially lowering the concentration of hydrogen fluoride in the electrolyte.

[0364] When LiPF6 is used as the lithium salt, hydrolysis may generate hydrogen fluoride. Furthermore, the reaction between polyvinylidene fluoride (PVDF), used as a component of the positive electrode, and alkali may also generate hydrogen fluoride. Reducing the hydrogen fluoride concentration in the electrolyte may suppress corrosion of the current collector. Additionally, it may suppress the decrease in adhesion due to the insolubilization of PVDF.

[0365] If the positive electrode active material 200 contains phosphorus along with magnesium, Li x The stability is extremely high and preferable when x in CoO2 is small. When the positive electrode active material 200 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Or 1% to 10% is preferable. Or 1% to 8% is preferable. Or 2% to 20% is preferable. Or 2% to 8% is preferable. Or 3% to 20% is preferable. Or 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. Or 0.1% to 5% is preferable. Or 0.1% to 4% is preferable. Or 0.5% to 10% is preferable. Or 0.5% to 4% is preferable. Or 0.7% to 10% is preferable. Alternatively, a concentration of 0.7% or more and 5% or less is preferred. The concentrations of phosphorus and magnesium shown herein may be values ​​obtained by performing an overall elemental analysis of the positive electrode active material 200 using, for example, GC-MS, ICP-MS, etc., or they may be based on the values ​​of the raw material formulation during the manufacturing process of the positive electrode active material 200.

[0366] Furthermore, if the positive electrode active material 200 has cracks, the propagation of the cracks can be suppressed by the presence of phosphorus, or more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on its surface, for example, in the embedded portion 202 shown in Figure 13B.

[0367] Furthermore, if the surface layer 200a contains both magnesium and nickel, divalent magnesium may be able to exist more stably near divalent nickel. Therefore, Li x Even when x in CoO2 is small, magnesium elution can be suppressed. Therefore, this can contribute to the stabilization of the surface layer 200a.

[0368] Furthermore, if the additive element A is a combination of additive element X and additive element Y, the concentration distribution of additive element X and the concentration distribution of additive element Y are different, which is preferable because it can stabilize the crystal structure over a wider area. For example, if the positive electrode active material 200 contains both magnesium and nickel, which are part of additive element X, and aluminum, which is one of the additive elements Y, it can stabilize the crystal structure over a wider area than when it contains only one of the additive elements X or Y. In this way, when the positive electrode active material 200 contains both additive elements X and Y, surface stabilization can be sufficiently achieved by additive element X such as magnesium, so additive element Y such as aluminum is not essential for the surface. Rather, it is preferable for aluminum to be widely distributed in deeper regions, for example, in the region between 5 nm and 50 nm from the surface, as this can stabilize the crystal structure over a wider area.

[0369] As described above, when multiple additive elements A are present, the effects of each additive element A synergistically contribute to further stabilization of the surface layer 200a and the interior 200b. In particular, the presence of magnesium, nickel, and aluminum is highly preferable as it is more effective in creating a stable crystal structure.

[0370] However, it is undesirable if the surface layer 200a is occupied solely by compounds of additive element A and oxygen, as this makes lithium insertion and removal difficult. For example, it is undesirable for the surface layer 200a to be occupied solely by MgO, a structure in which MgO and NiO(II) are in solid solution, and / or a structure in which MgO and CoO(II) are in solid solution. Therefore, the surface layer 200a must contain at least cobalt, and in the discharge state, lithium as well, and have pathways for lithium insertion and removal.

[0371] To ensure a pathway for lithium insertion and removal, it is preferable that the surface layer 200a has a higher cobalt concentration than magnesium. For example, the number of atoms of magnesium A Mg and the number of atoms of cobalt A Co Ratio A Mg / A Co It is preferable that the cobalt concentration in the surface layer 200a is higher than that of nickel. It is also preferable that the cobalt concentration in the surface layer 200a is higher than that of aluminum. Furthermore, it is preferable that the cobalt concentration in the surface layer 200a is higher than that of fluorine.

[0372] Furthermore, since too much nickel may inhibit lithium diffusion, it is preferable that the surface layer 200a has a higher magnesium concentration than nickel. For example, it is preferable that the number of nickel atoms be 1 / 6 or less of the number of magnesium atoms.

[0373] Furthermore, while it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, are present at higher concentrations in the surface layer 200a than in the interior 200b, it is also preferable that they be present randomly and dilutely in the interior 200b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites of the interior 200b, it has the effect of making it easier to maintain a layered rock salt-type crystal structure, similar to the above. Also, when nickel is present at an appropriate concentration in the interior 200b, the displacement of the octahedral layer of cobalt and oxygen (e.g., the CoO2 layer) due to charging and discharging can be suppressed, similar to the above. In addition, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, so a synergistic effect of suppressing magnesium leaching can be expected.

[0374] Furthermore, due to the concentration gradient of the added element A as described above, it is preferable that the crystal structure changes continuously from the interior 200b toward the surface. Alternatively, it is preferable that the crystal orientations of the surface layer 200a and the interior 200b are roughly the same.

[0375] For example, it is preferable that the crystal structure changes continuously from the interior 200b, which has a layered rock salt type crystal structure, toward the surface 200a (i.e., the surface), which has a rock salt type crystal structure, or features of both a rock salt type crystal structure and a layered rock salt type crystal structure. Alternatively, it is preferable that the orientation of the surface 200a, which has a rock salt type crystal structure, or features of both a rock salt type crystal structure and a layered rock salt type crystal structure, and the interior 200b with a layered rock salt type crystal structure are roughly the same.

[0376] ≪Li x When x is small during CoO2≫ In one aspect of the present invention, the positive electrode active material 200 has the above-described distribution and / or crystal structure of the additive element A in the discharge state, resulting in Li x The crystal structure of CoO2 when x is small differs from that of conventional cathode active materials. Here, x is small when it is 0.1 <x≦0.24をいうこととする。

[0377] First, Figure 17 shows the change in the crystal structure of conventional positive electrode active materials. Conventional lithium cobalt oxide is Li x When the x-value of CoO2 is around 0.5 (when x=0.5), the symmetry of lithium increases, and it is known that it has a crystal structure belonging to the monoclinic space group P2 / m. In this structure, there is one CoO2 layer in the unit cell. For this reason, it is sometimes called the O1 type or monoclinic O1 type. In Figure 17, the crystal structure when x=0.5 is labeled P2 / m (monoclinic O1).

[0378] Also Li x When x=0 in CoO2, conventional lithium cobalt oxide has a crystal structure of the trigonal space group P-3m1, and there is one CoO2 layer in the unit cell. For this reason, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, the trigonal structure is sometimes converted to a composite hexagonal lattice and called hexagonal O1 type. In Figure 17, the crystal structure when x=0 is labeled P-3m1 (trigonal O2).

[0379] Also Li xConventional lithium cobalt oxide has a crystal structure with space group R-3m when x is around 0.24 (when x = 0.24). This structure can be described as a structure in which the trigonal O1 type CoO2 structure and the R-3m(O3) LiCoO2 structure are alternately stacked. For this reason, this crystal structure is sometimes called the H1-3 type crystal structure. In Figure 17, the crystal structure when x = 0.12 is labeled R-3m(H1-3).

[0380] In reality, the H1-3 type crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in this specification, including Figure 17, the H1-3 type crystal structure is shown with the c-axis halved compared to the unit cell in order to facilitate comparison with other crystal structures.

[0381] As an example, in the H1-3 type crystal structure, as described in Non-Patent Literature 3, the coordinates of cobalt and oxygen in the unit cell can be represented as follows: Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045). O1 and O2 are oxygen atoms, respectively. The unit cell to be used to represent the crystal structure of lithium cobalt oxide can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, the unit cell that yields the smallest GOF (goodness of fit) value should be adopted.

[0382] Li x When the CoO2 is repeatedly charged to a value of 0.24 or less and discharged to a value of 1, the conventional lithium cobalt oxide undergoes repeated changes in its crystal structure (i.e., non-equilibrium phase transitions) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state.

[0383] However, these two crystal structures exhibit a large displacement of the CoO2 layer. As shown by the dotted line and arrow in Figure 17, in the H1-3 type crystal structure, the CoO2 layer is significantly displaced from the R-3m(O3) layer in the discharge state. Such dynamic structural changes can negatively affect the stability of the crystal structure.

[0384] Furthermore, these two crystal structures also have a large volume difference. When comparing them per the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the R-3m(O3) type crystal structure in the discharged state exceeds 3.5%, and is typically 3.9% or more.

[0385] In addition, the H1-3 crystal structure, which consists of continuous CoO2 layers like the trigonal O1 type, is likely to be unstable.

[0386] Therefore, if charging is repeated such that x becomes 0.24 or less, and discharging is repeated such that x becomes 1, the conventional crystal structure of lithium cobalt oxide will break down. This breakdown of the crystal structure leads to a deterioration of the cycle characteristics. This is because the breakdown of the crystal structure reduces the number of sites where lithium can exist stably, and also makes it more difficult for lithium to be inserted and removed.

[0387] On the other hand, in the positive electrode active material 200 of one embodiment of the present invention shown in Figure 16, when x = approximately 0.2, it has a crystal structure that belongs to the trigonal space group R-3m. This is because the symmetry of the CoO2 layer is the same as that of O3. Therefore, this crystal structure will be called an O3' type crystal structure. In Figure 16, the crystal structure when x = 0.2 is labeled R-3m(O3').

[0388] The lithium cobalt oxide used in the positive electrode active material 200 according to one embodiment of the present invention is Li xThe change in crystal structure during discharge (where x is 1) and charging (where x is 0.24 or less) in CoO2 is less than that of conventional lithium cobalt oxide. Specifically, as shown by the dotted line in Figure 16, there is almost no displacement of the CoO2 layer between the discharged R-3m(O3) and the O3' type crystal structure. Furthermore, the change in volume per cobalt atom can be reduced. Specifically, the difference in volume per the same number of cobalt atoms between the discharged R-3m(O3) and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, and typically 1.8%. Therefore, the positive electrode active material 200 in one aspect of the present invention is less prone to crystal structure collapse even when repeatedly subjected to charging (where x is 0.24 or less) and discharging (where x is 1), and can achieve excellent cycle characteristics.

[0389] Thus, in the positive electrode active material 200 of one aspect of the present invention, Li x Unlike conventional positive electrode active materials, the crystal structure of CoO2 with x values ​​of 0.24 or less suppresses changes from the crystal structure in the discharge state where x is 1. Furthermore, the change in volume per unit of the same number of cobalt atoms is also suppressed in the positive electrode active material 200 according to one aspect of the present invention. Therefore, the crystal structure of the positive electrode active material 200 is less likely to collapse even when repeatedly charging and discharging in a state where x is 0.24 or less. As a result, the decrease in discharge capacity during charge-discharge cycles is suppressed in the positive electrode active material 200. Also, the lithium cobalt oxide used in the positive electrode active material 200 according to one aspect of the present invention is Li x When x in CoO2 is 0.24 or less, it can adopt a more stable crystal structure than conventional lithium cobalt oxide. Therefore, the positive electrode active material 200 in one aspect of the present invention is Li x When the x value in CoO2 is maintained at 0.24 or less, short circuits are less likely to occur, improving the safety of the secondary battery. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 200 of one aspect of the present invention has a large discharge capacity per unit weight and per unit volume. Therefore, by using the positive electrode active material 200 of one aspect of the present invention, a secondary battery with a high discharge capacity per unit weight and per unit volume can be manufactured.

[0390] The O3' type crystal structure of lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention can be expressed with the coordinates of cobalt and oxygen in the unit cell as follows: Co(0,0,0.5), O(0,0,x), and 0.20≦x≦0.25. The lattice constant of the unit cell is preferably 0.2797≦a≦0.2837(nm) for the a axis, more preferably 0.2807≦a≦0.2827(nm), and typically a=0.2817(nm). The c axis is preferably 1.3681≦c≦1.3881(nm), more preferably 1.3751≦c≦1.3811(nm), and typically c=1.3781(nm).

[0391] In the O3' type crystal structure, ions such as cobalt, nickel, and magnesium occupy the six-coordinate positions of oxygen. Lighter elements such as lithium may occupy the four-coordinate positions of oxygen.

[0392] In one embodiment of the present invention, the positive electrode active material 200 is Li x It has been confirmed that when x in CoO2 is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is estimated that it also has an O3' type crystal structure when x is greater than 0.24 and less than or equal to 0.27. However, the crystal structure is Li x Because x in CoO2 is affected not only by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the range of x mentioned above.

[0393] Therefore, the positive electrode active material 200 is Li x When x in CoO2 is greater than 0.1 and less than or equal to 0.24, the entire interior 200b of the positive electrode active material 200 does not have to have an O3' type crystal structure. It may contain other crystal structures, or a portion may be amorphous.

[0394] Also Li x To reduce x in CoO2, it is generally necessary to charge with a high charging voltage. xA state where x in CoO2 is small can be rephrased as a state where it has been charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6V or higher relative to the potential of lithium metal at a temperature of 25°C, the H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6V or higher relative to the potential of lithium metal can be considered a high charging voltage. Furthermore, unless otherwise specified in this specification, the charging voltage is expressed relative to the potential of lithium metal.

[0395] Therefore, the positive electrode active material 200 according to one aspect of the present invention is preferable because it can maintain its crystal structure even when charged at a high charging voltage, for example, at a voltage of 4.6V or higher in a 25°C environment. Furthermore, when charged at a higher charging voltage, for example, at a voltage of 4.65V to 4.7V in a 25°C environment, the positive electrode active material 200 according to one aspect of the present invention is preferable because it can adopt an O3' type crystal structure.

[0396] In one embodiment of the present invention, the positive electrode active material 200 may exhibit an H1-3 type crystal structure when the charging voltage is further increased. Furthermore, as described above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, electrolyte, etc., so even at lower charging voltages, for example, when the charging voltage is 4.5V or higher and less than 4.6V in a 25°C environment, the positive electrode active material 200 in one embodiment of the present invention may adopt an O3' type crystal structure.

[0397] Furthermore, in the case of a secondary battery, if graphite is used as the negative electrode active material, for example, the voltage of the secondary battery will decrease by the amount of the graphite's potential compared to the above. The potential of graphite is approximately 0.05V to 0.2V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is observed when the voltage obtained by subtracting the graphite's potential from the above voltage is obtained.

[0398] Furthermore, while Figure 16 shows the O3' type crystal structure with lithium present at all lithium sites with equal probability, this is not the only option. Lithium may be concentrated at some lithium sites, or, for example, in the monoclinic O1(Li) crystal structure shown in Figure 17. 0.5It may have symmetry similar to that of CoO2. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0399] Furthermore, the O3' type crystal structure can be described as a crystal structure similar to the CdCl2 type crystal structure, although it has lithium randomly placed between the layers. This crystal structure similar to the CdCl2 type is formed when lithium nickelate is used. 0.06 Although the crystal structure is similar to that of NiO2 when charged to this level, it is known that pure lithium cobalt oxide, or layered rock salt-type cathode active materials containing a large amount of cobalt, do not usually adopt a CdCl2-type crystal structure.

[0400] Furthermore, it is preferable that the concentration gradient of additive element A is similar at multiple locations on the surface layer 200a of the positive electrode active material 200. In other words, it is preferable that the reinforcement derived from additive element A is uniformly present on the surface layer 200a. Even if there is reinforcement in a part of the surface layer 200a, if there are parts without reinforcement, stress may concentrate in those parts. If stress concentrates in a part of the positive electrode active material 200, defects such as cracks may occur from that point, which may lead to cracking of the positive electrode active material and a decrease in discharge capacity.

[0401] However, it is not necessarily required that the additive element A has a similar concentration gradient across the entire surface layer 200a of the positive electrode active material 200. An example of the distribution of additive element X near CD in Figure 13A is shown in Figure 13E, and an example of the distribution of additive element Y near CD is shown in Figure 13F.

[0402] Here, the region near CD has a layered rock salt crystal structure of R-3m, and the surface is (001) oriented. The (001) oriented surface may have a different distribution of additive element A than the other surfaces. For example, the (001) oriented surface and its surface layer 200a may have one or more concentration distributions or peak tops selected from additive elements X and Y located in a shallower portion of the surface compared to the other oriented surfaces. Alternatively, the (001) oriented surface and its surface layer 200a may have lower concentrations of one or more selected from additive elements X and Y compared to the other oriented surfaces. Alternatively, the (001) oriented surface and its surface layer 200a may have one or more selected from additive elements X and Y below the detection limit.

[0403] In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This indicates a structure in which CoO2 layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion pathways for lithium ions also exist in a direction parallel to the (001) plane.

[0404] Since the CoO2 layer is relatively stable, the (001) plane on which the CoO2 layer is present is also relatively stable. The main diffusion pathways of lithium ions during charging and discharging are not exposed on the (001) plane.

[0405] On the other hand, the lithium ion diffusion pathways are exposed on surfaces other than those oriented in the (001) direction. Therefore, the surfaces and surface layer 200a other than those oriented in the (001) direction are important regions for maintaining the lithium ion diffusion pathways, but at the same time, they are prone to instability because they are the regions where lithium ions first desorb. For this reason, reinforcing the surfaces and surface layer 200a other than those oriented in the (001) direction is important for maintaining the overall crystal structure of the positive electrode active material 200.

[0406] Therefore, in another embodiment of the positive electrode active material 200 of the present invention, it is preferable that the distribution of additive element A on surfaces other than (001) and its surface layer 200a is as shown in Figures 13C and 13D. On the other hand, on the (001) surface and its surface layer 200a, the concentration of additive element A may be low or absent, as described above.

[0407] In the manufacturing method described in a later embodiment, in which high-purity LiCoO2 is produced and then additive element A is mixed in and heated, the additive element A spreads mainly through the diffusion pathway of lithium ions. Therefore, it is easy to set the distribution of additive element A on surfaces other than (001) and its surface layer 200a to a desirable range.

[0408] Furthermore, while it is preferable that the surface of the positive electrode active material 200 be smooth and have few irregularities, it is not necessarily required that the entire positive electrode active material 200 be so. In composite oxides having a layered rock salt-type crystalline structure of R-3m, slip is likely to occur on planes parallel to the (001) plane, such as the plane where lithium is arranged. For example, if a (001) plane exists as shown in Figure 18A, slip may occur parallel to the (001) plane and deformation may occur as shown by the arrow in Figure 18B after going through processes such as pressing.

[0409] In this case, the newly formed surface and its surface layer 200a resulting from the slip may either not contain additive element A, or may be below the detection limit. EF in Figure 18B is an example of the newly formed surface and its surface layer 200a resulting from the slip. Enlarged views of the area around EF are shown in Figures 18C1 and 18C2. Unlike Figures 13C to 13F, additive elements X and Y are not distributed in Figures 18C1 and 18C2.

[0410] However, since slips tend to occur parallel to the (001) plane, the newly formed surface and its surface layer 200a tend to be (001) oriented. In this case, the diffusion pathway of lithium ions is not exposed and it is relatively stable, so there is little problem even if the additive element A is absent or below the detection limit.

[0411] As mentioned above, in composite oxides with the composition LiCoO2 and the layered rock salt type crystal structure R-3m, cobalt atoms are arranged parallel to the (001) plane. Furthermore, in HAADF-STEM images, the brightness of cobalt, which has the highest atomic number in LiCoO2, is the highest. Therefore, in HAADF-STEM images, the arrangement of atoms with high brightness can be considered to be the arrangement of cobalt. The repetition of this high-brightness arrangement is synonymous with crystal fringes or lattice fringes.

[0412] ≪Grain Boundaries≫ In one embodiment of the present invention, it is more preferable that, in addition to the distribution described above, at least a portion of the additive element A in the positive electrode active material 200 is concentrated in and near the grain boundaries 201.

[0413] In this specification, "non-uniformity" refers to a situation where the concentration of an element in one region differs from that in other regions. It is synonymous with segregation, precipitation, heterogeneity, bias, or a mixture of areas with high and low concentrations.

[0414] For example, it is preferable that the magnesium concentration at and near the grain boundary 201 of the positive electrode active material 200 is higher than that of other regions of the interior 200b. It is also preferable that the fluorine concentration at and near the grain boundary 201 is higher than that of other regions of the interior 200b. Furthermore, it is preferable that the nickel concentration at and near the grain boundary 201 is higher than that of other regions of the interior 200b. It is also preferable that the aluminum concentration at and near the grain boundary 201 is higher than that of other regions of the interior 200b.

[0415] The grain boundary 201 is a type of surface defect. Therefore, it is prone to instability, similar to the surface, and is susceptible to initiation of changes in the crystal structure. For this reason, if the concentration of additive element A at and near the grain boundary 201 is high, changes in the crystal structure can be suppressed more effectively.

[0416] Furthermore, if the magnesium and fluorine concentrations are high at and near the grain boundary 201, even if a crack occurs along the grain boundary 201 of the positive electrode active material 200 in one embodiment of the present invention, the magnesium and fluorine concentrations will be high near the surface created by the crack. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the crack has occurred.

[0417] <Particle size> In one embodiment of the present invention, if the particle size of the positive electrode active material 200 is too large, problems arise such as difficulty in lithium diffusion and excessive roughness of the surface of the active material layer when coated onto the current collector. On the other hand, if it is too small, problems arise such as difficulty in supporting the active material layer when coating onto the current collector and excessive reaction with the electrolyte. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Or preferably 1 μm or more and 40 μm or less. Or preferably 1 μm or more and 30 μm or less. Or preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Or preferably 5 μm or more and 100 μm or less. Or preferably 5 μm or more and 40 μm or less.

[0418] <Analysis method> A certain positive electrode active material is Li x Whether or not the positive electrode active material 200 of one embodiment of the present invention has an O3' type crystal structure when x in CoO2 is small depends on Li x The presence of a positive electrode active material with a small x value in CoO2 can be determined by analyzing it using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0419] XRD is particularly favored because it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, compare the crystallinity and crystal orientation, analyze the periodic distortion of the lattice and the crystallite size, and obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling a secondary battery. Among XRD methods, powder XRD provides diffraction peaks that reflect the crystal structure of the interior 200b of the positive electrode active material 200, which occupies most of the volume of the positive electrode active material 200.

[0420] In one aspect of the present invention, the positive electrode active material 200 is Li x A key characteristic is that the crystal structure changes little when x in CoO2 is 1 and when it is 0.24 or less. If the crystal structure with large changes accounts for more than 50%, it is undesirable because it cannot withstand high-voltage charging and discharging.

[0421] It is also important to note that simply adding element A may not result in the formation of an O3' type crystal structure. For example, even if lithium cobalt oxide has magnesium and fluorine, or lithium cobalt oxide has magnesium and aluminum, the Li structure may vary depending on the concentration and distribution of element A. x There are two cases in CoO2: one where x is 0.24 or less and the O3' type crystal structure accounts for more than 60%, and another where the H1-3 type crystal structure accounts for more than 50%.

[0422] Furthermore, even with the positive electrode active material 200 according to one aspect of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9V, an H1-3 type or trigonal O1 type crystal structure may occur. Therefore, in order to determine whether or not the positive electrode active material 200 is according to one aspect of the present invention, analysis of the crystal structure, including XRD, and information such as charging capacity or charging voltage are necessary.

[0423] Furthermore, positive electrode active materials with a small x value may undergo changes in their crystal structure when exposed to air. For example, they may change from an O3' type crystal structure to an H1-3 type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0424] Furthermore, whether the distribution of additive element A in a given positive electrode active material is in the state described above can be determined by analyzing it using methods such as XPS, energy dispersive X-ray spectroscopy (EDX), and EPMA (electron probe microanalysis).

[0425] Furthermore, the crystal structure of the surface layer 200a, grain boundaries 201, etc., can be analyzed by electron diffraction of the cross-section of the positive electrode active material 200.

[0426] ≪Charging method≫ Whether a certain composite oxide is the positive electrode active material 200 according to one embodiment of the present invention can be determined by performing high-voltage charging. For example, a coin cell (CR2032 type, 20 mm in diameter and 3.2 mm in height) can be manufactured using the composite oxide as the positive electrode and lithium metal as the negative electrode (also referred to as the counter electrode), and then high-voltage charging can be performed.

[0427] More specifically, the positive electrode can be made by coating an aluminum foil positive electrode current collector with a slurry of a mixture of positive electrode active material, conductive material, and binder.

[0428] Lithium metal can be used for the counter electrode. However, if a material other than lithium metal is used for the counter electrode, the potential of the secondary battery and the potential of the positive electrode will differ. In this specification, the voltage and potential refer to the potential of the positive electrode unless otherwise specified.

[0429] The electrolyte in the electrolyte solution may be 1 mol / L lithium hexafluoride phosphate (LiPF6), and the electrolyte solution may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7, and vinylene carbonate (VC) at 2 wt%.

[0430] The positive electrode and negative electrode cans can be made of stainless steel (SUS).

[0431] The coin cell prepared under the above conditions is charged with a constant current of 10 mA / g to an arbitrary voltage (e.g., 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V). It is desirable to charge with such a small current to observe the phase change of the positive electrode active material. The ambient temperature for the coin cell is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere, and the positive electrode is removed to obtain a positive electrode active material with an arbitrary charge capacity. When performing various analyses afterward, it is preferable to seal the cell under an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to remove the positive electrode and subject it to analysis as soon as possible after charging is complete. Specifically, it is preferable to do this within 1 hour, and more preferably within 30 minutes.

[0432] Furthermore, when analyzing the crystal structure of the charged state after multiple charge-discharge cycles, the conditions for these multiple charge-discharge cycles may differ from the charging conditions described above. For example, charging can be performed by constant current charging at a current value of 100 mA / g up to an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V), then constant voltage charging until the current value becomes 10 mA / g, and discharging can be performed by constant current discharge at 2.5V and 100 mA / g.

[0433] Furthermore, when analyzing the crystal structure of the discharged state after multiple charge-discharge cycles, a constant current discharge can be performed, for example, at 2.5V and a current value of 100mA / g.

[0434] ≪XRD≫ The equipment and conditions for XRD measurement are not particularly limited. For example, measurements can be taken using the following equipment and conditions. XRD system: Bruker AXS D8 ADVANCE X-ray source:Cu Output: 40KV, 40mA Slit width: Div.Slit, 0.5° Detector: LynxEye Scanning method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0435] If the sample to be measured is a powder, it can be set up by placing it in a glass sample holder or by sprinkling the sample onto a grease-coated silicone anti-reflective plate. If the sample to be measured is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set up to match the measurement surface required by the device.

[0436] Figures 19 and 20 show the ideal powder XRD patterns calculated using CuKα1 lines, based on the O3' type crystal structure and the H1-3 type crystal structure model. For comparison, Li x The ideal XRD patterns calculated from the crystal structures of LiCoO2O3 at x=1, H1-3 type, and trigonal O1 at x=0 in CoO2 are also shown. The patterns for LiCoO2(O3) and CoO2(O1) were created using the Reflex Powder Diffraction module in Materials Studio (BIOVIA) from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 3). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength λ1 = 1.540562 × 10⁻¹⁰. -10m and λ2 were not set, and the Monochromator was set to single. The H1-3 type crystal structure pattern was similarly created from the crystal structure information described in Non-Patent Literature 3. For the O3' type crystal structure pattern, the crystal structure was estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitted using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and the XRD pattern was created in the same manner as the others.

[0437] As shown in Figure 19, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0438] However, as shown in Figure 20, peaks do not appear at these positions in the H1-3 type crystal structure and trigonal O1. Therefore, Li x The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°) when x in CoO2 is small is a characteristic feature of the positive electrode active material 200 in one embodiment of the present invention.

[0439] This can also be described as the positions where XRD diffraction peaks appear being close together in the crystal structure at x=1 and x≦0.24. More specifically, for the main diffraction peaks in the crystal structure at x=1 and x≦0.24 where 2θ is between 42° and 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0440] In one embodiment of the present invention, the positive electrode active material 200 is Li xWhen x in CoO2 is small, it has an O3' type crystal structure, but it does not have to be entirely an O3' type crystal structure. It may contain other crystal structures, and part of it may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3' type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3' type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be made into a cathode active material with sufficiently excellent cycle characteristics.

[0441] Furthermore, even after more than 100 charge-discharge cycles from the start of measurement, it is preferable that the O3' type crystal structure accounts for 35% or more, more preferably 40% or more, and even more preferably 43% or more when Rietveld analysis is performed.

[0442] Furthermore, the sharpness of diffraction peaks in the XRD pattern indicates high crystallinity. Therefore, it is preferable for each diffraction peak after charging to be sharp, i.e., have a narrow full width at half maximum (FWHM). The FWHM differs even for peaks arising from the same crystalline phase, depending on the XRD measurement conditions or the value of 2θ. Under the measurement conditions described above, for peaks observed between 2θ = 43° and 46°, the FWHM is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. It is not necessary for all peaks to satisfy this requirement. If some peaks satisfy this requirement, it can be said that the crystal phase has high crystallinity. High crystallinity contributes to the stabilization of the crystal structure after charging.

[0443] Furthermore, the crystallite size of the O3'-type crystal structure of the positive electrode active material 200 decreases to only about 1 / 20th of that of LiCoO2(O3) in the discharged state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, Li xWhen x is small in CoO2, a clear peak of the O3'-type crystal structure can be observed. On the other hand, in conventional LiCoO2, even if some parts can adopt a structure similar to the O3'-type crystal structure, the crystallite size becomes small, and the peak becomes broad and small. The crystallite size can be determined from the full width at half maximum of the XRD peak.

[0444] ≪XPS≫ In X-ray photoelectron spectroscopy (XPS), when using monochromatic aluminum Kα rays as the X-ray source for inorganic oxides, it is possible to analyze regions from the surface to a depth of approximately 2 to 8 nm (typically less than 5 nm). Therefore, the concentration of each element can be quantitatively analyzed in a region that is about half the depth of the surface layer (200a). Furthermore, narrow-scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often around ±1 atomic percent, and the detection limit is also around 1 atomic percent, although this varies depending on the element.

[0445] In one embodiment of the present invention, it is preferable that the concentration of one or more elements A selected from the additive elements A is higher in the surface layer 200a than in the interior 200b. This is equivalent to saying that it is preferable that the concentration of one or more elements A selected from the additive elements A in the surface layer 200a is higher than the average concentration of the entire positive electrode active material 200. For example, it can be said that it is preferable that the concentration of one or more elements A selected from the surface layer 200a, as measured by XPS, is higher than the average concentration of elements A of the entire positive electrode active material 200, as measured by ICP-MS or GD-MS. For example, it is preferable that the concentration of magnesium in at least a portion of the surface layer 200a, as measured by XPS, is higher than the magnesium concentration of the entire positive electrode active material 200. It is also preferable that the concentration of nickel in at least a portion of the surface layer 200a is higher than the nickel concentration of the entire positive electrode active material 200. Furthermore, it is preferable that the concentration of aluminum in at least a portion of the surface layer 200a is higher than the aluminum concentration of the entire positive electrode active material 200. Furthermore, it is preferable that the fluorine concentration in at least a portion of the surface layer 200a is higher than the fluorine concentration of the entire positive electrode active material 200.

[0446] In one embodiment of the present invention, the surface and surface layer 200a of the positive electrode active material 200 do not contain carbonates, hydroxyl groups, etc., that have been chemically adsorbed after the positive electrode active material 200 was manufactured. Furthermore, electrolyte, binder, conductive material, or compounds derived therefrom that are not included on the surface of the positive electrode active material 200. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc., which can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds through analysis, and corrections may be made to exclude CF bonds derived from the binder.

[0447] Furthermore, before subjecting the sample to various analyses, the positive electrode active material and positive electrode active material layer may be washed to remove electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve into the solvent used for washing, but even in that case, the additive element A is unlikely to dissolve, so it will not affect the atomic ratio of additive element A.

[0448] Furthermore, the concentration of additive element A may be compared in ratio to cobalt. Using the ratio to cobalt is preferable because it reduces the influence of carbon dioxide and other substances chemically adsorbed after the production of the positive electrode active material. For example, the ratio of magnesium to cobalt atoms, Mg / Co, determined by XPS analysis is preferably 0.4 or more and 1.5 or less. On the other hand, the Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0449] Similarly, in order to ensure sufficient pathways for lithium insertion and deinsertion, it is preferable that the concentrations of lithium and cobalt in the surface layer 200a of the positive electrode active material 200 are higher than those of each additive element A. This means that it is preferable that the concentrations of lithium and cobalt in the surface layer 200a are higher than the concentrations of one or more additive elements A selected from the additive elements A present in the surface layer 200a as measured by XPS, etc. For example, it is preferable that the concentration of at least a portion of cobalt in the surface layer 200a as measured by XPS, etc. is higher than the concentration of magnesium in at least a portion of the surface layer 200a as measured by XPS, etc. Similarly, it is preferable that the concentration of lithium is higher than the concentration of magnesium. It is also preferable that the concentration of cobalt is higher than the concentration of nickel. Similarly, it is preferable that the concentration of lithium is higher than the concentration of nickel. It is also preferable that the concentration of cobalt is higher than that of aluminum. Similarly, it is preferable that the concentration of lithium is higher than that of aluminum. It is also preferable that the concentration of cobalt is higher than that of fluorine. Similarly, it is preferable that the concentration of lithium is higher than that of fluorine.

[0450] Furthermore, it is more preferable that the additive elements Y, including aluminum, are widely distributed in deeper regions, for example, in the region between 5 nm and 50 nm from the surface. Therefore, it is more preferable that, although additive elements Y, including aluminum, are detected in the analysis of the entire cathode active material 200 using ICP-MS, GD-MS, etc., they are below the detection limit in XPS, etc.

[0451] Furthermore, when XPS analysis was performed on the positive electrode active material 200 according to one embodiment of the present invention, the number of magnesium atoms is preferably 0.4 to 1.2 times the number of cobalt atoms, and more preferably 0.65 to 1.0 times. Also, the number of nickel atoms is preferably 0.15 times or less, and more preferably 0.03 to 0.13 times the number of cobalt atoms. Also, the number of aluminum atoms is preferably 0.12 times or less, and more preferably 0.09 times or less. Also, the number of fluorine atoms is preferably 0.3 to 0.9 times the number of cobalt atoms, and more preferably 0.1 to 1.1 times.

[0452] For XPS analysis, for example, monochromatic aluminum Kα radiation can be used as the X-ray source. The extraction angle can be set to, for example, 45°. Measurements can be performed using, for example, the following apparatus and conditions. Measurement device: PHI QuanteraII X-ray source: Monochromatic Al Kα (1486.6eV) Detection area: 100 μmφ Detection depth: Approximately 4-5 nm (extraction angle 45°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0453] Furthermore, when the positive electrode active material 200 according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably around 684.3 eV. This value is different from both the bond energy of lithium fluoride, which is 685 eV, and the bond energy of magnesium fluoride, which is 686 eV. In other words, when the positive electrode active material 200 according to one embodiment of the present invention contains fluorine, it is preferable that the bond is with something other than lithium fluoride and magnesium fluoride.

[0454] Furthermore, when the positive electrode active material 200 according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably around 1303 eV. This value is different from the bond energy of magnesium fluoride, which is 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the positive electrode active material 200 according to one embodiment of the present invention contains magnesium, it is preferable that the bond is with an element other than magnesium fluoride.

[0455] ≪EDX≫ It is preferable that the additive element A in the positive electrode active material 200 has a concentration gradient. More preferably, the concentration distribution or the position of the peak top differs depending on the additive element A. The concentration distribution includes a concentration gradient. The concentration distribution of additive element A can be evaluated, for example, by exposing the cross-section of the positive electrode active material 200 using FIB (Focused Ion Beam) and analyzing the cross-section using EDX, EPMA (Electron Probe Microanalysis), etc.

[0456] In EDX measurements, the method of scanning within a region to evaluate it in two dimensions is called area analysis. The method of scanning linearly to evaluate the distribution of atomic concentration within the positive electrode active material is called line analysis. Furthermore, sometimes the extraction of linear region data from EDX area analysis is also called line analysis. Finally, measuring a region without scanning is called point analysis.

[0457] EDX surface analysis (e.g., elemental mapping) allows for semi-quantitative analysis of the concentration of additive element A in the surface layer 200a, interior 200b, and near grain boundaries 201 of the positive electrode active material 200. EDX radiation analysis can also analyze the concentration distribution or peak tops of additive element A. Furthermore, analysis methods that thin the sample, such as STEM-EDX, are preferable because they allow for analysis of the depth-direction concentration distribution from the surface to the center of the positive electrode active material in a specific region, with less influence from the depth-direction distribution.

[0458] Therefore, when EDX surface analysis or EDX point analysis is performed on the positive electrode active material 200 according to one embodiment of the present invention, it is preferable that the concentration of each additive element A, particularly additive element X, in the surface layer 200a is higher than that of the interior 200b.

[0459] For example, when EDX surface analysis or EDX point analysis is performed on a positive electrode active material 200 having magnesium as the additive element X, it is preferable that the magnesium concentration in the surface layer 200a is higher than the magnesium concentration in the interior 200b. Furthermore, when EDX radiation analysis is performed, it is preferable that the peak top of the magnesium concentration in the surface layer 200a is located within a depth of 3 nm from the surface toward the center of the positive electrode active material 200, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. In addition, it is preferable that the magnesium concentration distribution has a concentration gradient such that the concentration attenuates to 60% or less of the peak top at a position shifted 1 nm from the peak top. It is also preferable that the concentration gradient attenuates to 30% or less of the peak top at a position shifted 2 nm from the peak top. The shifted position may be toward the surface side or toward the interior side from the peak top. The above concentration gradient may be located at a position shifted toward either the surface side or the interior side.

[0460] Furthermore, in a positive electrode active material 200 having magnesium and fluorine as additive elements X, it is preferable that the distribution of fluorine overlaps with the distribution of magnesium. For example, it is preferable that the difference in position between the peak top of the fluorine concentration and the peak top of the magnesium concentration is within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0461] Furthermore, when EDX radiation analysis is performed, the peak of fluorine concentration in the surface layer 200a is preferably located within a depth of 3 nm from the surface toward the center of the positive electrode active material 200, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. It is also preferable that the peak of fluorine concentration be located slightly closer to the surface than the peak of magnesium concentration, as this increases resistance to hydrofluoric acid. For example, it is more preferable that the peak of fluorine concentration be located 0.5 nm or more closer to the surface than the peak of magnesium concentration, and even more preferable that it be located 1.5 nm or more closer to the surface.

[0462] Furthermore, in the positive electrode active material 200 having nickel as the additive element X, the peak top of the nickel concentration in the surface layer 200a is preferably located within a depth of 3 nm from the surface toward the center of the positive electrode active material 200, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Furthermore, in the positive electrode active material 200 having magnesium and nickel, the distribution of nickel is preferably superimposed on the distribution of magnesium. For example, the difference in position between the peak tops of magnesium concentration and magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0463] Furthermore, when the positive electrode active material 200 contains aluminum as an additive element Y, it is preferable that the peak top of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak top of the aluminum concentration in the surface layer 200a when EDX radiation analysis is performed. For example, it is preferable that the peak top of the aluminum concentration is located at a depth of 0.5 nm to 50 nm from the surface toward the center of the positive electrode active material 200, and more preferably at a depth of 5 nm to 50 nm.

[0464] Furthermore, when EDX radiation analysis, surface analysis, or point analysis is performed on the positive electrode active material 200, the ratio of the number of atoms of magnesium Mg to cobalt Co (Mg / Co) at the peak top of the magnesium concentration is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum Al to cobalt Co (Al / Co) at the peak top of the aluminum concentration is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel Ni to cobalt Co (Ni / Co) at the peak top of the nickel concentration is preferably 0 or more and 0.2 or less, and more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine F to cobalt Co (F / Co) at the peak top of the fluorine concentration is preferably 0 or more and 1.6 or less, and more preferably 0.1 or more and 1.4 or less.

[0465] Furthermore, the surface of the positive electrode active material 200 in the EDX radiation analysis results can be estimated as follows: For elements uniformly present in the interior 200b of the positive electrode active material 200, such as oxygen or cobalt, the point where the detected amount in the interior 200b becomes half is defined as the surface.

[0466] Since the positive electrode active material 200 is a composite oxide, the surface can be estimated using the amount of oxygen detected. Specifically, first, the average value of the oxygen concentration is obtained from the region where the amount of oxygen detected in the interior 200b is stable. ave We determine the amount of oxygen O in the region that is clearly outside the surface, which is thought to be due to chemiadsorption or background. background If detected, O background Subtracting this gives the average oxygen concentration O ave This can be done. This average value O ave Half of that value, that is, 1 / 2O ave The measurement point that shows the closest measurement value can be estimated to be the surface of the positive electrode active material.

[0467] The surface can also be estimated in the same way as described above using the amount of cobalt detected. Alternatively, it can be estimated similarly using the sum of the detected amounts of multiple transition metals. The detected amounts of transition metals, including cobalt, are suitable for surface estimation because they are less affected by chemiadsorption.

[0468] Furthermore, when line analysis or surface analysis is performed on the positive electrode active material 200, the ratio of added element A to cobalt Co (A / Co) near the grain boundary 201 is preferably 0.020 or more and 0.50 or less. More preferably 0.025 or more and 0.30 or less. More preferably 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less.

[0469] For example, when the additive element X is magnesium, when line analysis or surface analysis is performed on the positive electrode active material 200, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) near the grain boundary 201 is preferably 0.020 or more and 0.50 or less. Furthermore, 0.025 or more and 0.30 or less is preferred. Furthermore, 0.030 or more and 0.20 or less is preferred. Or 0.020 or more and 0.30 or less is preferred. Or 0.020 or more and 0.20 or less is preferred. Or 0.025 or more and 0.50 or less is preferred. Or 0.025 or more and 0.20 or less is preferred. Or 0.030 or more and 0.50 or less is preferred. Or 0.030 or more and 0.30 or less is preferred.

[0470] ≪EPMA≫ EPMA (Electron Probe Microanalysis) can also quantify elements. Surface analysis allows for the analysis of the distribution of each element.

[0471] When EPMA surface analysis is performed on a cross-section of the positive electrode active material 200 according to one embodiment of the present invention, it is preferable that one or more elements selected from the additive elements A have a concentration gradient, similar to the results of EDX analysis. Furthermore, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element A. The preferred range of the concentration peak of each additive element A is also the same as in the case of EDX.

[0472] However, EPMA analyzes only the area from the surface down to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from those obtained using other analytical methods. For example, when surface analysis of cathode active material 200 is performed using EPMA, the concentration of each additive element A present in the surface layer 200a may be lower than that obtained with XPS.

[0473] ≪Charging curve and dQ / dV curve≫ In one embodiment of the present invention, the positive electrode active material 200 may exhibit characteristic voltage changes during charging. These voltage changes can be read from the dQ / dV curve, obtained by differentiating the capacitance (Q) with respect to voltage (V) from the charging curve (dQ / dV). For example, around the peak in the dQ / dV curve, a non-equilibrium phase change is thought to occur, indicating a significant change in the crystal structure. In this specification, a non-equilibrium phase change refers to a phenomenon that causes a nonlinear change in a physical quantity.

[0474] In one embodiment of the present invention, the positive electrode active material 200 may have a broad peak around 4.55V in the dQ / dV curve. The peak around 4.55V reflects the voltage change during the phase change from O3 type to O3' type. Therefore, a broad peak means that the change in energy required to extract lithium is smaller than when the peak is sharp, i.e., the change in crystal structure is smaller. Smaller changes are preferable because they reduce the influence of shift and volume changes in the CoO2 layer.

[0475] More specifically, in the dQ / dV curve of the charging curve, when the maximum value appearing between 4.5V and 4.6V is defined as the first peak, a half-width of 0.10V or more of the first peak is considered sufficiently broad and preferable. In this specification, the half-width of the first peak is defined as the difference between HWHM1, the average value of the first peak and the first minimum value when the minimum value of the dQ / dV value appearing between 4.3V and 4.5V is defined as the first minimum value, and HWHM2, the average value of the first peak and the second minimum value when the minimum value of the dQ / dV value appearing between 4.6V and 4.8V is defined as the second minimum value.

[0476] When acquiring the dQ / dV curve, charging can be performed using a constant current charge of 10mA / g up to, for example, 4.9V. Furthermore, when acquiring the dQ / dV for the initial charge, it is preferable to discharge the battery to 2.5V at 100mA / g before starting the above charging process.

[0477] The data acquisition interval during charging can be set to, for example, every second or to acquire voltage and current when there is a voltage fluctuation of 1 mV. The charging capacity is calculated by integrating the current value and time.

[0478] The difference between the nth and (n+1)th data points of the above charging capacity data is taken as the nth value of the capacity change dQ. Similarly, the difference between the nth and (n+1)th data points of the above voltage data is taken as the nth value of the voltage change dV.

[0479] However, since the above data is susceptible to the influence of minute noise, dQ / dV may be calculated from a moving average of a certain number of intervals for the difference between voltage and charging capacity. The number of intervals can be, for example, 500.

[0480] Specifically, the average value of dQ from the nth to the n+500th value is calculated, and similarly, the average value of dV from the nth to the n+500th value is calculated. dQ(average of 500 values) / dV(average of 500 values) can be expressed as dQ / dV. Similarly, the voltage on the horizontal axis of the dQ / dV curve can also be calculated using a moving average of 500 intervals. However, when using a moving average of 500 intervals as described above, it is preferable not to use the data from the 501st to the last data point to the last data point in the dQ / dV curve, as the noise influence becomes significant.

[0481] Furthermore, when analyzing the dQ / dV curve after multiple charge-discharge cycles, the conditions for these multiple cycles may differ from the charging conditions described above. For example, charging can be performed at an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V) with a constant current of 100mA / g, followed by constant voltage charging until the current reaches 10mA / g, and then discharging can be performed at 2.5V with a constant current of 100mA / g.

[0482] Furthermore, at around 4.55V, the phase changes from O3 type to O3' type, and at this time, the O3 type is Li xThe x value in CoO2 is approximately 0.3. This exhibits the same symmetry as the O3 type with x=1, as explained in Figure 17, but the distance between CoO2 layers is slightly different. In this specification, when distinguishing between O3 types with different x values, the O3 type with x=1 is referred to as O3(2θ=18.85), and the O3 type with x=0.3 is referred to as O3(2θ=18.57). This is because the position of the peak where 2θ appears around 19° in XRD measurements corresponds to the distance between CoO2 layers.

[0483] ≪Discharge curve and dQ / dV curve≫ Furthermore, in one embodiment of the present invention, when the positive electrode active material 200 is charged at a high voltage and then discharged at a low current of, for example, 40 mA / g or less, a characteristic voltage change may appear near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the dQ / dV curve obtained from the discharge curve, at a voltage lower than the peak that appears around 3.9 V, and in the range up to 3.5 V.

[0484] ≪ESR≫ In one embodiment of the present invention, the positive electrode active material 200 has cobalt, and preferably has nickel and magnesium as additive element A. As a result, some Co 3+ Ni 3+ It is replaced by, and also some Li + is Mg 2+ It is preferable that it be replaced with Li. + is Mg 2+ As a result of being replaced by, the Ni 3+ It is reduced to Ni 2+ This can happen. Also, some Li + is Mg 2+ It is replaced by Mg 2+ Nearby Co 3+ It is reduced to Co 2+ This can happen. Also, some Co 3+ is Mg 2+ It is replaced by Mg 2+ Nearby Co 3+ It is oxidized to Co 4+ This can happen.

[0485] Therefore, the positive electrode active material 200 is Ni 2+ Ni 3+ Co 2+ and Co 4+ It is preferable to have one or more of the following. Also, Ni per unit weight of positive electrode active material 200 2+ Ni 3+ Co 2+ and Co 4+ The spin density resulting from one or more of the following is 2.0 × 10 17 spins / g or more 1.0×10 21 It is preferable that the spin density is less than or equal to spins / g. Using the above-mentioned positive electrode active material 200 is preferable because it stabilizes the crystal structure, especially in the charged state. Note that if the magnesium concentration is too high, Ni 2+ Ni 3+ Co 2+ and Co 4+ A decrease in spin density may occur due to one or more of the following factors.

[0486] The spin density in the positive electrode active material can be analyzed using methods such as electron spin resonance (ESR).

[0487] ≪Surface roughness and specific surface area≫ In one embodiment of the present invention, the positive electrode active material 200 preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of the flux described later has been fully exerted, and the surface of the composite oxide and the source of the added element A have melted. This is one factor indicating that the distribution of the added element A in the surface layer 200a is good. Good distribution means, for example, that the concentration distribution of the added element A in the surface layer 200a is uniform.

[0488] The smoothness and minimal irregularities of the surface can be determined, for example, from a cross-sectional SEM image or TEM image of the positive electrode active material 200, or from the specific surface area of ​​the positive electrode active material 200.

[0489] For example, the surface smoothness of the positive electrode active material 200 can be quantified from a cross-sectional SEM image, as shown below.

[0490] First, the positive electrode active material 200 is processed using FIB or the like to expose its cross-section. At this time, it is preferable to cover the positive electrode active material 200 with a protective film, protective agent, etc. Next, an SEM image of the interface between the protective film, etc. and the positive electrode active material 200 is taken. Noise processing is performed on the SEM image using image processing software. For example, Gaussian blurring (σ=2) is performed, followed by binarization. Interface extraction is then performed using image processing software. Furthermore, the interface line between the protective film, etc. and the positive electrode active material 200 is selected using an automatic selection tool, etc., and the data is extracted into spreadsheet software, etc. Using the functions of the spreadsheet software, correction is performed from the regression curve (quadratic regression), parameters for roughness calculation are obtained from the slope-corrected data, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness of the positive electrode active material at least at the outer circumference of 400 nm.

[0491] In this embodiment, the root mean square (RMS) surface roughness of the positive electrode active material 200 is preferably less than 3 nm, more preferably less than 1 nm, and more preferably less than 0.5 nm.

[0492] The image processing software used for noise reduction, interface extraction, etc., is not particularly limited, but for example, "ImageJ" can be used. Similarly, the spreadsheet software is not particularly limited, but for example, Microsoft Office Excel can be used.

[0493] For example, the actual specific surface area S measured by the gas adsorption method using the constant-volume method. R And the ideal specific surface area S i The surface smoothness of the positive electrode active material 200 can also be quantified from this ratio.

[0494] Ideal specific surface area S i This is calculated by assuming that all positive electrode active materials have the same diameter as D50, the same weight, and are ideally spherical in shape.

[0495] The median diameter (D50) can be measured using a particle size analyzer that employs laser diffraction and scattering methods. The specific surface area can be measured using a specific surface area measuring device that employs, for example, a gas adsorption method using a constant volume method.

[0496] In one embodiment of the present invention, the positive electrode active material 200 has an ideal specific surface area A determined from the median diameter (D50). i And the actual specific surface area S R Ratio S R / S i It is preferable that the value is 2.1 or less.

[0497] Alternatively, the surface smoothness of the positive electrode active material 200 can be quantified from a cross-sectional SEM image using the following method.

[0498] First, a surface SEM image of the positive electrode active material 200 is obtained. A conductive coating may be applied as a pretreatment before observation. It is preferable that the observation surface is perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area should be the same.

[0499] Next, using image processing software (for example, "ImageJ"), the above SEM image is converted to, for example, an 8-bit image (this is called a grayscale image). A grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be represented by 2 to the power of 8 = 256 gradations. Darker areas have a lower number of gradations, and brighter areas have a higher number of gradations. The change in luminance can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it becomes possible to numerically evaluate the unevenness of the positive electrode active material.

[0500] Furthermore, it becomes possible to represent the brightness changes of the target area using a histogram. A histogram is a three-dimensional representation of the grayscale distribution in a target area, and is also called a brightness histogram. By obtaining a brightness histogram, it becomes possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0501] In one embodiment of the present invention, the positive electrode active material 200 preferably has a difference of 120 or less between the maximum and minimum values ​​of the grayscale values, more preferably 115 or less, and even more preferably 70 or more and 115 or less. Furthermore, the standard deviation of the grayscale values ​​is preferably 11 or less, more preferably 8 or less, and even more preferably 4 or more and 8 or less.

[0502] ≪Current pause method≫ In one embodiment of the present invention, the distribution of additive elements A, including magnesium, present in the surface layer of the positive electrode active material 200 may change slightly during repeated charging and discharging cycles. For example, the distribution of additive elements A may improve, leading to a decrease in electron conduction resistance. As a result, the electrical resistance, i.e., the fast-responding resistance component R(0.1s) measured by the current pause method, may decrease in the initial stages of the charge-discharge cycle.

[0503] For example, when comparing the nth charge (where n is an integer greater than 1) with the (n+1)th charge, the fast-responding resistance component R(0.1s), measured by the current pause method, may be lower in the (n+1)th charge than in the nth charge. Consequently, the discharge capacity in the (n+1)th charge may be higher than that in the nth charge. When n is 1, that is, when comparing the first charge with the second charge, the second charge capacity may be larger, which can occur even with positive electrode active materials that do not contain additive elements. Therefore, it is preferable that n is, for example, between 2 and 10. However, this is not limited to the initial stages of the charge-discharge cycle. A charge-discharge capacity that is approximately equal to the rated capacity, for example, 97% or more of the rated capacity, can be considered the initial stage of the charge-discharge cycle.

[0504] <Additional features> The positive electrode active material 200 may have depressions, cracks, pits, or V-shaped cross-sections. These are defects, and repeated charging and discharging may lead to the leaching of transition metal M, collapse of the crystal structure, cracking of the main body, and desorption of oxygen. However, if there are embedded portions 202 as shown in Figure 13B to fill these defects, the leaching of transition metal M and other defects can be suppressed. Therefore, a positive electrode active material 200 with excellent reliability and cycle characteristics can be obtained.

[0505] Furthermore, the positive electrode active material 200 may have convex portions 203 as shown in Figure 13B, which are regions where the added element A is unevenly distributed.

[0506] As mentioned above, if the additive element A in the positive electrode active material 200 is in excess, it may adversely affect the insertion and removal of lithium. It may also lead to an increase in internal resistance and a decrease in charge / discharge capacity when used in a secondary battery. On the other hand, if it is insufficient, it may not be distributed throughout the entire surface layer 200a, and the effect of suppressing the deterioration of the crystal structure may be insufficient. Thus, the additive element A in the positive electrode active material 200 needs to be at an appropriate concentration, but adjusting it is not easy.

[0507] Therefore, if the positive electrode active material 200 has regions where the additive element A is unevenly distributed, some of the excess additive element A is removed from the interior 200b of the positive electrode active material 200, allowing for an appropriate concentration of additive element A in the interior 200b. This suppresses the increase in internal resistance and the decrease in charge / discharge capacity when used as a secondary battery. The ability to suppress the increase in the internal resistance of a secondary battery is an extremely desirable characteristic, especially when charging and discharging at high currents, such as 400 mA / g or higher.

[0508] Furthermore, in positive electrode active material 200 having regions where additive element A is unevenly distributed, it is permissible to mix additive element A in excess to some extent during the manufacturing process. This is preferable because it widens the margin in production.

[0509] Furthermore, the positive electrode active material may develop progressive defects that penetrate deep from the surface into the interior when charged under conditions such as charging at 4.5V or higher, or when charged and discharged in high-temperature environments, such as an ambient temperature of 45°C or higher. The phenomenon in which defects in the positive electrode active material progress to form holes can also be called pitting corrosion, and the holes generated by this phenomenon are referred to as pits in this specification.

[0510] Figure 21 shows a schematic cross-sectional view of the positive electrode active material 51 having pits. A crystal plane 55 parallel to the arrangement of cations is also shown. Since Figure 21 is a cross-sectional view, pits 54 and 58 are shown as holes, but the shape of these openings is not circular but has depth and a groove-like shape. Also, as shown in pits 54 and 58, unlike recesses 52, they tend to occur parallel to the arrangement of lithium ions.

[0511] Furthermore, the surface layers of the positive electrode active material 51 where the added element A is present are shown as 53 and 56. In the surface layers where pits have formed, the amount of added element A is less than in 53 and 56 or below the detection limit, suggesting that the function of the barrier film is reduced. In addition, it is thought that the crystal structure of the composite oxide collapses near where the pits form, resulting in a crystal structure different from that of the layered rock salt type. Since the collapse of the crystal structure inhibits the diffusion and release of lithium ions, which are carrier ions, the pits are considered to be a factor in the deterioration of cycle characteristics.

[0512] The source of the pits may be point defects. It is thought that point defects in the positive electrode active material change with repeated charging and discharging, and are chemically or electrochemically eroded by the surrounding electrolyte, or that the material deteriorates. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather occurs locally and concentrated.

[0513] Furthermore, as shown in crack 57 in Figure 21, defects such as cracks (also called fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification, cracks and pits are different. Cracks may exist immediately after the positive electrode active material is manufactured, but pits do not. A pit can be described as a hole where several layers of transition metal M and oxygen have been removed due to charging and discharging under high voltage conditions of 4.5V or higher or high temperature (45°C or higher), and can also be described as a location where the transition metal M has dissolved. A crack refers to a new surface created by, for example, the application of physical pressure, or a fissure caused by a grain boundary 201. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In addition, pits may occur from cracks and / or cavities inside the positive electrode active material.

[0514] [Method for preparing positive electrode active material] A method for producing a positive electrode active material 200 having the distribution, composition, and / or crystal structure of additive element A as described in the previous embodiment will now be explained.

[0515] In the process of preparing the positive electrode active material 200, it is preferable to first synthesize a composite oxide having lithium and a transition metal, and then mix in the additive element A source and perform a heat treatment.

[0516] In a method of synthesizing a composite oxide containing additive element A, lithium, and transition metal M by mixing a transition metal M source and a lithium source simultaneously with an additive element A source, it is difficult to increase the concentration of additive element A in the surface layer 200a. Furthermore, if the additive element A source is only mixed after synthesizing the composite oxide containing lithium and transition metal M without heating, the additive element will only adhere to the composite oxide without solid dissolution. Without sufficient heating, it is also difficult to properly distribute the additive element A. Therefore, it is preferable to synthesize the composite oxide, then mix in the additive element A source, and then perform a heat treatment. This heat treatment after mixing in the additive element A source is sometimes called annealing.

[0517] However, if the annealing temperature is too high, cation mixing occurs, increasing the likelihood that the added element A, for example, magnesium, will enter the transition metal M site. Magnesium present at the transition metal M site is Li x When x in CoO2 is small, it does not maintain the layered rock salt crystal structure of R-3m. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent cobalt and the evaporation or sublimation of lithium.

[0518] Therefore, it is preferable to mix a material that functions as a flux together with the source of additive element A. Any material that has a lower melting point than the composite oxide having lithium and transition metal M can be considered to function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux causes a melting point depression in both the source of additive element A and the composite oxide having lithium and transition metal M. By lowering the melting point, it becomes easier to distribute the additive element A well at a temperature where cation mixing is less likely to occur.

[0519] Furthermore, it is even more preferable to heat the composite oxide containing lithium and transition metal M before mixing in the added element A. This heating is sometimes referred to as initial heating.

[0520] Initial heating causes lithium to desorb from a portion of the surface layer 200a of the composite oxide containing lithium and transition metal M, which further improves the distribution of additive element A.

[0521] More specifically, the following mechanism is thought to make it easier to differentiate the distribution of added element A through initial heating. First, lithium is desorbed from a portion of the surface layer 200a by initial heating. Next, a composite oxide containing lithium and a transition metal M, which has this lithium-deficient surface layer 200a, is mixed with a source of added element A, including nickel, aluminum, and magnesium, and heated. Of the added element A, magnesium is a divalent typical element, and nickel is a transition metal but is prone to becoming a divalent ion. Therefore, Mg 2+ and Ni 2+ And, due to lithium deficiency, Co 2+ A rock salt-type phase is formed, which has the following characteristics.

[0522] Of the added element A, nickel readily dissolves in the composite oxide containing layered rock salt-type lithium and transition metal M in the surface layer 200a, and diffuses to the interior 200b. However, if a portion of the surface layer 200a is rock salt-type, nickel tends to remain in the surface layer 200a.

[0523] Furthermore, in these rock salt types, the bond distance between metallic Me and oxygen (Me-O distance) tends to be longer than in layered rock salt types.

[0524] For example, rock salt type 0.5 Mg 0.5 The Me-O distance in O is 0.209 nm, and the Me-O distance in rock salt type MgO is 0.211 nm. Furthermore, even if a spinel-type phase is formed in a part of the surface layer 200a, the Me-O distance of spinel-type NiAl2O4 is 0.20125 nm, and the Me-O distance of spinel-type MgAl2O4 is 0.202 nm. In both cases, the Me-O distance exceeds 0.2 nm.

[0525] On the other hand, in layered rock salt, the bond distance between metals other than lithium and oxygen is shorter than described above. For example, in layered rock salt LiAlO2, the Al-O distance is 0.1905 nm (Li-O distance is 0.211 nm). Also, in layered rock salt LiCoO2, the Co-O distance is 0.19224 nm (Li-O distance is 0.20916 nm).

[0526] According to Shannon et al., Acta A 32 (1976) 751, the ionic radius of 6-coordinate aluminum is 0.0535 nm, and the ionic radius of 6-coordinate oxygen is 0.14 nm, with their sum being 0.1935 nm.

[0527] From the above, it is considered that aluminum exists more stably at sites other than lithium in the layered rock salt type than in the rock salt type. Therefore, aluminum is more likely to be distributed in deeper regions with the layered rock salt type and / or in the interior 200b of the surface layer 200a than in the region closer to the surface with the rock salt type phase.

[0528] Furthermore, initial heating is expected to enhance the crystallinity of the layered rock salt-type crystalline structure within the 200b layer.

[0529] However, initial heating is not always necessary. By controlling the atmosphere, temperature, time, etc., in other heating processes, such as annealing, Li xWhen x in CoO2 is small, it may be possible to produce a positive electrode active material 200 having the O3' type.

[0530] An example of the production flow for the positive electrode active material 200, which undergoes initial heating, will be explained using Figures 22A to 22C.

[0531] <Step S11> In step S11 shown in Figure 22A, lithium sources (Li sources) and transition metal M sources (M sources) are prepared as the starting materials, lithium and transition metal M, respectively.

[0532] As a lithium source, it is preferable to use a lithium-containing compound, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source should preferably have high purity; for example, a material with a purity of 99.99% or higher is preferable.

[0533] The transition metal M can be selected from elements listed in groups 4 to 13 of the periodic table, for example, at least one of manganese, cobalt, and nickel can be used. The transition metal M may be cobalt only, nickel only, cobalt and manganese, cobalt and nickel, or cobalt, manganese, and nickel. When cobalt only is used, the resulting positive electrode active material contains lithium cobalt oxide (LCO), and when cobalt, manganese, and nickel are used, the resulting positive electrode active material contains nickel-cobalt-lithium manganese oxide (NCM).

[0534] As the transition metal M source, it is preferable to use a compound having the above-mentioned transition metal M. For example, oxides of the metals exemplified above as the transition metal M, or hydroxides of the exemplified metals can be used. If it is a cobalt source, cobalt oxide, cobalt hydroxide, etc. can be used. If it is a manganese source, manganese oxide, manganese hydroxide, etc. can be used. If it is a nickel source, nickel oxide, nickel hydroxide, etc. can be used. Although it is not a transition metal, it is also possible to use an aluminum source. If it is an aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0535] The transition metal M source is preferably of high purity; for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N5 (99.995%) or higher, and even more preferably 5N (99.999%) or higher is used. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0536] In addition, it is preferable that the transition metal M source has high crystallinity, for example, having single crystal grains. The crystallinity of the transition metal M source can be evaluated by TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle scattering annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, etc., or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. The above methods for evaluating crystallinity can be applied not only to transition metal M sources but also to the evaluation of other materials.

[0537] Furthermore, when using two or more transition metal M sources, it is preferable to prepare them in a proportion (mixing ratio) such that the two or more transition metal M sources can adopt a layered rock salt type crystalline structure.

[0538] <Step S12> Next, as step S12 shown in Figure 22A, the lithium source and the transition metal M source are crushed and mixed to prepare a mixed material. Crushing and mixing can be done dry or wet. Wet crushing is preferred because it allows for finer crushing. If wet crushing is used, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is more preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the transition metal M source with dehydrated acetone with a purity of 99.5% or higher, with a water content of 10 ppm or less, and then crush and mix them. By using dehydrated acetone of such purity as described above, the amount of impurities that may be introduced can be reduced.

[0539] A ball mill or bead mill can be used for mixing and other processes. When using a ball mill, it is preferable to use aluminum oxide balls or zirconium oxide balls as the grinding media. Zirconium oxide balls are preferable because they produce less impurity. When using a ball mill or bead mill, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0540] <Step S13> Next, in step S13 shown in Figure 22A, the mixed material is heated. The heating temperature is preferably between 800°C and 1100°C, more preferably between 900°C and 1000°C, and even more preferably around 950°C. If the temperature is too low, the decomposition and melting of the lithium source and the transition metal M source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to the evaporation of lithium from the lithium source and / or the excessive reduction of the metal used as the transition metal M source. Such defects include, for example, when cobalt is used as the transition metal M, excessive reduction can cause the cobalt to change from trivalent to divalent, inducing oxygen defects.

[0541] If the heating time is too short, LiMO2 will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time should ideally be between 1 hour and 100 hours, and preferably between 2 hours and 20 hours.

[0542] The heating rate depends on the target temperature, but a rate between 80°C / h and 250°C / h is generally recommended. For example, when heating to 1000°C for 10 hours, a heating rate of 200°C / h is appropriate.

[0543] The heating atmosphere should preferably be a dry air atmosphere with low moisture content, for example, an atmosphere with a dew point of -50°C or lower, more preferably -80°C or lower. In this embodiment, heating will be carried out in an atmosphere with a dew point of -93°C. In order to suppress impurities that may be mixed into the material, the concentrations of impurities such as CH4, CO, CO2, and H2 in the heating atmosphere should be kept below 5 ppb (parts per billion) each.

[0544] An atmosphere containing oxygen is preferred as the heating atmosphere. For example, one method is to continuously introduce dry air into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flowing through the chamber is called flow.

[0545] When the heating atmosphere is an oxygen-containing atmosphere, a method that does not involve flowing oxygen is also acceptable. For example, the reaction chamber can be depressurized and then filled with oxygen, preventing the oxygen from entering or leaving the reaction chamber; this method is called purging. For example, the reaction chamber can be depressurized to -970 hPa and then filled with oxygen up to 50 hPa.

[0546] After heating, natural cooling is acceptable, but it is preferable that the cooling time from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary; it is sufficient if it cools to a temperature acceptable for the next step.

[0547] Heating in this process may be carried out using a rotary kiln or a roller hearth kiln. When using a rotary kiln, heating can be performed while stirring, whether in a continuous or batch system.

[0548] The crucible or sheath used during heating is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite / cordierite, magnesia, or zirconia. Furthermore, since aluminum oxide is a material that is less prone to impurities, the purity of the alumina crucible or sheath should be 99% or higher, preferably 99.5% or higher. In this embodiment, a crucible made of aluminum oxide with a purity of 99.9% is used. It is preferable to place a lid on the crucible or sheath before heating. This can prevent the volatilization or sublimation of the material.

[0549] After heating is complete, the material may be crushed and sieved as needed. When collecting the heated material, it may be transferred from the crucible to a mortar before collection. It is preferable to use a mortar made of aluminum oxide. Mortars made of aluminum oxide are less likely to release impurities. Specifically, a mortar made of aluminum oxide with a purity of 90% or higher, preferably 99% or higher, should be used. In addition, heating conditions equivalent to those in step S13 can be applied to the heating processes described later, other than step S13.

[0550] <Step S14> Through the above process, a composite oxide (LiMO2) containing a transition metal M can be obtained in step S14 shown in Figure 22A. The composite oxide only needs to have the crystal structure of a lithium composite oxide represented as LiMO2, and its composition is not strictly limited to Li:M:O=1:1:2. When cobalt is used as the transition metal M, it is called a composite oxide containing cobalt and is represented as LiCoO2. The composition is not strictly limited to Li:Co:O=1:1:2.

[0551] Although examples of producing composite oxides by solid-phase methods have been shown as in steps S11 to S14, composite oxides may also be produced by coprecipitation or by hydrothermal methods.

[0552] <Step S15> Next, as step S15 shown in Figure 22A, the composite oxide is heated. Because this is the first heating of the composite oxide, the heating in step S15 is sometimes called initial heating. Alternatively, because it is heated before step S20 shown below, it may be called preheating or pretreatment.

[0553] As described above, initial heating causes lithium to be desorbed from a portion of the surface layer 200a of the composite oxide. It is also expected to improve the crystallinity of the interior 200b. Furthermore, the lithium source and / or transition metal M prepared in step S11, etc., may contain impurities. Initial heating makes it possible to reduce impurities from the composite oxide completed in step 14.

[0554] Furthermore, initial heating has the effect of smoothing the surface of the composite oxide. A smooth surface means that there are few irregularities, the composite oxide is generally rounded, and the corners are also rounded. In addition, a smooth surface is defined as having few foreign substances adhering to it. Foreign substances are thought to be a cause of irregularities, so it is preferable that they do not adhere to the surface.

[0555] For this initial heating, it is not necessary to prepare a lithium compound source, nor is it necessary to prepare a source of additive element A, nor is it necessary to prepare a material that functions as a flux.

[0556] If the heating time in this process is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, it can be performed by selecting from the heating conditions described in step S13. To add to those heating conditions, the heating temperature in this process should be lower than the temperature in step S13 in order to maintain the crystal structure of the composite oxide. Also, the heating time in this process should be shorter than the time in step S13 in order to maintain the crystal structure of the composite oxide. For example, heating at a temperature of 700°C to 1000°C for 2 to 20 hours is recommended.

[0557] Furthermore, the effect of increasing the crystallinity of the internal 200b is, for example, the effect of mitigating strain, displacement, etc., that originate from the difference in shrinkage of the composite oxide fabricated in step S13.

[0558] The above-mentioned composite oxide may develop a temperature difference between its surface and interior due to heating in step S13. This temperature difference can induce a difference in shrinkage. It is thought that the difference in shrinkage occurs because the fluidity of the surface and interior differs due to the temperature difference. The energy associated with the difference in shrinkage gives the composite oxide a difference in internal stress. This difference in internal stress is also called strain, and the energy associated with it is sometimes called strain energy. The internal stress is removed by the initial heating in step S15, or in other words, the strain energy is considered to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain of the composite oxide is relieved. Therefore, after step S15, the surface of the composite oxide may become smoother. This is also referred to as the surface being improved. In other words, it is thought that after step S15, the difference in shrinkage that occurred in the composite oxide is relieved, and the surface of the composite oxide becomes smoother.

[0559] Furthermore, differences in shrinkage can cause microscopic displacements in the composite oxide, such as crystal displacements. This process is also recommended to reduce such displacements. This process makes it possible to homogenize the displacements in the composite oxide. When the displacements are homogenized, the surface of the composite oxide may become smoother. This can also be described as the crystal grains being aligned. In other words, it is believed that step S15 alleviates the crystal displacements and other issues that have occurred in the composite oxide, resulting in a smoother surface.

[0560] Using a composite oxide with a smooth surface as the positive electrode active material reduces degradation during charging and discharging in a secondary battery and prevents cracking of the positive electrode active material.

[0561] A smooth surface of a composite oxide can be defined as having a surface roughness of at least 10 nm or less, when the surface irregularities information is quantified from measurement data in a cross-section of the composite oxide. This cross-section is, for example, the cross-section obtained during STEM observation.

[0562] Furthermore, a pre-synthesized composite oxide containing lithium, a transition metal M, and oxygen may be used as step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on the pre-synthesized composite oxide, a composite oxide with a smooth surface can be obtained.

[0563] It is possible that the lithium in the composite oxide decreases due to initial heating. The reduced lithium, which will be explained in the next step S20, may make it easier for additive element A to enter the composite oxide.

[0564] <Step S20> Additive element A may be added to a composite oxide with a smooth surface, as long as it can adopt a layered rock salt-type crystalline structure. Adding additive element A to a composite oxide with a smooth surface allows for uniform addition of additive element A. Therefore, it is preferable to add additive element A after initial heating. The step of adding additive element A will be explained using Figures 22B and 22C.

[0565] <Step S21> In step S21 shown in Figure 22B, a source of additive element A (source A) to be added to the composite oxide is prepared. A lithium source may also be prepared along with the additive element source A.

[0566] As additive element A, one or more can be selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic. Alternatively, one or more can be selected from bromine and beryllium as additive elements. However, since bromine and beryllium are toxic to living organisms, it is preferable to use the additive elements mentioned above.

[0567] When magnesium is selected as additive element A, the source of additive element A can be called a magnesium source. Examples of such magnesium sources include magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate. Multiple magnesium sources may also be used.

[0568] When fluorine is selected as additive element A, the additive element A source can be called a fluorine source. Examples of suitable fluorine sources include lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, lanthanum fluoride (LaF3), or sodium aluminum hexafluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating process described later.

[0569] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Another lithium source used in step S21 is lithium carbonate.

[0570] The fluorine source may also be a gas, such as fluorine (F2), carbon fluoride, sulfur fluoride, or oxygen fluoride, which may be mixed into the atmosphere during the heating process described later. Multiple fluorine sources may also be used.

[0571] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as both the fluorine source and the magnesium source. When lithium fluoride and magnesium fluoride are mixed at a molar ratio of LiF:MgF2 = 65:35 or near thereto, the gel effect is highest when the melting point is lowered. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be in excess and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2 = x:1 (0 ≤ x ≤ 1.9), more preferably LiF:MgF2 = x:1 (0.1 ≤ x ≤ 0.5), and even more preferably LiF:MgF2 = x:1 (x = 0.33 or near thereto). In this specification, "near thereto" means a value greater than 0.9 times and less than 1.1 times the value.

[0572] At the same time, the amount of magnesium added is preferably more than 0.1 atomic% and 3 atomic% or less based on LiCoO2, more preferably 0.5 atomic% to 2 atomic% or less, and even more preferably 0.5 atomic% to 1 atomic% or less. If the amount of magnesium added is 0.1 atomic% or less, the initial discharge capacity is high, but the discharge capacity decreases rapidly when repeated charge-discharge cycles with high charge depths are performed. If the amount of magnesium added is more than 0.1 atomic% and 3 atomic% or less, both the initial discharge characteristics and charge-discharge cycle characteristics are good even when repeated charge-discharge cycles with high charge depths are performed. On the other hand, if the amount of magnesium added exceeds 3 atomic%, both the initial discharge capacity and charge-discharge cycle characteristics tend to gradually deteriorate.

[0573] <Step S22> Next, in step S22 shown in Figure 22B, the magnesium source and the fluorine source are crushed and mixed. This step can be performed by selecting from the crushing and mixing conditions described in step S12.

[0574] A heating step may be performed after step S22 if necessary. The heating step can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800°C to 1100°C.

[0575] <Step S23> Next, in step S23 shown in Figure 22B, the material that has been crushed and mixed above is recovered to obtain the additive element A source (A source). The additive element A source shown in step S23 has multiple starting materials and can be called a mixture.

[0576] The particle size of the above mixture is preferably such that the median diameter (D50) is 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. Even when one material is used as the source of additive element A, the median diameter (D50) is preferably 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less.

[0577] When such a finely powdered mixture (including cases where only one element A is added) is used, it is easier to uniformly adhere the mixture to the surface of the composite oxide when it is mixed with the composite oxide in a later process. It is preferable that the mixture, such as the source of element A, is uniformly adhered to the surface of the composite oxide, as this facilitates the uniform distribution or diffusion of fluorine and magnesium to the surface layer of the composite oxide after heating. The region where fluorine and magnesium are distributed can also be called the surface layer. If there are regions in the surface layer that do not contain fluorine and magnesium, it may be difficult to form an O3' type crystal structure in the charged state. Although fluorine was used in this explanation, fluorine may also be chlorine, and the term "halogen" can be used interchangeably if these are included.

[0578] <Step S21> A process different from that shown in Figure 22B will be explained using Figure 22C. In step S21 shown in Figure 22C, four types of additive element A sources are prepared to be added to the composite oxide. In other words, Figure 22C shows a different type of additive element A source than Figure 22B. A lithium source may also be prepared along with the additive element A sources.

[0579] Four types of additive element A sources are prepared: a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source). The magnesium and fluorine sources can be selected from the compounds described in Figure 22B. Nickel sources such as nickel oxide and nickel hydroxide can be used. Aluminum sources such as aluminum oxide and aluminum hydroxide can be used.

[0580] <Step S22> and <Step S23> Next, steps S22 and S23 shown in Figure 22C are the same as the steps described in Figure 22B.

[0581] <Step S31> Next, in step S31 shown in Figure 22A, the composite oxide and the additive element A source (A source) are mixed. The ratio of the number of atoms M of the transition metal M in the composite oxide having lithium, transition metal M, and oxygen to the number of atoms Mg of magnesium in the additive element A is preferably M:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0582] The mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the complex oxide. For example, it is preferable to use conditions with a lower rotation speed or shorter time than the mixing in step S12. Also, dry mixing is generally milder than wet mixing. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0583] In this embodiment, the mixing is performed dry using a ball mill with zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour. The mixing is carried out in a dry room with a dew point of -100°C or higher and -10°C or lower.

[0584] <Step S32> Next, in step S32 of Figure 22A, the materials mixed above are recovered to obtain mixture 903. During recovery, if necessary, the materials may be crushed and then sieved.

[0585] In this embodiment, a method is described in which lithium fluoride is added as a fluorine source and magnesium fluoride as a magnesium source to the composite oxide after initial heating. However, the present invention is not limited to the above method. At step S11, that is, at the stage of the starting materials of the composite oxide, the magnesium source and fluorine source can be added to the lithium source and the transition metal M source. Then, in step S13, heating is performed to obtain LiMO2 with added magnesium and fluorine. In this case, it is not necessary to separate the processes of steps S11 to S14 from the processes of steps S21 to S23. This can be said to be a simple and highly productive method.

[0586] Alternatively, a composite oxide with magnesium and fluorine added beforehand may be used. Using a composite oxide with magnesium and fluorine added allows for the omission of steps S11 to S32 and step S20. This can be considered a simple and highly productive method.

[0587] Alternatively, to a composite oxide to which magnesium and fluorine have been added in advance, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added in accordance with step S20.

[0588] <Step S33> Next, in step S33 shown in Figure 22A, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. A heating time of 2 hours or more is preferable.

[0589] Let me add some information about the heating temperature here. The lower limit of the heating temperature in step S33 must be above the temperature at which the reaction between the composite oxide (LiMO2) and the additive element A source proceeds. The temperature at which the reaction proceeds is the temperature at which interdiffusion of the elements present in LiMO2 and the additive element A source occurs, and it may be lower than the melting temperature of these materials. Let me explain using an oxide as an example, but the melting temperature T m 0.757 times (Tammann temperature T) d It is known that solid-phase diffusion occurs from ). Therefore, the heating temperature in step S33 should be 500°C or higher.

[0590] Of course, the reaction proceeds more easily if the temperature is above the melting point of at least a portion of the mixture 903. For example, if LiF and MgF2 are used as the source of additive element A, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the lower limit of the heating temperature in step S33 to 742°C or higher.

[0591] Furthermore, when mixture 903 is obtained by mixing LiCoO2:LiF:MgF2 in a molar ratio of 100:0.33:1, an endothermic peak is observed around 830°C in differential scanning calorimetry (DSC measurement). Therefore, a lower limit of the heating temperature of 830°C or higher is more preferable.

[0592] A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0593] The upper limit of the heating temperature should be below the decomposition temperature of LiMO2 (the decomposition temperature of LiCoO2 is 1130°C). At temperatures near the decomposition temperature, there is a concern that LiMO2 may decompose, albeit in small amounts. Therefore, it is more preferable to be 1000°C or lower, even more preferable to be 950°C or lower, and even more preferable to be 900°C or lower.

[0594] Based on these considerations, the heating temperature in step S33 is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Furthermore, 742°C to 1130°C is preferred, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, 800°C to 1100°C, 830°C to 1130°C are preferred, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C. The heating temperature in step S33 should be lower than that in step 13.

[0595] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride, which may be caused by the fluorine source, to an appropriate range.

[0596] In the manufacturing method described in this embodiment, some materials, such as LiF, a fluorine source, may function as a flux. This function allows the heating temperature to be lowered to below the decomposition temperature of the composite oxide (LiMO2), for example, between 742°C and 950°C, enabling the distribution of magnesium and other additive elements A to the surface layer and the production of a positive electrode active material with good properties.

[0597] However, since LiF is less dense than oxygen in its gaseous state, heating may cause LiF to volatilize or sublimate, and if it volatilizes, the amount of LiF in mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the volatilization or sublimation of LiF. Even if LiF is not used as a fluorine source, Li on the surface of LiMO2 may react with F from the fluorine source to produce LiF, which may then volatilize or sublimate. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization or sublimation.

[0598] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 while the partial pressure of LiF in the heating furnace is high. Such heating can suppress the volatilization or sublimation of LiF in the mixture 903.

[0599] In this process, it is preferable to heat the mixture 903 so that it does not stick together. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the diffusion pathway of added element A (e.g., fluorine) is obstructed, which may worsen the distribution of added element A (e.g., magnesium and fluorine) to the surface layer.

[0600] Furthermore, it is believed that if the additive element A (e.g., fluorine) is uniformly distributed on the surface, a smooth positive electrode active material with few irregularities can be obtained. Therefore, in order to maintain or further improve the smooth surface after heating in step S15 of this process, it is preferable that the mixture 903 does not adhere to each other.

[0601] Furthermore, when heating with a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere inside the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to purge the atmosphere initially and then not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. If oxygen flows, the fluorine source may volatilize or sublimate, which is undesirable for maintaining surface smoothness.

[0602] When heating by roller hearth kiln, for example, the mixture 903 can be heated in an atmosphere containing LiF by placing a lid on the container containing the mixture 903.

[0603] A note regarding heating time: The heating time varies depending on conditions such as the heating temperature, the size of the LiMO2 in step S14, and its composition. When the LiMO2 is small, a lower temperature or shorter time may be preferable than when it is large.

[0604] When the median diameter (D50) of the composite oxide (LiMO2) in step S14 of Figure 22A is approximately 12 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably, for example, 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0605] On the other hand, if the median diameter (D50) of the composite oxide (LiMO2) in step S14 is about 5 μm, the heating temperature is preferably, for example, 600°C to 950°C. The heating time is preferably, for example, 1 hour to 10 hours, and more preferably about 2 hours. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0606] <Step S34> Next, in step S34 shown in Figure 22A, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 200. At this time, it is preferable to further sift the recovered positive electrode active material 200. By the above steps, a positive electrode active material 200 according to one embodiment of the present invention can be produced. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0607] This embodiment can be used in combination with other embodiments.

[0608] (Embodiment 5) This embodiment describes an example of the configuration of a secondary battery as described in the previous embodiment.

[0609] <Laminated rechargeable battery> An example of the external view of a laminate-type secondary battery 100 is shown in Figures 23A and 23B. Figures 23A and 23B show a positive electrode layer 106, a negative electrode layer 107, an electrolyte layer 103, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0610] <Method for manufacturing laminated rechargeable batteries> An example of a method for manufacturing a laminate-type secondary battery, whose external view is shown in Figure 23A, will be explained using Figures 24B and 24C.

[0611] As shown in Figure 24A, a positive electrode layer 106 and a negative electrode layer 107 are prepared. In the positive electrode layer 106, the positive electrode active material layer 102 is formed on one surface of the positive electrode current collector. The positive electrode active material layer 102 may also be formed on the other surface of the positive electrode current collector. Furthermore, the positive electrode layer 106 has a region where the positive electrode current collector is partially exposed (hereinafter referred to as the tab region), and this tab region will be denoted as the positive electrode tab 501. In the negative electrode layer 107, the negative electrode active material layer 104 is formed on one surface of the negative electrode current collector. The negative electrode active material layer 104 may also be formed on the other surface of the negative electrode current collector. Furthermore, the negative electrode layer 107 has a region where the negative electrode current collector is partially exposed, i.e., a tab region, and this tab region will be denoted as the negative electrode tab 504. The area and shape of each tab region are not limited to the example shown in Figure 24A.

[0612] Next, the negative electrode layer 107, electrolyte layer 103, and positive electrode layer 106 are laminated. Figure 24B shows the laminated negative electrode layer 107, electrolyte layer 103, and positive electrode layer 106. The area of ​​the electrolyte layer 103 should be larger than the area of ​​the negative electrode layer 107 and the positive electrode layer 106. Figure 24B shows an example configuration in which five sets of negative electrodes and four sets of positive electrodes are laminated. Next, the positive electrode tabs 501 are joined together, and the positive electrode lead electrodes 510 are joined to the outermost tab region. For joining, ultrasonic welding, for example, can be used. Similarly, the negative electrode tabs 504 are joined together, and the negative electrode lead electrodes 511 are joined to the outermost tab region.

[0613] Next, as shown in Figure 24C, the negative electrode layer 107, electrolyte layer 103, and positive electrode layer 106 are placed on the outer casing 509, and the outer casing 509 is folded along the dashed line. Then, the outer periphery of the outer casing 509 is bonded. The area used for bonding is referred to as the bonded area. For bonding, for example, heat bonding may be used.

[0614] Next, it is also possible to inject the ionic liquid into the inside of the outer casing 509 through an inlet provided in the outer casing 509. The introduction of liquid materials such as ionic liquid is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this way, a laminate-type secondary battery 100 can be manufactured.

[0615] As described in the above embodiments, in one aspect of the present invention, the secondary battery has an electrolyte layer 103 in which the solid electrolyte holds the ionic liquid. In other words, at least in the electrolyte layer 103, the ionic liquid is impregnated into the solid electrolyte. Such an electrolyte layer 103 is preferable because the ionic liquid does not seep out.

[0616] This embodiment can be used in combination with other embodiments.

[0617] (Embodiment 6) This embodiment describes an example of the configuration of a secondary battery as described in the previous embodiment.

[0618] For example, the laminated secondary battery 100 described above can be bent. In other words, the secondary battery 100 is flexible.

[0619] Figure 25A shows a bent secondary battery 100. Figure 25A shows a secondary battery 100 having a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107, bent toward the positive electrode layer 106. Of course, the secondary battery 100 can also have a bent shape toward the negative electrode layer 107. The electrolyte layer 103, which is one embodiment of the present invention, is suitable for a bent secondary battery 100 because the electrolyte, such as an ionic liquid, does not leak out. Note that a bent shape includes a shape in which one cross-section of the secondary battery 100 has an arc-shaped portion.

[0620] The smallest unit constituting a secondary battery is referred to as a battery unit, and the battery unit has a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107. In one embodiment of the present invention, the secondary battery 100 may have a plurality of such battery units. That is, the secondary battery 100 may have a configuration in which a plurality of such battery units are stacked. The electrolyte layer 103 in one embodiment of the present invention is suitable when stacking units because it does not leak out of electrolytes such as ionic liquids.

[0621] Figure 25A shows a single battery unit, but a configuration with multiple battery units stacked on top of each other is also possible.

[0622] Although the secondary battery 100 also has an outer casing, the outer casing described in the above embodiment can conform to the curved battery unit. Therefore, the outer casing is not shown in Figure 25A.

[0623] Next, we will describe the bent state in detail. As shown in Figure 25A, in the secondary battery 100, the radius of curvature 1802 of the layer closer to the center of curvature 1800, for example, the positive electrode layer 106, is smaller than the radius of curvature 1804 of the layer further away from the center of curvature 1800, for example, the negative electrode layer 107. To make it easier to bend, it is preferable to make the thickness of the layer with a smaller radius of curvature, for example, the positive electrode layer 106, smaller than that of the negative electrode layer 107.

[0624] As shown in Figure 25B, when the secondary battery 100 is bent as shown in Figure 25A, compressive stress is applied to the surface of the positive electrode layer 106 and tensile stress is applied to the surface of the negative electrode layer 107, as indicated by the arrows. To alleviate the compressive stress, the layer with a smaller radius of curvature, such as the positive electrode layer 106, may be made thicker than the negative electrode layer 107.

[0625] As one way to alleviate the compressive and tensile stresses mentioned above, a configuration in which recesses and protrusions are provided on the exterior body will be explained using Figures 26A and 26B.

[0626] The recesses and protrusions are formed on the surface of the exterior body 1805, creating a pattern-like appearance. As can be seen in one cross-section of the exterior body 1805, when protrusions are provided on the exterior body, recesses are also formed simultaneously, and when recesses are provided on the exterior body, protrusions are also formed simultaneously. In other words, it is not necessary to provide both recesses and protrusions on the exterior body; providing one will simultaneously provide the other.

[0627] The outer casing 1805 can alleviate the compressive and tensile stresses mentioned above. In other words, the secondary battery 100 can be deformed within a range where the radius of curvature of the outer casing on the side closer to the center of curvature is 30 mm or more, preferably 10 mm or more.

[0628] The ends of the outer casing 1805 shown in Figures 26A and 26B have adhesive regions 1807. The adhesive region 1807 is the area where the outer casing 1805 is bonded by heat compression or the like. In the adhesive region 1807, an adhesive layer 1803 is preferably located between the outer casing 1805.

[0629] In the bonding region 1807, it is preferable that the recesses or protrusions provided on the upper and lower parts of the outer casing 1805 overlap. To ensure that the recesses or protrusions overlap, recesses or protrusions may be formed on the outer casing 1805 again when bonding the outer casing. This configuration can increase the bonding strength.

[0630] Figure 26A shows a secondary battery 100 where the end portion of the outer casing 1805, region 1808 (which is not the adhesive region 1807), has a space 1810.

[0631] In Figure 26B, the end portion of the outer casing 1805, region 1808 (which is not the adhesive region 1807), shows a secondary battery 100 containing ionic liquid 118. The ionic liquid 118 is held in place by the electrolyte layer 103, but in the case of a bendable secondary battery, leakage is possible. Note that if the region 1808 in Figure 26B is not filled with ionic liquid 118, there may be a configuration in which region 1808 has space between the ionic liquid 118 and the region 1808. Due to the high adhesive strength of the outer casing 1805, the ionic liquid 118 will not leak from the outer casing 1805.

[0632] The shape of the curved secondary battery 100 is not limited to a simple arc shape in cross-sectional view, but may also be a shape in which part is an arc. For example, it can be the shape shown in Figure 27A, or the wavy or S-shaped shape shown in Figure 27B. The secondary battery 100 shown in Figures 27A and 27B can also be fitted with the casing having the recesses or protrusions described above, and multiple stacked battery units can be fitted.

[0633] As shown in Figure 27A or Figure 27B, if the curved surface of the secondary battery 100 has a shape with multiple centers of curvature, the secondary battery can be bent in a range where the radius of curvature of the outer casing closest to the center of curvature is 10 mm or more, preferably 30 mm or more.

[0634] In one embodiment of the present invention, a secondary battery has an electrolyte layer 103 in which a solid electrolyte holds an ionic liquid. In other words, at least in the electrolyte layer 103, the ionic liquid is impregnated into the solid electrolyte. Such an electrolyte layer 103 is preferable because the ionic liquid does not seep out.

[0635] This embodiment can be used in combination with other embodiments.

[0636] (Embodiment 7) This embodiment describes an electronic device having a secondary battery.

[0637] As described above, a secondary battery according to one aspect of the present invention is bendable (or may be referred to as having flexibility). That is, a secondary battery according to one aspect of the present invention can be made flexible. A secondary battery according to one aspect of the present invention can be fixed in a bent state. Furthermore, a secondary battery according to one aspect of the present invention can be changed from a bent state.

[0638] [Configuration Example 1] One aspect of the present invention will be described in which a rechargeable battery is installed in a wristwatch-type electronic device.

[0639] Figure 28A shows a wristwatch-type electronic device 70. The wristwatch-type electronic device 70 comprises a frame 71 (also referred to as a case), a display unit 72, a belt 21, a buckle 27, a sensor 74, operation buttons 77, etc. The wristwatch-type electronic device 70 can perform various applications such as mobile phone calls, email, document viewing and creation, music playback, internet communication, or computer games.

[0640] A belt 21 is a component used to attach a watch to the wrist, and is also called a band, strap, or bracelet.

[0641] The display unit 72 may have a curved display surface. The display can be performed along the curved surface. The display unit 72 may also be equipped with a touch sensor, which can be positioned along the curved surface. Furthermore, the application can be operated by touching the touch sensor with a finger or stylus. For example, touching an icon 73 displayed on the display unit 72 can launch the application associated with that icon.

[0642] The operation button 77 can be assigned various functions, such as turning the power on and off, turning wireless communication on and off, activating and deactivating silent mode, and activating and deactivating power saving mode. The functions of the operation button 77 can be freely configured by the operating system built into the wristwatch-type electronic device 70.

[0643] The wristwatch-type electronic device 70 is capable of performing standardized short-range wireless communication. For example, it can communicate with a wireless communication-enabled headset to enable hands-free calling. The wristwatch-type electronic device 70 has an antenna for communication. The antenna can be provided on the display unit 72 or the belt 21.

[0644] The wristwatch-type electronic device 70 preferably has a...

Claims

1. In a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, The positive electrode layer has a positive electrode active material containing lithium cobalt oxide, The negative electrode layer has a negative electrode active material, The electrolyte layer has a structure in which a liquid material is impregnated into a solid material. The positive electrode active material has an O3' type crystal structure when charged at a voltage of 4.6V with reference to the potential of lithium metal. A lithium-ion secondary battery wherein the lithium-ion conductivity of the liquid material is 0.1 mS / cm or more and 20 mS / cm or less at 25°C.

2. In a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, The positive electrode layer has a positive electrode active material containing lithium cobalt oxide, The negative electrode layer has a negative electrode active material, The electrolyte layer has a structure in which a liquid material is impregnated into a solid material. When the positive electrode active material is charged at a voltage of 4.6V with reference to the potential of lithium metal, and the positive electrode is subjected to XRD measurement, the XRD pattern due to CuKα1 line shows diffraction peaks at 2θ = 19.25 ± 0.12° and 2θ = 45.47 ± 0.10°. A lithium-ion secondary battery wherein the lithium-ion conductivity of the liquid material is 0.1 mS / cm or more and 20 mS / cm or less at 25°C.

3. In a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, The positive electrode layer has a positive electrode active material containing lithium cobalt oxide, The negative electrode layer has a negative electrode active material, The electrolyte layer has a structure in which a liquid material is impregnated into a solid material. The proportion of the liquid material in the electrolyte layer is 5% by volume or more and 30% by volume or less. The positive electrode active material has an O3' type crystal structure when charged at a voltage of 4.6V with reference to the potential of lithium metal. A lithium-ion secondary battery wherein the lithium-ion conductivity of the liquid material is 0.1 mS / cm or more and 20 mS / cm or less at 25°C.

4. In a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, The positive electrode layer has a positive electrode active material containing lithium cobalt oxide, The negative electrode layer has a negative electrode active material, The electrolyte layer has a structure in which a liquid material is impregnated into a solid material. The proportion of the liquid material in the electrolyte layer is 5% by volume or more and 30% by volume or less. The positive electrode active material is LixCoO ２ When expressed as such, it has an O3' type crystal structure when x is 0.

2. A lithium-ion secondary battery wherein the lithium-ion conductivity of the liquid material is 0.1 mS / cm or more and 20 mS / cm or less at 25°C.

5. In a lithium-ion secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, The positive electrode layer has a positive electrode active material containing lithium cobalt oxide, The negative electrode layer has silicon, The electrolyte layer has a structure in which a liquid material is impregnated into a solid material. The proportion of the liquid material in the electrolyte layer is 5% by volume or more and 30% by volume or less. The positive electrode active material is LixCoO ２ When expressed as such, it has an O3' type crystal structure when x is 0.

2. A lithium-ion secondary battery wherein the lithium-ion conductivity of the liquid material is 0.1 mS / cm or more and 20 mS / cm or less at 25°C.

6. In any one of claims 1 to 5, A lithium-ion secondary battery in which lithium ions can move through the electrolyte layer from the positive electrode layer to the negative electrode layer during charging.

7. In any one of claims 1, 3 to 5, The positive electrode active material is a lithium-ion secondary battery in which the difference in volume per equal number of cobalt atoms between the O3-type crystal structure in the discharged state and the O3'-type crystal structure in the charged state is 2.5% or less.

8. In claim 7, The aforementioned charge state is when the lithium-ion secondary battery has been charged to 219.2 mAh / g, wherein the lithium-ion secondary battery is a lithium-ion secondary battery.

9. In any one of claims 1 to 5, The electrolyte layer is in the form of a sheet, in a lithium-ion secondary battery.

10. In any one of claims 1 to 5, The electrolyte layer has a stacked structure of two or more layers in this lithium-ion secondary battery.

11. In any one of claims 1 to 5, A lithium-ion secondary battery wherein the electrolyte layer comprises a layer without the solid material and a layer having the solid material.

12. In any one of claims 1 to 5, The solid material comprises a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or a polymer-based solid electrolyte in a lithium-ion secondary battery.

13. In any one of claims 1 to 5, A lithium-ion secondary battery in which the transport rate of lithium ions in the solid material is higher than that of the liquid material.

14. In any one of claims 1 to 5, A lithium-ion secondary battery in which lithium ions are capable of moving between the solid material and the liquid material.

15. In any one of claims 1 to 5, A lithium-ion secondary battery in which lithium ions are capable of moving only within the solid material.

16. In any one of claims 1 to 5, A lithium-ion secondary battery in which lithium ions are capable of moving only within the liquid material.

17. In any one of claims 1 to 5, A lithium-ion secondary battery in which the positive electrode layer comprises the solid material and the liquid material.

18. In any one of claims 1 to 5, A lithium-ion secondary battery in which the positive electrode layer has the solid material or the liquid material.

19. In any one of claims 1 to 5, A lithium-ion secondary battery in which the negative electrode layer comprises the solid material and the liquid material.

20. In any one of claims 1 to 5, A lithium-ion secondary battery in which the negative electrode layer has the solid material or the liquid material.

21. In any one of claims 1 to 5, The aforementioned liquid material is a gel-like ionic liquid or a gel-like organic solvent in a lithium-ion secondary battery.

22. In any one of claims 1 to 5, A lithium-ion secondary battery in which the proportion of the solid material in the electrolyte layer decreases as it approaches the positive electrode layer and also decreases as it approaches the negative electrode layer.

23. In any one of claims 1 to 5, A lithium-ion secondary battery in which the proportion of the solid material in the electrolyte layer decreases as it approaches the positive electrode layer, or as it approaches the negative electrode layer.

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