Secondary battery

Abstract

Provided is a separator and a secondary battery exhibiting good wettability to an electrolyte. This secondary battery has a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte. The separator has a first member, a second member positioned on one surface of the first member, and a third member positioned on the other surface of the first member. The second member has higher wettability to the electrolyte than the first member, and the third member has higher wettability to the electrolyte than the first member.

Description

secondary battery

[0001] The present invention relates to a secondary battery. However, the technical field of the present invention is not limited to secondary batteries, and examples of the technical field of the present invention include semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, vehicles, and manufacturing methods thereof. For example, the secondary battery of the present invention can be applied as a necessary power source in semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, and vehicles. The above-mentioned electronic devices include information terminal devices equipped with a secondary battery, and the power storage devices include stationary power storage devices.

[0002] In recent years, demand for high-power, high-capacity lithium-ion secondary batteries has rapidly expanded in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0003] In lithium-ion secondary batteries, separators are used to prevent short circuits between the positive and negative electrodes. Although separators are not involved in the chemical reactions of lithium-ion secondary batteries, they are components that play an important role in lithium-ion secondary batteries and have been extensively studied. For example, Patent Document 1 proposes a configuration in which electrodes and separators are bonded together with an adhesive resin layer to solve the problem of insufficient battery performance when manufacturing large-area secondary batteries, due to the inability to maintain a constant inter-electrode distance and variations in internal resistance of the secondary battery.

[0004] JP 2011-60470 A

[0005] In order to ensure the separator's electrolyte retention capacity, the separator is required to exhibit good wettability with respect to the electrolyte. However, while Patent Document 1 examines the wettability of the electrodes with respect to the electrolyte, it does not examine the wettability of the separator with respect to the electrolyte. Furthermore, Patent Document 1 describes a configuration in which the electrodes and the separator are bonded by an adhesive resin layer, and when the secondary battery is curved, this can lead to poor battery characteristics.

[0006] Therefore, an object of the present invention is to provide a separator that has good wettability with respect to an electrolyte. Another object of the present invention is to provide a separator that can suppress poor battery characteristics even when a secondary battery is curved. A further object of the present invention is to provide a secondary battery having the above-mentioned separator.

[0007] Note that the description of these problems does not preclude the existence of other problems. Furthermore, one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of this specification, drawings, claims, etc.

[0008] In view of the above problems, one embodiment of the present invention is a secondary battery that includes a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte solution, wherein the separator includes a first member, a second member located on one surface of the first member, and a third member located on the other surface of the first member, and the second member has higher wettability with respect to the electrolyte solution than the first member, and the third member has higher wettability with respect to the electrolyte solution than the first member.

[0009] Another aspect of the present invention is a secondary battery comprising a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte solution, wherein the separator comprises a first member, a second member positioned on one surface of the first member, and a third member positioned on the other surface of the first member, wherein the second member has a higher wettability with respect to the electrolyte solution than the first member, and the third member has a higher wettability with respect to the electrolyte solution than the first member, and the second member has a region in contact with the positive electrode, and the third member has a region in contact with the negative electrode.

[0010] Another aspect of the present invention is a secondary battery comprising a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte solution, wherein the separator comprises a first member, a second member positioned on one surface of the first member, and a third member positioned on the other surface of the first member, wherein the second member has higher wettability to the electrolyte solution than the first member, the third member has higher wettability to the electrolyte solution than the first member, the first member is thicker than the second member, and the first member is thicker than the third member.

[0011] Another aspect of the present invention is a secondary battery having a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte solution, wherein the separator has a first member, a second member positioned on one surface of the first member, and a third member positioned on the other surface of the first member, wherein the second member has a higher porosity than the first member, and the third member has a higher porosity than the first member.

[0012] In the present invention, it is preferable that the first member comprises polypropylene, and the second member and the third member each comprise polyimide.

[0013] In the present invention, the electrolyte preferably contains FEC and MTFP.

[0014] In the present invention, the electrolyte preferably contains an ionic liquid.

[0015] In the present invention, it is preferable that the second member has an area bonded to the first member, and the third member has an area bonded to the first member.

[0016] In the present invention, it is preferable that the interface between the first member and the second member has a region where the first member and the second member are mixed, and that the interface between the first member and the third member has a region where the first member and the third member are mixed.

[0017] In the present invention, it is preferable that the secondary battery is bendable.

[0018] According to one embodiment of the present invention, a separator having good wettability with an electrolyte solution can be provided. Furthermore, a separator that does not cause poor secondary battery characteristics even when the secondary battery is curved can be provided. Furthermore, according to another embodiment of the present invention, a secondary battery including the separator can be provided.

[0019] FIG. 1A is a diagram illustrating a separator included in a secondary battery according to one embodiment of the present invention, and FIGS. 1B to 1E are diagrams illustrating a method for manufacturing the separator. FIGS. 2A and 2B are diagrams illustrating a secondary battery according to one embodiment of the present invention. FIG. 3A is a diagram illustrating a secondary battery according to one embodiment of the present invention, and FIG. 3B is a diagram illustrating a positive electrode active material layer according to one embodiment of the present invention. FIG. 4 is a diagram illustrating an electrolyte injection device. FIGS. 5A and 5B are diagrams illustrating a flexible secondary battery according to one embodiment of the present invention. FIGS. 6A to 6D are diagrams illustrating a method for manufacturing a positive electrode active material. FIG. 7 is a diagram illustrating a method for manufacturing a positive electrode active material. FIGS. 8A to 8C are diagrams illustrating a method for manufacturing a positive electrode active material. FIGS. 9A and 9B are cross-sectional views illustrating a positive electrode active material. FIGS. 10A to 10F are cross-sectional views illustrating a positive electrode active material. FIG. 11 is a diagram illustrating the crystalline structure of a positive electrode active material. FIG. 12 is a diagram illustrating the crystalline structure of a conventional positive electrode active material. FIG. 13 is a diagram illustrating an XRD pattern calculated from the crystalline structure. FIG. 14 is a diagram showing an XRD pattern calculated from the crystal structure. FIGS. 15A to 15G are diagrams explaining the positional relationship of distribution in EDX-ray analysis. FIG. 16A is an exploded perspective view of a coin-type secondary battery, FIG. 16B is a perspective view of the coin-type secondary battery, and FIG. 16C is a cross-sectional perspective view thereof. FIG. 17A shows an example of a cylindrical secondary battery. FIG. 17B shows an example of the internal structure of a cylindrical secondary battery. FIG. 17C shows an example of a plurality of cylindrical secondary batteries. FIG. 17D shows an example of an energy storage system having a plurality of cylindrical secondary batteries. FIGS. 18A and 18B are diagrams explaining an example of a secondary battery, and FIG. 18C is a diagram showing the internal structure of a secondary battery. FIGS. 19A to 19C are diagrams explaining an example of a secondary battery. FIGS. 20A and 20B are diagrams explaining an example of a secondary battery. FIG. 21A shows an example of the configuration of a battery pack, FIG. 21B shows an example of the circuit of the battery pack, and FIG. 21C shows an example of the configuration of an electric vehicle. Figures 22A to 22D are diagrams illustrating an example of a transportation vehicle. Figure 23A is a diagram illustrating an electric bicycle, Figure 23B is a diagram illustrating a secondary battery for the electric bicycle, and Figure 23C is a diagram illustrating a scooter. Figures 24A to 24D are diagrams illustrating an example of an electronic device.Fig. 25A shows an example of a wearable device, Fig. 25B shows a perspective view of a wristwatch-type device, and Fig. 25C is a diagram illustrating a side view of the wristwatch-type device. Figs. 26A to 26D are diagrams showing an example of space equipment. Fig. 27 is a photograph showing the results of wettability measurements in the example.

[0020] The following description will explain the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0021] In this specification, ordinal numbers such as "first" and "second" are used for convenience and do not limit the number of components or the order of the components. The order of the components includes, for example, the order of processes or the order of stacking. In other words, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the examples of this specification may not match the ordinal numbers used in the claims. Furthermore, the ordinal numbers used in the embodiments of this specification may not match the ordinal numbers used in the examples of this specification.

[0022] In this specification and the like, a lithium ion secondary battery may be referred to as a lithium ion battery, and refers to a secondary battery that uses lithium ions as carrier ions. However, the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions may be used as carrier ions of the present invention, and specifically, sodium ions may be used. In this case, the present invention can be understood by replacing lithium ions with sodium ions. Furthermore, when describing a configuration in which there is no limitation on the carrier ions, the term "secondary battery" or "battery" may be used.

[0023] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode substance, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, or the like.

[0024] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will also be expressed as a composite hexagonal lattice in this specification. Miller indices may also be expressed as (hkil) rather than (hkl). Here, i is −(h+k).

[0025] In this specification and the like, the space group of the positive electrode active material and the like is identified by XRD (X-ray diffraction), electron beam diffraction, neutron beam diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0026] In this specification, if the anions have a structure in which three layers are stacked with a mutually shifted arrangement, such as ABCABC, it is referred to as a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. Furthermore, since real crystals always have defects, the analysis results do not necessarily conform to the theory. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation between the theoretical position and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0027] In this specification, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C-plane, the basal plane, or the like. In addition, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. In other words, it can be said that the lithium diffusion path exists along the (00l) plane. In this specification, a surface on which the lithium diffusion path is exposed, that is, a surface other than the surface where lithium is inserted and extracted (specifically, the (00l) plane), may also be referred to as an edge plane.

[0028] In this specification and the like, the cross-sectional shape of a particle is not limited to a circle, i.e., a sphere. The cross-sectional shape of a particle includes an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, an asymmetric shape, etc. When there are multiple particles, the cross-sectional shapes of the particles may be different from each other.

[0029] In this specification, etc., when describing the characteristics of individual particles of a positive electrode active material in embodiments, etc., it is not necessary for all particles to have that characteristic. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of a positive electrode active material have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material and a secondary battery including the same.

[0030] In this specification, particle size can be measured using a particle size distribution meter (laser diffraction particle size distribution analyzer) using a laser diffraction / scattering method. In this specification, median diameter (D50) can be used as the average particle size. D50 is the particle size at which the cumulative amount accounts for 50% on the cumulative curve of the particle size distribution measurement results.

[0031] In this specification, the measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may be measured by analysis such as SEM (Scanning Electron Microscope) or TEM. For example, as a method for measuring D50 by analysis such as SEM or TEM, 20 or more particles are measured, a cumulative curve is created, and the particle diameter when the cumulative amount accounts for 50% can be taken as D50. In this specification, the maximum particle size can be the particle size that can be confirmed in a cross section of a positive electrode measuring 100 μm square.

[0032] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g per weight. ２ The theoretical capacity of LiMn is 275 mAh / g per weight. ２ O ４ The theoretical capacity of the battery is 148 mAh / g by weight.

[0033] In this specification, the amount of lithium remaining in the positive electrode active material that can be inserted or removed is determined by x in the composition formula, for example, Li ｘ MO ２ In this specification, M represents a transition metal, and unless otherwise specified, M is cobalt and / or nickel. In the case of a positive electrode active material in a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, when a lithium ion secondary battery using lithium cobalt oxide as the positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ MO ２ The small value of x in the formula means, for example, 0.1<x≦0.24.

[0034] In this specification and the like, when properly synthesized lithium cobalt oxide before use in a positive electrode approximately satisfies the stoichiometric ratio, LiCoO ２and x = 1. The lithium cobalt oxide contained in the lithium ion secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The state where discharge is completed (discharged state) here refers to a state where the voltage is 3.0 V or less or 2.5 V or less at a current of, for example, 100 mA / g or less.

[0035] In this specification, Li ｘ MO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data from a lithium-ion secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0036] In this specification, the distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain continuous analytical method. The region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is detected at or above the lower detection limit.

[0037] Unless otherwise specified, in this specification and the like, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in a secondary battery are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging and burn-in treatments during the secondary battery manufacturing stage is not considered to be degradation. For example, a secondary battery consisting of a single secondary cell or a secondary battery pack can be said to be in its pre-degradation state when it has a discharge capacity of 97% or more of its rated capacity. For secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other secondary batteries, the rated capacity conforms not only to the above JIS standard but also to various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0038] In this specification, etc., secondary particles refer to particles formed by aggregation of primary particles. Also in this specification, etc., primary particles refer to particles that do not have grain boundaries on their appearance. Also in this specification, etc., single particles refer to particles that do not have grain boundaries on their appearance. Also in this specification, etc., single crystals refer to crystals in which there are no grain boundaries inside the particles, and polycrystals refer to crystals in which there are grain boundaries inside the particles. Polycrystals can be said to be an aggregate of multiple crystallites, and grain boundaries can be said to be the interface that exists between two or more crystallites. In polycrystals, it is preferable that the crystallites are aligned in the same direction.

[0039] In this specification, the term "supported amount" refers to the weight of the active material per unit surface area of ​​the current collector. The supported amount of the negative electrode active material can be adjusted according to the capacity of the positive electrode. In the case of double-sided coating, in which a slurry containing the active material is applied to both sides of the current collector, the supported amount is considered per side.

[0040] In this specification and the like, the term "slurry" refers to a material liquid used to form an active material layer on a current collector, and contains an active material, a binder, and a solvent, and preferably further contains a conductive material. Note that the slurry is also called an electrode slurry or an active material slurry, and the slurry for forming a positive electrode active material layer is also called a positive electrode slurry, and the slurry for forming a negative electrode active material layer is also called a negative electrode slurry.

[0041] Here, we will explain the flow of electrons and lithium ions in a secondary battery during charging. When a charger is connected and charging of the secondary battery begins, electrons are released from the positive electrode, causing an oxidation reaction, and electrons are supplied to the negative electrode, causing a reduction reaction. Then, lithium ions (Li ＋ ) is released into the electrolyte, and lithium ions move to the negative electrode. During discharge, a reduction reaction occurs at the positive electrode and an oxidation reaction occurs at the negative electrode.

[0042] In other words, in a secondary battery, the anode (positive electrode) and cathode (negative electrode) are interchanged during charging and discharging, and the oxidation reaction and reduction reaction are interchanged. Therefore, the electrode with a high reaction potential is called the positive electrode, and the electrode with a low reaction potential is called the negative electrode. Therefore, in this specification, whether during charging or discharging, the positive electrode is called the "positive electrode" or "+ electrode (plus electrode)," and the negative electrode is called the "negative electrode" or "- electrode (minus electrode)." If the terms anode (positive electrode) or cathode (negative electrode) related to the oxidation reaction or reduction reaction are used, they will be reversed during charging and discharging, which may lead to confusion.

[0043] As used herein, a full cell refers to a cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a cell assembled with lithium metal as the negative electrode (counter electrode).

[0044] Unless otherwise specified, in this specification, the charge voltage is expressed based on the potential of lithium metal. Furthermore, in this specification, a high charge voltage is, for example, a charge voltage of 4.6 V or higher, preferably 4.65 V or higher, more preferably 4.7 V or higher, even more preferably 4.75 V or higher, and most preferably 4.8 V or higher. In other words, in the case of a half cell using lithium metal as the counter electrode, a charge voltage of 4.6 V or higher is referred to as a high charge voltage.

[0045] In this specification, a high charging voltage is defined as a charging voltage of 4.5 V or higher, based on the potential when the negative electrode is made of a carbon material (e.g., graphite). That is, in the case of a full cell in which a carbon material (e.g., graphite) is used as the negative electrode, a charging voltage of 4.5 V or higher is referred to as a high charging voltage.

[0046] In this specification and the like, carbonate refers to a compound having at least one carbonate ester in its molecular structure, and includes cyclic carbonates and chain carbonates unless otherwise specified. Furthermore, chain carbonates include both linear and branched chain carbonates.

[0047] In this specification, the porosity can be a value calculated from the volume, density, and mass. Also, in this specification, the porosity can be calculated based on an observation image of an object that has voids, is filled with an organic material, and then processed into a thin film. In this specification, the processing can be performed using a focused ion beam (FIB) or ion milling.

[0048] In this specification and the like, flexibility refers to the property that an object is soft and deformable. In this specification, an object having flexibility refers to at least a part of the object having flexibility. In other words, a flexible object may have a non-flexible part (also called a hard part).

[0049] In this specification, a secondary battery that can deform to follow a deformable electronic device is referred to as a deformable secondary battery, a flexible secondary battery, or a flexible battery. In this specification, "deformable" means that the shape of an object changes, and includes the shape of an object deforming in response to an external force applied to the object. In this specification, "the shape of an object deforming in response to an external force" means that the shape of an object can be deformed by the hand of an average adult without requiring excessive force.

[0050] In this specification, the shape of an object deformed in response to an external force includes a shape of an object bent in response to an external force. In this specification, a secondary battery that can be bent to follow a bendable electronic device is referred to as a bendable secondary battery, foldable battery, or bendable battery. A foldable battery includes a secondary battery that is deformed in response to an external force and has a folded shape.

[0051] In this specification, bendable electronic devices and bendable secondary batteries include a state in which they are bent and fixed, and also include a state in which they are repeatedly bent and straightened. In this specification, a state in which they are repeatedly bent and straightened includes a state in which they are repeatedly bent and straightened. In this specification, a state in which they are unbent includes a straight state.

[0052] In this specification and the like, the phrase "having A and / or B" may be used, which means having A, having B, and having A and B.

[0053] Embodiment 1 A separator included in a secondary battery that is one embodiment of the present invention will be described.

[0054] [Separator] As shown in FIG. 1A , the separator 105 includes a first member 15. The first member 15 is preferably a porous substrate. The porous substrate is preferably made of an insulating material, and one or more insulating materials selected from organic and inorganic materials can be used. The porous substrate is preferably made of a thermoplastic resin as the organic material to provide the separator 105 with a shutdown function. The shutdown function is a function that closes the pores of the separator 105 when the secondary battery 100 generates abnormal heat. Thermoplastic resins soften when heated, thereby closing the pores. Using a thermoplastic resin in this way can provide the separator 105 with a shutdown function. A material with a softening point or melting point below 200°C is preferably used as the thermoplastic resin, and typically one or more selected from polypropylene (PP), polyethylene (PE), acrylic, and polyamide (PA) can be used. Polypropylene has higher heat resistance than polyethylene, with a softening point of 140°C or higher and a melting point of 164°C to 170°C. Since the softening point of polypropylene is close to the temperature at which abnormal heat generation occurs, polypropylene can be said to be a material with good shutdown function. Therefore, it is preferable that the first member 15 has polypropylene as a porous substrate. Using polypropylene allows the pore ratio of the first member 15, i.e., the porosity, to be 35% to 60%, preferably 35% to 45%. It is preferable that the porosity satisfies the above range in a dry state. If the porosity is too small, the ion permeability of lithium ions and the like becomes low, which is undesirable. Furthermore, if the porosity is too large, the separator strength may be insufficient or lithium metal deposition at the negative electrode or the like may cause a short circuit between the negative electrode and the positive electrode, which is undesirable. Therefore, when polypropylene is used, it is preferable that the porosity of the first member 15 satisfies 35% to 60%, preferably 35% to 45%.

[0055] In the separator 105, it is preferable that the second member 16 is provided on one side (one surface) of the first member 15. A configuration in which the second member 16 is provided on one side (one surface) of the first member 15 is sometimes referred to as a separator in which two types of members are combined. The second member 16, which can form an interface with the electrolyte, is preferably made of a material that has higher wettability to the electrolyte than the first member 15. In this specification, wettability can be evaluated by contact angle. The contact angle is preferably measured according to JIS R3257, for example, in an environment of 25°C, after 30 to 60 seconds have elapsed since 10 μμL to 25 μL of electrolyte is dropped onto the member. The contact angle can be measured from an image observed from a horizontal direction. It is also preferable to use the average value of the contact angle measured at multiple locations. In this specification, good wettability is defined as a contact angle of less than 10 degrees. It is preferable that the contact angle of the second member 16 is smaller than that of the first member 15, or that the contact angle is less than 10 degrees. A typical material that can be used for the second member 16 is polyimide. Since polyimide can have a higher porosity than polypropylene, using polyimide as the second member 16 allows the porosity of the second member 16 to be 60% to 85%, preferably 75% to 85%. It is preferable that the porosity be within the above range in a dry state. A porosity that is too small is undesirable because it reduces the permeability of ions such as lithium ions. A porosity that is too large is also undesirable because it can cause the separator to lack strength or cause lithium metal deposition at the negative electrode, etc., to short-circuit the negative electrode and positive electrode. Therefore, when polyimide is used, it is preferable that the porosity of the third member 17 be 60% to 85%, preferably 75% to 85%. Furthermore, when polyimide is used as the second member 16, the melting point of the second member 16 can be set to 150° C. or higher, preferably 180° C. or higher.

[0056] The separator 105 preferably has a third member 17 on the other side (other surface) of the first member 15, and the third member 17 can be made of the same material as the second member 16. When the third member 17 is on the other side (other surface) of the first member 15 and is made of the same material as the second member 16, it is sometimes called a separator made of a composite of two members. In other words, the third member 17, which can form an interface with the electrolyte, preferably has a material that is more wettable to the electrolyte than the first member 15. The contact angle of the third member 17 is preferably lower than that of the first member 15, or is preferably 10 degrees or less. A typical material that can be used for the third member 17 is polyimide. When polyimide is used as the third member 17, the porosity of the third member 17 can be 60% or more and 85% or less, preferably 75% or more and 85% or less. It is preferable that the porosity satisfy the above range in a dry state. If the porosity is too small, the ion permeability of lithium ions and the like will be reduced, which is undesirable. On the other hand, if the porosity is too large, the separator may lack strength, or lithium metal deposition in the negative electrode or the like may cause a short circuit between the negative electrode and the positive electrode, which is undesirable. Therefore, when polyimide is used, the porosity of the third member 17 is preferably 60% to 85%, and more preferably 75% to 85%. Furthermore, when polyimide is used as the third member 17, the melting point of the third member 17 can be set to 150°C or higher, preferably 180°C or higher.

[0057] It is preferable that the member located on the surface of the separator 105 exhibits high wettability with respect to the electrolyte solution, because this improves the injection of the electrolyte solution into the exterior body. Furthermore, it is preferable that the member located on the surface of the separator 105 exhibits high wettability with respect to the electrolyte solution, because this ensures the retention of the electrolyte solution in the separator 105 even when the electrode expands and contracts during charge and discharge.

[0058] Next, the film thickness of the separator 105 will be described. The film thickness of the separator 105 is preferably 10 μm or more and 80 μm or less, and more preferably 20 μm or more and 60 μm or less. If the separator film thickness is too thin, it may cause a short circuit between the positive electrode and the negative electrode, which is not preferable. Furthermore, if the separator film thickness is too thick, it may cause a low energy density per volume of the secondary battery, which is not preferable. The film thickness of the separator 105 can be, for example, a value measured at the center of a cross-sectional observation image of a secondary battery including the separator 105.

[0059] Next, the thicknesses of the first member 15 to the third member 17 will be described. The first member 15 is a member that imparts a shutdown function to the separator 105, and is preferably thicker than the second member 16. The first member 15 is a member that imparts a shutdown function to the separator 105, and is preferably thicker than the third member 17. The polyimide used for the second member 16 and the third member 17 can have a porosity of 60% or more and 85% or less, preferably 75% or more and 85% or less. In other words, by using a polyimide that can have a higher porosity than polypropylene, it is easy to make the second member 16 and the third member 17 thinner than the first member 15. Furthermore, even if the second member 16 and the third member 17 are thinner than the first member 15, they can still function sufficiently as the separator 105. The thickness of the first member 15 to the third member 17 can be, for example, a value measured at the center of a cross-sectional observation image of a secondary battery including the first member 15 to the third member 17 .

[0060] Furthermore, when the second member 16 is disposed close to the negative electrode side of the separator 105, it is preferable that the thickness of the second member 16 is greater than the thickness of the third member 17. This can prevent internal short circuits in the secondary battery caused by dendrites that may occur in the negative electrode.

[0061] In the secondary battery of one embodiment of the present invention, when a multilayer structure such as the separator 105 is applied, the safety of the secondary battery can be maintained even when the thickness of the entire separator is reduced, typically, when the thickness is 20 μm to 40 μm. Furthermore, when a multilayer structure such as the separator 105 is applied, the thickness of the entire separator can be reduced, so that the proportions of the positive electrode and the negative electrode can be increased, and the capacity per volume of the secondary battery can be increased.

[0062] Furthermore, by using a separator that combines two types of components, even if the second component 16 is made of relatively expensive polyimide, it may be possible to reduce costs compared to a separator made only of polyimide.

[0063] Furthermore, the second member 16 may have a recess on its surface. A recess is a region that is thinner than the region of the image in a cross-sectional observation image, and the first member 15 may be exposed by removing the second member 16. The recess is preferably arranged in a stripe pattern. Similarly, the third member 17 may have an uneven surface. A recess is a region that is thinner than the region of the image in a cross-sectional observation image, and the first member 15 may be exposed by removing the third member 17. The recess is preferably arranged in a stripe pattern.

[0064] Next, an example of a method for manufacturing (producing) the separator 105 using polypropylene for the first member 15 and polyimide for the second member 16 and the third member 17 will be described.

[0065] First, as shown in FIG. 1B, a first member 15 is prepared. The first member 15 can be produced using a wet method or a dry method. The dry method is more versatile and preferable than the wet method because it does not use a solvent. Furthermore, either a uniaxial stretching method or a biaxial stretching method can be used as the dry method.

[0066] As shown in FIG. 1C, a polyimide precursor 16a is applied to one surface of the first member 15 as the second member 16. The polyimide precursor is one that imidizes at a temperature below the temperature at which the polypropylene, which is the first member 15, begins to melt, and preferably contains polyamic acid. After the polyimide precursor 16a is applied, heating is performed to form a polyimide as the second member 16, as shown in FIG. 1D. The polyimide can be bonded to the first member 15. When the polyamic acid is subjected to a heat treatment, imidization (dehydration and cyclization) progresses, resulting in the production of polyimide. Alternatively, a soluble polyimide may be used as the polyimide precursor. Soluble polyimide is polyimide dissolved in a solvent, and when this is subjected to a heat treatment, the solvent is removed to produce the polyimide.

[0067] Next, a polyimide precursor is applied to the other surface of the second member 16 as a third member 17. This precursor can be the same as that of the second member 16. After the polyimide precursor is applied, heating is performed to imidize the precursor, thereby forming a polyimide as the third member 17 as shown in FIG. 1E. The polyimide can be bonded to the first member 15. Heating the polyimide of the third member 17 to imidize it further promotes imidization of the polyimide of the previously formed second member 16. As a result, the polyimide of the second member 16 can have a higher hardness than the polyimide of the third member 17. By arranging such polyimide of the second member 16 in proximity to the negative electrode, defects due to dendrites and the like can be suppressed.

[0068] Note that the method for manufacturing the separator of one embodiment of the present invention is not limited to the above. For example, the separator can also be manufactured by a method in which, as shown in FIG. 1C , one surface of a first member 15 is coated with a second member 16, the first members are stacked so that the first members are in contact with each other and the second members are on the outside, and then the stacked members are heated.

[0069] In the separator 105, materials other than polypropylene (PP), polyethylene (PE), acrylic, and polyamide (PA) can also be used for the first member 15. Insulating materials that can be used for the first member 15 include cellulose-containing fibers, nonwoven fabrics, glass fibers, ceramics, and synthetic fibers made from nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. While the above-mentioned materials may have a lower shutdown function than polypropylene, the separator 105 includes the second member 16 and the third member 17, which broadens the range of materials that can be used for the first member 15.

[0070] In the separator 105, one or more materials selected from cellulose-containing fibers, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane may be provided between the first member 15 and the second member 16.

[0071] In the separator 105, one or more materials selected from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane may be provided between the first member 15 and the third member 17.

[0072] The separator 105 may further be coated with a ceramic material, a fluorine-based material, a polyamide-based material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide-based materials include nylon and aramid (meta-aramid, para-aramid). To ensure high wettability with the electrolyte in the second member 16 and the third member 17 of the separator 105, the ceramic material, the fluorine-based material, the polyamide-based material, or a mixture thereof is selectively formed. It is preferable that a portion of the second member 16 and a portion of the third member 17 are exposed in the separator 105.

[0073] The shape of the separator 105 is not limited, and it may be, for example, a sheet. The separator 105 may also be in a bag shape, and a form in which either the positive electrode or the negative electrode is housed in the bag is also suitable for the separator 105.

[0074] [Secondary Battery] Next, a secondary battery having the separator 105 will be described with reference to Figs. 2A to 3B. Fig. 2A shows the components of the secondary battery 100 stacked together, and Fig. 2B shows the components of the secondary battery 100 separated from each other. Fig. 3A shows a cross-sectional view of the secondary battery 100, and Fig. 3B shows the positive electrode active material layer 22 of the secondary battery 100.

[0075] The secondary battery 100 has a plurality of positive electrodes. In FIG. 2B , a first positive electrode 103 a and a second positive electrode 103 b are illustrated as the plurality of positive electrodes. The first positive electrode 103 a and the second positive electrode 103 b are collectively referred to as the positive electrode 103. However, the number of positive electrodes in the secondary battery 100 is not limited to two layers, and the secondary battery 100 may have a single layer or three or more layers.

[0076] The secondary battery 100 has a plurality of negative electrodes. In FIG. 2B , a first negative electrode 106 a, a second negative electrode 106 b, and a third negative electrode 106 c are illustrated as the plurality of negative electrodes. The first negative electrode 106 a, the second negative electrode 106 b, and the third negative electrode 106 c are collectively referred to as the negative electrode 106. However, the number of negative electrodes in the secondary battery 100 is not limited to three layers, and the secondary battery 100 may have a single layer, two layers, or four or more layers.

[0077] The secondary battery 100 has a separator between the negative electrode and the positive electrode. In FIG. 2B , the first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are indicated by dashed lines as multiple separators. The first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are collectively referred to as separator 105. However, the number of separators in the secondary battery 100 is not limited to four layers, and may be a single layer, two or three layers, or five or more layers. The multiple separators may be independent sheets as shown in FIG. 1B , or a continuous separator may also be used. The continuous separator is formed by preparing a separator having a larger area than the positive electrode and the negative electrode, and folding the separator appropriately to arrange the separator portions at positions corresponding to the first separator 105 a to the fourth separator 105 d. Since the separator is arranged in greater numbers than the positive electrode or the negative electrode, the thickness of the entire separator becomes thinner, which increases the capacity per volume of the secondary battery.

[0078] Fig. 3A is an example of a cross-sectional view of the secondary battery 100 taken along the dashed line AB in Fig. 2B. Fig. 3A will be described using the positive electrode 103, the separator 105, the negative electrode 106, the protrusion 31t, and the like.

[0079] The positive electrode 103 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is a layer containing positive electrode active material particles and has a region in contact with the positive electrode current collector 21. The manufacturing process of the positive electrode 103 includes a press process, and in a positive electrode that has undergone this press process, recesses into which the positive electrode active material particles are pressed may be formed in a portion of the positive electrode current collector 21. As shown in FIG. 3A , the positive electrode active material layer 22 can be formed on both sides of the positive electrode current collector 21. This is called a double-sided coated structure. Furthermore, although not shown, the positive electrode active material layer 22 can be formed on only one side of the positive electrode current collector 21. This is called a single-sided coated structure.

[0080] The protrusion 21t shown in FIG. 2A is a part of the positive electrode current collector 21. In other words, the protrusion 21t is a region of the positive electrode current collector 21 where the positive electrode active material layer 22 is not provided. As shown in FIG. 2A , in the secondary battery 100, multiple protrusions 21t overlap each other to form an assembly. The assembly of protrusions 21t is called a positive electrode tab. The positive electrode tab is joined to the positive electrode lead 107a at the joint 109a. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The positive electrode lead 107a can be made of a material selected from aluminum, nickel, titanium, or an alloy thereof. An insulating seal may be placed to surround the joint 109a and / or the positive electrode lead 107a. Kapton tape can be used as the insulating seal.

[0081] 3B shows an example of a cross-sectional view of the positive electrode active material layer 22. The positive electrode active material layer 22 has at least a positive electrode active material 10. The positive electrode active material layer 22 may have a second positive electrode active material 20. The positive electrode active material layer 22 may have a conductive material 41. The positive electrode active material layer 22 has an electrolyte 108. Although not shown, the positive electrode active material layer 22 may have a binder. The positive electrode active material layer 22 may not have the second positive electrode active material 20. The positive electrode active material layer 22 may not have the conductive material 41. The positive electrode active material layer 22 may not have a binder.

[0082] The positive electrode active material 10 preferably has an average particle size of 9 μm or more and less than 20 μm, and a maximum particle size of less than 30 μm, and satisfies the large diameter (also referred to as large particle size). Secondary particles may be used as the positive electrode active material 10, and the secondary particles preferably have an average particle size of 9 μm or more and less than 20 μm, and a maximum particle size of less than 30 μm. The positive electrode active material 10 may be LiMO having a layered rock salt crystal structure. ２ (M1 is one or more selected from Fe, Ni, Co, Mn, and Al) can be used, and lithium cobalt oxide can be used as a representative example. Lithium cobalt oxide, which has good high-voltage charging characteristics, will be described in the second embodiment and onward.

[0083] Furthermore, the positive electrode active material 10 preferably has an average particle size of 5 μm or less, more preferably 0.1 μm or more and 5 μm or less, and a maximum particle size of less than 9 μm. It is also preferable to use an active material that satisfies the small diameter (also referred to as small particle size). The positive electrode active material 10 is LiM2PO4 having an olivine-type crystal structure. ４ (M2 is one or more selected from Fe, Ni, Co, and Mn). ４ As an example, LiFePO ４ , LiNiPO ４ , LiCoPO ４ , LiMnPO ４ , LiFe ａ Ni ｂ P.O. ４ , LiFe ａ Co ｂ P.O. ４ , LiFe ａ Mn ｂ P.O. ４ , LiNi ａ Co ｂ P.O. ４ , LiNi ａ Mn ｂ P.O. ４ (a+b is 1 or less, 0<a<1, 0<b<1), LiFe ｃ Ni ｄ Co ｅ P.O. ４ , LiFe ｃ Ni ｄ Mn ｅ P.O. ４ , LiNi ｃCo ｄ Mn ｅ P.O. ４ (c+d+e is 1 or less, 0<c<1, 0<d<1, 0<e<1), LiFe ｆ Ni ｇ Co ｈ Mn ｉ P.O. ４ (f+g+h+i is 1 or less, 0<f<1, 0<g<1, 0<h<1, 0<i<1), etc., and lithium iron phosphate can be used as a representative example. In addition, it is preferable that the particle surfaces of the positive electrode active material 10 have a carbon layer.

[0084] The second positive electrode active material 20 contained in the positive electrode active material layer 22 preferably has an average particle size of 5 μm or less, preferably 0.1 μm or more and 5 μm or less, and a maximum particle size of less than 9 μm. It is preferable to use an active material that satisfies the small diameter (also referred to as small particle size). The second positive electrode active material 20 is LiM2PO4 having an olivine-type crystal structure. ４ (M2 is one or more selected from Fe, Ni, Co, and Mn). ４ As an example, LiFePO ４ , LiNiPO ４ , LiCoPO ４ , LiMnPO ４ , LiFe ａ Ni ｂ P.O. ４ , LiFe ａ Co ｂ P.O. ４ , LiFe ａ Mn ｂ P.O. ４ , LiNi ａ Co ｂ P.O. ４ , LiNi ａ Mn ｂ P.O. ４ (a+b is 1 or less, 0<a<1, 0<b<1), LiFe ｃ Ni ｄ Co ｅ P.O. ４ , LiFe ｃ Ni ｄ Mn ｅ P.O. ４ , LiNi ｃ Co ｄ Mn ｅ P.O.４ (c+d+e is 1 or less, 0<c<1, 0<d<1, 0<e<1), LiFe ｆ Ni ｇ Co ｈ Mn ｉ P.O. ４ (f+g+h+i is 1 or less, 0<f<1, 0<g<1, 0<h<1, 0<i<1), etc., and lithium iron phosphate can be used as a representative example. In addition, it is preferable that the particle surfaces of second positive electrode active material 20 have a carbon layer.

[0085] The negative electrode 106 includes a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode active material layer 32 is a layer containing negative electrode active material particles and has a region in contact with the negative electrode current collector 31. The manufacturing process for the negative electrode 106 includes a press process, and in a negative electrode that has undergone this press process, recesses into which the negative electrode active material particles are pressed may be formed in a portion of the negative electrode current collector 31. As shown in FIG. 3A , the negative electrode active material layer 32 can be formed on only one side of the negative electrode current collector 31. In particular, in a negative electrode disposed in the outermost layer of the secondary battery 100, the negative electrode active material layer that is not disposed facing the positive electrode does not need to have a negative electrode active material layer because carrier ions are not inserted or removed or are difficult to insert or remove. In other words, a single-sided coating is often used for the outermost negative electrode. Although not shown, a double-sided coating structure in which the negative electrode active material layer 32 is formed on both sides of the negative electrode current collector 31 may also be used. Preparing all negative electrodes with a double-sided coating structure is preferable because it increases productivity. In this case, a negative electrode having a double-sided coating structure can also be disposed on the outermost layer.

[0086] Furthermore, the negative electrode current collector 31 has a protrusion 31t. The protrusion 31t is a region where the negative electrode active material layer 32 is not provided. In FIG. 2B, a first protrusion 31ta, a protrusion 31tb, and a third protrusion 31tc are illustrated as multiple protrusions. The first protrusion 31ta, the protrusion 31tb, and the third protrusion 31tc are collectively referred to as the protrusion 31t. The protrusion 31t is also illustrated in FIG. 2A. The multiple protrusions 31t overlap each other to form an assembly. The assembly of the protrusions 31t is called a negative electrode tab. Note that in FIG. 3A, the protrusion 31t is shown separated from the other protrusions.

[0087] As shown in FIG. 2A , the negative electrode tab (protrusion 31t) is joined to the negative electrode lead 107b at a joint 109b. Ultrasonic bonding can be used for the joining. As a result of the joining, they are electrically connected to each other. The negative electrode lead 107b can be made of a material selected from nickel, copper, titanium, or an alloy thereof. An insulating seal may be placed so as to surround the joint 109b and / or the negative electrode lead 107b. Kapton tape can be used as the insulating seal.

[0088] The negative electrode active material layer 32 may further contain a binder. The negative electrode active material layer 32 may further contain a conductive material. Of course, the negative electrode active material layer 32 does not necessarily have to contain a binder or a conductive material. The binder and the conductive material will be described later.

[0089] In this specification and the like, a structure in which a plurality of positive electrodes, a plurality of negative electrodes, and a plurality of separators are stacked as shown in FIG. 2B is referred to as a stacked electrode.

[0090] [Electrolyte] The electrolyte contains a solvent and a lithium salt. The solvent is preferably an aprotic organic solvent, and examples thereof include 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, and sultone, and any combination and ratio of two or more of these can be used. The electrolytic solution according to one embodiment of the present invention is preferably a liquid at room temperature (25° C.), but may be a solid, and may be simply referred to as an electrolyte.

[0091] [Mixed Solvent 1] A solvent containing two or more solvents is called a mixed solvent. When the mixed solvent contains ethylene carbonate (EC) and diethyl carbonate (DEC), the volume ratio of ethylene carbonate to diethyl carbonate is x:100-x (where 20≦x≦40) when the total content of ethylene carbonate and diethyl carbonate is 100 vol%. More specifically, a mixed solvent containing EC and DEC in an EC:DEC ratio of 30:70 (volume ratio) can be used.

[0092] [Mixed Solvent 2] As the electrolyte solution, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) is preferably used. In this case, when the total content of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate can be x:y:100-x-y (where 5≦x≦35 and 0<y<65). More specifically, a mixed solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 can be used.

[0093] [Mixed Solvent 3: Fluorinated Cyclic Carbonate and Fluorinated Chain Carbonate] As the electrolyte, it is preferable to use a mixed solvent containing a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) and a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate). Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have a substituent that exhibits electron-withdrawing properties, which is preferable because it reduces the solvation energy of lithium ions. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for the electrolyte, and a mixed solvent containing them is suitable.

[0094] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, they are believed to have low solvation energies for lithium ions.

[0095] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0096]

[0097] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0098]

[0099] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The following structural formula (H23) is the structural formula of trifluoromethyl 3,3,3-trifluoropropionate. The electron-withdrawing substituent is CF ３ It is the base.

[0100]

[0101] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The following structural formula (H24) is the structural formula of trifluoromethyl propionate. The electron-withdrawing substituent is CF ３ It is the base.

[0102]

[0103] An example of a fluorinated chain carbonate is methyl 2,2-difluoropropionate. The following structural formula (H25) is the structural formula of methyl 2,2-difluoropropionate. The electron-withdrawing substituent is CF ２ It is the base.

[0104]

[0105] The mixed solvent of the electrolyte solution according to one embodiment of the present invention may contain at least one selected from the fluorinated cyclic carbonates and at least one selected from the fluorinated chain carbonates. For example, the mixed solvent described in this embodiment may contain FEC and MTFP. The reason for this will be described below.

[0106] [FEC and MTFP] FEC is a cyclic carbonate with a high dielectric constant, and therefore, when used in an organic solvent, it promotes the dissociation of lithium salts. Furthermore, since FEC has electron-withdrawing substituents, it is more likely to desolvate with lithium ions than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is lower than that of ethylene carbonate (EC), which does not have electron-withdrawing substituents. Therefore, it is easier to separate lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, FEC is thought to have a deep highest occupied molecular orbital (HOMO) level. A deep HOMO level makes it less susceptible to oxidation and improves oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is preferable to use a mixed solvent containing not only FEC but also MTFP in the electrolyte. MTFP is a chain carbonate, and it can reduce the viscosity of the electrolyte or maintain the viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). Furthermore, MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, and therefore may form a solvation with lithium ions when used in the electrolyte.

[0107] The HOMO levels, solvation energies, and melting points (measured or literature values) for FEC, MTFP, EC, and MP are summarized in the table below.

[0108]

[0109] When the total content of a mixed solvent containing FEC and MTFP having such physical properties is taken as 100 vol%, it is preferable to mix and use the FEC and MTFP so that the volume ratio of FEC:MTFP is x:100-x (where 5≦x≦30, preferably 10≦x≦20). In other words, it is preferable to mix so that the amount of MTFP is greater than that of FEC in the mixed solvent.

[0110] Furthermore, it is preferable that the solvent used in the electrolyte solution has almost no peaks due to impurities that can be confirmed by NMR (Nuclear Magnetic Resonance) measurement or the like. "Almost no peaks due to impurities" means that the ratio of the integrated area of ​​the peak due to the main component to the integrated area of ​​the peak due to the impurity (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less. The device used for NMR measurement is not particularly limited, but for example, Bruker's "AVANCE III 400" can be used. １ Of the five peaks of acetonitrile derived from acetonitrile-d3 used as a solvent in H-NMR measurement, the central peak can be located at 1.94 ppm.

[0111] For example, in the case of MTFP, acetonitrile-d3 solvent was used. １ It is known that when measuring H-NMR, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost not observed.

[0112] The solvent used in the above-mentioned electrolyte solution is free of granular dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities"), including oxygen (O ２ ), water (H ２ The electrolyte preferably contains a low content of impurities such as 0 or 100 ppm, preferably 50 or less ppm, and more preferably less than 10 ppm. For example, the amount of water can be detected by Karl Fischer titration.

[0113] [Ionic Liquid] By using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0114] [Lithium Salt] Examples of the lithium salt include LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in any combination and ratio of two or more of these. The lithium salt is preferably used in an amount of 0.5 mol / L or more and 3.0 mol / L or less relative to the solvent. ６ , LiBF ４ The use of such improves the safety of lithium ion secondary batteries.

[0115] [Additives] The electrolyte preferably contains an additive in addition to a solvent. The concentration of the additive is preferably 0.1 wt % or more and 10 wt % or less relative to the total solvent. When distinguishing between a solvent and an additive, an additive that satisfies this concentration range can be considered an additive. The organic materials listed above as solvents can be used as the additive. As other organic materials, the additive preferably contains one or more selected from vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), lithium bis(oxalate)borate (LiBOB), and dinitrile compounds such as succinonitrile or adiponitrile. Vinylene carbonate (VC) or lithium bis(oxalate)borate (LiBOB) is preferred because it is easy to form a good coating portion. 1,3-propane sultone (PS) has HOMO and LUMO levels equivalent to those of ethylene carbonate (EC) and diethyl carbonate (DEC), making it resistant to oxidation and reduction even at high cutoff voltages. Furthermore, when decomposed on the surface of the positive electrode active material, it tends to form polymers. Therefore, it has the advantage of being less likely to gasify into low-molecular-weight decomposition products. It is more preferable for the electrolyte to contain 0.25 wt% to 7.5 wt% of PS. In this way, the electrolyte can be made safer by mixing multiple solvents and additives.

[0116] FIG. 4 shows a liquid injection device 280 for the electrolyte 180. The liquid injection device 280 includes a processing chamber 281, a liquid injection nozzle 282 located on the upper surface of the processing chamber 281, a pump 283 for sending the electrolyte 180 to the liquid injection nozzle 282, and a tank 284 for storing the electrolyte 180. The pump 283 preferably has a function for adjusting the liquid delivery speed. The processing chamber 281 includes a fixing device 285 for fixing the secondary battery 100 (which includes a stacked battery housed in an outer casing, and a portion of the outer casing is thermocompression bonded). Although not shown, the processing chamber 281 preferably includes a vacuum pump or the like, and liquid injection is preferably performed in a vacuum atmosphere. Examples of the vacuum pump include a dry pump, turbomolecular pump, oil-sealed rotary pump, cryopump, and mechanical booster pump. The vacuum atmosphere includes an atmosphere reduced in pressure so that a differential pressure gauge included in the processing chamber 281 indicates a pressure of −0.1 MPa or more but less than −0.08 MPa. It is preferable that a heating mechanism 286 is provided in a portion of the injection nozzle 282 that is close to the processing chamber 281. The viscosity of the electrolytic solution 180 can be controlled by the heating mechanism 286. When the injection of the electrolytic solution 108 is completed, the injection nozzle 282 moves up, and thermocompression bonding of the exterior body can be performed in the processing chamber 281.

[0117] Since the separator 105 has a larger area than the positive electrode and the negative electrode, the separator 105 is the first to come into contact with the electrolyte when the electrolyte is poured in. Therefore, it is preferable to use a separator 105 that has good wettability with the electrolyte, as this facilitates the pouring of the electrolyte.

[0118] In the separator 105, when stripe-shaped recesses are provided in the second member 16, the stripe-shaped recesses preferably extend from the injection nozzle 282 side as a starting point toward the opposing side. The third member 17 can also be provided with stripe-shaped recesses similar to those in the second member 16.

[0119] [Exterior Body] Although not shown, the secondary battery 100 has an exterior body, and the laminated electrodes are housed in the exterior body. The exterior body of the secondary battery 100 can be made of a metal material such as aluminum, stainless steel, or titanium, or a resin material. A film-like exterior body can also be used. Examples of films include a three-layer structure in which a flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the metal thin film as the outer surface of the exterior body. Such a multilayer structure film can be called a laminate film. In this case, the laminate film may be referred to as an aluminum (aluminum) laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, or the like, using the name of the material of the metal layer.

[0120] The material or thickness of the metal layer of the laminate film may affect the flexibility, i.e., the ease of bending, of the secondary battery 100. For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer is preferably used as an exterior body for a secondary battery 100 that prioritizes flexibility or lightweight design. Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the aluminum layer is thinner than 10 μm, there is a concern that pinholes in the aluminum layer may reduce the gas barrier properties, so the thickness of the aluminum layer is desirably 10 μm or more.

[0121] For example, a stainless steel laminate film having a polypropylene layer, a stainless steel layer, and a nylon layer is preferably used as an exterior body for a secondary battery 100 that emphasizes physical strength or safety. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the stainless steel layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the stainless steel layer is thinner than 10 μm, there is a concern that pinholes in the stainless steel layer may reduce the gas barrier property, so the thickness of the stainless steel layer is desirably 10 μm or more. Note that, in this specification, stainless steel refers to steel (an alloy of iron and carbon) containing approximately 12% or more chromium, and can be broadly classified into martensitic, ferritic, and austenitic types based on composition. Furthermore, stainless steel also includes stainless steels containing one or more elements selected from Ti, Nb, Mo, Cu, Ni, and Si.

[0122] Alternatively, for example, a titanium laminate film having a polypropylene layer, a titanium layer, and a nylon layer is preferably used. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the titanium layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the titanium layer is thinner than 10 μm, there is a concern that pinholes in the titanium layer may reduce the gas barrier properties, so the thickness of the titanium layer is preferably 10 μm or more.

[0123] A secondary battery that uses a film as an exterior body is called a laminated secondary battery. Although not shown in the present embodiment, a can case may be used as the exterior body. For example, a secondary battery that uses a circular case is called a coin-type secondary battery. A secondary battery that uses a cylindrical case is called a cylindrical secondary battery.

[0124] [Bendable Secondary Battery] Next, a bendable secondary battery 100x will be described using FIGS. 5A and 5B. FIG. 5A is a cross-sectional view of the secondary battery 100x, and FIG. 5B is a perspective view of the secondary battery 100x. Like the secondary battery 100, the secondary battery 100x has a positive electrode 103, a separator 105, and a negative electrode 106. The positive electrode 103, the separator 105, and the negative electrode 106 are preferably made of flexible materials. Regarding the positive electrode 103, the separator 105, and the negative electrode 106, descriptions of the same components as those of the secondary battery 100 will be omitted. As a difference from the secondary battery 100, the bendable secondary battery 100x may have two single-sided coated positive electrodes 103, with the positive electrode current collectors in contact with each other. A structure in which the positive electrode current collectors are in contact with each other is called a current collector overlapping structure. Similarly, two single-sided coated negative electrodes 106 may be prepared to prepare a negative electrode having a current collector overlapping structure in which the negative electrode current collectors are in contact with each other. In a secondary battery having a current collector overlapping structure, the contacting current collectors are likely to come off when the secondary battery is bent. Therefore, a current collector overlapping structure is suitable for a bendable secondary battery 100x.

[0125] Furthermore, in a bendable secondary battery 100x, the protrusion 31t may wrinkle, potentially causing the protrusion 31t to break. To prevent the protrusion 31t from wrinkling, FIGS. 5A and 5B show a secondary battery 100x bent so that the ends of the stacked battery on the protrusion 31t side are aligned. As a result, in the secondary battery 100x, misalignment occurs in the stacked battery on the side facing the protrusion 31t, and this misalignment increases toward the opposite side. In this case, using a current collector mating structure allows appropriate misalignment to occur, since the current collectors that are in contact with each other are likely to misalign. In a secondary battery 100x bent so that the ends of the stacked battery on the protrusion 31t side are aligned, there is a possibility that separators other than the outermost layer may come into contact with the exterior body.

[0126] In the case where the protrusion 31t is wrinkled and breaks, it is preferable to bend the secondary battery 100x (i.e., apply an external force) at a position farther from the protrusion 31t than the center of the stacked electrode. Of course, the bend position of the secondary battery 100x (i.e., apply an external force) may be the center of the stacked electrode.

[0127] In this specification and the like, a secondary battery 100x having a curved region (curved region) as shown in Figures 5A and 5B may be referred to as a curved secondary battery. It is preferable that a laminated secondary battery be used as the secondary battery 100x. The exterior body used in a laminated secondary battery is flexible, and therefore is preferable because it easily follows the deformation of the secondary battery, specifically the bending of the secondary battery. The secondary battery 100x can be bent and fixed as shown in Figures 5A and 5B and charged and discharged.

[0128] The bendable secondary battery 100x also includes a secondary battery that can be repeatedly changed between the straight state shown in Fig. 3A and the bent state shown in Fig. 5A and Fig. 5B. The secondary battery 100x can be discharged while changing from the straight state to the bent state or from the bent state to the straight state. Charging instead of discharging is also possible.

[0129] The separator 105 of the bendable secondary battery 100x preferably employs the separator 105 described above with reference to FIG. 1 . However, in the bendable secondary battery 100x, it is preferable that no adhesive layer is provided on the outermost surface of the separator 105, and that the separator 105 has a region in contact with the positive electrode 103. This allows the separator 105 and the positive electrode 103 to be appropriately misaligned. Similarly, it is preferable that no adhesive layer is provided, and that the separator 105 has a region in contact with the negative electrode 106. This allows the separator 105 and the negative electrode 106 to be appropriately misaligned. Specifically, a configuration without an adhesive layer is preferable in which the second member 16 is positioned on one of the outermost surfaces of the separator 105, and this surface is not coated with a ceramic material, a fluorine-based material, a polyamide-based material, or a mixture thereof. Similarly, it is preferable that the third member 17 is positioned on the outermost surface of the separator 105 and that this surface is not coated with a ceramic material, a fluorine material, a polyamide material, a mixture of these, etc. Note that the second member 16 and the third member 17 may be coated with a ceramic material, a fluorine material, a polyamide material, or a mixture of these that does not exhibit adhesive properties.

[0130] The flexible secondary battery 100x has a separator with good wettability with the electrolyte, which allows for good injection of the electrolyte into the exterior body. This is also preferable because it ensures the retention of the electrolyte in the separator 105 even when the electrodes expand and contract during charging and discharging of the flexible secondary battery 100x. Furthermore, the use of a separator with a multilayer structure allows for a large capacity per volume of the flexible secondary battery 100x.

[0131] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0132] Embodiment 2 In this embodiment, an example of a method for manufacturing the positive electrode active material 10 included in the secondary battery 100 of one embodiment of the present invention will be described with reference to FIGS. 6A to 8C.

[0133] In a manufacturing process of the positive electrode active material 10 according to one embodiment of the present invention, the first step preferably involves synthesizing lithium cobalt oxide by mixing a first lithium source and a cobalt source and performing a first heat treatment. When pre-synthesized lithium cobalt oxide is prepared, the first step can be omitted. Next, the second step preferably involves mixing the lithium cobalt oxide and the second lithium source and performing a second heat treatment on the resulting mixture. Then, the third step preferably involves mixing the mixture with an additive element source and performing a third heat treatment.

[0134] In the manufacturing process of the positive electrode active material 10 according to one embodiment of the present invention, the temperature of the second heat treatment and the third heat treatment is preferably 950° C. or lower. This is because heating at a temperature higher than 950° C. causes cation mixing, increasing the possibility that magnesium will enter the cobalt site of lithium cobalt oxide. As will be described later, magnesium present at the lithium site is Li ｘ CoO ２When x in the formula is small, it has the effect of maintaining the layered rock salt crystal structure of R-3m. However, magnesium present in the cobalt site does not have the above effect, so a temperature at which cation mixing does not occur is preferable. Furthermore, if the second and third heat treatments are performed at temperatures higher than 950°C, there are concerns that the cobalt in the lithium cobalt oxide will be reduced to divalent cobalt, resulting in a rock salt crystal structure, and that lithium will evaporate, which may have other adverse effects.

[0135] In a process for preparing the positive electrode active material 10 according to one embodiment of the present invention, the step of mixing the additive element source while maintaining the layered rock-salt crystal structure of the lithium cobalt oxide allows the additive element to be appropriately segregated in the surface layer portion of the positive electrode active material. In other words, it is difficult to appropriately segregate the additive element in the surface layer portion of the positive electrode active material using a method in which the additive element source is mixed in the same step as the first lithium source and the cobalt source to synthesize lithium cobalt oxide containing the additive element. Therefore, it is preferable to synthesize lithium cobalt oxide, then mix the additive element source, and then perform a heat treatment at an appropriate temperature.

[0136] [Initial Heating] In order to maintain the layered rock salt crystal structure of lithium cobalt oxide, it is preferable to heat the lithium cobalt oxide (also referred to as initial heating) in the process of producing the positive electrode active material 10 of one embodiment of the present invention. Specifically, it is preferable to heat the mixture after mixing the lithium cobalt oxide with the second lithium source. For example, lithium fluoride is suitable as the second lithium source. Lithium fluoride is preferable because it can function as a flux.

[0137] By adding a flux such as lithium fluoride in the initial heating step, a melting point drop occurs near the surface of the lithium cobalt oxide when the lithium cobalt oxide and the additive element source are mixed and heated after the initial heating. This melting point drop allows the additive element to be well distributed in the surface layer at a temperature where cation mixing is unlikely to occur. Therefore, the heating temperature in the initial heating is preferably a temperature equal to or higher than the melting point of the second lithium source. For example, when lithium fluoride is used as the second lithium source, the temperature is preferably higher than the melting point of lithium fluoride, 848°C (e.g., 850°C or higher), and more preferably 900°C or higher. Furthermore, the heating temperature in the initial heating is preferably equal to or lower than the temperature used to synthesize lithium cobalt oxide (e.g., 950°C or lower). That is, the initial heating is preferably performed by mixing lithium cobalt oxide and lithium fluoride and heating the mixture at a temperature between 900°C and 950°C.

[0138] Furthermore, adding a material that functions as a flux has the effect of suppressing re-adhesion of lithium cobalt oxide particles when the lithium cobalt oxide and the additive element source are mixed and heated after the initial heating. If the additive element is added and heated while the lithium cobalt oxide particles are still stuck together, the additive element may not be sufficiently distributed in the stuck areas. Therefore, when the sticking is resolved in a later process, such as a pressure process after coating on a positive electrode current collector, a surface with insufficient additive element may be exposed, and degradation may occur from that surface when used in a secondary battery. Therefore, it is preferable to suppress the sticking of lithium cobalt oxide particles before mixing and heating the additive element source.

[0139] In order to prevent the lithium cobalt oxide particles from adhering to each other, it is preferable to thoroughly mix the lithium cobalt oxide and the second lithium source before the initial heating. The mixing method can be, for example, by using a ball mill.

[0140] By using lithium cobalt oxide that has undergone the above initial heating, it is possible to suppress lithium deficiency that occurs due to lithium evaporation in the subsequent process of mixing additional elements and heating.

[0141] The lithium cobalt oxide that has undergone the above-mentioned initial heating preferably has a higher proportion of layered rock salt type crystal structures in the surface layer compared to the lithium cobalt oxide before the initial heating. For example, the lithium cobalt oxide that has undergone the above-mentioned initial heating preferably has layered rock salt type crystal structures in a portion within 2 nm from the surface of 35% or more, more preferably 45% or more. Furthermore, when EELS analysis is performed, the valence of cobalt is preferably 2.35 or more, more preferably 2.45 or more. The abundance of layered rock salt type crystal structures is one factor indicating that lithium deficiency is suppressed.

[0142] On the other hand, it is preferable that the entire surface layer of the positive electrode active material particles does not have a layered rock-salt type crystal structure. For example, the solid solubility limit of magnesium in pure lithium cobalt oxide is extremely low. In order to dissolve magnesium and other additive elements in a sufficient concentration, it is preferable that the surface layer of the positive electrode active material particles also have a rock-salt type crystal structure. Therefore, it is preferable that the positive electrode active material particles that have undergone the above-mentioned initial heating have a layered rock-salt type crystal structure in a portion within 2 nm from the surface, which is less than 100%, and more preferably 90% or less.

[0143] In other words, in the positive electrode active material particles that have undergone the above-mentioned initial heating, the layered rock salt type crystal structure is preferably 35% or more but less than 100%, more preferably 35% or more but less than 90%, and even more preferably 45% or more but less than 90%, in the portion within 2 nm from the surface.

[0144] <<Manufacturing Method 1 of Positive Electrode Active Material 10>> Manufacturing method 1 of positive electrode active material 10, which involves initial heating, will be described with reference to FIGS. 6A to 6D. FIG.

[0145] <Step S10> As shown in FIG. 6A, step S10 is ２ Step S10 includes at least steps S11 to S14.

[0146] <Step S11> In step S11 shown in FIG. 6A, a first lithium source (Li source 1) and a cobalt source (Co source) are prepared as starting materials.

[0147] As the first lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0148] As the cobalt source, it is preferable to use a compound containing cobalt, for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, or the like.

[0149] The cobalt source preferably has a 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.

[0150] <Step S12> Next, in step S12 shown in FIG. 6A , the first lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react 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 cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix the mixture.

[0151] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, it is preferable to use aluminum oxide balls or zirconium oxide balls as media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media.

[0152] 6A, the mixed material is heated at a temperature of preferably 800° C. to 1100° C., more preferably 900° C. to 1000° C., and even more preferably about 950° C. If the temperature is too low, the lithium source and the cobalt source may not be sufficiently decomposed and melted.

[0153] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0154] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate is preferably 200° C. / h.

[0155] The heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of −50° C. or less, more preferably an atmosphere with a dew point of −80° C. or less. In order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ It is preferable that the impurity concentrations of the above are each 5 ppb (parts per billion) or less.

[0156] The heating atmosphere is preferably an atmosphere containing oxygen, and dry air can be used. For example, there is a method of continuously introducing dry air into the furnace. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the furnace and having oxygen flow through the furnace is called flow.

[0157] When the heating atmosphere is an oxygen-containing atmosphere, a method of preventing oxygen from flowing may be used. For example, the furnace may be depressurized and then filled with oxygen (or purged) to prevent oxygen from entering or leaving the furnace. For example, the furnace may be depressurized until a differential pressure gauge indicates -970 hPa, and then oxygen may be filled to 50 hPa.

[0158] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0159] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0160] A crucible, setter, or sheath can be used as a container used during heating. The container preferably contains aluminum oxide. Aluminum oxide has the property of being less likely to release impurities. In this embodiment, it is advisable to use a sheath made of aluminum oxide with a purity of 99.5% or more, preferably 99.9% or more. In addition, for mass production, mullite-cordierite (Al ２ O ３ , SiO ２ It is recommended to use a setter or sheath made of MgO. It is preferable to place a lid on the container before heating, which can prevent the material from volatilizing.

[0161] Furthermore, it is preferable to use a container that has been used multiple times rather than a new one when heating. In this specification, a new container refers to one that has undergone two or fewer heating steps containing lithium, a transition metal M, and / or an additive element. A container that has been used multiple times refers to one that has undergone three or more heating steps containing lithium, a transition metal M, and / or an additive element. This is because using a new container may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the container during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of the additive element, particularly in the surface layer of the positive electrode active material, may not fall within a desirable range. On the other hand, this risk is less likely with a container that has been used multiple times.

[0162] After heating, the recovered material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the container to a mortar and then recovered. Furthermore, it is preferable to use aluminum oxide or zirconium oxide for the mortar. Aluminum oxide is a material that does not easily release impurities. Specifically, it is preferable to use a mortar made of aluminum oxide with a purity of 90% or more, preferably 99% or more. Note that heating conditions equivalent to those of step S13 can be applied to heating steps other than step S13, which will be described later.

[0163] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized. In this way, lithium cobalt oxide is prepared in step S10 of FIG. 6B.

[0164] Although an example of producing lithium cobalt oxide by a solid-phase method as shown in steps S11 to S14 has been shown, lithium cobalt oxide may also be produced by a coprecipitation method, a hydrothermal method, or a composite oxide other than lithium cobalt oxide.

[0165] It is also possible to use lithium cobalt oxide synthesized in advance in step S14, in which case steps S11 to S13 can be omitted.

[0166] 6B, a second lithium source (Li source 2) is prepared. As the second lithium source, for example, lithium fluoride is preferably used.

[0167] <Step S16> In step S16 shown in Fig. 6B, lithium cobalt oxide and the second lithium source are mixed. The mixing in step S16 is preferably performed under milder conditions than in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than in step S12. It can also be said that a dry method provides milder conditions than a wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium, for example.

[0168] <Step S17> Next, in step S17 shown in Fig. 6A, the lithium cobalt oxide is heated. Heating is preferably performed at a temperature of 850°C or higher and 950°C or lower. The heating time is preferably 1 hour or higher and 60 hours or lower, more preferably 2 hours or higher and 20 hours or lower, more preferably 2 hours or higher and 10 hours or lower, and even more preferably 2 hours or higher and 6 hours or lower. Since this is the first heating of the lithium cobalt oxide, the heating in step S17 is referred to as initial heating. By performing steps S16 and S17, lithium cobalt oxide with a smooth surface can be obtained.

[0169] <Step S20> Next, as shown in step S20, it is preferable to add the additional element A to the lithium cobalt oxide that has undergone the initial heating. When the additional element A is added to the lithium cobalt oxide that has undergone the initial heating, the additional element A can be added evenly. Therefore, it is preferable to add the additional element A after the initial heating. The step of adding the additional element A will be described with reference to FIGS. 6C and 6D.

[0170] 6C and 6D, the steps of preparing a source of additional element A (A source) will be described. Although not shown, in step S21, a lithium source may be prepared together with the source of additional element A.

[0171] The additional element A can be, for example, one or more selected from magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0172] <Step S21> Step S21 shown in Fig. 6C will be described. When magnesium is selected as the additional element A, the additional element A source (A source) can be called a magnesium source (Mg source). As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0173] When fluorine is selected as the additional element A, the additional element A source (A source) can be called a fluorine source (F source). Examples of the fluorine source include lithium fluoride (LiF), magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ ) can be used. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below. When lithium fluoride is used as the fluorine source, the lithium fluoride can also be called a third lithium source.

[0174] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0175] The fluorine source is preferably a gas. For example, fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ It is preferable to use a fluorine source such as fluorine fluoride, and mix it into the atmosphere during the heating step described below. It is more preferable to use a plurality of the above-mentioned fluorine sources.

[0176] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and the magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２ The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0177] 6C, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0178] 6C, the pulverized and mixed materials are collected to obtain a source of the additional element A. The source of the additional element A shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0179] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 10 μm, more preferably 1 μm to 5 μm. Even when a single material is used as the source of the additional element A, the D50 (median diameter) is preferably 600 nm to 10 μm, more preferably 1 μm to 5 μm.

[0180] Such a finely powdered mixture (including the case where only one type of additive element A is contained) can be easily adhered to the surfaces of lithium cobalt oxide particles when mixed with lithium cobalt oxide in a later step. Uniform adhesion of the mixture to the surfaces of lithium cobalt oxide particles is preferable because it can easily distribute or diffuse additive element A evenly in the surface layer portion of positive electrode active material 10 after heating.

[0181] <Step S21> A step different from that shown in FIG. 6C will be described with reference to FIG. 6D. In step S21 shown in FIG. 6D, four types of additive element A sources to be added to lithium cobalt oxide are prepared. That is, the types of additive element A sources in FIG. 6D are different from those in FIG. 6C. A lithium source may be prepared together with the additive element A sources.

[0182] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described with reference to FIG. 6C . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0183] <Steps S22 and S23> Steps S22 and S23 shown in FIG. 6D are similar to the steps described in FIG. 6C.

[0184] <Step S31> Next, in step S31 shown in FIG. 6B, the lithium cobalt oxide that has been initially heated is mixed with an additional element A source (A source).

[0185] In the present embodiment, the number of magnesium atoms contained in the additive element A source is preferably 0.50% or more and 3.0% or less, more preferably 0.75% or more and 2.0% or less, and still more preferably 0.75% or more and 1.0% or less, relative to the number of cobalt atoms contained in the lithium cobalt oxide.

[0186] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter mixing time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0187] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0188] <Step S32> Next, in step S32 of Fig. 6B, the mixed materials are collected to obtain a mixture 903. When collecting the materials, sieving may be performed as necessary.

[0189] 6A to 6D illustrate a manufacturing method in which the additional element A is added only after initial heating, but the present invention is not limited to the above method. The additional element A may be added at a different timing or multiple times. The timing may vary depending on the element.

[0190] <Step S33> Next, in step S33 shown in FIG. 6B, the mixture 903 is heated.

[0191] For example, as the additive element A source (A source), MgF ２ When LiF and MgF２ Since the eutectic point of [the relevant substances] is around 742°C, it is preferable that the heating temperature in step S33 is 742°C or higher.

[0192] Also, LiCoO ２ :LiF:MgF ２ A mixture obtained by mixing so that [the ratio] = 100:0.33:1 (molar ratio) shows an endothermic peak around 830°C in differential scanning calorimetry (DSC test). Therefore, it is more preferable that the heating temperature is 830°C or higher. Thus, the heating in step S33 is preferably performed at 800°C or higher and 1000°C or lower, more preferably at 830°C or higher and 950°C or lower, and even more preferably at 850°C or higher and 950°C or lower. Also, the heating time is preferably 1 hour or longer and 60 hours or shorter, and more preferably 2 hours or longer and 20 hours or shorter.

[0193] In the manufacturing method described in this embodiment, LiF, which is the second lithium source added in step S15 of FIG. 6B, may function as a flux. By this function, the heating temperature in step S33 can be lowered to less than the melting point of lithium cobaltate, for example, 742°C or higher and 950°C or lower, and an additive element A such as magnesium can be present in the surface layer portion, and a positive electrode active material with good characteristics can be produced.

[0194] Supplementary explanation about the heating time. The heating time varies depending on conditions such as the heating temperature, the size of lithium cobaltate in step S14, and the composition. When the lithium cobaltate is small, a lower heating temperature or a shorter heating time may be more preferable than when it is large.

[0195] <Step S34> Next, in step S34 shown in FIG. 6B, the heated material is recovered to obtain the positive electrode active material 10. At this time, the recovered particles can be crushed by sieving them as necessary. Through the above steps, the positive electrode active material 10 of one aspect of the present invention can be produced. The surface of the positive electrode active material of one aspect of the present invention is smooth.

[0196] <<Manufacturing Method 2 for Positive Electrode Active Material 10>> Next, a manufacturing method 2 for positive electrode active material 10, which is an embodiment of the present invention and differs from manufacturing method 1 for positive electrode active material 10, will be described with reference to FIGS. 7 to 8C . Manufacturing method 2 for positive electrode active material differs from manufacturing method 1 mainly in the number of times additive element A is added and the mixing method. For other descriptions, the description of manufacturing method 1 can be referred to.

[0197] In FIG. 7, steps S11 to S17 are carried out in the same manner as in FIGS. 6A and 6B to prepare lithium cobalt oxide that has undergone initial heating.

[0198] <Step S20a> Next, as shown in step S20a, it is preferable to add an additional element A1 to the lithium cobalt oxide that has undergone the initial heating.

[0199] <Step S21> In steps S21 to S23 shown in Fig. 8A, a first additive element source (A1 source) is prepared. The first additive element source can be selected from the additive elements A described in step S21 shown in Fig. 6C and used. For example, the additive element A1 can be any one or more selected from magnesium, fluorine, and calcium. Fig. 8A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source (A1 source).

[0200] Steps S21 to S23 shown in Fig. 8A can be performed under the same conditions as steps S21 to S23 shown in Fig. 6C. As a result, an additional element source (Al source) can be obtained in step S23.

[0201] Furthermore, steps S31 to S33 shown in FIG. 7 can be performed in the same manner as steps S31 to S33 shown in FIG. 6A.

[0202] <Step S34a> Next, the material heated in step S33 is recovered to produce lithium cobalt oxide containing the additional element A1. To distinguish it from the lithium cobalt oxide in step S14, this is called a composite oxide.

[0203] <Step S40> In step S40 shown in Fig. 7, a second additive element source (A2 source) is prepared. This will be described with reference to Figs. 8B and 8C.

[0204] <Step S41> In steps S41 to S43 shown in FIG. 8B, a second additive element source (A2 source) is prepared. The additive element A2 contained in the second additive element source can be selected from the additive elements A described in step S21 shown in FIG. 6C. For example, the additive element A2 can be one or more selected from nickel, boron, zirconium, and aluminum. FIG. 8B illustrates an example in which a nickel source (Ni source) and an aluminum source (Al source) are used as the second additive element source (A2 source). Examples of nickel sources that can be used include nickel hydroxide and nickel fluoride. Examples of aluminum sources that can be used include aluminum hydroxide and aluminum fluoride.

[0205] The number of nickel atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms contained in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when nickel hydroxide is used as the nickel source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of nickel hydroxide contained in the second additive element source (A2 source) is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0206] The number of aluminum atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms contained in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when aluminum hydroxide is used as the aluminum source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of aluminum hydroxide contained in the second additive element source (A2 source) is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0207] Steps S41 to S43 shown in Fig. 8B can be performed under the same conditions as steps S21 to S23 shown in Fig. 6C. As a result, an additional element source (A2 source) can be obtained in step S43.

[0208] 8C shows a modified example of the steps described with reference to FIG. 8B. In step S41 shown in FIG. 8C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are pulverized. That is, in step S43 shown in FIG. 8C, a plurality of second additive element sources (A2 sources) are prepared.

[0209] <Steps S51 to S53> Next, steps S51 to S53 shown in FIG. 7 can be performed under the same conditions as steps S31 to S34 shown in FIG. 6A. The heating in step S53 is preferably performed at a temperature of 800°C to 950°C, and even more preferably at a temperature of 800°C to 900°C. The heating time is preferably 1 hour to 60 hours, more preferably 2 hours to 20 hours, and even more preferably 2 hours to 10 hours. The heating in step S53 is preferably performed at a lower temperature and for a shorter time than in step S33. Through the above steps, the cathode active material 10 of one embodiment of the present invention can be produced in step S54. The cathode active material of one embodiment of the present invention has a smooth surface.

[0210] 7 to 8C , in manufacturing method 2, the addition of the additional element A to lithium cobalt oxide is carried out separately as additional element A1 and additional element A2. By separately adding the additional element A1 and the additional element A2, the distribution of each additional element in the depth direction can be changed. For example, it is possible to distribute the additional element A1 so that its concentration is higher in the surface layer portion of the positive electrode active material than in the interior of the positive electrode active material, and to distribute the additional element A2 so that its concentration is higher in the interior than in the surface layer portion.

[0211] High purity LiCoO ２ In the manufacturing method of preparing the silicon nitride film, and then subsequently mixing and heating the additional element A, the additional element A spreads mainly through the diffusion path of lithium ions. Therefore, it is easy to make the distribution of the additional element A in the planes other than the (001) plane and in the surface layer portion 10 a thereof fall within a preferred range.

[0212] The characteristics of the positive electrode active material 10 produced through the above-described steps will be described with reference to FIGS. 9A to 10F.

[0213] 9A and 9B are cross-sectional views of a cathode active material 10 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 9B are shown in FIGS. 10A to 10C. Enlarged views of the vicinity of C-D in FIG. 9B are shown in FIGS. 10D to 10F.

[0214] 9A, the positive electrode active material 10 has a surface layer portion 10a and an inner portion 10b. In FIGS. 9A and 9B, a dashed line indicates an example of the boundary between the surface layer portion 10a and the inner portion 10b.

[0215] The surface layer 10a of the positive electrode active material 10 refers to, for example, a region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 10a is synonymous with the near-surface, near-surface region, or shell.

[0216] The region deeper than the surface layer 10a of the positive electrode active material is referred to as the inner portion 10b, which is synonymous with the inner region or core.

[0217] Furthermore, when the positive electrode active material 10 has a layered rock salt crystal structure of space group R-3m, as shown in FIG. 9B , the surface layer portion 10a has an edge region 10a1 and a basal region 10a2. In FIGS. 9A and 9B , the straight line labeled (00l) represents the (00l) plane. Here, the edge region 10a1 is a region that intersects with the (00l) plane, and is defined as a region extending from the surface of the edge region 10a1 to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular to the surface. Note that "intersecting with the (00l) plane" means that the angle between the perpendicular to the (00l) plane and the normal to the surface of the positive electrode active material 10 is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less.

[0218] Basal region 10a2 has a surface parallel to the (00l) plane, and the region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior is referred to as basal region 10a2. Note that "parallel to the (00l) plane" means that the angle between the perpendicular to the (00l) plane and the normal to the surface of positive electrode active material 10 is 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0219] The surface of the positive electrode active material 10 refers to the surface of the composite oxide including the surface layer portion 10a and the inner portion 10b. Therefore, the positive electrode active material 10 is made of aluminum oxide (Al ２ O ３ The term "adhered metal oxide" does not include metal oxides attached to the surface of the positive electrode active material, such as the inner surface 10b, carbonates chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. Note that the "adhered metal oxide" refers to, for example, metal oxides whose crystal orientation does not match that of the inner surface 10b.

[0220] The fact that the crystal orientations of the two regions roughly coincide can be determined from a TEM (Transmission Electron Microscope) image, a STEM (Scanning Transmission Electron Microscope) image, a HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) image, an ABF-STEM (Annular Bright-field Scanning Transmission Electron Microscopy) image, an electron beam diffraction pattern, etc. It can also be determined from an FFT (Fast Fourier Transform) pattern of a TEM image and an FFT pattern of a STEM image, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for judgment.

[0221] It also does not include the electrolyte solution, decomposition products of the electrolyte solution, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 10.

[0222] Since the positive electrode active material 10 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between the region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that oxidizes and reduces with lithium insertion / extraction and the region where oxygen is present and the region where oxygen is not present is defined as the surface of the positive electrode active material. Surfaces created by slips, cracks, and / or cracks can also be considered the surface of the positive electrode active material. When analyzing the positive electrode active material, a protective film may be applied to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0223] <Containing Elements> The positive electrode active material 10 contains lithium, cobalt, oxygen, and an additional element A. Alternatively, the positive electrode active material 10 contains lithium cobalt oxide (LiCoO ２ ) to which an additional element A is added. However, the positive electrode active material 10 of one embodiment of the present invention may have any crystal structure described below, and the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0224] The positive electrode active material must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 10 of one embodiment of the present invention preferably uses cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or more selected from nickel and manganese may also be used. It is preferable for the positive electrode active material 10 to contain 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt among the transition metals contained therein, as this has many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0225] Furthermore, when the cobalt content of the transition metals in the positive electrode active material 10 is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal, ｘ CoO ２The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) accounts for the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel accounts for the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0226] The additive element A contained in the positive electrode active material 10 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium.

[0227] That is, the positive electrode active material 10 can be one or more of lithium cobalt oxide having magnesium, lithium cobalt oxide having magnesium and aluminum, lithium cobalt oxide having magnesium and nickel, lithium cobalt oxide having magnesium, aluminum, and nickel, lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium, fluorine, and nickel, lithium cobalt oxide having magnesium, fluorine, nickel, and aluminum, and the like.

[0228] Furthermore, it can also be said that one or more of the following can be used as the positive electrode active material 10 in a lithium ion secondary battery: a positive electrode active material having cobalt, oxygen, and magnesium; a positive electrode active material having cobalt, oxygen, magnesium, and aluminum; a positive electrode active material having cobalt, oxygen, magnesium, and nickel; a positive electrode active material having cobalt, oxygen, magnesium, aluminum, and nickel; a positive electrode active material having cobalt, oxygen, magnesium, and fluorine; a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and aluminum; a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and nickel; and a positive electrode active material having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum.

[0229] The additive element A is preferably present as a solid solution in the positive electrode active material 10. For example, when performing a line analysis using STEM-EDX, the position of the rising edge at which the additive element A is detected in the depth direction is preferably located deeper than the position of the rising edge at which the transition metal M is detected, i.e., closer to the interior of the positive electrode active material 10.

[0230] These additional elements A further stabilize the crystal structure of the positive electrode active material 10 as will be described later.

[0231] The additional element A does not necessarily have to include magnesium, fluorine, nickel, aluminum, zirconium, titanium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, or beryllium.

[0232] For example, if the cathode active material 10 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, etc., are further enhanced. The weight amount of manganese contained in the cathode active material 10 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0233] The surface layer 10a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 10b. In addition, the atoms on the surface of the positive electrode active material 10 in the surface layer 10a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 10a is likely to become unstable, and can be said to be a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 10a can be made sufficiently stable, Li ｘ CoO ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 10b made of cobalt and oxygen octahedra can be made less likely to break.Furthermore, displacement of the layers made of cobalt and oxygen octahedra in the inner portion 10b can be suppressed.

[0234] In order to give the surface layer portion 10a a stable crystal structure, the surface layer portion 10a preferably contains an additive element A, and more preferably contains a plurality of additive elements A. Furthermore, the surface layer portion 10a preferably has a higher concentration of one or more selected from the additive elements A than the interior portion 10b. Furthermore, it is preferable that the one or more selected from the additive elements A contained in the positive electrode active material 10 have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive element A in the positive electrode active material 10 differs depending on the additive element A. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element A. Here, the concentration peak refers to the maximum value of the concentration in the surface layer portion 10a or within 50 nm from the surface.

[0235] [Distribution] The distribution of the additional element A will be described. Figures 10A to 10C are diagrams illustrating the edge region 10a1 of the positive electrode active material 10. Figures 10D to 10F are diagrams illustrating the basal region 10a2 of the positive electrode active material 10.

[0236] For example, some of the additive elements A, such as magnesium, fluorine, silicon, phosphorus, titanium, boron, and calcium, preferably have a concentration gradient that increases from the interior 10b toward the surface, as shown by the gradation in Figures 10A and 10D. The additive element A having such a concentration gradient will be referred to as additive element X. Although additive element X often corresponds to additive element A1, it does not necessarily have to correspond to additive element A1. A concentration gradient such as that shown by the gradation in Figures 10A and 10D can be achieved depending on the diffusion rate rather than the timing of addition.

[0237] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient and a concentration peak in a region deeper than the additive element X shown in FIGS. 10A and 10D, as shown by the hatch density in FIGS. 10B and 10E. The concentration peak may be present in the surface layer 10a, or may be deeper than the surface layer 10a. For example, it is preferable that the peak be in a region of 5 nm to 30 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Y. Although additive element Y often corresponds to additive element A2, it does not necessarily correspond to additive element A2. The concentration gradient shown by the hatch density in FIGS. 10B and 10E depends on the diffusion rate rather than the timing of addition.

[0238] As shown by the presence or absence of hatching and the density of the hatching in FIGS. 10C and 10F , another additive element, such as nickel or barium, may be clearly present in the edge region 10a1 but substantially absent in the basal region 10a2. Here, "clearly present" refers to a case in which the characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. "Substantially absent" refers to a case in which the characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 10. This also refers to the element being below the detection limit in STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Z. While additive element Z often corresponds to additive element A2, it does not necessarily correspond to additive element A2. The concentration gradient shown by the density of the hatching in FIGS. 10B and 10E is determined depending on the diffusion rate rather than the timing of addition.

[0239] For example, magnesium ions, which are one of the additive elements X, are divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 10a makes it easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x in the formula (10a) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 10. Furthermore, a high magnesium concentration in the surface layer portion 10a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0240] At an appropriate concentration, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides or fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become a resistance component in the secondary battery. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0241] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 10 is appropriate. For example, the number of magnesium atoms is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the entire positive electrode active material 10 here may be a value obtained by performing elemental analysis of the entire positive electrode active material 10 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 10.

[0242] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum has the effect of suppressing the elution of surrounding cobalt and improving continuous charging durability. Furthermore, because 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, the presence of aluminum as the additive element Y can improve the safety of the positive electrode active material 10 when used in a secondary battery. Furthermore, the positive electrode active material 10 can be made to have a crystal structure that is less likely to collapse even with repeated charging and discharging.

[0243] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0244] Therefore, it is preferable that the total amount of aluminum contained in the cathode active material 10 is appropriate. For example, the number of aluminum atoms contained in the entire cathode active material 10 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire cathode active material 10 here may be, for example, a value obtained by performing elemental analysis of the entire cathode active material 10 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the cathode active material 10.

[0245] Nickel, which is one of the additional elements Z, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower oxidation-reduction potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0246] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the change in volume caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site can also be prevented from shifting to CoO ２This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0247] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect the insertion and extraction of lithium.

[0248] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 10 is appropriate. For example, the number of nickel atoms in the positive electrode active material 10 is preferably greater than 0% and not greater than 7.5% of the total number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably greater than 0% and not greater than 4%. Alternatively, it is preferably greater than 0% and not greater than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0249] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer 10a is substituted with fluorine, the lithium desorption energy decreases. This is because the valence of cobalt ions changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different oxidation-reduction potentials. Therefore, when a portion of the oxygen in the surface layer 10a of the positive electrode active material 10 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 10 is used in a secondary battery, charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer 10a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as described in the first embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of the other additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0250] 10A and 10C, when the surface layer 10a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer 10a.

[0251] Furthermore, having additive elements with different distributions, such as additive element X, additive element Y, and additive element Z, in combination is preferable because it can stabilize the crystal structure in a wider region. For example, when the positive electrode active material 10 contains magnesium, which is one of the additive elements X, aluminum, which is one of the additive elements Y, and nickel, which is one of the additive elements Z, it can stabilize the crystal structure in a wider region than when it contains only one or two of the additive elements X, Y, and Z. In this way, when the positive electrode active material 10 contains additive element X, additive element Y, and additive element Z in combination, the surface can be sufficiently stabilized by additive element X such as magnesium and additive element Z such as nickel, so additive element Y such as aluminum is not essential to the surface. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from the surface to a depth of 1 nm to 25 nm. In this way, a wider distribution of aluminum is preferable because it can stabilize the crystal structure in a wider region.

[0252] 10C and 10F , when the additive element Z is contained in a larger amount in the edge region 10a1 than in the basal region 10a2 (also referred to as being contained preferentially or selectively), this is preferable because it improves the stability of the crystal structure of the edge region 10a1, where lithium ions enter and exit the positive electrode active material 10 during charging and discharging of the lithium ion secondary battery. Furthermore, when the additive element Z has the above-described distribution, for example, when the positive electrode active material 10 is lithium cobalt oxide, this is preferable because it minimizes the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0253] As described above, when multiple additive elements are present, the effects of each additive element are synergistic, which can contribute to further stabilization of the surface layer portion 10a. In particular, the presence of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, making it preferable. In particular, it is preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where magnesium is distributed closer to the surface than aluminum. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 10a of the positive electrode active material 10 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 10a1.

[0254] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the positive electrode active material 10 of one embodiment of the present invention, the positive electrode active material 10 is in a discharged state, that is, Li ｘ CoO ２ In the case where x = 1 in the formula (I), it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. The layered rock-salt type composite oxide has a high discharge capacity, has two-dimensional lithium ion diffusion paths, is suitable for lithium ion insertion / extraction reactions, and is excellent as a positive electrode active material for secondary batteries. Therefore, it is particularly preferable that the inner portion 10b, which occupies the majority of the volume of the positive electrode active material 10, has a layered rock-salt type crystal structure.

[0255] On the other hand, the surface layer portion 10a of the cathode active material 10 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 10b, which is made up of an octahedron of the transition metal M and oxygen, so that the layered structure formed by the octahedron of the transition metal M and oxygen is not destroyed even when lithium is released from the cathode active material 10 upon charging. Alternatively, the surface layer portion 10a preferably functions as a barrier film for the cathode active material 10. Alternatively, the surface layer portion 10a, which is the outer periphery of the cathode active material 10, preferably reinforces the cathode active material 10. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 10a and inner portion 10b of the cathode active material 10, such as oxygen release, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 10.

[0256] Therefore, the surface layer portion 10a preferably has a different crystal structure from the interior portion 10b. Furthermore, the surface layer portion 10a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 10b. For example, at least a portion of the surface layer portion 10a of the positive electrode active material 10 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 10a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 10a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0257] Furthermore, although it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 10a than in the interior 10b, they are also preferably present randomly and dilutely in the interior 10b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 10b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 10b, it can suppress the deviation of the layered structure consisting of the transition metal M and oxygen octahedra, as described above. Furthermore, 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 elution can be expected.

[0258] It is also preferable that the crystal structure continuously changes from the interior 10b toward the surface due to the concentration gradient of the added element A. Alternatively, it is preferable that the crystal orientation of the surface layer 10a and the interior 10b are substantially the same.

[0259] For example, it is preferable that the crystal structure continuously change from the layered rock salt type interior 10b toward the surface and surface layer 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 10a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure is approximately the same as that of the layered rock salt type interior 10b.

[0260] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice structure, which may result in a lower symmetry than the rock-salt crystal structure.

[0261] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0262] The presence of both the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, and the like.

[0263] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal M. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the rock salt type in an ideal state, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２ The bright spots on the (003) plane of MgO are observed at a distance about half the distance between the bright spots on the (111) plane of MgO. ２In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0264] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0265] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3'-type crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0266] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0267] However, the space group of the layered rock salt type crystal and the O3' type crystal is R-3m, which is different from the space group Fm-3m of the rock salt type crystal (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal and the O3' type crystal and the rock salt type crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal, the O3' type and the rock salt type crystal, it may be said that the crystal orientations are approximately the same.

[0268] <Li ｘ CoO ２ The positive electrode active material 10 according to one embodiment of the present invention has the above-described distribution of the additive element A and / or the crystal structure in a discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small differs from that of conventional positive electrode active materials in that the magnesium present at the lithium site maintains the R-3m layered rock salt type crystal structure. Note that a small x here means that 0.1<x≦0.24.

[0269] 11 to 14, Li ｘ CoO ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 10 according to one embodiment of the present invention will be described by comparing a conventional positive electrode active material with the positive electrode active material 10 according to one embodiment of the present invention.

[0270] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 12. The conventional positive electrode active material shown in FIG. 12 is a lithium cobalt oxide (LiCoO ２ )

[0271] In Figure 12, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0272] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0273] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0274] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in this specification, including Figure 12, the c-axis of the H1-3 crystal structure is shown as half the unit cell to make it easier to compare with other crystal structures.

[0275] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as 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 each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, a unit cell with a small GOF (goodness of fit) value can be adopted.

[0276] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0277] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 12, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0278] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0279] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0280] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and extract lithium.

[0281] On the other hand, in the positive electrode active material 10 according to one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the volume change per cobalt atom can be reduced. Therefore, the positive electrode active material 10 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 10 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 10 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0282] Li ｘ CoO ２ 9 shows the crystal structure of the interior 10b of the positive electrode active material 10 when x is approximately 1 or 0.2. The interior 10b occupies the majority of the volume of the positive electrode active material 10 and is the part that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0283] When x=1, the positive electrode active material 10 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0284] However, the positive electrode active material 10 has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.12, which results in an H1-3 type crystal structure.

[0285] When x is about 0.2, the positive electrode active material 10 according to one embodiment of the present invention has a crystal structure belonging to the trigonal space group R-3m. ２The layer symmetry is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 11 with the notation R-3m O3'.

[0286] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0287] In the O3' type crystal structure, ions of cobalt, nickel, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0288] As shown by the dotted line in FIG. 11, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0289] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0290] As described above, in the positive electrode active material 10 according to one embodiment of the present invention, Li ｘ CoO ２When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the crystal structure of the positive electrode active material 10 is resistant to collapse even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 10 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 10, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0291] The positive electrode active material 10 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is presumed that the O3' type crystal structure is present even when x is greater than 0.24 and 0.27 or less. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0292] Therefore, the positive electrode active material 10 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the entire interior 10b of the positive electrode active material 10 does not have to have the O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0293] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２A state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when a battery is charged at a constant current / constant voltage (CC / CV) at a voltage of 4.6 V or higher relative to the potential of lithium metal in a 25°C environment, a H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be considered a high charging voltage.

[0294] Therefore, in other words, positive electrode active material 10 of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it is preferable because it can adopt an O 3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C.

[0295] In some cases, the H1-3 crystal structure is finally observed when the charge voltage is further increased, even in positive electrode active material 10. As described above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher but lower than 4.6 V at 25° C., positive electrode active material 10 of one embodiment of the present invention may be able to adopt the O3′ crystal structure.

[0296] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect 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 maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0297] In addition, in O3' of FIG. 11, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, in the monoclinic O1 (Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0298] The O3' type crystal structure has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0299] Furthermore, it is preferable that the concentration gradient of the additive element A be similar at multiple locations in the surface layer portion 10a of the positive electrode active material 10. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 10a. Even if a portion of the surface layer portion 10a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the positive electrode active material 10, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0300] However, the additional element A does not necessarily have the same concentration gradient throughout the entire surface layer portion 10a of the positive electrode active material 10. It is preferable that the additional element A has the distribution of the additional element X shown in Fig. 10D and the distribution of the additional element Y shown in Fig. 10E.

[0301] Here, the area C-D has an R-3m layered rock salt type crystal structure, and the surface has a (00l) orientation. The (00l)-oriented surface may have a different distribution of the additional element A than the other surfaces. For example, the (00l)-oriented surface and its surface layer 10a may have a distribution of one or more concentration peaks selected from the additional element A limited to a shallower portion from the surface compared to surfaces other than the (00l) orientation. Alternatively, the (00l)-oriented surface and its surface layer 10a may have a lower concentration of one or more elements selected from the additional element A compared to the other orientations. Alternatively, the (00l)-oriented surface and its surface layer 10a may have one or more elements selected from the additional element A below the lower detection limit.

[0302] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is such that the lithium ion diffusion path is parallel to the (00l) plane.

[0303] CoO ２ Since the layer is relatively stable, the surface of the positive electrode active material 10 is more stable when it is oriented in the (00l) direction. The main diffusion path of lithium ions during charge and discharge is not exposed on the (00l) plane.

[0304] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (00l) orientation. Therefore, the surface and the surface layer 10a other than the (00l) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer 10a other than the (00l) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 10.

[0305] <Grain Boundary> In addition to the distribution described above, it is more preferable that at least a portion of the additive element A contained in the positive electrode active material 10 of one embodiment of the present invention is unevenly distributed, that is, present at a high concentration, in and near the grain boundaries.

[0306] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of areas with high concentration and areas with low concentration.

[0307] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 10 is higher than that in other regions of the interior 10b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in other regions of the interior 10b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in other regions of the interior 10b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in other regions of the interior 10b.

[0308] Grain boundaries are a type of planar defect. Therefore, like surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of the added element A at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0309] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of positive electrode active material 10 of one embodiment of the present invention, the magnesium concentration and fluorine concentration become high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid of the positive electrode active material after the cracks occur can be improved.

[0310] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 10 of one embodiment of the present invention has an O3′-type crystal structure by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0311] In particular, XRD is preferred in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 10b of the positive electrode active material 10, which occupies the majority of the volume of the positive electrode active material 10.

[0312] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0313] As described above, the positive electrode active material 10 according to one embodiment of the present invention is ｘ CoO ２The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0314] It should also be noted that simply adding an additional element A may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, Li ｘ CoO ２ In this case, x is 0.24 or less and the O3' type crystal structure accounts for 60% or more, and in other cases the H1-3 type crystal structure accounts for 50% or more.

[0315] Furthermore, even in the case of positive electrode active material 10 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not a material is positive electrode active material 10 of one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0316] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it 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.

[0317] Whether the distribution of the additive element A in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, EPMA (Electron Probe Micro Analyzer), or the like.

[0318] The crystal structure, such as the grain boundaries, can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 10 .

[0319] <Charging Method> Charging for determining whether a composite oxide is the positive electrode active material 10 of one embodiment of the present invention can be performed by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using the composite oxide as a positive electrode and lithium metal as a counter electrode. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can. The coin cell used to determine whether the composite oxide is the positive electrode active material 10 does not need to include the separator and electrolyte of one embodiment of the present invention.

[0320] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0321] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0322] The lithium salt contained in the electrolyte solution is 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0323] The separator may be a 25 μm thick porous polypropylene film.

[0324] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0325] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging using CC / CV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, charging at such a low current value is desirable. The temperature is set to 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 the desired charge capacity. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container under an argon atmosphere. After charging is completed, the positive electrode is preferably removed and analyzed promptly. Specifically, within 1 hour, and more preferably within 30 minutes, after charging is completed.

[0326] Furthermore, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, for example, charging can be performed by constant current charging at a current value of 20 mA / g to 100 mA / g up to a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V), followed by constant voltage charging until the current value reaches 2 mA / g to 10 mA / g, and then discharging at a constant current of 20 mA / g to 100 mA / g at 2.5 V. Alternatively, discharging can be performed by constant current discharging at 3.0 V at a current value of 20 mA / g to 200 mA / g.

[0327] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed, for example, at 2.5 V and a current value of 20 mA / g or more and 200 mA / g or less, or at 3.0 V and a current value of 20 mA / g or more and 200 mA / g or less.

[0328] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS, X-ray: CuKα1, output: 40 kV, 40 mA, slit width: Div. Slit, 0.5°, detector: Lynx Eye, scan method: 2θ / θ continuous scan, measurement range (2θ): 15° to 90°, step width (2θ): 0.01°, set counting time: 1 second / step, sample stage rotation: 15 rpm.

[0329] If the measurement sample is a powder, it can be set by placing it in a glass sample holder or by sprinkling the sample on a greased silicone anti-reflective plate. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0330] CuKα calculated from the O3' type crystal structure and the H1-3 type crystal structure model １ The ideal powder XRD patterns by the line are shown in Figures 13 and 14. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structure of LiCoO3 and the trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD. The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The XRD pattern of the H1-3 type crystal structure was created by the same method as above, based on the information on the H1-3 type crystal structure shown in Fig. 12. The XRD pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same manner as the others.

[0331] As shown in FIG. 13, in the O3′ type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0332] However, as shown in FIG. 14, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and 19.37° or less) and 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) when x is small can be said to be a characteristic of positive electrode active material 10 of one embodiment of the present invention.

[0333] 13, for example, when charging is performed with an upper limit of the charging voltage being slightly lower than 4.60 V (4.56 V, 4.57 V, 4.58 V, or 4.59 V), the above peak appears shifted to a lower angle. For example, when charging is performed with an upper limit of the charging voltage being 4.58 V, positive electrode active material 10 has diffraction peaks at 2θ = 18.85 ± 0.20 ° and 2θ = 45.15 ± 0.10 °, which are diffraction peaks derived from the O3'-type crystal structure.

[0334] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0335] The positive electrode active material 10 according to one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0336] Furthermore, even after 5 or more, 30 or more, 50 or more, or 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3'-type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0337] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43° or more and 46° or less, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0338] The crystallite size of the O3'-type crystal structure of the positive electrode active material 10 is approximately equal to that of LiCoO ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the graph is small, a clear peak of the O3' type crystal structure can be confirmed. ２ In this case, even if a part of the crystal structure resembles the O3'-type crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0339] <XPS> In XPS (X-ray Photoelectron Spectroscopy), in the case of inorganic oxides, if monochromatic aluminum Kα rays are used as X-rays, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so that the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 10a. Furthermore, narrow scan analysis can be performed to analyze the bonding state of the elements. Note that the quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0340] In the cathode active material 10 according to one embodiment of the present invention, the concentration of one or more selected from additive elements A is preferably higher in the surface layer portion 10a than in the interior portion 10b. This is equivalent to saying that the concentration of one or more selected from additive elements A in the surface layer portion 10a is preferably higher than the average concentration throughout the cathode active material 10. Therefore, for example, it can be said that the concentration of one or more selected additive elements A in the surface layer portion 10a measured by XPS or the like is preferably higher than the average concentration of additive elements A throughout the cathode active material 10 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration of at least a portion of the surface layer portion 10a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 10. Furthermore, the nickel concentration of at least a portion of the surface layer portion 10a is preferably higher than the average nickel concentration throughout the cathode active material 10. Furthermore, the aluminum concentration of at least a portion of the surface layer portion 10a is preferably higher than the average aluminum concentration throughout the cathode active material 10. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 10 a is higher than the average fluorine concentration in the entire positive electrode active material 10 .

[0341] Note that the surface and surface layer 10a of the positive electrode active material 10 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 10. Furthermore, they also do not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 10. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0342] Furthermore, before being subjected to various analyses, samples such as the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent used for washing, but even in this case, the additional element A is unlikely to dissolve, and therefore the atomic ratio of the additional element A is not affected.

[0343] The concentration of the additive element A may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparison while reducing the influence of carbonates and the like that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more and 1.20 or less, more preferably 0.500 or more and 1.00 or less, even more preferably 0.500 or more and 1.00 or less, even more preferably 0.500 or more and 0.900 or less, and even more preferably 0.500 or more and 0.700 or less.

[0344] Furthermore, for example, the ratio of the number of atoms of nickel to cobalt (Ni / Co) determined by XPS analysis is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, even more preferably 0.050 or more and 0.100 or less, and even more preferably 0.050 or more and 0.070 or less.

[0345] Furthermore, the ratio of the number of aluminum atoms to the number of cobalt atoms (Al / Co) determined by XPS analysis, for example, is preferably 0.010 or more and 0.100 or less, more preferably 0.010 or more and 0.050 or less, and even more preferably 0.010 or more and 0.040 or less.

[0346] Furthermore, the ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) as determined by, for example, XPS analysis is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, still more preferably 0.100 or more and 0.500 or less, still more preferably 0.100 or more and 0.300 or less, and still more preferably 0.100 or more and 0.200 or less.

[0347] The above range indicates that the additive element A is not attached to a narrow area on the surface of the positive electrode active material 10, but is widely distributed at a preferred concentration in the surface layer 10a of the positive electrode active material 10. In other words, as a result of XPS analysis of the positive electrode active material 10, the above range indicates that the crystalline structure is less likely to collapse even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and desorption is possible in the positive electrode active material 10, and excellent rate characteristics can be achieved.

[0348] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-rays. Furthermore, it is recommended to use an XPS instrument with an energy resolution such that the half-width of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV±0.1 eV. The take-off angle can be, for example, 45°. For example, the following XPS instrument and measurement conditions can be used for the measurement: Measurement instrument: PHI Quantera II X-ray: Monochromated Al Kα (1486.6 eV) Energy resolution: Half-width of the Ag3d5 / 2 peak is 1.0 eV±0.1 eV Detection area: 100 μmφ Detection depth: Approximately 4-5 nm (take-off angle 45°) Measurement spectrum: Wide scan, narrow scan for each detected element

[0349] When positive electrode active material 10 according to one embodiment of the present invention is analyzed by XPS, the peak (Mg1s peak) representing the bond energy between magnesium and other elements is preferably equal to or greater than 1303.0 eV and less than 1305.0 eV, and more preferably about 1304.0 eV, which is different from the bond energy of magnesium fluoride (1306.0 eV) and is close to the bond energy of magnesium oxide.

[0350] In the XPS analysis of the positive electrode active material 10 according to one embodiment of the present invention, the measured XPS spectrum is preferably corrected so that the C1s peak is aligned with the reference value (284.8 eV), i.e., the entire spectrum is preferably shifted, which can reduce the influence of differences in the XPS apparatus, differences in measurement conditions, etc. on the XPS measurement.

[0351] Furthermore, in XPS analysis of the positive electrode active material 10 of one embodiment of the present invention, when the ratios of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond are analyzed by analyzing the Mg1s peak, it is preferable that the positive electrode active material 10 has a peak component derived from the O-Mg-O bond. Furthermore, the positive electrode active material 10 may contain a peak component derived from the "O-Mg-F" bond, but the peak component preferably accounts for 30% or less of the total of the three peak components, more preferably 20% or less, still more preferably 20% or less, and preferably 10% or less. Furthermore, the positive electrode active material 10 may contain a peak component derived from the "F-Mg-F" bond, but preferably accounts for 10% or less of the total.

[0352] That is, when the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond are analyzed in an XPS analysis of the positive electrode active material 10 of one embodiment of the present invention, the proportion of the peak component derived from the "O-Mg-O" bond is preferably 70% or more, more preferably 80% or more, still more preferably 90% or more, and particularly preferably 100%.

[0353] A method for analyzing the Mg1s peak of an XPS spectrum in XPS analysis will be described. In analyzing the Mg1s peak, it is preferable to define the peak component derived from the O-Mg-O bond as fit peak 1, the peak component derived from the O-Mg-F bond as fit peak 2, and the peak component derived from the F-Mg-F bond as fit peak 3, synthesize these three fit peaks, and calculate the peak synthesis ratio that minimizes the difference from the Mg1s peak of the XPS spectrum obtained by XPS analysis. The analysis results can be output by assuming that the area ratio of fit peak 1, fit peak 2, and fit peak 3 at this time represents the abundance ratio of O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds.

[0354] In the above-mentioned XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called peak top) of fit peak 1 is determined by the MgO-coated LiCoO２ The energy value at the maximum value of the Mg1s peak when measured separately as a standard sample can be referenced. The energy value at the maximum value of fit peak 3 (Ep3) can be referenced to the energy value at the maximum value of magnesium fluoride (MgF ２ The energy value at the maximum value of the Mg1s peak when a standard sample (99.9% purity (3N) up) is separately measured can be referenced. The energy value at the maximum value of fit peak 2 (Ep2) can be set to an intermediate value between Ep1 and Ep3. EP1 is located on the lower energy side compared to EP3. The energy value at the maximum value of a peak is also referred to as the peak position.

[0355] In the XPS analysis of the positive electrode active material 10 according to one embodiment of the present invention, the half width of the Mg1s peak is preferably 1.0 eV to 3.0 eV, more preferably 1.0 eV to 2.8 eV, and particularly preferably 1.0 eV to 2.6 eV. Note that the peak position of the Mg1s peak is on the lower energy side than the energy value of the maximum value of the Mg1s peak measured separately using magnesium fluoride as a standard sample.

[0356] <EDX> It is preferable that one or more selected from the additive elements A contained in the positive electrode active material 10 have a concentration gradient. Furthermore, when two or more additive elements A are used, it is more preferable that the depth from the surface of the concentration peak of each additive element A is different. For example, the concentration gradient, concentration peak, etc. of the additive element A can be evaluated by exposing a cross section of the positive electrode active material 10 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, EPMA (electron probe microanalysis), or the like.

[0357] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0358] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element A in the surface layer 10a, the interior 10b, and near the grain boundaries of the positive electrode active material 10. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the additive element A. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by elements present in the depth direction of the sample.

[0359] Since the positive electrode active material 10 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0360] STEM-EDX analysis can measure the energy value and intensity of characteristic X-rays generated from the analysis target by irradiating an electron beam onto the analysis target region. The region where characteristic X-rays are generated can be expanded due to scattering of the electron beam inside the sample. Therefore, when the analysis target region includes an interface or a surface, half of the detected amount of characteristic X-rays of the elements contained in the analysis target can be used as the interface or surface. Furthermore, in a profile when the analysis target region includes an interface or a surface, half of the detected amount can be used as the reference point.

[0361] Elemental quantification in STEM-EDX analysis can employ a standardless quantification method using the k-factor, an element-specific coefficient built into the analytical instrument and / or analytical software. Furthermore, when calculating element concentrations from quantitative results, the target elements (also referred to as denominator elements) preferably include the target material, raw materials, the mesh on which the thin section sample is placed, the instrument components, and the elements used in thin section processing. For example, the target elements are preferably the 15 elements C, O, F, Mg, Al, Si, P, S, Ca, Ti, Fe, Co, Ni, Mn, and Ga. The count number and concentration can be output as desired. Furthermore, when simply referring to the detected amount of a certain element, the count number and concentration of characteristic X-rays are included, and the concentration includes weight % and atomic %. When determining the reference point, the concentration of each element shown in the profile can also be used instead of the detected amount.

[0362] Specifically, the reference point in the STEM-EDX analysis is the average value M of the detected amount of the transition metal M1 inside the positive electrode active material. ＡＶＥ When the left side of the reference point is described as the outside of the positive electrode active material and the right side of the reference point is described as the inside of the positive electrode active material, the reference point may be referred to as the position of the surface of the positive electrode active material. In addition, in STEM-EDX-ray analysis or the like, when the amount of detected characteristic X-rays of the transition metal M1 does not decrease sufficiently to the left of the reference point, the amount of detected characteristic X-rays of the transition metal M1 to the left of the reference point is called background, and the average value M of the amount of detected characteristic X-rays of the transition metal M1 in the background is used. ＢＧ and the average value M of the detected amount of characteristic X-rays of the transition metal M1 inside ＡＶＥ The reference point may be the point where the average value M of the detected amount of characteristic X-rays of oxygen inside the positive electrode active material is 50% of the sum of the transition metal M1 and the transition metal M1. However, since oxygen is an element that is easily affected by the outside of the positive electrode active material, the reference point is the average value M of the detected amount of characteristic X-rays of the transition metal M1. ＡＶＥ It is preferable to calculate it from 50% of the average value M of the detected amount of characteristic X-rays of the transition metal M1. ＡＶＥ 50% of the detected amount of characteristic X-rays of oxygen, MＡＶＥ If the difference is not 50% of the average value M of the detected amount of characteristic X-rays of the transition metal M1, it is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen that adhere to the surface of the positive electrode active material. ＡＶＥ In the case of a positive electrode active material having a plurality of transition metals M1, it is preferable to adopt the point where the amount of characteristic X-rays detected inside the element M1 is 50% of the amount of characteristic X-rays detected inside the element M1. ＡＶＥ The reference point can be found using

[0363] The average value M of the detected amount of characteristic X-rays of the internal transition metal M1 ＡＶＥ can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 20 nm or more, preferably 30 nm or more, from a region where the detected amount of characteristic X-rays of the transition metal M1 is saturated and stable, for example, a region where the detected amount of characteristic X-rays of the transition metal M1 starts to increase. ＢＧ can be obtained by averaging the amount of detected characteristic X-rays of the transition metal M1 over a range of 2 nm or more, preferably 3 nm or more, outside the area where the amount of detected characteristic X-rays of the transition metal M1 starts to increase. ＢＧ and the average value of the detected amount of characteristic X-rays of oxygen in the background O ＡＶＥ can also be found in the same way.

[0364] Furthermore, the surface of the positive electrode active material 10 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystal structure of the positive electrode active material is observed and an area where an image is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are confirmed.

[0365] In addition, a peak in STEM-EDX-ray analysis refers to a convex maximum value that appears in a graph of the detected amount of characteristic X-rays for each element, or the maximum value of characteristic X-rays for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0366] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured by two scans can be used as the detection value for each element. The number of scans is not limited to two, and more scans can be performed and the average can be used as the detection value for each element.

[0367] STEM-EDX analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using a carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0368] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0369] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W (Dual EDS) EDX detector. An example of the conditions for EDX ray analysis using the Hitachi High-Tech HD-2700 is to set the acceleration voltage of the STEM device to 200 kV and the emission current to 6 μA or more and 10 μA or less, and measure a portion of the thinned sample with minimal depth and unevenness. The magnification is, for example, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0370] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used in STEM-EDX ray analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0371] In addition, in positive electrode active material 10 having magnesium and fluorine as added element A, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in depth between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and even more preferably within 0.5 nm.

[0372] Furthermore, in the positive electrode active material 10 containing nickel as the additive element A, the peak of the nickel concentration or detectable amount in the surface layer 10a is preferably present at the surface of the positive electrode active material 10 or at a depth of up to 3 nm from the reference point toward the center, and more preferably at a depth of up to 1 nm. Furthermore, in the positive electrode active material 10 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, and more preferably within 1 nm.

[0373] Furthermore, when the positive electrode active material 10 contains aluminum as the additive element A, it is preferable that, when EDX-ray analysis is performed, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 10a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 10a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material 10 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0374] Here, how to express the positional relationship of element distribution when EDX-ray analysis is performed will be explained using Figures 15A to 15G. Figures 15A to 15F are schematic diagrams showing the concentration distribution or detection amount distribution of a first element e1 and a second element e2. Figure 15G is a schematic diagram showing the concentration distribution or detection amount distribution of a first element e1, a second element e2, and a third element e3.

[0375] For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15A, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15B, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15C, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15D, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15E, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 15F, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum.

[0376] The expression "having an overlapping region" will be explained using an example in which the concentration distributions or detection amount distributions of a first element e1, a second element e2, and a third element e3 have the positional relationship shown in Figure 15G. In this specification and the like, "having an overlapping region" means, for example, that the position of maximum value in the concentration distribution or detection amount distribution of at least one element is located in a range in which the concentration or detection amount in the concentration distribution or detection amount distribution of the other element is 1 / 5 or more of the maximum value.

[0377] 15G, the position (P2) at which the second element e2 has a maximum value in its concentration distribution or detectable amount distribution is located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the second element e2 have an overlapping distribution area. Also, the position (P3) at which the third element e3 has a maximum value in its concentration distribution or detectable amount distribution is not located in the range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the third element e3 do not have an overlapping distribution area.

[0378] 15G, the distribution of the second element e2 and the distribution of the third element e3 can be said to be located more inward than the distribution of the first element e1. Alternatively, the distribution of the second element e2 and the distribution of the third element e3 can be said to be biased more inward than the distribution of the first element e1.

[0379] <Powder Resistivity Measurement> The positive electrode active material 10 according to one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge. A feature of the positive electrode active material 10 having the above-described excellent properties is that, in the above-described <XRD>, Li ｘ CoO ２ It has been explained that when x in the formula (I) is small, the cathode active material 10 has an O3'-type and / or monoclinic O1(15)-type crystal structure. Furthermore, in the above <EDX>, a preferred abundance distribution of the additive element A (e.g., Mg, Al, Ni) when subjected to STEM-EDX analysis of the cathode active material 10 has been explained. Furthermore, in the above <XPS>, a preferred abundance ratio of the additive element A (e.g., Mg, Al, Ni) when subjected to XPS analysis of the cathode active material 10 has been explained. Furthermore, the cathode active material 10 of one embodiment of the present invention is also characterized by the volume resistivity of the powder.

[0380] A feature of the positive electrode active material 10 according to one embodiment of the present invention is that the volume resistivity of the powder of the positive electrode active material 10 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ Positive electrode active material 10 having the above volume resistivity has a stable crystal structure even at high voltage, and can be used as an indicator that surface layer portion 10 a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been well formed.

[0381] The volume resistivity of the powder of the positive electrode active material 10 is 1.0×10 ８ Ω・cm or more 1.0×10 ９ It is more preferable that the resistance is Ω cm or less, and 1.0 × 10 ８ Ω・cm or more 5.0×10 ８ The positive electrode active material 10 having the above volume resistivity has a stable crystal structure even at high voltages, and can be used as an indicator that the surface layer portion 10 a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been well formed, and can also be used as an indicator that good lithium intercalation and deintercalation is possible in the positive electrode active material.

[0382] A method for measuring the volume resistivity of the powder of the positive electrode active material 10 according to one embodiment of the present invention will be described.

[0383] The measurement of the volume resistivity of a powder preferably includes an instrument having a terminal for resistance measurement and a mechanism for applying pressure to the powder to be measured. The terminal for resistance measurement preferably has four terminals (also called four-probe). For example, the MCP-PD600 manufactured by Nitto Seiko Analytech Co., Ltd. can be used as a measuring device having a terminal for resistance measurement and a mechanism for applying pressure to the powder (sample) to be measured. The Loresta GXII or Hiresta UX can be used as the instrument for the four-probe method. The Loresta GXII can be used to measure low-resistivity samples, and the Hiresta UX can be used to measure high-resistivity samples. The measurement environment is preferably a stable environment such as a dry room. The dry room environment is preferably, for example, a temperature environment of 25°C and a dew point environment of -40°C or below.

[0384] Measurement of the volume resistivity of powder using the measuring device shown above will be described. First, a powder sample is set in the measuring unit. The measuring unit is structured so that the powder sample and a terminal for resistance measurement are in contact with each other and can apply pressure to the powder sample. The measuring unit also has a structure for measuring the volume of the powder sample. Specifically, the measuring unit has a cylindrical space in which the powder sample is set. The structure for measuring the volume of the powder sample described above can measure the volume occupied by the powder at that time by measuring the height of the powder set in the space.

[0385] In measuring the volume resistivity of a powder, the electrical resistance of the powder and the volume of the powder are measured while pressure is applied to the powder. The pressure applied to the powder can be measured under a variety of conditions. For example, the electrical resistance and volume of the powder can be measured under pressure conditions of 13 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa. The volume resistivity of the powder can be calculated from the measured electrical resistance and volume of the powder.

[0386] When the measurement described above is performed, the volume resistivity of the powder of the positive electrode active material 10 according to one embodiment of the present invention is 1.0 × 10 at a pressure of 64 MPa.８ Ω・cm or more 1.0×10 １０ When the capacitance is 1.0×10 Ω cm or less, favorable cycle characteristics are exhibited in a charge-discharge cycle test under high voltage conditions, and ８ Ω・cm or more 1.0×10 ９ When the volume resistivity is 1.0×10 Ω·cm or less, the battery exhibits favorable cycle characteristics in a charge-discharge cycle test under voltage conditions, and also exhibits favorable discharge characteristics in a discharge rate test. ８ Ω・cm or more 5.0×10 ８ When the resistance is Ω·cm or less, more preferable discharge characteristics are exhibited in the discharge rate test.

[0387] <EPMA> The concentration of the additive element A in the positive electrode active material 10 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has a higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where the additive element A is present in trace amounts.

[0388] In the EPMA analysis, a cross section of the positive electrode active material 10 is exposed by mechanical polishing, ion polishing, FIB, etc., and the cross section is analyzed. As an EPMA device, for example, an electron probe microanalyzer JXA-iHP200F manufactured by JEOL Ltd. can be used.

[0389] EPMA uses a wavelength-dispersive detector, so it has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of the additive element A in the surface layer 10a of the positive electrode active material 10, while EPMA analysis is suitable for analysis of trace amounts of the additive element A in the interior 10b. Note that EPMA and EDX use different analytical methods, so the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0390] <Microelectron Beam Diffraction Pattern> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the microelectron beam diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the microelectron beam diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer portion 10a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because the presence of an additive element A such as magnesium in the lithium layer while maintaining the layered rock salt-type crystal structure can ensure a lithium diffusion path and have a stronger function of stabilizing the crystal structure, rather than the outermost surface being covered with a rock salt-type crystal structure.

[0391] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0392] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is 0.1 × 10 for the a-axis. −１０ m or less, and the c-axis is preferably 1.0 × 10 −１０ m or less. −１０ m or less, and the c-axis is preferably 0.6×10 −１０ It is more preferable that the a-axis is 0.04×10 −１０ m or less, and the c-axis is more preferably 0.3 × 10 −１０ It is more preferable that the length is m or less.

[0393] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0394] Embodiment 3 In this embodiment, elements constituting a secondary battery will be described.

[0395] [Positive Electrode] The secondary battery has a positive electrode, which is as described in the first and second embodiments.

[0396] <Positive Electrode Current Collector> The positive electrode has a positive electrode current collector. Materials with high conductivity, such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof, can be used as the positive electrode current collector. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. The positive electrode current collector may also be formed from a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector may be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable that the thickness of the current collector be 5 μm or more and 30 μm or less.

[0397] <Binder> The positive electrode preferably contains a binder. As the binder, for example, a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer is preferably used. Furthermore, fluororubber can be used as the binder.

[0398] As the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides can be used. As the polysaccharide, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, etc., or starch can be used. Furthermore, it is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0399] As the binder, it is preferable to use materials such as polystyrene, polymethyl acrylate, 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, and nitrocellulose.

[0400] The binder may be a combination of two or more of the above binders. For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with another material. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as a water-soluble polymer with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0401] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurries. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0402] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0403] When the binder covers the surface of the active material or contacts the surface and forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the secondary battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0404] <Conductive Material> The positive electrode preferably contains a conductive material. The conductive material is also called a conductivity imparting agent or a conductive auxiliary, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the active material is electrically connected even when not in contact with each other.

[0405] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds can be used.

[0406] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0407] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0408] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0409] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0410] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the regions between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. A secondary battery obtained by a manufacturing method according to one embodiment of the present invention has high capacity density and stability, making it effective as an in-vehicle secondary battery.

[0411] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive material and a binder.

[0412] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0413] In addition, the negative electrode active material can be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These 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. Alternatively, compounds containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３, LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0414] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0415] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0416] 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, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0417] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety than metallic lithium.

[0418] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0419] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0420] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0421] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ , CuO, Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ Fluorides such as:

[0422] Another example of the negative electrode is a negative electrode that does not have a negative electrode active material at the end of the secondary battery production. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the secondary battery production, in which lithium ions released from the positive electrode active material upon charging the secondary battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A secondary battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) secondary battery, a negative electrode-less (anode-less) secondary battery, or the like.

[0423] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making them suitable as films for uniforming lithium deposition.

[0424] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0425] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0426] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0427] [Separator] The secondary battery has a separator, which is as described in the first embodiment and the like.

[0428] [Exterior Body] The secondary battery has an exterior body, which is as described in the first embodiment and the like.

[0429] [Electrolyte] The secondary battery has an electrolyte containing carrier ions. The electrolyte is as described in the first embodiment and the like.

[0430] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0431] (Embodiment 4) In this embodiment, a secondary battery including a separator and a positive electrode according to one embodiment of the present invention will be described. A secondary battery including a separator according to one embodiment of the present invention is preferable because it can secure the amount of electrolyte solution retained in the separator even when the electrode expands and contracts during charging and discharging. Furthermore, a separator having a multilayer structure is preferable because it can increase the capacity per volume of the secondary battery. A secondary battery including a positive electrode according to one embodiment of the present invention is preferable because it exhibits good high-voltage charging characteristics.

[0432] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 16A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 16B is an external view, and Fig. 16C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0433] 16A is a schematic diagram showing the overlapping of components (upper and lower positions and positional relationships) for ease of understanding, and therefore, FIGS. 16A and 16B are not completely identical corresponding views.

[0434] In Fig. 16A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. A separator according to one embodiment of the present invention can be used for the separator 310. These are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in Fig. 16A. The spacer 322 and the washer 312 are used to protect the interior or to fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0435] A positive electrode 304 has a stacked structure in which a positive electrode active material layer 306 is formed over a positive electrode current collector 305. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 306.

[0436] FIG. 16B is a perspective view of the completed coin-type secondary battery.

[0437] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0438] Note that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 can each have an active material layer formed on only one surface.

[0439] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0440] 16C , the positive electrode 304, the separator 310, the negative electrode 307, and the negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downward, and the positive electrode can 301 and the negative electrode can 302 are pressure-bonded together via a gasket 303 to produce a coin-shaped secondary battery 300. The mixed solvent of one embodiment of the present invention is preferably used as the solvent for the electrolyte.

[0441] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 17A. As shown in Fig. 17A, a cylindrical secondary battery 616 has a positive electrode cap (secondary battery lid) 601 on the top surface and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0442] 17B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in Fig. 17B has a positive electrode cap (secondary battery lid) 601 on the top surface and a secondary battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and secondary battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0443] A secondary battery element is provided inside a hollow cylindrical secondary battery can 602. The secondary battery element includes a strip-shaped positive electrode 604 and a strip-shaped negative electrode 606 wound with a separator 605 sandwiched therebetween. A separator according to one embodiment of the present invention can be used for the separator 605. Although not shown, the secondary battery element is wound around a central axis. The secondary battery can 602 is closed at one end and open at the other end. The secondary battery can 602 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to an electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, the secondary battery can 602 is preferably coated with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the secondary battery can 602, the wound secondary battery element including the positive electrode, negative electrode, and separator is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) is poured into the secondary battery can 602 in which the secondary battery element is provided. The electrolyte may be the same as that used in a coin-type secondary battery.

[0444] Since the positive and negative electrodes used in cylindrical secondary batteries are wound, it is preferable to form active materials on both sides of the current collector.

[0445] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as a positive electrode active material of the positive electrode 604, and the cylindrical secondary battery 616 can have favorable high-voltage charging characteristics.

[0446] A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. The positive electrode terminal 603 can be made of a metal material such as aluminum. The negative electrode terminal 607 can be made of a metal material such as copper. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the secondary battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the secondary battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases when the temperature rises, and the increase in resistance limits the amount of current to prevent abnormal heat generation. ３ )-based semiconductor ceramics, etc. can be used.

[0447] 17C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of the respective secondary batteries are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of the respective secondary batteries are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0448] 17D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0449] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0450] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0451] 17D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0452] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 18 and 19. FIG.

[0453] A secondary battery 913 shown in FIG. 18A includes a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte solution inside the housing 930. The mixed solvent of one embodiment of the present invention is preferably used as a solvent for the electrolyte solution. The terminal 952 is in contact with the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that for convenience, the housing 930 is shown separated in FIG. 18A , but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a laminate of a metal material and a resin material.

[0454] 18B, the housing 930 shown in Fig. 18A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 18B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0455] The housing 930a can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material. An organic resin or the like can be used as the resin material. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to suppress shielding of the electric field by the secondary battery 913. Note that if the shielding of the electric field by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of a metal material (such as aluminum) or a laminate of a metal material and a resin material.

[0456] 18C shows the structure of a wound body 950. The wound body 950 includes a negative electrode 931, a positive electrode 932, and a separator 933. The separator 933 can be a separator according to one embodiment of the present invention. The wound body 950 is formed by stacking the negative electrode 931 and the positive electrode 932 with the separator 933 sandwiched therebetween, and winding the laminated sheet. Note that a plurality of stacks of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0457] 19A may be a secondary battery 913 having a wound body 950a as shown in Fig. 19A. The wound body 950a shown in Fig. 19A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0458] Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 932a.

[0459] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0460] 19B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0461] 19C , the wound body 950a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the secondary battery from exploding.

[0462] As shown in Fig. 19B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 19A and 19B, refer to the descriptions of the secondary battery 913 shown in Figs. 18A to 18C.

[0463] <Laminated Secondary Battery> Next, examples of external views of an example of a laminated secondary battery are shown in FIGS. 20A and 20B . Each of FIGS. 20A and 20B includes a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. The separator 507 can be a separator according to one embodiment of the present invention. Although not shown, the mixed solvent according to one embodiment of the present invention is preferably used as a solvent for the electrolyte solution.

[0464] FIG. 20A shows an external view of a positive electrode 103 and a negative electrode 106. The positive electrode 103 has a positive electrode current collector 21, and a positive electrode active material layer 22 is formed on the surface of the positive electrode current collector 21. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the positive electrode active material layer 22. The positive electrode 103 also has a region where the positive electrode current collector 21 is partially exposed (hereinafter referred to as a tab region). The negative electrode 106 has a negative electrode current collector 31, and the negative electrode active material layer 32 is formed on the surface of the negative electrode current collector 31. The negative electrode 106 also has a region where the negative electrode current collector 31 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 20A .

[0465] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0466] Embodiment 5 In this embodiment, an example of a vehicle including a secondary battery of one embodiment of the present invention will be described.

[0467] The secondary battery can be applied to a typical vehicle, such as an automobile. Examples of the automobile include next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHEVs or PHVs). The secondary battery can be used as one of the power sources mounted on the automobile. The vehicle is not limited to an automobile. Examples of the vehicle include trains, monorails, ships, submersibles (deep-sea exploration vessels, unmanned submersibles), aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, rockets, and artificial satellites), electric bicycles, and electric motorcycles. The secondary battery of one embodiment of the present invention can be applied to these vehicles.

[0468] As shown in Fig. 21C , an electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0469] The internal structure of the first battery 1301a may be a wound type as shown in FIG. 18C or FIG. 19A, or a stacked type as shown in FIG. 20A or FIG. 20B.

[0470] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a secondary battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a secondary battery pack.

[0471] In addition, in a secondary battery for vehicle use, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple secondary batteries.

[0472] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0473] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0474] Next, the first battery 1301a will be described with reference to FIG. 21A.

[0475] FIG. 21A shows an example in which nine prismatic secondary batteries 1300 are used as one secondary battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a secondary battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple secondary batteries using the fixing portions 1413 and 1414 and the secondary battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0476] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a secondary battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0477] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M-Zn oxide (wherein the element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) may be used as the metal oxide. In particular, the In-M-Zn oxide that can be used as the metal oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the metal oxide. The CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis oriented in a specific direction. The specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. The crystalline regions are regions in which the atomic arrangement is periodic. When the atomic arrangement is considered as a lattice arrangement, the crystalline regions are also regions in which the lattice arrangement is aligned.

[0478] Note that "CAC-OS" has a mosaic structure in which a material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter, also referred to as a cloud structure). That is, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0479] For example, in the case of a CAC-OS made of an In—Ga—Zn oxide, EDX mapping obtained using EDX confirms that the CAC-OS has a structure in which a region containing In as the main component (first region) and a region containing Ga as the main component (second region) are unevenly distributed and mixed.

[0480] When a CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act complementarily, thereby providing the CAC-OS with a switching function (a function of turning on / off). That is, a CAC-OS has a conductive function in a part of the material and an insulating function in a part of the material, and functions as a semiconductor as a whole. By separating the conductive function and the insulating function, both functions can be maximized. Therefore, by using a CAC-OS in a transistor, a high on-current (I ｏｎ ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0481] Oxide semiconductors have a variety of structures, each of which has different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0482] Furthermore, the control circuit unit 1320 preferably uses a transistor using an oxide semiconductor because it can be used in a high-temperature environment. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. Transistors using an oxide semiconductor for the semiconductor layer have a wider operating ambient temperature range (-40°C to 150°C) than single-crystal Si transistors, and their characteristics change less even when the secondary battery overheats than single-crystal Si transistors. The off-current of a transistor using an oxide semiconductor is extremely low regardless of temperature, even at 150°C, whereas the off-current characteristics of a single-crystal Si transistor are highly temperature-dependent. For example, at 150°C, the off-current of a single-crystal Si transistor increases, and the current on / off ratio is not sufficiently large. The control circuit unit 1320 can improve safety. A separator according to one embodiment of the present invention can be used as a separator for the secondary battery. Lithium cobalt oxide according to one embodiment of the present invention can be used as a positive electrode active material for the secondary battery. The secondary battery and the control circuit unit 1320 can significantly contribute to eliminating accidents, such as fires, caused by secondary batteries.

[0483] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for a secondary battery to address 10 causes of instability, such as micro-short circuits. The functions for addressing the 10 causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of degradation, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for a secondary battery can be miniaturized.

[0484] Furthermore, a "micro-short" refers to a minute short circuit within a secondary battery, which is not so severe that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather refers to a phenomenon in which a small amount of short-circuit current flows at the minute short-circuited part. Even if the short-circuit occurs in a relatively short period of time and in a small location, a large voltage change occurs, and this abnormal voltage value may affect subsequent estimations.

[0485] One of the causes of micro-short circuits is said to be that multiple charge and discharge cycles cause uneven distribution of the positive electrode active material, resulting in localized current concentration in parts of the positive electrode and negative electrode, causing parts of the separator to stop functioning, or the generation of by-products due to side reactions, resulting in micro-short circuits.

[0486] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the secondary battery and manage the charge / discharge state of the secondary battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0487] Next, an example of a block diagram of the secondary battery pack 1415 shown in FIG. 21A is shown in FIG. 21B.

[0488] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range between the lower limit and the upper limit of the secondary battery's voltage is within the recommended voltage range, and when the secondary battery falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and / or overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0489] The switch portion 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch portion 1324 is not limited to a switch having a Si transistor using single crystal silicon. For example, the switch portion 1324 may be formed using a power transistor having Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), or GaOx (gallium oxide; x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on circuits using Si transistors, and therefore integration can be easily achieved. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, and therefore can be manufactured at low cost. That is, the control circuit portion 1320 using OS transistors can be stacked on the switch portion 1324 and integrated into a single chip. The volume occupied by the control circuit section 1320 can be reduced, which allows for miniaturization.

[0490] The first batteries 1301a and 1301b mainly supply power to on-board equipment of the 42V system (high-voltage HV system), while the second battery 1311 supplies power to on-board equipment of the 14V system (low-voltage LV system). Lead-acid secondary batteries are often used as the second battery 1311 due to their cost advantages. Lead-acid secondary batteries have the disadvantage of being more self-discharged than lithium-ion secondary batteries and being prone to deterioration due to a phenomenon known as sulfation. Using a lithium-ion secondary battery as the second battery 1311 has the advantage of being maintenance-free, but after long-term use, such as three years or more, there is a risk of abnormalities occurring that are difficult to identify during manufacturing. In particular, if the second battery 1311 that starts the inverter becomes inoperable, in order to prevent the motor from being unable to start even if the first batteries 1301a and 1301b have remaining capacity, if the second battery 1311 is a lead-acid secondary battery, power is supplied from the first battery to the second battery, and the second battery is charged to always maintain a fully charged state.

[0491] In this embodiment, an example is shown in which lithium ion secondary batteries are used as both the first battery 1301a and the second battery 1311. The second battery 1311 may be a lead-acid secondary battery, an all-solid-state secondary battery, or an electric double layer capacitor.

[0492] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 via the motor controller 1303 or the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0493] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the secondary battery used, and can perform rapid charging.

[0494] Although not shown, when the electric vehicle is connected to an external charger, the charger plug or charger connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via a control circuit unit 1320 to prevent overcharging. The charger plug or charger connection cable may also be provided with a control circuit. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU also includes a microcomputer. The ECU uses a CPU or a GPU.

[0495] External chargers installed at charging stations and the like include 100V-200V outlets, or three-phase 200V and 50kW. Charging can also be performed by receiving power from external charging equipment using a contactless power supply system or the like.

[0496] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0497] Furthermore, by using graphene as a conductive material, a secondary battery with significantly improved electrical characteristics can be realized, as a synergistic effect of suppressing capacity decline and maintaining high capacity even when the electrode layer is thickened and the amount of graphene supported is increased. This is particularly effective for secondary batteries used in vehicles, and it is possible to provide a vehicle with a long driving range, specifically a driving distance of 500 km or more per charge, without increasing the ratio of the weight of the secondary battery to the total weight of the vehicle.

[0498] In particular, the separator of one embodiment of the present invention can be applied to a separator of a secondary battery, and lithium cobalt oxide of one embodiment of the present invention can be applied to a positive electrode active material of the secondary battery.

[0499] Next, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0500] 17D, 19C, and 21A can be installed in a vehicle to realize next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Furthermore, the secondary battery can also be installed in agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft. The secondary battery of one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0501] 22A to 22D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 illustrated in FIG. 22A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 illustrated in FIG. 22A includes a secondary battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile further include a charge control device electrically connected to the secondary battery module.

[0502] Furthermore, the automobile 2001 can charge its secondary battery by receiving power from an external charging facility using a plug-in system, a contactless power supply system, or the like. The charging method or connector standard, etc., may be appropriately determined using a predetermined system such as CHAdeMO (registered trademark) or Combo. The charging facility is preferably a charging station installed in a commercial facility, or it is also preferable to use a household power source. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter.

[0503] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, a solar secondary battery can be installed on the exterior of the vehicle, and the secondary battery can be charged while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0504] 22B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 is, for example, a four-cell unit of secondary batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series for a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of the secondary battery pack 2201, the transport vehicle has the same functions as those shown in FIG. 22A, and therefore a description thereof will be omitted.

[0505] FIG. 22C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, in which one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less are connected in series. Therefore, a secondary battery with little variation in characteristics is required. Lithium cobalt oxide, which is one embodiment of the present invention, can be used as the positive electrode active material of the secondary battery. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the secondary battery pack 2202, the secondary battery pack 2202 has the same functions as those shown in FIG. 22A , and therefore a description thereof will be omitted.

[0506] Fig. 22D shows, as an example, an aircraft 2004 having an engine that burns fuel. The aircraft 2004 shown in Fig. 22D has wheels for takeoff and landing, and can therefore be considered a type of transport vehicle, and has a secondary battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries and includes the secondary battery module and a charge control device.

[0507] The secondary battery module of the aircraft 2004 has, for example, eight 4 V secondary batteries connected in series to produce a maximum voltage of 32 V. Apart from the number of secondary batteries constituting the secondary battery module of the secondary battery pack 2203, it has the same functions as those in Fig. 22A, and therefore a description thereof will be omitted.

[0508] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0509] Embodiment 6 In this embodiment, an example in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described as an example in which a secondary battery is mounted on a vehicle.

[0510] 23A illustrates an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 23A. The power storage device of one embodiment of the present invention includes, for example, a plurality of secondary storage batteries and a protection circuit.

[0511] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state removed from the bicycle in FIG. 23B . The power storage device 8702 includes a plurality of built-in secondary batteries 8701, which are included in the power storage device of one embodiment of the present invention, and a display unit 8703 can display the remaining battery charge and other information. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery. The control circuit 8704 is electrically connected to the positive and negative electrodes of the secondary battery 8701. The separator of one embodiment of the present invention can be used as a separator of the secondary battery. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery. The secondary battery and the control circuit 8704 can significantly contribute to eliminating accidents such as fires caused by secondary batteries.

[0512] 23C illustrates an example of a two-wheeled vehicle using a power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 23C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602, which includes a plurality of secondary batteries each using the positive electrode active material 10 described in Embodiment 2 or the like for a positive electrode, can have a high capacity and contribute to miniaturization.

[0513] 23C can store a power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0514] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0515] Embodiment 7 In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0516] 24A illustrates an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, an operation button 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a secondary battery 2107. By using lithium cobalt oxide, which is one embodiment of the present invention, as a positive electrode active material of the secondary battery 2107, the battery can have a high capacity, and a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0517] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0518] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0519] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0520] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0521] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0522] FIG. 24B illustrates an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 of one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery 2301. This secondary battery has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0523] Fig. 24C shows an example of a robot. A robot 6400 shown in Fig. 24C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0524] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0525] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0526] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0527] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. Lithium cobalt oxide according to one embodiment of the present invention can be used as a positive electrode active material of the secondary battery 6409. The secondary battery has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as the secondary battery 6409 to be mounted on the robot 6400.

[0528] 24D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, operation buttons 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0529] The cleaning robot 6300 can analyze images captured by the camera 6303 and determine the presence or absence of obstacles such as walls, furniture, or steps. Furthermore, if an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. Lithium cobalt oxide according to one embodiment of the present invention can be used as a positive electrode active material of the secondary battery 6306. The secondary battery has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery 6306 is suitable as the secondary battery 6306 to be mounted on the cleaning robot 6300.

[0530] 25A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0531] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 25A . The eyeglasses-type device 4000 includes a frame 4000 a and a display portion 4000 b. Mounting the secondary battery on the temples of the curved frame 4000 a makes it possible to provide the eyeglasses-type device 4000 that is lightweight, has a good weight balance, and has a long continuous use time. Lithium cobalt oxide of one embodiment of the present invention can be used as the positive electrode active material of the secondary battery. The secondary battery has high energy density and can be configured to accommodate space saving associated with a miniaturized housing.

[0532] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b or the earphone unit 4001c. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery. The secondary battery has high energy density and can achieve a space-saving configuration that can be achieved by miniaturizing the housing.

[0533] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery 4002b. The secondary battery has high energy density and can be configured to save space due to the miniaturization of the housing.

[0534] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery 4003b. The secondary battery has high energy density and can be configured to be space-saving due to the miniaturization of the housing.

[0535] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted in an inner region of the belt portion 4006a. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery. The secondary battery has high energy density and can be configured to be space-saving due to a miniaturized housing.

[0536] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. Lithium cobalt oxide of one embodiment of the present invention can be used as a positive electrode active material of the secondary battery. The secondary battery has high energy density and can achieve a space-saving configuration due to a miniaturized housing.

[0537] The display unit 4005a can display not only the time but also various other information such as incoming emails or phone calls.

[0538] Furthermore, since the wristwatch device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0539] FIG. 25B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0540] 25C shows a side view of the display portion 4005a. Fig. 25C shows a state in which a secondary battery 913 is built in the internal region. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided in a position overlapping with the display portion 4005a, and can have high density and ...

Claims

a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the separator has a first member, a second member located on one surface of the first member, and a third member located on the other surface of the first member; the second member has higher wettability with respect to the electrolyte solution than the first member; The third member has higher wettability with respect to the electrolyte solution than the first member. Secondary battery. 　 a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the separator has a first member, a second member located on one surface of the first member, and a third member located on the other surface of the first member; the second member has higher wettability with respect to the electrolyte solution than the first member; the third member has higher wettability with respect to the electrolytic solution than the first member; the second member has a region in contact with the positive electrode, the third member has a region in contact with the negative electrode; Secondary battery. 　 a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the separator has a first member, a second member located on one surface of the first member, and a third member located on the other surface of the first member; the second member has higher wettability with respect to the electrolyte solution than the first member; the third member has higher wettability with respect to the electrolytic solution than the first member; The first member has a greater thickness than the second member. The first member has a greater thickness than the third member. Secondary battery. 　 a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte; the separator has a first member, a second member located on one surface of the first member, and a third member located on the other surface of the first member; The second member has a higher porosity than the first member, The third member has a higher porosity than the first member. Secondary battery. 　 In any one of claims 1 to 4, the first member comprises polypropylene; the second member and the third member each include polyimide; Secondary battery. 　 In any one of claims 1 to 4, The secondary battery, wherein the electrolyte solution comprises FEC and MTFP. 　 In any one of claims 1 to 4, The secondary battery, wherein the electrolyte solution contains an ionic liquid. 　 In any one of claims 1 to 4, the second member has an area bonded to the first member, The third member has a region bonded to the first member. 　 In any one of claims 1 to 4, an interface between the first member and the second member has a region where the first member and the second member are mixed; a secondary battery, wherein an interface between the first member and the third member has a region where the first member and the third member are mixed; 5. The secondary battery according to claim 1, wherein the secondary battery has a curved region.

Images