Secondary battery

Abstract

The present invention provides a highly safe secondary battery. The secondary battery comprises a positive electrode, wherein: the positive electrode includes positive electrode active material particles and a fibrous conductive material; the positive electrode active material particles contain lithium cobalt oxide; the positive electrode active material particles each have a surface layer part that has a rock salt type crystal structure and an internal part that has a layered rock salt type crystal structure; the surface layer part contains cobalt, nickel, magnesium, and fluorine; the internal part contains cobalt and aluminum, and in a planar view of the positive electrode or a cross-sectional view of the positive electrode, there is a region where the fibrous conductive material surrounds substantially all of the positive electrode active material particles.

Description

secondary battery

[0001] The present invention relates to a secondary battery. However, the present invention is not limited to the above fields and may also relate to semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, vehicles, and manufacturing methods thereof. For example, a secondary battery according to one embodiment of the present invention can be applied as a power source required in semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, and vehicles. Electronic devices include information terminal devices equipped with a secondary battery. 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] The cathode active material used in secondary batteries is an oxide, and research into conductive materials to improve the conductivity of the cathode has been active. Carbon nanotubes (CNTs) are known as a conductive material for cathodes, but it is difficult to obtain a liquid (CNT dispersion) in which CNTs, which have strong cohesion, are dispersed. For example, a method has been proposed in which CNTs are dispersed in a water-soluble organic solvent using zirconia beads and polyvinylpyrrolidone as a resin-type dispersant (see Patent Document 1).

[0004] JP 2020-105316 A

[0005] In Patent Document 1, dispersion is performed using zirconia beads, which raises concerns about crushing of CNTs, leading to insufficient strength of an electrode using the CNTs and a risk of reduced safety. Therefore, an object of one embodiment of the present invention is to provide a new method for dispersing CNTs. Another object of one embodiment of the present invention is to provide a positive electrode including CNTs with a novel structure and a positive electrode active material. Another object of one embodiment of the present invention is to provide a secondary battery including the above positive electrode.

[0006] 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.

[0007] In view of the above problems, one embodiment of the present invention is a secondary battery that has a positive electrode, the positive electrode including positive electrode active material particles and a fibrous conductive material, the positive electrode active material particles including lithium cobalt oxide, the positive electrode active material particles including a surface layer portion having a rock salt crystal structure and an interior portion having a layered rock salt crystal structure, the surface layer portion including cobalt, nickel, magnesium, and fluorine, and the interior portion including cobalt and aluminum, and the positive electrode includes a region in which the fibrous conductive material substantially entirely surrounds the positive electrode active material particles in a top view or a cross-sectional view of the positive electrode.

[0008] In one embodiment of the present invention, the fibrous conductive material preferably has at least a region that wraps around the edge surfaces of the positive electrode active material particles.

[0009] Another aspect of the present invention is a secondary battery including a positive electrode, the positive electrode including first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material, the second positive electrode active material particles including lithium cobalt oxide, the second positive electrode active material particles including a surface layer portion having a rock salt crystal structure and an interior portion having a layered rock salt crystal structure, the surface layer portion including cobalt, nickel, magnesium, and fluorine, and the interior portion including cobalt and aluminum, and the fibrous conductive material including a region in which the fibrous conductive material substantially entirely surrounds the second positive electrode active material particles in a top view or a cross-sectional view of the positive electrode.

[0010] Another aspect of the present invention is a secondary battery including a positive electrode, the positive electrode including first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material, the second positive electrode active material particles including lithium cobalt oxide, the second positive electrode active material particles including a surface layer portion having a rock salt crystal structure and an interior portion having a layered rock salt crystal structure, the surface layer portion including cobalt, nickel, titanium, magnesium, and fluorine, and the interior portion including cobalt and aluminum, and the fibrous conductive material including a region in which the fibrous conductive material substantially entirely surrounds the second positive electrode active material particles in a top view or a cross-sectional view of the positive electrode.

[0011] Another aspect of the present invention is a secondary battery including a positive electrode, the positive electrode including first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material, the first positive electrode active material particles including lithium cobalt oxide, the first positive electrode active material particles including a surface layer portion having a rock salt crystal structure and an interior portion having a layered rock salt crystal structure, the surface layer portion including cobalt, nickel, magnesium, and fluorine, and the interior portion including cobalt and aluminum, and the fibrous conductive material including a region that substantially entirely surrounds the first positive electrode active material particles in a top view or a cross-sectional view of the positive electrode.

[0012] Another aspect of the present invention is a secondary battery including a positive electrode, the positive electrode including first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material, the first positive electrode active material particles including lithium cobalt oxide, the first positive electrode active material particles including a surface layer portion having a rock salt crystal structure and an interior portion having a layered rock salt crystal structure, the surface layer portion including cobalt, nickel, titanium, magnesium, and fluorine, and the interior portion including cobalt and aluminum, and the fibrous conductive material including a region that substantially entirely surrounds the first positive electrode active material particles in a top view or a cross-sectional view of the positive electrode.

[0013] In another embodiment of the present invention, the fibrous conductive material preferably comprises carbon fibers.

[0014] In another embodiment of the present invention, the fibrous conductive material preferably comprises carbon nanotubes.

[0015] According to one embodiment of the present invention, a new method for dispersing CNTs can be provided. Furthermore, according to another embodiment of the present invention, a positive electrode including CNTs and a positive electrode active material can be provided. Furthermore, according to another embodiment of the present invention, a highly safe secondary battery including the above positive electrode can be provided.

[0016] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these include those that become clear from the description in the specification, drawings, claims, etc., and effects other than these can be extracted from the description in the specification, drawings, claims, etc.

[0017] FIGS. 1A and 1B are diagrams illustrating a secondary battery. FIGS. 2A to 2D are diagrams illustrating a positive electrode. FIGS. 3A and 3B are diagrams illustrating a positive electrode, positive electrode active material particles, or conductive material. FIGS. 4A to 4C are diagrams illustrating positive electrode active material particles or conductive material. FIG. 5A is a cross-sectional view of a positive electrode active material particle, and FIGS. 5B to 5E are diagrams illustrating the distribution of added elements. FIG. 6 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 7A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 7B is an FFT pattern of the rock salt crystal RS region, and FIG. 7C is an FFT pattern of the layered rock salt crystal LRS region. FIG. 8 is a diagram illustrating a change in the crystal structure of a positive electrode active material. FIG. 9 is a diagram illustrating a change in the crystal structure of a conventional positive electrode active material. FIG. 10 is an XRD (X-ray diffraction) pattern. FIG. 11 is an XRD pattern. FIGS. 12A and 12B are XRD patterns. FIG. 13 is a cross-sectional view of a positive electrode active material particle. FIGS. 14A and 14B are diagrams illustrating a nail penetration test. FIG. 15 is a diagram illustrating a method for fabricating a positive electrode. FIG. 16 is a diagram illustrating a method for fabricating a positive electrode. FIG. 17 is a diagram illustrating a method for fabricating a positive electrode active material. FIG. 18 is a diagram illustrating a method for fabricating a positive electrode active material. FIGS. 19A to 19C are diagrams illustrating a method for fabricating a positive electrode active material. FIGS. 20A and 20B are diagrams illustrating a method for fabricating a positive electrode active material. FIGS. 21A to 21C are diagrams illustrating a secondary battery. FIGS. 22A to 22D are diagrams illustrating a secondary battery and a power storage system. FIGS. 23A to 23C are diagrams illustrating a secondary battery. FIGS. 24A to 24C are diagrams illustrating a secondary battery. FIGS. 25A to 25C are diagrams illustrating an electric vehicle. FIGS. 26A to 26D are diagrams illustrating a transportation vehicle. 27A to 27C are diagrams illustrating a motorcycle or the like according to one embodiment of the present invention. FIGS. 28A to 28D are diagrams illustrating an electronic device or the like according to one embodiment of the present invention. FIGS. 29A to 29D are diagrams illustrating an example of space equipment. FIG. 30 is an SEM image of an example. FIGS. 31A and 31B are cycle test results of an example. FIGS. 32A and 32B are cycle test results of an example. FIGS. 33A and 33B are cycle test results of an example.34A and 34B show cycle test results for an example. 35A and 35B show cycle test results for an example. 36A and 36B show cycle test results for an example. 37A and 37B show discharge capacity measurement results by rate for an example. 38A and 38B show discharge capacity measurement results by rate for an example. 39A and 39B show discharge capacity measurement results by rate for an example.

[0018] 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.

[0019] In this specification and the like, a lithium ion secondary battery is sometimes called a lithium ion battery, and refers to a 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 can be used as carrier ions of the present invention, and specifically, sodium ions can be used. In this case, the present invention can be understood by reading lithium ions as sodium ions. Furthermore, when describing a configuration in which there is no limitation on the carrier ions, the battery may be referred to as a secondary battery.

[0020] In this specification, an electrode with a high reaction potential is called a positive electrode, and an electrode with a low reaction potential is called a negative electrode. Therefore, whether during charging or discharging, the positive electrode is called a "positive electrode" or "+ electrode (plus electrode)," and the negative electrode is called a "negative electrode" or "- electrode (minus electrode)." In secondary batteries, the oxidation reaction and reduction reaction alternate during charging and discharging.

[0021] In this specification, a full cell refers to a battery cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a battery cell assembled with lithium metal as the negative electrode (counter electrode).

[0022] 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.

[0023] In this specification, the term "supported amount" refers to the weight of the active material per unit area. 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 to be per side.

[0024] In this specification, the median diameter (D50) is one of the powder characteristics, and is the particle diameter at which the cumulative amount accounts for 50% in a cumulative curve of the particle size distribution measurement results. There is a method for measuring the median diameter (D50) by image analysis using SEM or TEM, for example. For example, 20 or more particles are measured, a cumulative particle amount curve is created, and the particle diameter at which the cumulative amount accounts for 50% can be taken as the median diameter (D50).

[0025] Unless otherwise specified, the charge voltage is expressed relative to the potential of lithium metal in this specification. Furthermore, in this specification, a high charge voltage is, for example, 4.6 V or higher, preferably 4.65 V or higher, and more preferably 4.7 V or higher.

[0026] In this specification, the C rate is the ratio of the charge / discharge current value to the secondary battery capacity, and serves as a criterion for judging the charge / discharge characteristics of a secondary battery. In other words, the characteristics of secondary batteries with different capacities can be compared under the same conditions by using the C rate. 1 C represents the current value required to change from a fully charged state to a discharged state in 1 hour. Unless otherwise specified, in this specification, the current value corresponding to 1 C is 200 mA / g per weight of the positive electrode active material.

[0027] 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 {}. Trigonal crystals expressed in the space group R-3m are generally sometimes expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure. Miller indices may also be used in place of (hkl) or (hkil). Here, i is -(h+k). In this specification and the like, for the space group R-3m, unless otherwise specified, crystal planes and the like are expressed as a composite hexagonal lattice.

[0028] In this specification, etc., the space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification, etc., "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."

[0029] In this specification, if the arrangement of anions is roughly close to cubic close packing, it can be considered cubic close packing. Cubic close packing of anions refers to a state in which a second layer of anions is arranged above the voids of the first layer of anions, and a third layer of anions is arranged directly above the voids of the second layer of anions, but not directly above the first layer of anions. Therefore, the anions do not necessarily have to be in a cubic lattice. Furthermore, since real crystals always have defects, analytical results do not necessarily conform to 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, a cubic close packing structure can be said to be present if the deviation from the theoretical position and orientation is 5 degrees or less, or 2.5 degrees or less.

[0030] In this specification etc., the shape of the active material or conductive material may be referred to as particulate, and particulate shapes include spherical shapes (circular cross-sectional shapes). In this specification etc., the shape of the active material or conductive material is not limited to spherical, and may include cross-sectional shapes such as oval, rectangular, trapezoidal, triangular, square with rounded corners, and asymmetrical shapes, and individual particles may also have an irregular shape.

[0031] In this specification, the term "smooth surface of an active material" refers to a cross-section of the active material having a surface roughness of at least 10 nm or less, when surface irregularity information is quantified from measurement data. In this specification, the cross-section is, for example, a cross-section obtained when observing with a scanning transmission electron microscope (STEM) image.

[0032] In this specification and the like, when simply referring to a positive electrode active material, there are cases where the term "positive electrode active material" refers to a plurality of positive electrode active material particles or a single positive electrode active material particle, depending on the analytical method, etc. For example, in the case of descriptions relating to line analysis using a scanning transmission electron microscope-energy dispersive X-ray fluorescence detector (STEM; Scanning Transmission Electron Microscope-EDX: Energy Dispersive X-ray Spectroscopy), STEM-electron energy loss spectroscopy (STEM-EELS: Electron Energy Loss Spectroscopy), and electron diffraction, the description refers to a single positive electrode active material particle unless otherwise specified. On the other hand, in the case of X-ray photoelectron spectroscopy (XPS), XRD, various mass analyses, and the like, unless otherwise specified, the description is of a plurality of positive electrode active material particles.

[0033] In this specification and the like, when describing individual characteristics of positive electrode active material particles, it is not necessary for all particles in a secondary battery to have that characteristic. For example, as long as three or more randomly selected positive electrode active material particles have that characteristic, it can be said that the effect of improving the characteristics of the positive electrode active material and the secondary battery containing it is sufficient.

[0034] In this specification, the aging treatment includes a process of performing one or more charge / discharge cycles at a temperature equal to or higher than room temperature (25°C), or maintaining the secondary battery at a temperature close to the upper limit of the operating temperature range (typically 60°C) for a long period of time. The aging treatment may further include a process of releasing gas generated in the area covered by the exterior body. The aging treatment can detect initial defects in the secondary battery. Furthermore, when one or more charge / discharge cycles are performed, a stable coating can be formed on the negative electrode active material.

[0035] 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 treatment during the secondary battery manufacturing stage is not considered to be degradation. For example, a lithium-ion secondary battery having a discharge capacity of 97% or more of its rated capacity can be said to be in its pre-degradation state. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0036] In this specification and the like, the state of the materials of a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of the rated capacity) is sometimes referred to as a product in use or in use state, or a used product or used state.

[0037] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the lithium ions that can be inserted and removed from the positive electrode active material are removed. ２The theoretical capacity of LiNiO is 274 mAh / g per active material weight. ２ The theoretical capacity of LiMn is 275 mAh / g per active material weight. ２ O ４ The theoretical capacity of the active material is 148 mAh / g.

[0038] In this specification and the like, the amount of lithium ions remaining in the positive electrode active material that can be inserted and removed is determined by x in the composition formula, for example, Li ｘ CoO ２ In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ In the above formula, "x" is small, for example, when 0.1<x≦0.24. The degree to which lithium ions have been released from the positive electrode active material relative to the theoretical capacity is sometimes referred to as the depth of charge. In this specification and elsewhere, the depth of charge is defined as 1−x.

[0039] For example, if properly synthesized lithium cobalt oxide before use in the positive electrode approximately satisfies the stoichiometric ratio, LiCoO ２ and x = 1 (the depth of charge is 0 at this time). The lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The completion of discharge here refers to a state in which the voltage reaches 3.0 V or 2.5 V at a current of 100 mA / g or less, for example.

[0040] Li ｘ CoO 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 on a 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.

[0041] In this specification and the like, the distribution of a certain element may refer to a region in which the element is continuously detected within a range that is not noise. The detection of a continuous distribution of an element within a range that is not noise includes the detection of a continuous distribution of an element within a range that is not noise when an analysis is performed multiple times.

[0042] In this specification, "ignition" in a nail penetration test refers to the observation of a flame outside the exterior body within one minute of the nail being inserted, or the occurrence of thermal runaway in the secondary battery. For example, thermal runaway is said to have occurred if, after the completion of the nail penetration test, thermal decomposition products of the positive electrode and / or negative electrode are observed at a location 2 cm or more away from the insertion point. Examples of thermal decomposition products of the positive electrode and / or negative electrode include aluminum oxide, which is oxidized aluminum from the positive electrode current collector, and copper oxide, which is oxidized copper from the negative electrode current collector.

[0043] On the other hand, even if a flame, spark, and / or smoke is observed in the nail penetration test, if the flame remains at the point of penetration, i.e., the fire does not spread and the secondary battery does not go into thermal runaway, it is not considered to have ignited. For example, if a secondary battery is subjected to a nail penetration test and the above-mentioned ignition does not occur, it can be said to be a non-igniting secondary battery.

[0044] 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 ions have two-dimensional diffusion paths. In other words, it can be said that the diffusion paths of lithium ions exist along the plane. In this specification, a plane on which the lithium diffusion path is exposed, that is, a plane other than the plane on which lithium ions are inserted and desorbed (specifically, the (00l) plane), may be referred to as an edge plane.

[0045] In this specification and the like, the phrase "having A and / or B" may be used, which means having A, having B, or having A and B.

[0046] Embodiment 1 Secondary Battery A secondary battery and the like which are one embodiment of the present invention will be described with reference to FIGS. 1A and 1B. FIG.

[0047] FIG. 1A shows a laminated secondary battery 100, with each component shown separated for clarity. The secondary battery 100 has multiple positive electrodes. The multiple positive electrodes shown in the figure are a first positive electrode 103a and a second positive electrode 103b. However, the number of positive electrodes in the secondary battery 100 is not limited, and a single positive electrode may be used. The first positive electrode 103a and the second positive electrode 103b are collectively referred to as positive electrode 103.

[0048] The secondary battery 100 has a plurality of negative electrodes. The multiple negative electrodes are shown as a first negative electrode 106 a, a second negative electrode 106 b, and a third negative electrode 106 c. However, the number of negative electrodes in the secondary battery 100 is not limited, and a single negative electrode may be included. The first negative electrode 106 a, the second negative electrode 106 b, and the third negative electrode 106 c are collectively referred to as negative electrode 106.

[0049] The secondary battery 100 has a separator between the negative electrode and the positive electrode. For clarity, the separators are shown in FIG. 1A with dotted lines. In FIG. 1A, the separators are shown as a first separator 105a, a second separator 105b, a third separator 105c, and a fourth separator 105d. However, the number of separators in the secondary battery 100 is not limited and may be a single separator. While the separators may be independent as shown in the figure, a continuous separator can also be used. By folding the continuous separator, the separators can be positioned corresponding to the first separator 105a to the fourth separator 105d. The first separator 105a, the second separator 105b, the third separator 105c, and the fourth separator 105d are collectively referred to as separator 105.

[0050] FIG. 1B shows a state in which the components of the secondary battery 100 are stacked. The first positive electrode 103a and the second positive electrode 103b each have a positive electrode current collector, and each positive electrode current collector has a protrusion 103t. The protrusions 103t of each positive electrode current collector overlap to form an assembly. The assembly of protrusions 103t is called a positive electrode tab. The first negative electrode 106a, the second negative electrode 106b, and the third negative electrode 106c each have a negative electrode current collector, and each negative electrode current collector has a protrusion 106t. The protrusions 106t of each negative electrode current collector overlap to form an assembly. The assembly of protrusions 106t is called a negative electrode tab.

[0051] As shown in FIG. 1B , multiple positive electrodes, multiple negative electrodes, and multiple separators are collectively referred to as a stacked electrode. In the stacked electrode, the positive electrode tab (protruding portion 103t) is joined to the positive electrode lead 107a at a joint 109a. In the stacked electrode, the negative electrode tab (protruding portion 106t) 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 positive electrode lead 107a can be made of a material selected from aluminum, nickel, titanium, or an alloy thereof. The negative electrode lead 107b can be made of a material selected from nickel, copper, titanium, or an alloy thereof.

[0052] The secondary battery 100 further includes an exterior body (not shown), in which the laminated electrode shown in FIG. 1B is housed. A film may be used for the exterior body to reduce weight. A secondary battery using a film for the exterior body is called a laminated secondary battery. The secondary battery according to one embodiment of the present invention is a coin-shaped laminated secondary battery.

[0053] Although not illustrated in this embodiment, a can case may be used as the exterior body, and a circular can case can be used, which is called a coin-type secondary battery (coin cell). The secondary battery of one embodiment of the present invention can be a coin-type secondary battery (coin cell). That is, the secondary battery of one embodiment of the present invention is not limited in appearance. In other words, the secondary battery of one embodiment of the present invention is not limited in material of the exterior body.

[0054] Next, the positive electrode of one embodiment of the present invention will be described.

[0055] <Positive Electrode Configuration Example 1> Fig. 2A shows a cross-sectional view of a positive electrode 103. The positive electrode 103 has a positive electrode current collector 110 and a positive electrode active material layer 111. The positive electrode active material layer 111 is a layer having first positive electrode active material particles 120, second positive electrode active material particles 121, etc., and has a region in contact with the positive electrode current collector 110. The positive electrode active material layer is preferably formed on both sides of the positive electrode current collector. This is called a double-sided coated structure. The positive electrode active material layer may also be formed on only one side of the positive electrode current collector. This is called a single-sided coated structure.

[0056] The positive electrode active material layer 111 shown in FIG. 2A further includes a first conductive material 123 and voids 125. The voids 125 in the positive electrode 103 are preferably impregnated with an electrolyte solution. The electrolyte solution may be in contact with the first positive electrode active material particles 120 and the second positive electrode active material particles 121, or may be impregnated into a portion of the first positive electrode active material particles 120 or a portion of the second positive electrode active material particles 121. The electrolyte solution may be in contact with the first conductive material 123, or may be impregnated into a portion of the first conductive material 123. To remove the electrolyte solution from the positive electrode 103, washing with an organic solvent may be performed.

[0057] In the positive electrode 103, the first conductive material 123 may be typically fibrous, and in the fibrous form, the first conductive material 123 has a long axis. While the surface of the positive electrode active material layer 111 is illustrated as being uniform, the surface has an uneven shape that conforms to the shapes of the first positive electrode active material particles 120 and the second positive electrode active material particles 121. Furthermore, while the interface between the positive electrode current collector 110 and the positive electrode active material layer 111 is illustrated as being uniform, the surface of the positive electrode current collector 110 may have recesses in regions where the first positive electrode active material particles 120 and the second positive electrode active material particles 121 overlap. The positive electrode active material layer 111 coated on the positive electrode current collector 110 may undergo a press processing process, and through this process, the first positive electrode active material particles 120 and the second positive electrode active material particles 121 are pressed into the surface of the positive electrode current collector 110, forming the recesses.

[0058] In the positive electrode 103, the first positive electrode active material particles 120 preferably have a different particle diameter from the second positive electrode active material particles 121. Specifically, the first positive electrode active material particles 120 preferably have a particle diameter of less than 10 μm, and the second positive electrode active material particles 121 preferably have a particle diameter of 10 μm or more and 50 μm or less. More preferably, the particle diameter of the first positive electrode active material particles 120 is 1 / 12 times or more and 1 / 8 times or less the particle diameter of the second positive electrode active material particles 121. As long as the above-mentioned particle diameters are satisfied, the composition of the first positive electrode active material particles 120 may be the same as the composition of the second positive electrode active material particles 121. Furthermore, as long as the above-mentioned particle diameters are satisfied, the composition of the first positive electrode active material particles 120 may be different from the composition of the second positive electrode active material particles 121.

[0059] <Particle size analysis and particle size distribution analysis using cross-sectional SEM image of positive electrode> The particle size of each particle can be calculated from a cross-sectional SEM image including the first positive electrode active material particles 120 and the second positive electrode active material particles 121 by the following method.

[0060] First, an analysis region is cut out from the acquired cross-sectional SEM image. A range having a sufficient area for image analysis can be cut out, for example, a range of 50 μm or more × 100 μm or more, but this is not limited to this. The cross-sectional SEM image may be cut out using the functions of image processing software. For example, ImageJ may be used as the image processing software, and the image may be cut out using its crop function.

[0061] Next, the cut-out first image is binarized using image processing software, and particle analysis is performed.

[0062] ImageJ, for example, can be used as the image processing software. The binarization process will be described below. A first image displayed on a 256-value grayscale is used as a frequency graph excluding black (value 0) and white (value 255), and the low-value side (HWHM_L) and high-value side (HWHM_H) are determined as the half-width at half maximum (HWHM) of the maximum peak in the frequency graph. Next, a minimum value a in a range twice the width of HWHM_L on the low-value side from the value corresponding to the peak top (maximum frequency) of the maximum peak, and a maximum value b in a range twice the width of HWHM_H on the high-value side are determined.

[0063] Next, binarization is performed so that values ​​less than a are white, values ​​greater than a and less than b are black, and values ​​greater than b are white. Specifically, the threshold function of ImageJ is used to perform binarization as Threshold (AB). After that, random bright spots thought to be caused by the conductive material are removed using the Gray Morphology (radius = 3, operator = open, type = circle) and Gray Morphology (radius = 1, operator = close, type = circle) conditions, and a second image can be obtained.

[0064] Next, using the second image, the particle size (projected area) was determined to be 0.5 μm using the Analyze Particles function of ImageJ. ２ 700 μm or more ２ The following particles are detected, and the area S of each particle is obtained. Next, based on the area S of each particle, the particle size (diameter) r of each particle is calculated (Equation 1).

[0065]

[0066] In this way, the particle size of each particle can be obtained from the cross-sectional SEM image. This is called particle size analysis using a cross-sectional SEM image of the positive electrode. Furthermore, the particle size distribution of each particle can also be determined from the particle size. This is called particle size distribution analysis using a cross-sectional SEM image of the positive electrode.

[0067] As described above, when the particle diameters of the positive electrode active material particles are different, the electrode density of the positive electrode 103 can be increased. The electrode density here is the weight of the electrode layer including the first positive electrode active material particles 120 and the second positive electrode active material particles per unit volume of the positive electrode active material layer 111. The electrode layer contains a conductive material. The electrode layer may also contain a binder.

[0068] <Positive Electrode Configuration Example 2> Fig. 2B shows a cross-sectional view of the positive electrode 103. The positive electrode active material layer 111 has first positive electrode active material particles 120, second positive electrode active material particles 121, a first conductive material 123, a second conductive material 124, and voids 125. In Fig. 4B, the first conductive material 123 is fibrous and the second conductive material 124 is particulate. Note that the first conductive material 123 and the second conductive material 124 may be made of the same material as long as they have different shapes. Furthermore, the first conductive material 123 and the second conductive material 124 may be made of different materials as long as they have different shapes.

[0069] The second conductive material 124 is preferably present in an agglomerated state near the first positive electrode active material particles 120. The second conductive material 124 is preferably present in an agglomerated state near the second positive electrode active material particles 121. The second conductive material 124 is preferably present in an agglomerated state between the first positive electrode active material particles 120 and the second positive electrode active material particles 121. The above-mentioned agglomerated second conductive material 124 can also be said to be located in the voids 125. In other words, the agglomerated second conductive material 124 located in the voids 125 may hold an electrolyte.

[0070] Furthermore, the weight of the first conductive material 123 may be different from the weight of the second conductive material 124. The weight may be the amount charged in the step of preparing the positive electrode 103, or may be the weight that can be analyzed from the positive electrode 103. In one embodiment of the present invention, the weight of the second conductive material 124 is equal to or greater than the weight of the first conductive material 123. Typically, the weight of the second conductive material 124 is two to five times, preferably two to three times, the weight of the first conductive material 123. This configuration can ensure appropriate conductivity of the positive electrode 103. Furthermore, this configuration can increase the conductivity of the positive electrode 103 compared to when a single type of conductive material is used. As a result, the secondary battery 100 can exhibit good rate characteristics.

[0071] Furthermore, by varying the particle diameter of the positive electrode active material particles, the electrode density of the positive electrode 103 can be increased.

[0072] 2C shows a cross-sectional view of the positive electrode 103. The positive electrode active material layer 111 has first positive electrode active material particles 120, a first conductive material 123, a second conductive material 124, and voids 125. In other words, the positive electrode active material layer 111 does not necessarily have to have the second positive electrode active material particles 121.

[0073] <Positive Electrode Configuration Example 4> Although not shown, in the positive electrode 103 shown in FIG. 2C , the positive electrode active material layer 111 does not necessarily have to include the second conductive material 124 in addition to the second positive electrode active material particles 121 .

[0074] 2D shows a cross-sectional view of the positive electrode 103. The positive electrode active material layer 111 has second positive electrode active material particles 121, a first conductive material 123, a second conductive material 124, and voids 125. In other words, the positive electrode active material layer 111 does not necessarily have to have the first positive electrode active material particles 120.

[0075] <Positive Electrode Configuration Example 6> Although not shown, in the positive electrode 103 shown in FIG. 2D , the positive electrode active material layer 111 does not necessarily have to include the second conductive material 124 in addition to the first positive electrode active material particles 120 .

[0076] Next, the characteristics of the positive electrode active material particles and the conductive material according to one embodiment of the present invention will be described.

[0077] <Positive electrode active material particles and conductive material> The following description will be given using first positive electrode active material particles 120 as the positive electrode active material particles and a first conductive material 123 as the conductive material. The first positive electrode active material particles 120 may be replaced with second positive electrode active material particles 121 to understand the characteristics of the positive electrode active material particles and the conductive material. In other words, this characteristic can be applied to the relationship between the positive electrode active material particles and the fibrous conductive material in the above-described positive electrode configuration examples 1 to 6.

[0078] FIG. 3A is an example of a surface SEM image of a positive electrode 103 including first positive electrode active material particles 120 and a first conductive material 123, and FIG. 3B is a schematic diagram of the first positive electrode active material particles 120 and the first conductive material 123 according to FIG. 3A.

[0079] 3A and 3B show that the first positive electrode active material particles 120 are substantially entirely surrounded by the first conductive material 123. "Substantially entirely surrounded" refers to a configuration in which 60% or more, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the first positive electrode active material particles 120 overlap with the first conductive material 123 in a top view of the positive electrode, typically a surface SEM image. "Substantially entirely surrounded" also refers to a configuration in which 60% or more, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the first positive electrode active material particles 120 overlap with the first conductive material 123 in a cross-sectional view of the positive electrode, typically a cross-sectional SEM image or a cross-sectional TEM image. "Wrapped" in a cross-sectional view may be equivalent to the first conductive material 123 being positioned along the first positive electrode active material particles 120. Incidentally, the word "wrap" can also be expressed as "cover," "cover," "stick," "adhere," "stick," "stick around," "bind," "surround and bind," or "wind around."

[0080] The first conductive material 123 may have a plurality of fibrous conductive materials. In other words, the first conductive material 123 may have an aggregate of fibrous conductive materials. That is, the aggregate of conductive materials can encase substantially the entire first positive electrode active material particle 120. In a portion of the electrode, a plurality of fibrous conductive materials may be entangled with each other. Furthermore, in a portion of the electrode, aggregates of conductive materials may be entangled with each other. To achieve the entangled state, the surface of the fibrous conductive material may have irregularities (scratches). In the entangled state, the length in the major axis direction is longer than the major axis of a single fibrous conductive material. Therefore, by utilizing this entangled state, substantially the entire first positive electrode active material particle 120 may be enclosed in an aggregate of fibrous conductive material.

[0081] The terms "enveloping," "covering," "sticking," "attaching," "clinging," "binding," "surrounding and binding," or "winding" refer to physical adhesion between the first positive electrode active material particle 120 and the first conductive material 123. However, in one embodiment of the present invention, this is not limited to physical adhesion between the first positive electrode active material particle 120 and the first conductive material 123, and may include cases where a covalent bond is formed, bonding by van der Waals forces, or the first conductive material 123 is embedded in the surface irregularities of the first positive electrode active material particle 120. Specifically, states such as "enveloping," "covering," "sticking," "attaching," "clinging," "binding," "surrounding and binding," or "winding" can be confirmed in a surface SEM image of the positive electrode or a cross-sectional SEM image of the positive electrode. It is sufficient that at least the first positive electrode active material particle 120 and the first conductive material 123 are in contact in the surface SEM image of the positive electrode or the cross-sectional SEM image of the positive electrode. As long as this is the case, the type and strength of the force that attracts them are not important. Furthermore, a binder may be located at the interface or part of the interface between the first positive electrode active material particles 120 and the first conductive material 123. This is because the binder does not interfere with the crack suppression effect described below. In this specification, etc., cracks are sometimes referred to as regions where the crystal plane is displaced or regions where the crystal plane is broken, and often occur along the (00l) plane. It is preferable that no cracks are observed in the first positive electrode active material particles 120.

[0082] The first positive electrode active material particles 120 and the first conductive material 123 as shown in FIGS. 3A and 3B can be obtained by preparing a positive electrode slurry using a dispersion liquid in which the first conductive material 123 is well dispersed. In other words, when the above-described positive electrode slurry is used, a configuration in which the first positive electrode active material particles 120 are surrounded by the first conductive material 123 may be obtained. Of course, the manufacturing method is not limited in any way as long as the configuration shown in FIGS. 3A and 3B is obtained. When the first positive electrode active material particles 120 are surrounded by the first conductive material 123, the conductivity of the positive electrode 103 can be increased. As a result, the secondary battery 100 can exhibit good rate characteristics.

[0083] Furthermore, when the first positive electrode active material particles 120 are wrapped in the first conductive material 123, the safety of the secondary battery 100 is improved. This is preferable because the first conductive material 123 can suppress the occurrence of cracks in the first positive electrode active material particles 120. In addition to cracks, cleavage, fractures, and displacement may occur in the first positive electrode active material particles 120. These are collectively referred to as cracks, etc., and the occurrence of cracks, etc. is suppressed by wrapping the first positive electrode active material particles 120 in the first conductive material 123 (crack suppression effect).

[0084] The following describes a case where the first positive electrode active material particles 120 contain lithium cobalt oxide. Lithium cobalt oxide having a basal plane and an edge plane is prone to cracks and the like from the edge plane. Therefore, the first conductive material 123 is configured to wrap around the edge plane of the first positive electrode active material particles 120. In other words, the first conductive material 123 wraps around the edge plane of the first positive electrode active material particles 120 to prevent cracks from progressing, thereby improving the safety of the secondary battery. Wrapping around the edge plane includes a configuration in which 60% or more, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the edge plane of the first positive electrode active material particles 120 overlaps with the first conductive material 123 in a top view of the edge-facing positive electrode, typically in a surface SEM image. Furthermore, "covering the edge surface" includes a configuration in which 60% or more, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the edge surface of the first positive electrode active material particle 120 overlaps with the first conductive material 123 in a cross-sectional view of a positive electrode having an edge surface, typically in a cross-sectional SEM image or a cross-sectional TEM image.

[0085] In this case, aligning the major axis of the first conductive material 123 approximately parallel to the c-axis of the first positive electrode active material particles 120 can have the effect of suppressing the progression of cracks. Furthermore, the first conductive material 123 may have a state in which multiple conductive materials are entangled with each other. When the first conductive material 123 wrapping the edge surfaces is in the entangled state, the progression of cracks in the first positive electrode active material particles 120 can be further suppressed, thereby improving safety.

[0086] Furthermore, the occurrence of cracks and the like described above is also suppressed by the first positive electrode active material particles 120 being wrapped in the first conductive material 123 via the binder.

[0087] 4A , the first conductive material 123 can surround the second positive electrode active material particles 121 instead of the first positive electrode active material particles 120. When a positive electrode slurry is prepared using a dispersion liquid in which the first conductive material 123 is well dispersed, the second positive electrode active material particles 121 may be surrounded by the first conductive material 123. When the second positive electrode active material particles 121 are surrounded by the first conductive material 123, the conductivity of the positive electrode 103 can be increased. As a result, the secondary battery 100 can exhibit good rate characteristics.

[0088] Furthermore, when the second positive electrode active material particles 121 are wrapped with the first conductive material 123, the safety of the secondary battery 100 is improved. One configuration that improves safety is a state in which the major axis of the first conductive material 123 wraps around the second positive electrode active material particles 121 along the c-axis direction. This configuration suppresses the occurrence of cracks and the like in the second positive electrode active material particles 121. Furthermore, the occurrence of cracks and the like is suppressed by wrapping the second positive electrode active material particles 121 with the first conductive material 123 via the binder.

[0089] 4B , the first conductive material 123 can actively surround the first positive electrode active material particles 120 relative to the first positive electrode active material particles 120 and the second positive electrode active material particles 121. A positive electrode slurry is prepared by mixing the first positive electrode active material particles 120 with a dispersion liquid in which the first conductive material 123 is well dispersed, and then the second positive electrode active material particles 121 are mixed with the positive electrode slurry, resulting in a configuration in which the first conductive material 123 actively surrounds the first positive electrode active material particles 120. A configuration in which the first conductive material 123 actively surrounds the first positive electrode active material particles 120 includes a configuration in which the area overlapped by the first conductive material 123 is larger on the first positive electrode active material particles 120 than on the second positive electrode active material particles 121 in a surface SEM image of the positive electrode or a cross-sectional SEM image of the positive electrode. When the first positive electrode active material particles 120 are actively wrapped with the first conductive material 123, the conductivity of the positive electrode 103 can be increased. As a result, the secondary battery 100 can exhibit good rate characteristics. Furthermore, when the first positive electrode active material particles 120 are actively wrapped with the first conductive material 123, the occurrence of cracks and the like is suppressed. Furthermore, when the first positive electrode active material particles 120 are actively wrapped with the first conductive material 123 via the binder, the occurrence of cracks and the like is suppressed. In other words, the safety of the secondary battery 100 is improved.

[0090] 4C , the first conductive material 123 can actively surround the second positive electrode active material particles 121 relative to the first positive electrode active material particles 120 and the second positive electrode active material particles 121. A positive electrode slurry is prepared by mixing the second positive electrode active material particles 121 with a dispersion liquid in which the first conductive material 123 is well dispersed, and the first positive electrode active material particles 120 are subsequently mixed into the positive electrode slurry, which may result in a configuration in which the first conductive material 123 actively surrounds the second positive electrode active material particles 121. A configuration in which the first conductive material 123 actively surrounds the second positive electrode active material particles 121 includes a configuration in which the area overlapped by the first conductive material 123 is larger on the second positive electrode active material particles 121 than on the first positive electrode active material particles 120 in a surface SEM image or a cross-sectional SEM image of the positive electrode. When the second positive electrode active material particles 121 are actively wrapped with the first conductive material 123, the conductivity of the positive electrode 103 can be increased. As a result, the secondary battery 100 can exhibit good rate characteristics. Furthermore, when the second positive electrode active material particles 121 are actively wrapped with the first conductive material 123, the occurrence of cracks and the like is suppressed. Furthermore, when the second positive electrode active material particles 121 are actively wrapped with the first conductive material 123 via the binder, the occurrence of cracks and the like is suppressed. In other words, the safety of the secondary battery 100 is improved.

[0091] <Positive Electrode Active Material> The positive electrode active material used for the first positive electrode active material particles 120 may be a lithium composite oxide having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. The positive electrode active material is not particularly limited as long as it is a lithium composite oxide, but the surrogate target may be lithium cobalt oxide (LiCoO ２ However, the composition is not limited thereto, and it is also called LCO), lithium iron phosphate (LiFePO ４ However, the composition is not limited thereto, and it is also called LFP), lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMn ２ O ４ However, the composition is not limited thereto), lithium manganese phosphate (LiMnPO ４ However, the composition is not limited thereto), lithium manganese iron phosphate (LiFe ａ Mnｂ P.O. ４ (a+b is 1 or less, 0<a<1, 0<b<1) can be used. When lithium iron phosphate is used, it is preferable to use one coated with carbon.

[0092] The positive electrode active material used for the second positive electrode active material particles 121 may be a lithium composite oxide having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. The positive electrode active material is not particularly limited as long as it is a lithium composite oxide, but examples of the substitute target include lithium cobalt oxide, lithium iron phosphate, lithium nickel oxide, and lithium manganese oxide (LiMn ２ O ４ ), lithium manganese phosphate, and lithium manganese iron phosphate can be used. When lithium iron phosphate is used, it is preferable to use one coated with carbon.

[0093] <Conductive Material> The first conductive material 123 and the second conductive material 124 can each be a metal material or a carbon material. Carbon fiber or the like can be used as the first conductive material 123. Carbon fiber can be considered a fibrous conductive material. Instead of carbon fiber, graphene or a graphene compound can be used as the first conductive material 123. Graphene or a graphene compound can be considered a sheet-like conductive material. Furthermore, graphene or a graphene compound can be added to the positive electrode 103 as a third conductive material.

[0094] In this specification and the like, graphene refers to a substance that contains carbon, has a shape such as a plate or sheet, and has a two-dimensional structure formed by six-membered carbon rings. In this specification and the like, graphene includes multilayer graphene. Graphene exhibits excellent electrical properties, such as high conductivity, and is therefore suitable as a conductive material.

[0095] In this specification and the like, graphene compounds include graphene oxide, multilayer graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, etc. In other words, graphene compounds may have a functional group, and examples of the functional group include an epoxy group, a carboxy group, or a hydroxy group. Graphene compounds are suitable as conductive materials because they exhibit excellent electrical properties, such as high flexibility and high conductivity.

[0096] As the second conductive material 124, particulate conductive materials such as carbon black, Ketjen Black (registered trademark), and acetylene black (hereinafter, sometimes referred to as AB) can be used. Such particulate conductive materials may be aggregated in the positive electrode 103. The aggregated state may be called an aggregate.

[0097] <Carbon Fiber> Carbon fibers that function as conductive materials include CNT, VGCF (registered trademark), or carbon fiber. CNTs have a layer of carbon atoms; when there is a single layer, they are called single-wall nanotubes; when there are multiple layers, they are called multi-wall nanotubes; and multi-wall nanotubes include double-wall nanotubes, which have two layers. Carbon fibers can form aggregates. Furthermore, because carbon fibers have a long axis or fiber length, they can become entangled, and this state is called an aggregate. An entangled state includes a state in which one carbon fiber is entangled, or a state in which multiple carbon fibers are entangled. To form an entangled state, the carbon fiber may have unevenness (scratches) on its surface.

[0098] The specific surface area of ​​VGCF (registered trademark) carbon fiber is 100 m ２ / g or more, preferably 60m ２ / g or more, more preferably 20m ２ The specific surface area of ​​the CNT in the carbon fiber is preferably 500 m / g or more. ２ / g or more, preferably 650m ２ / g or more, more preferably 800m ２ The specific surface area is preferably a value measured by the Brunauer Emmett Teller method (BET method), for example.

[0099] Among carbon fibers, the long axis or fiber length of VGCF (registered trademark) is preferably 1 μm or more and 100 μm or less, and more preferably 2 μm or more and 20 μm or less. Among carbon fibers, the long axis or fiber length of CNT is preferably 100 μm or more and 600 μm or less, and more preferably 200 μm or more and 500 μm or less. Furthermore, if the long axis or fiber length is larger than the particle diameter of the positive electrode active material, the carbon fiber will encase multiple positive electrode active materials, so in one embodiment of the present invention, it is not limited to the above numerical values. Furthermore, since carbon fibers are entangled, the long axis when entangled becomes important as a conductive material. The entangled carbon fibers can encase multiple positive electrode active materials.

[0100] Furthermore, when the cross section of one carbon fiber can be considered as a circle, the average diameter of the carbon fiber is preferably 1 nm or more and 180 nm or less, and preferably 2 nm or more and 150 nm or less. VGCF (registered trademark) can satisfy the average diameter of 100 nm or more and 180 nm or less, preferably 130 nm or more and 160 nm or less, and can be said to have a large average diameter. VGCF (registered trademark) with a large average diameter exhibits high dispersibility. CNT can satisfy the average diameter of 1 nm or more and 100 nm or less, preferably 1 nm or more and 50 nm or less, and more preferably 3 nm or more and 5 nm or less, and can be said to have a small average diameter.

[0101] Carbon fibers are preferred because they easily aggregate due to the length and average diameter described above. The aggregated carbon fibers can suppress the occurrence of cracks in the positive electrode active material while exhibiting the function of a conductive path for the positive electrode active material, and can suppress deterioration of the positive electrode active material due to charge / discharge cycles.

[0102] As can be seen from their appearance, carbon fibers or carbon fiber aggregates are preferred as conductive materials because they provide a long-distance conductive path. For example, they can ensure a conductive path between the positive electrode current collector 110 and the first positive electrode active material particles 120 located far away, enabling rapid charge and discharge. Carbon fibers or carbon fiber aggregates that can ensure a conductive path can achieve good charge and discharge cycle characteristics even at a low content. This is preferable because it allows the content of positive electrode active material particles in the positive electrode 103 to be increased.

[0103] In the case of a positive electrode active material layer 111 having first positive electrode active material particles 120 and / or second positive electrode active material particles 121, a first conductive material 123, and a second conductive material 124, the total content of the first conductive material 123 and the second conductive material 124 relative to the total amount of the positive electrode active material layer 111 is preferably 0.1 wt % to 10 wt %, more preferably 1 wt % to 5 wt %. Since the first conductive material 123 and the second conductive material 124 have different shapes, this is preferable because it can further increase the proportion of the positive electrode active material in the positive electrode active material layer 111. Furthermore, the proportion of the first conductive material 123 and the second conductive material 124 is not limited, but it is preferable that the proportion of the first conductive material 123 is equal to or greater than the proportion of the second conductive material 124.

[0104] Furthermore, the carbon fiber or carbon fiber aggregate can wrap multiple positive electrode active materials together, as can be seen from the appearance. The carbon fiber or carbon fiber aggregate can also bind the positive electrode active material so as to surround it, as can be seen from the appearance. This configuration improves the safety of the secondary battery 100.

[0105] The above-mentioned carbon fibers may be dispersed in water or an organic solvent using a dispersant. Even when carbon fibers are mixed with a particulate conductive material, they can be dispersed in water or an organic solvent using the above-mentioned dispersant. A specific organic solvent that can be used is N-methyl-2-pyrrolidone (NMP). When NMP is used as the organic solvent, it is preferable to use polyvinylpyrrolidone (PVP) as the dispersant. PVP can be adsorbed onto the surface of CNTs, and can suppress re-aggregation of the CNTs after dispersion.

[0106] <Binder (binding agent)> The positive electrode active material layer 111 may further contain a binder. The binder can strengthen the positive electrode 103. Of course, the positive electrode 103 may not require a binder. For example, if the first conductive material 123 is entangled and can hold the first positive electrode active material particles 120 and / or the second positive electrode active material particles 121, the binder may not be required. Eliminating the binder is preferable because it allows the proportion of the positive electrode active material in the positive electrode active material layer 111 to be further increased.

[0107] The binder functions to bind various components, such as positive electrode active materials together, conductive materials together, and positive electrode active materials and a positive electrode current collector. Furthermore, the binder should be thermally stable and electrochemically stable at the positive electrode potential. For example, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer as the binder. Fluorine rubber can also be used as the binder.

[0108] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0109] Alternatively, polymer 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 are preferably used as binders. Polymer materials are preferred because they are flexible and therefore make it easy to increase electrode density. PVDF has C—H bonds and C—F bonds, but is not polar overall, making it easy to disperse appropriately within the positive electrode.

[0110] The binder may be a combination of two or more of the above. In addition, since the binder is not a material that directly contributes to the battery characteristics, the binder may be unnecessary.

[0111] <Thickener> The binder may be a combination of a material with particularly excellent viscosity adjustment effects (sometimes referred to as a thickener) and other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it can be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix a rubber material with a material with particularly excellent viscosity adjustment effects. For example, a water-soluble polymer may be used as a material with particularly excellent viscosity adjustment effects. Furthermore, as water-soluble polymers with particularly excellent viscosity adjustment effects, the aforementioned polysaccharides, such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0112] 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.

[0113] Water-soluble polymers stabilize viscosity by dissolving in water, allowing the active material and other materials used as binders to be dispersed stably in the aqueous solution. Furthermore, their functional groups are expected to facilitate stable adsorption to the surface of the active material. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the functional groups are expected to allow interactions between the polymers, resulting in widespread coverage of the active material surface.

[0114] The binder material is preferably prepared in a state of being dissolved in an organic solvent, such as methyl alcohol, ethyl alcohol, propyl alcohol, diethylformamide, dimethylacetamide, or N-methyl-2-pyrrolidone (NMP).

[0115] <Electrolyte Solution> The electrolyte solution may include an organic solvent and a lithium salt (also referred to as an electrolyte) dissolved in the organic solvent. The organic 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 may also be used as a mixed solvent of two or more of these.

[0116] The mixed solvent may contain ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and the volume ratio of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total content of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%. More specifically, a mixed solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 may be used. Note that the above volume ratio may be the volume ratio before the mixed solvent is mixed, and the outside air temperature when the mixed solvent is mixed may be room temperature (typically, 25°C).

[0117] EC is a cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts. However, EC has a high viscosity and a high freezing point (melting point) of 38°C, making it difficult to use EC alone as a solvent in low-temperature environments. Therefore, a solvent specifically described as one embodiment of the present invention further contains EMC and DMC, rather than EC alone. EMC is a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −54°C. DMC is also a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −43°C. An electrolyte prepared using a mixed solvent containing EC, EMC, and DMC with such physical properties, in a volume ratio of x:y:100−x−y (where 5≦x≦35 and 0<y<65), assuming a total content of the three mixed solvents as 100 vol%, is characterized by a freezing point of −40°C or lower.

[0118] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the organic solvent, it is possible to prevent the power storage device from exploding or catching fire even if the internal temperature of the power storage device rises due to an internal short circuit or overcharging. 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.

[0119] The lithium salt (electrolyte) dissolved in the solvent is, for example, 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 ４ F 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 , lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), or two or more of these can be used in any combination and ratio.

[0120] The electrolyte solution may also contain additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive is preferably 0.1 wt % or more and 5 wt % or less relative to the solvent in which the electrolyte is dissolved.

[0121] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0122] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0123] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel. For example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0124] <Positive Electrode Current Collector> The positive electrode current collector 110 can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. The material used for the positive electrode current collector preferably does not leach at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector 110 may also be made of 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 positive electrode current collector 110 can be in any suitable shape, such as a foil, plate, sheet, mesh, punched metal, or expanded metal. The positive electrode current collector 110 preferably has a thickness of 5 μm to 30 μm.

[0125] <Negative Electrode> The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer contains a negative electrode active material and may further contain a conductive material and a binder. The negative electrode active material layer is a layer containing negative electrode active material particles and has a region in contact with the negative electrode current collector. The manufacturing process of a negative electrode includes pressing, and in a negative electrode that has undergone this pressing, recesses into a portion of the negative electrode current collector may be formed where the negative electrode active material particles are pressed. The negative electrode active material layer may have a double-coated structure formed on both sides of the negative electrode current collector. However, for a negative electrode arranged in the outermost layer, it is preferable to use a single-coated structure in which the negative electrode active material layer is formed on only one side of the negative electrode current collector. In a negative electrode arranged in the outermost layer, the negative electrode active material layer that is not arranged facing the positive electrode does not undergo insertion / desorption of carrier ions or is difficult to insert / desorb, so the negative electrode active material layer may not be formed. Since productivity is higher when all negative electrodes are prepared as double-sided coated structures, negative electrodes having a double-sided coated structure may be arranged as the outermost layers.

[0126] Furthermore, when the secondary battery 100 is used while being bent, a negative electrode with a single-sided coated structure is prepared. A configuration in which multiple negative electrodes with a single-sided coated structure are stacked so that the negative electrode current collectors are in contact with each other is called a back-to-back configuration. When a back-to-back configuration is used, the negative electrode current collectors, which have low contact resistance, are in contact with each other, making the secondary battery 100 easier to bend.

[0127] <Negative Electrode Active Material> The negative electrode active material can be a material that absorbs and releases lithium. Furthermore, the negative electrode active material can be a material that can undergo a charge-discharge reaction by alloying and dealloying with lithium. The negative electrode active material can be, for example, one or a composite material selected from lithium metal, carbon, and silicon. Silicon is preferred because it has a high theoretical capacity of 4200 mAh / g per active material weight. When lithium metal is used as the negative electrode active material, the negative electrode current collector can be omitted.

[0128] As the carbon, graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), CNT, graphene, carbon black, etc. can be used.

[0129] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), the graphite exhibits a potential as low as that of lithium metal (0.05 V to 0.3 V vs. Li / Li+). This allows the secondary battery to exhibit a high operating voltage. Furthermore, graphite is preferred because it has advantages such as a relatively high discharge capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0130] If the particle size or median diameter (D50) is small, the negative electrode active material becomes bulky and may inhibit improvement of electrode density. Therefore, the particle size or median diameter (D50) of the negative electrode active material should be 3 μm or more and 20 μm or less, preferably 7 μm or more and 12 μm or less. Typically, the median diameter (D50) should be in the above range as a powder characteristic of graphite.

[0131] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite that can be used include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. The artificial graphite may have a carbon coating layer, which is a low-crystalline layer. Since the shape of the artificial graphite is spherical, it is called spherical graphite. For example, MCMB is one of the preferred materials for spherical graphite. Furthermore, it is relatively easy to reduce the specific surface area of ​​MCMB. If the specific surface area is large, the decomposition reaction with the electrolyte on the surface of the negative electrode active material may become large, and good cycle characteristics may not be obtained. In order to suppress the decomposition reaction, the specific surface area of ​​the carbon is 0.8 m ２ / g or more 8m ２ / g, preferably 1m ２ / g or more 2m ２ / g is preferably satisfied. Typically, it is preferable that the spherical graphite has the specific surface area described above as a powder characteristic. The specific surface area can be measured by the BET method (Brunauer Emmett Teller method). The BET method is an analytical technique that extends the Langmuir theory to multilayer adsorption of adsorbed gas molecules, and is the most common method for calculating the specific surface area. The specific surface area by the BET method can be measured using an automatic specific surface area measuring device, Tristar 23020.

[0132] Examples of natural graphite include flake graphite, spheroidized natural graphite, etc. The natural graphite may have a carbon coating layer which is a low-crystalline layer.

[0133] The negative electrode active material can also be a silicon-carbon composite material containing carbon and silicon. In the silicon-carbon composite material, a mixture of carbon and silicon is preferably used, and it is even more preferable to confirm the sintered state through a heat treatment. In the silicon-carbon composite material, graphite particles are preferably used as the carbon, and the particle size or median diameter (D50) of the graphite particles is 1 μm or more and 20 μm or less, preferably 3 μm or more and 20 μm or less, and more preferably 7 μm or more and 12 μm or less. The particle size or median diameter of the graphite particles can be determined based on the particle size or median diameter (D50) of silicon.

[0134] The specific surface area of ​​the graphite particles is 0.5 m ２ / g or more 3m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Shimadzu Corporation).

[0135] In addition, in the silicon carbon composite material, it is preferable to use silicon particles as the silicon. The silicon particles preferably have a silicon material, and specifically, preferably contain one selected from silicon, silicon oxide, and silicon alloy. Silicon oxide includes silicon monoxide (SiO). In this specification, SiO refers to, for example, silicon monoxide. Silicon monoxide is SiO ｘ x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0136] The particle size or median diameter (D50) of the silicon particles is preferably less than 1 μm, typically 50 nm to 800 nm, preferably 100 nm to 500 nm. Silicon particles of this size are sometimes called nanosilicon particles. Silicon has problems with expansion and contraction during charge and discharge, but nanosilicon particles refined to the above particle size or median diameter (D50) are suitable because they improve charge and discharge degradation. It is preferable to grind the silicon raw material to obtain a uniform median diameter (D50).

[0137] The specific surface area of ​​silicon particles is 10 m ２ / g or more 35m ２ / g or less, preferably 10m ２ / g or more 15m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Shimadzu Corporation).

[0138] A secondary battery with a high discharge capacity can be realized by using a negative electrode active material containing both graphite particles and silicon particles. Furthermore, since the particle size or median diameter (D50) of graphite particles is different from, and specifically larger than, the particle size or median diameter (D50) of silicon particles, mixing these particles and using them in a negative electrode can increase the amount of negative electrode active material supported. A low supported amount can improve the output characteristics of a lithium-ion secondary battery, but a low supported amount reduces the discharge capacity. Therefore, the supported amount of negative electrode active material is set to 10 mg / cm. ２ The above is preferable.

[0139] In the negative electrode active material layer, the weight of the graphite particles is preferably higher than the weight of the silicon particles, and typically, the weight ratio of the graphite particles in the negative electrode active material layer is preferably 5 to 15 times the weight ratio of the silicon particles. In other words, the weight ratio of silicon to the total weight of the powder material constituting the negative electrode active material is preferably 7.5 wt % to 37.5 wt %.

[0140] Other examples of the negative electrode active material that can be used include materials containing one or more elements selected from tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, and the like.

[0141] The negative electrode active material may be a compound containing one or more elements selected from tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. A compound containing two or more elements may be called an alloy material, and for example, magnesium silicide (Mg ２ Si).

[0142] Other alloy materials include magnesium-germanium alloys (Mg ２ Ge), stannous oxide (SnO), stannic oxide (SnO ２ ), magnesium tin compounds (Mg ２ Sn), tin disulfide (SnS ２ ), other major binary alloys of tin (V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, LaSn ３ , La ３ Co ２ Sn ７ , SbSn), binary alloys of antimony (Ag ３ Sb, Ni ２ MnSb, CeSb ３ , CoSb ３ , InSb), etc.

[0143] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO２ ) and other oxides can be used.

[0144] 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 per active material weight) ３ ) and is preferred.

[0145] 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, is 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.

[0146] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the battery fabrication. 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 battery fabrication, in which lithium ions released from the positive electrode active material upon charging the battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] <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.

[0151] <Separator> When the electrolyte contains an electrolytic solution, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0152] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0153] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0154] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0155] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0156] <Exterior Body> The exterior body of the secondary battery can be made of, for example, a metal material such as aluminum and / or a resin material. A film-like exterior body can also be used. Examples of films that can be used include a three-layer film in which a highly flexible metal thin film such as aluminum, stainless steel, 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. A three-layer film containing aluminum is sometimes referred to as an aluminum laminate film.

[0157] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0158] 5 to 12 , a positive electrode active material particle 20 according to one embodiment of the present invention will be described. The positive electrode active material particle 20 is a particle that can be applied to one or more selected from first positive electrode active material particles 120 and second positive electrode active material particles 121.

[0159] 5A is a cross-sectional view of a positive electrode active material particle 20 according to one embodiment of the present invention. The positive electrode active material particle 20 preferably has a surface layer portion 20 a and an interior portion 20 b. In FIG. 5A, the dashed line indicates an example of the boundary between the surface layer portion 20 a and the interior portion 20 b.

[0160] In this specification, the surface layer 20a can be referred to as a region within 20 nm in the depth direction from the particle surface, a region within 10 nm in the depth direction from the particle surface, a region within 5 nm in the depth direction from the particle surface, or a region within 3 nm in the depth direction from the particle surface. Note that the depth direction is a direction perpendicular or approximately perpendicular to the particle surface. Perpendicular or approximately perpendicular refers to a range of 80° to 100° relative to the surface. Surfaces formed by cracks and / or fissures may also be referred to as the particle surface. The surface layer 20a is synonymous with the near-surface, near-surface region, or shell.

[0161] The region deeper than the surface layer 20a can be referred to as the inner portion 20b, which is synonymous with the inner region or the core.

[0162] The positive electrode active material particles 20 preferably contain lithium cobalt oxide as a main component and belong to the space group R-3m. The inner portion 20b preferably has a layered rock salt type crystal structure. The surface portion 20a preferably has a rock salt type crystal structure whose crystal orientation is identical or approximately identical to the layered rock salt type. Of course, the surface portion 20a must have a layered rock salt type crystal structure in part to enable the insertion and desorption of lithium ions. Therefore, it is preferable that the surface portion 20a have both a layered rock salt type crystal structure and a rock salt type crystal structure.

[0163] (001) in FIG. 5A indicates the (001) plane of lithium cobalt oxide. The surface layer portion 20a includes a region having a (001) plane (a (001)-oriented region, a region having a surface parallel to the (001) plane, called a basal region). The surface layer portion 20a also includes an edge region. The edge region is a region having a surface exposed in a direction intersecting the (001) plane, or a region other than a (001)-oriented region. In FIG. 5A, X1-X2 is a line along the cross section of the edge region, and Y1-Y2 is a line along the cross section of the basal region.

[0164] 5B to 5E are schematic diagrams showing the distribution of added elements in the edge region. In FIGS. 5B to 5E, the horizontal axis indicates the distance from the measurement point, and the vertical axis indicates the concentration of the element. By defining the particle surface on the horizontal axis, the distance from the particle surface can be determined. Schematic diagrams such as those in FIGS. 5B to 5E can be obtained from graphs of cross-sectional analysis results. The analysis direction of the cross-sectional analysis is from the surface of the particle toward the interior 20b, and analysis in this direction is called depth analysis. In other words, the distance on the horizontal axis can also be considered the depth of the particle. For cross-sectional analysis, for example, STEM-EDX line analysis can be used.

[0165] The positive electrode active material particles 20 do not contain any metal oxide, such as aluminum oxide, that does not have lithium sites that can contribute to charge and discharge, or any carbonate or hydroxyl group that is chemically adsorbed after the production of the positive electrode active material. Therefore, the surface of the positive electrode active material particles 20 refers to the surface of the particles including the surface layer portion 20a and the interior portion 20b. The attached metal oxide refers to, for example, a metal oxide whose crystal orientation does not match that of the interior portion 20b.

[0166] Furthermore, the positive electrode active material particles 20 do not include the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the positive electrode active material particles 20. The attached electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom can be removed by washing.

[0167] <Containing Elements> The positive electrode active material particles 20 contain lithium, a transition metal M, oxygen, and an additive element. The transition metal M is one or more selected from cobalt, nickel, and manganese. When cobalt is selected as the transition metal M, the positive electrode active material particles 20 can be said to contain lithium cobalt oxide and an additive element. However, the lithium cobalt oxide does not need to have a strict composition such as that shown in the chemical formula. In other words, the composition of the lithium cobalt oxide is not limited to Li:Co:O = 1:1:2 (atomic ratio).

[0168] The positive electrode active material needs to contain a transition metal M that can be oxidized and reduced in order to maintain charge neutrality even when lithium ions are inserted and extracted. If the transition metals contained in the positive electrode active material particles 20 contain cobalt at 75 atomic % (sometimes referred to as atomic %) or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, this has many advantages, such as being relatively easy to synthesize and handle, and having excellent charge-discharge cycle characteristics.

[0169] <Additive Element> The additive element contained in the positive electrode active material particles 20 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Nickel is sometimes used as a main component of the positive electrode active material particles 20, but nickel may also be added to the positive electrode active material particles 20 as an additive element.

[0170] [Magnesium] The presence of magnesium in the surface layer portion 20a makes it easier to maintain the layered rock salt type crystal structure. This phenomenon will be explained below.

[0171] Magnesium ions are divalent cations, and since magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, they tend to enter the lithium site. ２ Because they function as pillars supporting the layers, the layered rock salt crystal structure is easily maintained.

[0172] In addition, the presence of magnesium ｘ CoO ２ When x is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, if the magnesium concentration in the surface layer portion 20a is high, it can be expected that the corrosion resistance against hydrogen fluoride produced by decomposition of the electrolyte will be improved. In order to exert the effect of corrosion resistance against hydrogen fluoride, it is preferable that the magnesium concentration in the surface is high.

[0173] If the magnesium content is too low, the above-mentioned effects will not be fully achieved. On the other hand, if the magnesium content is too high, the capacity will decrease. Therefore, in the entire positive electrode active material particles 20, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably about 0.01 times. The amount of magnesium in the entire positive electrode active material particles 20 referred to here may be a value obtained by performing an elemental analysis of the entire positive electrode active material particles 20 using, for example, glow discharge mass spectrometry (GD-MS) or inductively coupled plasma mass spectrometry (ICP-MS), or may be based on the value of the composition of raw materials in the manufacturing process of the positive electrode active material particles 20.

[0174] [Aluminum] Aluminum can exist in the cobalt site of the layered rock salt crystal structure. That is, aluminum exists in the interior 20b. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is unlikely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Therefore, even if the positive electrode active material particles 20 are subjected to a force that causes them to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, i.e., even if a force that causes them to expand and contract in the c-axis direction due to a change in the charge depth or charge rate is applied, deterioration of the positive electrode active material particles 20 can be suppressed.

[0175] Aluminum also has the effect of suppressing the elution of surrounding cobalt, improving cycle characteristics. Furthermore, because the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen from the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum can improve the safety of secondary batteries.

[0176] On the other hand, excessive aluminum may adversely affect lithium insertion and desorption. Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material particles 20 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material particles 20 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 of aluminum contained in the entire positive electrode active material particles 20 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material particles 20 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the manufacturing process of the positive electrode active material particles 20.

[0177] [Fluorine] Fluorine is a monovalent anion, and when part of the oxygen in the surface layer portion 20a is substituted with fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, in the absence of fluorine, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when part of the oxygen in the surface layer portion 20a of the positive electrode active material particle 20 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 particle 20 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved.

[0178] Furthermore, the presence of fluorine on the surface that is in contact with the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material particles 20 and the electrolyte, and can also effectively improve corrosion resistance to hydrogen fluoride.

[0179] [Nickel] In lithium cobalt oxide having a layered rock-salt crystal structure, nickel can exist on both the cobalt site and the lithium site. In other words, aluminum can exist in the surface layer 20a. When nickel exists on the cobalt site, its oxidation-reduction potential is lower than that of cobalt, making it easier to release lithium and electrons during charging. This is expected to result in faster charge and discharge speeds.

[0180] Furthermore, when nickel is present at the lithium site, the deviation of the layer structure consisting of the octahedrons of cobalt and oxygen can be suppressed. This is because nickel present at the lithium site also acts as a CoO ２ This is thought to be because nickel functions as a pillar supporting the layers. Furthermore, the presence of nickel at the lithium sites is expected to make the crystal structure more stable when the material is charged at high temperatures, for example, above 45°C.

[0181] The ionization tendency of magnesium is lowest in the order of magnesium, aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, nickel is less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, nickel is preferred because it has a high effect of stabilizing the crystal structure of the surface layer portion 20a in a charged state.

[0182] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel may have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0183] On the other hand, excessive nickel is undesirable because it increases the influence of distortion due to the Jahn-Teller effect. Furthermore, excessive nickel may adversely affect lithium insertion and desorption. Therefore, it is preferable that the entire positive electrode active material particle 20 contains an appropriate amount of nickel. Specifically, in the entire positive electrode active material particle 20, the number of nickel atoms is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. The amount of nickel in the entire positive electrode active material particle 20 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the raw material composition in the manufacturing process of the positive electrode active material.

[0184] [Titanium] The presence of titanium in the surface layer portion 20a can promote the insertion and desorption of lithium ions into and from the positive electrode active material particles 20. This phenomenon will be described below.

[0185] Although titanium exists stably in the octahedral sites of hexavalent oxygen atoms in the oxide, titanium oxide and lithium titanate cannot form a stable layered rock salt type crystal structure. ２ The rutile crystal structure is the most stable, and lithium titanate, Li ４ Ti ５ O１２ has a spinel-type crystal structure. Therefore, when a small amount of titanium is dissolved in the surface layer 20a having a layered rock salt type or rock salt type crystal structure, defects occur in part of the crystal structure of the surface layer 20a. Lithium cobalt oxide is in a discharged state (the discharged state is referred to as Li ｘ CoO ２ It is known that the surface layer 20a has a large band gap and high resistance (also referred to as when x is 1). Therefore, by introducing titanium defects into a part of the surface layer 20a, the band gap can be narrowed and the resistance can be reduced.

[0186] Cobalt is also present in the oxide. ３＋ is the most stable, while titanium is Ti ４＋ Therefore, the charge on the lithium ion around titanium is relatively lower than that around cobalt. ４＋ Defects may be induced in the cation sites near the titanium, and such defects reduce the diffusion resistance of cations, particularly lithium ions. Therefore, the diffusion resistance of lithium ions is reduced around the titanium. Therefore, the presence of titanium in the surface layer portion 20 a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material particles 20.

[0187] If the amount of titanium is too small, the above-mentioned effects are not fully exhibited. On the other hand, if the amount of titanium is too large, it may form a different phase from other added elements such as magnesium (for example, MgTiO with an ilmenite-type crystal structure). ３ Furthermore, magnesium may be taken away by the formation of a different phase, which may reduce the magnesium concentration in the surface layer portion 20a. ＋When the battery is charged at a high voltage exceeding 100 kJ / s, it is preferable that the surface layer 20a contains magnesium to suppress phase change. Therefore, the loss of magnesium due to the formation of a different phase is a major disadvantage. Furthermore, if there are too many defects due to titanium, there is a concern that oxygen may be easily desorbed from the surface. Therefore, it is preferable that titanium is present in the surface layer 20a together with magnesium, fluorine, etc., or that titanium is present in the surface or near-surface region of the surface layer 20a at a concentration lower than that of magnesium.

[0188] Specifically, the number of Ti atoms is preferably 0.0001 to 0.005 times (0.01% to 0.5%), and more preferably 0.0005 to 0.0025 times (0.05% to 0.25%), the number of Co atoms throughout the entire positive electrode active material particles 20. The amount of titanium throughout the entire positive electrode active material particles 20 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material particles 20 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials during the manufacturing process of the positive electrode active material particles 20.

[0189] [Other additive elements] When phosphorus is contained in the surface layer portion 20a, Li ｘ CoO ２ When the value of x in the formula (1) is kept small, short-circuiting between the positive electrode and the negative electrode can be suppressed, which is preferable. For example, it is preferable that the compound containing phosphorus and oxygen exists in the surface layer portion 20a.

[0190] When the positive electrode active material particles 20 contain phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable. ６ If the electrolyte contains fluorine, it may react with water to generate hydrogen fluoride. Furthermore, hydrogen fluoride may also be generated by the reaction of polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, with an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 24 may be suppressed. Furthermore, it may be possible to suppress a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0191] Furthermore, when the positive electrode active material particles 20 have cracks or the like, the progression of the cracks or the like can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material particles with the cracks or the like on the surface, for example, in the embedded portion.

[0192] [Synergistic Effect of Multiple Added Elements] When the surface layer portion 20a 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 portion 20a.

[0193] For the above reasons, it is preferable that magnesium be added to the positive electrode active material in a process before nickel is added. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the process of addition, whereas nickel can diffuse widely into the interior of lithium cobalt oxide in the absence of magnesium. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of the lithium cobalt oxide and not remain in the desired amount in the surface layer.

[0194] Furthermore, when the surface layer portion 20a contains magnesium, nickel, and titanium, the presence of nickel results in the formation of MgTiO with an ilmenite-type crystal structure. ３ The formation of heterophases such as MgTiO is suppressed. The ilmenite crystal structure has hexagonal close-packed anions, which differs from the rock salt and layered rock salt crystal structures in that the anions are cubic close-packed. Therefore, a certain amount of activation energy is required for the phase transition. NiO(II) is a compound with low chemical activity, so MgTiO ３ It has the effect of suppressing the production of

[0195] For the above reasons, if titanium is added to the positive electrode active material in a manufacturing process subsequent to the process for adding magnesium and nickel, the effect of nickel in suppressing the formation of heterogeneous phases can be more pronounced.

[0196] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 20 a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0197] However, if the surface layer 20a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 20a to be occupied only by MgO or a structure in which MgO and CoO(II) are solid-solved. Therefore, the surface layer 20a must contain at least a transition metal M such as cobalt, and also contain lithium in the discharged state, so that it has a path for insertion and extraction of lithium.

[0198] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer portion 20a preferably has a higher cobalt concentration than magnesium. For example, when the surface of the positive electrode active material is measured by XPS, the ratio of the number of magnesium atoms Mg to the number of cobalt atoms Co (Mg / Co) is preferably 0.62 or less. The surface layer portion 20a preferably has a higher cobalt concentration than nickel. The surface layer portion 20a preferably has a higher cobalt concentration than aluminum. The surface layer portion 20a preferably has a higher cobalt concentration than fluorine.

[0199] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 20 a. For example, when measured from the surface of the positive electrode active material particle 20 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0200] Furthermore, although it is preferable that some of the added elements, particularly magnesium and nickel, have a higher concentration in the surface layer 20a than in the interior 20b, it is also preferable that they are present randomly and dilutely in the interior 20b. When magnesium and nickel are present at appropriate concentrations at the lithium sites in the interior 20b, 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 20b, it is possible to suppress the deviation of the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0201] The additive element indicates the detected amount of characteristic X-rays (sometimes simply referred to as the detected amount) that can be determined not to be noise in the STEM-EDX line analysis from the viewpoints of intensity, spatial resolution, etc. Furthermore, the state in which the detected amount is obtained continuously is called the distribution of the additive element. Furthermore, it is preferable that the distribution has a maximum value of the detected amount, that is, a peak of the detected amount. It is also possible that the distribution has multiple such peaks.

[0202] In the case where the additive element is magnesium, fluorine, nickel, or titanium, the peak of the detectable amount is preferably located in the surface layer portion 20a. In other words, the peak of the detectable amount of each of magnesium, fluorine, nickel, and titanium is preferably located 20 nm or less from the surface. The additive element may have multiple peaks of the detectable amount, but it is preferable that each of magnesium, fluorine, nickel, and titanium has the maximum value of the peak (maximum peak) in the surface layer portion 20a or 20 nm or less from the surface.

[0203] [Distribution] Desirable distributions of the additive elements magnesium, aluminum, nickel, and titanium are shown in Figures 5B to 5E, respectively. The distance on the horizontal axis in Figures 5B to 5E roughly corresponds to X1 to X2 of the positive electrode active material particle 20 shown in Figure 5A.

[0204] As shown in Figures 5B to 5E, magnesium is preferably distributed so that the concentration in the surface layer 20a is higher than the concentration in the interior 20b. As shown in Figures 5C and 5E, titanium is preferably distributed so that the concentration in the surface layer 20a is higher than the concentration in the interior 20b. When the concentration in the surface layer 20a is higher than the concentration in the interior 20b, the detected amount in the surface layer 20a is greater than the detected amount in the interior 20b.

[0205] As shown in Figures 5C and 5E, when the position where the magnesium distribution begins (referred to as the rising position) overlaps with the position where the titanium distribution begins, and when the position where the magnesium distribution ends (referred to as the falling position) overlaps with the position where the titanium distribution begins, it is said that the distributions overlap. In other words, it is preferable for the distributions of magnesium and titanium to overlap. Furthermore, when there is a position where the titanium distribution begins and ... magnesium distribution begins and a position where the titanium distribution ends are both offset, this also includes a state in which the distributions overlap.

[0206] 5B to 5E, the magnesium peak, typically the maximum peak, is preferably present in the surface layer 20a, and more preferably in a region closer to the surface of the surface layer 20a. For example, the magnesium concentration peak is preferably present on the surface or within 3 nm from the reference point.

[0207] As shown in Figures 5C and 5E, the titanium peak, typically the maximum peak, is also preferably present in the surface layer portion 20a, and more preferably in a region closer to the surface of the surface layer portion 20a, and even more preferably in a region closer to the surface of the surface layer portion 20a.

[0208] 5C and 5E, the magnesium peak position and the titanium peak position may overlap. Furthermore, if the magnesium peak position and the titanium peak position are different, the difference in peak position should be within 3 nm, preferably within 1 nm. If the full width at half maximum of the distribution based on the peak position can be calculated, it is preferable that the full width at half maximum of the titanium distribution be narrower than that of the magnesium distribution.

[0209] 5D and 5E, the nickel distribution preferably overlaps with the magnesium distribution and the titanium distribution. Alternatively, the nickel distribution preferably has an overlapping region with the magnesium distribution and the titanium distribution. The nickel peak position is preferably located in the surface layer 20a, and more preferably in a region of the surface layer 20a closer to the surface. For example, the nickel concentration peak position is preferably located on the surface or within 3 nm from the reference point. Furthermore, if the full width at half maximum based on the peak position can be determined, it is preferable that the full width at half maximum of the nickel distribution be narrower than that of the magnesium distribution.

[0210] 5E, the distribution of magnesium, nickel, and titanium is preferably present in the edge region of the surface layer 20a. On the other hand, the above distribution is not necessarily required in the basal region of the surface layer 20a.

[0211] Although it has been described that the amount of magnesium detected in the inner portion 20b is small compared to the surface layer portion 20a, it is preferable that magnesium be present in a dilute state in the inner portion 20b. The amount of titanium detected in the inner portion 20b may be very small compared to the surface layer portion 20a, may not be detected, or may be 1 atomic % or less. Furthermore, the amount of nickel detected in the inner portion 20b may be very small compared to the surface layer portion 20a, may not be detected, or may be 1 atomic % or less.

[0212] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer portion 20a is greater than the detectable amount in the interior portion 20b, as with magnesium. It is also preferable that the detectable amount peak in the surface layer portion 20a is closer to the surface. For example, it is preferable that the detectable amount peak is at the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of silicon, phosphorus, boron, and / or calcium is greater than the detectable amount in the interior portion 20b. It is also preferable that the detectable amount peak is at the surface or within 3 nm from the reference point.

[0213] Furthermore, it is preferable that the peak position of the detected amount of aluminum is located inside the distribution of magnesium or titanium, as shown in Figures 5B to 5E. In other words, it is preferable that at least aluminum, among the added elements, has a peak of detected amount inside magnesium or titanium. The distribution of magnesium or titanium may have an overlapping region with the distribution of aluminum, but the overlapping region may be almost nonexistent. The peak of detected amount of aluminum may be present in the surface layer portion 20a, or may be deeper than the surface layer portion 20a. For example, it is preferable that the peak position is located in a region of 5 nm to 30 nm from the surface or the reference point toward the interior.

[0214] The reason why aluminum is distributed deeper than magnesium or titanium is that the diffusion rate of aluminum is faster than that of magnesium, etc. On the other hand, the amount of aluminum detected in the region closest to the surface is low, presumably because aluminum exists more stably in regions where magnesium, etc. are not present than in regions where magnesium, etc. are present in solid solution at high concentrations.

[0215] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋From Co ２＋ However, since Al can only be trivalent, it is difficult for it to exist stably near magnesium in rock salt or layered rock salt structures.

[0216] The distribution of the additive elements in the basal region may differ from the distributions shown in FIGS. 5B to 5E . For example, the basal region and the surface layer portion 20a having the basal region may have a lower detectable amount of one or more selected from the additive elements compared to the edge region and the surface layer portion 20a having the basal region. Specifically, the detectable amount of one or more of magnesium, nickel, and titanium may be low. Alternatively, the basal region and the surface layer portion 20a having the basal region may have no detectable amount of one or more selected from the additive elements, or the detectable amount may be 1 atomic % or less. Specifically, the detectable amount of nickel may be no detectable amount of nickel, or 1 atomic % or less. In particular, with an analysis method that detects characteristic X-rays, such as EDX, the Kβ of cobalt and the Kα of nickel are close in energy, making it difficult to detect trace amounts of nickel in a material in which cobalt is the main element. Alternatively, the basal region and the surface layer portion 20a having the basal region may have a peak of detectable amount of one or more selected from the additive elements that is shallower from the surface compared to the edge region and the surface layer portion 20a having the edge region. Specifically, the peaks of the detected amounts of magnesium and aluminum may be shallower than the edge region and the surface layer portion 20a having the edge region.

[0217] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ It can be said that the structure is one in which the CoO layer and the lithium layer are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane. ２ Since the layer is relatively stable, it is more stable for the surface of the positive electrode active material particle 20 to have a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0218] On the other hand, the diffusion paths of lithium ions are exposed on surfaces other than the (001) orientation. Therefore, the surfaces other than the (001) orientation and the surface layer portion 20a are important regions for maintaining the diffusion paths 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 surfaces other than the (001) orientation and the surface layer portion 20a is extremely important for maintaining the crystal structure of the entire positive electrode active material particle 20. Therefore, in the positive electrode active material particle 20, it is preferable that the distribution of the additive element on the surfaces other than the (001) orientation and the surface layer portion 20a thereof is, for example, a distribution such as that shown in any one of Figures 5B to 5E.

[0219] High-purity LiCoO with low impurity concentration ２ In the manufacturing method in which the additive element is mixed and heated after the preparation of the silicon nitride film, the additive element spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element in the surface other than the (001) orientation and in the surface layer portion 20 a thereof can be easily controlled to a preferred range.

[0220] The additive element does not necessarily have to contain one selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0221] For example, positive electrode active material particles 20 that are substantially free of manganese have the advantages of being relatively easy to synthesize and handle, and having excellent cycle characteristics. Therefore, the weight of manganese contained in positive electrode active material particles 20 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0222] <Grain Boundary> In addition to the distribution described above, it is more preferable that at least a portion of the additive element contained in the positive electrode active material particles 20 is unevenly distributed in and near the grain boundaries. In this specification, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions. This is synonymous with segregation, precipitation, non-uniformity, bias, or the presence of a mixture of high-concentration and low-concentration regions.

[0223] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material particles 20 is higher than that in the interior 20b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in the interior 20b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in the interior 20b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in the interior 20b. It is also preferable that the titanium concentration at and near the grain boundaries is higher than that in the interior 20b.

[0224] Grain boundaries are a type of planar defect. Therefore, like particle surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added elements at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0225] 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 the positive electrode active material particles 20, subsequent heating or the like increases the magnesium concentration and fluorine concentration near the cracked surface. Therefore, even after cracks have occurred in the positive electrode active material, the corrosion resistance to hydrogen fluoride can be improved. Furthermore, even after cracks have occurred in the positive electrode active material, side reactions between the electrolyte and the positive electrode active material can be suppressed.

[0226] <Particle diameter> If the particle diameter of the positive electrode active material particles 20 is too large, problems arise such as difficulty in diffusing lithium, and the surface of the active material layer becomes too rough when coated on a current collector. On the other hand, if the particle diameter is too small, problems arise such as excessive reaction with the electrolyte solution.

[0227] The particle diameter of the positive electrode active material particles 20 can be measured, for example, by a laser diffraction particle size distribution analyzer. The particle diameter of the positive electrode active material measured by the laser diffraction particle size distribution analyzer is preferably 1 μm or more and 100 μm or less, and more preferably, particles less than 10 μm and particles 10 μm or more and 50 μm or more are mixed.

[0228] Furthermore, as in embodiment 1, when particles having different particle diameters or median diameters (D50) are mixed and used in the positive electrode, the electrode density can be increased, and a secondary battery with high energy density can be obtained, which is preferable. Positive electrode active material particles 20 having a relatively small particle diameter or median diameter (D50) are expected to have high charge / discharge rate characteristics. Positive electrode active material particles 20 having a relatively large particle diameter or median diameter (D50) are expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0229] The interior 20b preferably has a low density of defects, including dislocations. Dislocations in the interior 20b can be observed, for example, by TEM. If the density of defects, including dislocations, is sufficiently low, they may not be observed within a specific 1 μm square of the observation sample. Note that dislocations are a type of crystal defect and are different from vacancy defects.

[0230] The positive electrode active material preferably has a large crystallite size measured by XRD. In other words, the inner portion 20b preferably has high crystallinity. The larger the crystallite size, the greater the amount of Li, as will be described later. ｘ CoO ２ When x is small, the O3'-type crystal structure is easily maintained and the contraction of the c-axis length is easily suppressed. Furthermore, the fewer defects including dislocations observed by TEM, the larger the crystallite size measured by XRD, which is preferable.

[0231] The crystallite size is calculated using, for example, a Bruker D8 ADVANCE with a CuKα X-ray source. １ The diffraction pattern obtained using a LYNXEYE XE-T detector with a 2θ angle of 15° to 90° increments of 0.005 can be used, along with ICSD Coll. Code 172909 as the literature value for lithium cobalt oxide. Analysis can be performed using DIFFRAC. TOPAS ver. 6 crystal structure analysis software, and it is preferable to use the L Vol -IB value as the crystallite size. Note that if the calculated Preferred Orientation is less than 0.8, the sample may be too strongly oriented and therefore not suitable for determining the crystallite size.

[0232] XRD measurements for calculating the crystallite size are preferably performed on the positive electrode active material alone, but may also be performed on the positive electrode, which includes the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to pressure and other factors during the manufacturing process. Strong orientation can prevent accurate calculation of the crystallite size. Therefore, it is more preferable to obtain the sample by removing the positive electrode active material layer from the positive electrode, removing some of the binder and other materials in the positive electrode active material layer using a solvent, and then loading the sample into a sample holder. Alternatively, for powder samples, grease can be applied to a silicon non-reflective plate and the sample attached to it.

[0233] <Crystalline Structure> Due to the distribution of the added elements as described above, it is preferable that the crystalline structure continuously changes from the interior 20b toward the surface, or that the crystal orientation of the surface layer 20a and the interior 20b roughly coincide.

[0234] It is preferable that the crystal structure continuously changes from the layered rock salt type interior 20b toward the surface and surface layer portion 20a 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 portion 20a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure and the layered rock salt type interior 20b are approximately the same.

[0235] In this specification, a layered rock-salt crystal structure belonging to the space group R-3m refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which cobalt and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as vacancies of cations or anions may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted structure of the rock-salt crystal lattice, and may have lower symmetry than the rock-salt crystal structure.

[0236] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0237] The presence of both the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0238] In the rock salt crystal structure, there is no distinction in the cation sites, but in the layered rock salt crystal structure, there are two types of cation sites, one of which is mostly occupied by lithium and the other by cobalt. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same in both the rock salt crystal structure and the layered rock salt crystal structure. 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 an ideal rock salt crystal structure, and, for example, the (003) plane in a layered rock salt crystal structure. For example, MgO, which has a rock salt crystal structure, and LiCoO, which has a layered rock salt crystal structure, ２ When comparing the electron diffraction patterns of LiCoO ２ The distance between the bright spots on the (003) plane of MgO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２ When the two phases are present, 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 crystal structure and the layered rock salt type crystal structure have strong brightness, while bright spots occurring only in the layered rock salt type crystal structure have weak brightness.

[0239] 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. Rock-salt crystal structures do not exhibit this characteristic because there is no distinction between cation sites. When a material has both a rock-salt crystal structure and a layered rock-salt crystal structure, 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 greater than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0240] The layered rock salt crystal structure and the anions in the rock salt crystal structure have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type crystal structure and the monoclinic O1(15)-type crystal structure, which will be described later, also have a cubic close-packed structure. Therefore, when the layered rock salt crystal structure and the rock salt crystal structure come into contact, there are crystal planes where the cubic close-packed structures formed by the anions are aligned.

[0241] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. The layered rock salt crystal structure is in the space group R-3m and is a rhombohedral structure, but is generally expressed as a compound hexagonal lattice to make the structure easier to understand, and the (0001) plane of the layered rock salt crystal structure 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 crystal structure. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0242] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (the space group of a general rock salt type crystal structure). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal structure and the O3' type crystal structure and the rock salt type crystal structure. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure are aligned, it may be said that the crystal orientations are approximately the same. Furthermore, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0243] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM, high-angle scattering annular dark-field scanning transmission electron microscope) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope, annular bright-field scanning transmission electron microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for determination.

[0244] 6 shows an example of a TEM image in which the orientation of the layered rock salt-type crystal structure LRS and the rock salt-type crystal structure RS roughly coincides. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0245] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a composite hexagonal lattice of a layered rock-salt crystal structure, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in FIG. 6) are not clearly distinguishable from each other. ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0246] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atom positions, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate having a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0247] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0248] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0249] Figure 7A shows an example of an STEM image in which the orientations of the layered rock-salt-type crystal structure LRS and the rock-salt-type crystal structure RS are roughly the same. Figure 7B shows the FFT pattern of the region of the rock-salt-type crystal structure RS, and Figure 7C shows the FFT pattern of the region of the layered rock-salt-type crystal structure LRS. The left side of Figures 7B and 7C shows the composition, JCPDS card number, and the d value and angle calculated from the JCPDS card number. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0250] The spot marked A in Figure 7B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 7C is derived from the 0003 reflection of the layered rock salt type. From Figures 7B and 7C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type crystal structure are approximately consistent. In other words, it can be seen that the line passing through AO in Figure 7B is approximately parallel to the line passing through AO in Figure 7C. Here, "approximately consistent" and "approximately parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0251] In this way, in the FFT pattern and the electron diffraction pattern, when the orientations of the layered rock salt type crystal structure and the rock salt type crystal structure roughly coincide, the <0003> orientation of the layered rock salt type crystal structure and the <11-1> orientation of the rock salt type crystal structure may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point that is spot-like and not continuous with other reciprocal lattice points indicates high crystallinity.

[0252] Furthermore, as described above, when the orientation of the 11-1 reflection of a cubic crystal and the orientation of the 0003 reflection of a layered rock-salt crystal structure are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock-salt crystal structure may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock-salt crystal structure. For example, the spot marked B in FIG. 7C is originating from the 1014 reflection of the layered rock-salt crystal structure. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point (A in FIG. 7C) originating from the 0003 reflection of the layered rock-salt crystal structure, and at a location with a d value of 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, reciprocal lattice points equivalent to 0003 and 1014 may also be used.

[0253] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 7B is originating from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 7B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, reciprocal lattice points equivalent to the 11-1 and 200 reflections of the cubic crystal may also be used.

[0254] It is known that positive electrode active materials with a layered rock salt crystal structure, such as lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, when observing the (0003) plane using a TEM or the like, it is preferable to first select positive electrode active material particles whose shape is observed using a SEM or the like to reveal a crystal plane expected to be the (0003) plane, and then thin-section the positive electrode active material particles using a FIB (Focused Ion Beam) or the like so that the (0003) plane can be observed in a TEM or the like with an electron beam incident in the [12-10] direction. When determining whether the crystal orientation is consistent, it is preferable to thin-section the particles so that the (0003) plane of the layered rock salt crystal structure can be easily observed.

[0255] <Change in Crystal Structure> Using Figs. 8 to 12, ｘ CoO ２ The change in the crystal structure accompanying the change in x in the formula (I) will be described. Specifically, the description will be made by comparing a conventional positive electrode active material shown in FIG. 9 with a positive electrode active material according to one embodiment of the present invention shown in FIG. 8. Note that the conventional positive electrode active material is lithium cobalt oxide that does not particularly contain any additional element.

[0256] <Li ｘ CoO ２ When x is 1 in the positive electrode active material of one embodiment of the present invention shown in FIG. ｘ CoO ２ In the case where x=1 in the graph, the positive electrode active material has a layered rock salt type crystal structure belonging to the space group R-3m. ｘ CoO２ In the case where x=1, the crystal structure has a layered rock salt type belonging to the space group R-3m. ｘ CoO ２ The layered rock salt type crystal structure when x = 1 in the formula is designated as R-3m O3. R-3m O3 has a lattice constant of a = 2.81610 × 10 −１０ (m), b=2.81610×10 −１０ (m), c=14.05360×10 −１０ (m), α = 90.0000, β = 90.0000, γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), O(0,0,0.23951). In FIGS. 8 and 9, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may also exist disproportionately at some of the lithium sites. The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0257] A positive electrode active material having a layered rock salt crystal structure has a high discharge capacity, has two-dimensional lithium ion diffusion paths, and is suitable for lithium ion insertion / extraction reactions. Therefore, it is particularly preferable that the inner portion 20b, which occupies most of the volume of the positive electrode active material, has a layered rock salt crystal structure.

[0258] <Li ｘ CoO ２ The positive electrode active material of one embodiment of the present invention shown in FIG. 8 contains the above-described additional element, and therefore, ｘ CoO ２ The crystal structure when x is small differs from that of the conventional positive electrode active material shown in Fig. 9. Here, "small x" means that 0.1<x≦0.24.

[0259] First, it is known that when x = 0.5, the symmetry of lithium increases in the conventional lithium cobalt oxide shown in Figure 9, and the crystal structure belongs to the monoclinic space group P2 / m. This structure has CoO in the unit cell. ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0260] The conventional lithium cobalt oxide shown in FIG. 9 has a crystal structure of the trigonal space group P-3m1 when x = 0, and also contains CoO in the unit cell. 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.

[0261] The conventional lithium cobalt oxide shown in Figure 9 has a crystal structure of space group R-3m when x = 0.12. 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. Therefore, this crystal structure is sometimes referred to as an H1-3 crystal structure. In a unit cell of an H1-3 crystal structure, the number of cobalt atoms per unit cell is twice that of other structures. However, in Figure 9 and other parts of this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half that of the unit cell. Furthermore, since actual lithium insertion and desorption does not necessarily occur uniformly within the positive electrode active material and the lithium concentration can become uneven, an H1-3 crystal structure is experimentally observed from approximately x = 0.25.

[0262] 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), where O1 and O2 are oxygen atoms.

[0263] The unit cell that 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, the unit cell that results in the smallest goodness of fit (GOF) value is adopted.

[0264] Li ｘ CoO ２When charging and discharging are repeated so that x in the formula is 0.24 or less, the conventional lithium cobalt oxide shown in Figure 9 repeatedly changes its crystal structure (i.e., undergoes a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state. However, these two crystal structures are easily separated by the CoO ２ The layer displacement is large. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0265] Furthermore, there is a large difference in volume between these two crystal structures. The specific volume difference will be described later.

[0266] 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.

[0267] 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 for lithium to be inserted and extracted.

[0268] On the other hand, in the positive electrode active material of 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 change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 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 of one embodiment of the present invention is ｘ CoO ２ When x is 0.24 or less, a more stable crystal structure than that of conventional positive electrode active materials can be obtained, which is preferable as it further improves the safety of the secondary battery.

[0269] In the positive electrode active material of one embodiment of the present invention shown in FIG. ｘ CoO ２ The graph shows the crystal structures when x is 1, approximately 0.2, and approximately 0.15. When x = 1, the positive electrode active material particles 20 have the same R-3m O3 crystal structure as conventional lithium cobalt oxide. However, when x is 0.24 or less, for example, approximately 0.2 or 0.15, at which point conventional lithium cobalt oxide has an H1-3 type crystal structure, the positive electrode active material particles 20 have a different crystal structure.

[0270] In the positive electrode active material of one embodiment of the present invention shown in FIG. 8, lithium cobalt oxide has a crystal structure belonging to the trigonal space group R-3m when x is about 0.2. ２ The symmetry of the layers 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 8 with the notation R-3m O3'.

[0271] 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. Furthermore, the lattice constant of the unit cell is preferably 2.797≦a≦2.837 (Å), more preferably 2.807≦a≦2.827 (Å), typically a=2.817 (Å). The c-axis is preferably 13.681≦c≦13.881 (Å), more preferably 13.751≦c≦13.811 (Å), typically c=13.781 (Å).

[0272] In the positive electrode active material of one embodiment of the present invention shown in FIG. 8, when x is about 0.15, lithium cobalt oxide may have a crystal structure belonging to the monoclinic space group P2 / m. This is because CoO ２ There is one layer. In addition, the lithium present in the positive electrode active material particles 20 at this time is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 8 with the P2 / m monoclinic O1(15) symbol.

[0273] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constants of the unit cell are a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α = 90°, β = 109.6 ± 0.1°, and γ = 90°.

[0274] This crystal structure can also show the lattice constant in the space group R-3m if some error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constants of the unit cell are a = 2.817 ± 0.02 Å and c = 13.68 ± 0.1 Å.

[0275] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy hexacoordinated oxygen sites, although light elements such as lithium and magnesium may occupy tetracoordinated oxygen sites.

[0276] As shown by the dotted line in FIG. 8, the R-3m O3 in the discharged state, the O3'-type crystal structure, and the monoclinic O1(15)-type crystal structure show a difference in the CoO ２ There is almost no layer misalignment.

[0277] The table below shows the difference in volume per cobalt atom between discharged R-3m O3, the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, the H1-3-type, and the trigonal O1-type. The lattice constants of each crystal structure used in the calculation can be found in the literature for discharged R-3m O3 and trigonal O1 (ICSD coll.code.172909 and 88721). The O3'-type crystal structure and the monoclinic O1(15)-type crystal structure can be calculated from experimental XRD values.

[0278]

[0279] Using the volume per cobalt atom in the table above, the difference in volume between the H1-3 crystal structure and the R-3mO3 crystal structure in a discharged state can be calculated when comparing the volumes per cobalt atom. As shown in the volume change rate (%) in the table above, when the denominator is the R-3mO3 crystal structure in a discharged state, the difference in volume is 3.9%. This means that the difference in volume exceeds 3.5%, which is extremely large.

[0280] Using the volume per cobalt atom in the table above, the difference in volume between R-3m O3 in a discharged state and the O3'-type crystal structure can be calculated by comparing the volumes per the same number of cobalt atoms. As shown in the volume change rate (%) in the table above, when the denominator is the R-3m O3-type crystal structure in a discharged state, the difference in volume is 1.8%. In other words, the difference in volume is 2.5% or less, more specifically, 2.2% or less, which is very small.

[0281] Using the volume per cobalt atom in the table above, the difference in volume between R-3m O3 in a discharged state and the monoclinic O1(15) crystal structure can be calculated by comparing the volumes per cobalt atom. As shown in the volume change rate (%) in the table above, when the denominator is the R-3m O3 crystal structure in a discharged state, the difference in volume is 2.5%. In other words, the difference in volume is 3.3% or less, more specifically, 3.0% or less, which is very small.

[0282] As described above, in the positive electrode active material of 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 positive electrode active material is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material has a large discharge capacity per weight and per volume. Therefore, by using this positive electrode active material, secondary batteries with high discharge capacity per weight and per volume can be fabricated.

[0283] Note that the positive electrode active material of one embodiment of the present invention 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 even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ CoO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li 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.

[0284] Therefore, the positive electrode active material of one embodiment of the present invention is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, in the positive electrode active material of one embodiment of the present invention, not all of the particles in the interior 20b may have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or a portion may be amorphous.

[0285] 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 ２ In other words, a state where x is small can be referred to as a state where the material is charged at a high charging voltage. In other words, the positive electrode active material of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25°C. For example, when constant-current charging and constant-voltage charging (CCCV charging) are performed 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 referred to as a high charging voltage. Unless otherwise specified, charging voltages are expressed relative to the potential of lithium metal.

[0286] 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.

[0287] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x is small, whether the positive electrode active material of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure can be determined 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 diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0288] XRD is particularly 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, and it can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery as is.

[0289] When analyzing the crystallite size by powder XRD, it is preferable to measure the crystallite size while excluding the influence of orientation 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 sample.

[0290] Furthermore, even in the positive electrode active material of one embodiment of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, an H1-3 type or trigonal O1 type crystal structure may be formed. However, a positive electrode active material with a small x may undergo a change in crystal structure when exposed to air. For example, the O3' type and monoclinic O1(15) type crystal structures may change 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.

[0291] <<Coin Cell to be Charging>> Charging to determine whether a certain composite oxide is a positive electrode active material of one embodiment of the present invention can be performed by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging it. It is also possible to fabricate a coin cell using a positive electrode removed from a secondary battery. In other words, the positive electrode is prepared by coating a slurry containing a positive electrode active material, a conductive material, and a binder on an aluminum foil positive electrode current collector.

[0292] 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.

[0293] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte solution is 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) mixed with the electrolyte dissolved in the electrolyte solution.

[0294] The separator may be a 25 μm thick porous polypropylene film.

[0295] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0296] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 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 at a constant current / constant voltage (CCCV), 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 a current of 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material. To observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is considered that the current is being consumed for decomposition of the electrolyte rather than for charging the positive electrode active material. Therefore, CV charging may be terminated after a sufficient time has elapsed since the start of charging. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. 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 various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to quickly remove the positive electrode and subject it to analysis after charging is complete. Specifically, it is preferable to perform the analysis within one hour after charging is complete, and more preferably within 30 minutes.

[0297] <XRD> The XRD device and conditions are not particularly limited. The software for analyzing the crystal structure is also not particularly limited. It is preferable to use a highly accurate device, conditions, and corresponding software, as this allows for more accurate crystal structure analysis. For example, the following device, conditions, and analysis software can be used for measurement and analysis. XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm The obtained XRD pattern was analyzed using analysis software such as DIFFRAC.EVA to separate the background and CuKα ２ The peaks of the lines can be removed.

[0298] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ The ideal powder XRD patterns of the lines are shown in Figures 10, 11, 12A and 12B. ｘ CoO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 12A and 12B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 12A showing an enlarged view of the region where 2θ is in the range of 18° to 21°, and Figure 12B showing an enlarged view of the region where 2θ is in the range of 42° to 46°. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure DataBase). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０ The O3'-type and monoclinic O1(15)-type crystal structure patterns were estimated from the XRD pattern of the positive electrode active material, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker Corporation), and XRD patterns were created in the same manner as for the others.

[0299] As shown in FIGS. 10, 12A and 12B, in the O3' type crystal structure, diffraction peaks appear in the 2θ range of 19.13° or more and less than 19.37°, and in the 2θ range of 45.37° or more and less than 45.57°.

[0300] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear in the 2θ range of 19.37° or more and 19.57° or less, and in the 2θ range of 45.57° or more and 45.67° or less.

[0301] However, as shown in Figures 11, 12A and 12B, no diffraction peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at angles greater than or equal to 19.13° and less than 19.37° and / or greater than or equal to 19.37° and less than 19.57°, and greater than or equal to 45.37° and less than 45.57° and / or greater than or equal to 45.57° and less than 45.67° when x is small can be said to be a characteristic of the positive electrode active material of one embodiment of the present invention.

[0302] This can also be said to be because, in the positive electrode active material of one embodiment of the present invention, the positions at which XRD diffraction peaks appear are close between the crystal structures where x = 1 and where x ≦ 0.24. More specifically, the difference in 2θ between the main diffraction peaks of the crystal structures where x = 1 and where x ≦ 0.24 appears at a 2θ angle of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0303] When Rietveld analysis is performed on the XRD pattern of the positive electrode active material of one embodiment of the present invention, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. When the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0304] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably 50% or less, or preferably 34% or less, or more preferably substantially not observed.

[0305] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0306] 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 full width at half maximum is narrow. For example, a narrow full width at half maximum is preferable. Even for diffraction peaks arising from the same crystalline phase, the full width at half maximum varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for diffraction peaks observed at 2θ = 43° or more and 46° or less, the full width at half maximum is preferably, for example, 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all diffraction peaks necessarily meet this requirement. If some diffraction peaks meet this requirement, it can be said that the crystallinity of that crystalline phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

[0307] Furthermore, the crystallite size of the O3′-type and monoclinic O1(15) crystal structures of the positive electrode active material particles 20 of one embodiment of the present invention is approximately equal to that of LiCoO in a discharged state. 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 formula is small, the diffraction peaks of the O3' type and / or monoclinic O1(15) crystal structure can be clearly observed. ２ In this case, even if a portion of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the diffraction peaks will be broad and small. The crystallite size can be determined from the full width at half maximum of the diffraction peak.

[0308] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, 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 the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 20a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, with a lower limit of about 1 atomic %, depending on the element.

[0309] Note that the surface and surface layer 20a of the positive electrode active material particle 20 according to one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material particle 20. They are also assumed to be free of the electrolyte, binder, conductive material, and compounds derived therefrom that are attached to the surface of the positive electrode active material particle 20. 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.

[0310] Furthermore, before subjecting the sample to various analyses, the sample of 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. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0311] The concentration of the added element may also be compared with the cobalt concentration. Using the ratio to cobalt is preferable because it allows comparison while reducing the influence of carbonates and other substances chemisorbed 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 0.4 or more and 1.5 or less. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0312] Similarly, in order to ensure sufficient paths for lithium ion insertion and desorption, the positive electrode active material particles 20 preferably have higher concentrations of lithium and cobalt in the surface layer portion 20a than the respective additive elements. This means that the concentrations of lithium and cobalt in the surface layer portion 20a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 20a as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 20a as measured by XPS or the like is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 20a as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0313] Furthermore, it is more preferable that aluminum be widely distributed in a deep region, for example, on the surface or in a region at a depth of 5 nm to 50 nm from the reference point. Therefore, although aluminum is detected in an analysis of the entire cathode active material particle 20 using ICP-MS, GD-MS, or the like, it is more preferable that the concentration of aluminum is not detected by XPS or the like, or is 1 atomic % or less.

[0314] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 2 to 5 nm (take-off angle 15° to 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0315] Furthermore, when the positive electrode active material of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0316] Furthermore, when the positive electrode active material of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0317] Furthermore, in at least a portion of the surface layer portion 20 a measured by XPS, it is preferable that a bond between magnesium and fluorine is present, and it is more preferable that a bond between magnesium and fluorine and oxygen (O-Mg-F bond) is present rather than a bond between magnesium and fluorine (Mg-F bond). The bond between magnesium and fluorine in the surface layer portion 20 a is one factor indicating that the flux effect of the fluoride is fully exerted, the surfaces of the magnesium and lithium cobalt oxide are melted, and the magnesium is sufficiently segregated.

[0318] The excitation X-rays used in XPS measurements are monochromated Al, and the detection area can be 100 μmφ. The take-off angle (the angle between the tilt of the sample stage and the detection direction of the detector) can be 45° or 15°. The detection depth is approximately 4 to 5 nm when the take-off angle is 45°, and approximately 2 nm when the take-off angle is 15°.

[0319] The presence of an O-Mg-F bond in the XPS spectrum of Mg1s, which is one of the magnesium bonding states, means that when the XPS spectrum is waveform separated, the bond energy of the O-Mg-F bond is different from that of the MgO bond and the MgF bond. ２ The fitting is performed assuming that the bond is in the middle of the bond.

[0320] It is preferable that O-Mg-F bonds are confirmed at both take-off angles of 45° and 15°. Furthermore, if there is a difference in the peak values ​​of O-Mg-F bonds between take-off angles of 45° and 15°, it can be said that the proportion of O-Mg-F bonds present in the XPS detection region is different. For example, when the take-off angle of 15° is larger than 45°, it can be said that O-Mg-F bonds are mainly present in a very shallow region up to 2 nm from the surface.

[0321] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material particles 20 have a concentration gradient. Furthermore, it is more preferable that the position of the maximum peak of the detected amount of the additive element in the positive electrode active material particles 20 differs from a reference point, typically a depth from the surface, of the positive electrode active material particles 20. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material particles 20 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.

[0322] 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.

[0323] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 20a, the interior 20b, and near the grain boundaries of the positive electrode active material particle 20. Furthermore, EDX ray analysis can analyze the distribution of the added element and its maximum concentration. Furthermore, analysis that thins the 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 the distribution in the depth direction.

[0324] Since the positive electrode active material particles 20 are compounds 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 multi-layer film of carbon, metal, oxide, resin, etc.

[0325] In STEM-EDX analysis, the graph of the detected amount of characteristic X-rays of the element does not change sharply in principle or due to measurement errors, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX analysis, the transition metal M is considered to be the average value M of the detected amount inside. ＡＶＥ and the average background value M ＢＧ The point where the oxygen concentration is 50% of the sum of the two values, and the oxygen concentration is the average value O ＡＶＥ and the average background value O ＢＧ The reference point is the point where the sum of the internal and background is 50%. If the transition metal M and oxygen differ in the 50% point, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface. Therefore, the average value M of the detected amount inside the transition metal M is used. ＡＶＥ and the average background value M ＢＧ In the case of a positive electrode active material having a plurality of transition metals M, the M of the element with the largest count in the inner portion 20b can be used. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0326] The average value M of the background of the transition metal M ＢＧ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of transition metal M starts to increase. ＡＶＥcan be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the counts of the transition metal M and oxygen are saturated and stable, for example, the region where the detected amount of the transition metal M starts to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0327] Furthermore, the surface of the positive electrode active material particle 20 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where it is not observed, and is the outermost 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 observed. The surface in an STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0328] Furthermore, a peak in STEM-EDX-ray analysis refers to the maximum value (the apex of a convex shape) of characteristic X-rays that appears in a graph of the characteristic X-ray intensity for each element, and when multiple peaks are confirmed, refers to the maximum value of the characteristic X-rays. Note that, as noise in STEM-EDX-ray analysis, it is preferable to use a measured value of a full width at half maximum that is equal to or less than the spatial resolution (R), for example, R / 2 or less.

[0329] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured over six scans can be used as the detected amount of each element. The number of scans is not limited to six, and more scans can be performed, and the average can be used as the detected amount of each element.

[0330] A sample to be subjected to STEM-EDX ray analysis is prepared, 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 apparatus (MC1000 manufactured by Hitachi High-Technologies).

[0331] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-sectional sample. For example, the slice processing can be performed using an FIB-SEM (XVision 200TBS manufactured by Hitachi High-Technologies). In this case, pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0332] 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. In order to reduce the beam diameter, it is preferable to perform aberration correction. Specifically, an aberration correction mechanism (Cs-collector) is arranged in the optical system.

[0333] 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. During EDX ray analysis, the acceleration voltage of the STEM device is set to 200 kV, and the emission current is set to 6 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. The magnification is, for example, about 150,000 times.

[0334] <Coating Portion> A coating portion may be attached to at least a portion of the surface of the positive electrode active material particle 20. Fig. 13 shows a cross-sectional view of a positive electrode active material particle 20 to which a coating portion 24 is attached.

[0335] The coating portion 24 is preferably formed by the accumulation of decomposition products of the electrolyte and the electrolytic solution during charging and discharging. ｘ CoO ２When repeated charging is performed such that x in the formula is 0.24 or less, the presence of a coating portion derived from the electrolyte on the surface of the positive electrode active material particle 20 is expected to improve charge-discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the positive electrode active material surface or suppressing cobalt elution. The coating portion 24 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as the electrolyte, a high-quality coating portion is easily obtained. Therefore, a coating portion 24 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferred. Furthermore, the coating portion 24 does not have to cover the entire positive electrode active material particle 20. For example, it is sufficient for the coating portion 24 to cover 50% or more of the surface of the positive electrode active material particle 20, with 70% or more being more preferable and 90% or more being even more preferable.

[0336] <<DCIR (Direct Current Internal Resistance)>> In the above <<Charging Method>>, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) is prepared with a lithium counter electrode and charged as a charging method for determining whether a certain composite oxide is a positive electrode active material of one embodiment of the present invention. Using a coin cell prepared in a similar manner, it is possible to measure the low resistance DCIR, which is one of the characteristics of the positive electrode active material of one embodiment of the present invention.

[0337] A coin cell prepared under the same conditions as in the above charging method is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.60 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 can be performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be completed at 2 mA / g or more and 10 mA / g or less.

[0338] The charged coin cell is placed in a state where neither charging nor discharging is performed. This is called a rest period, and the rest period is preferably 5 minutes or more and 30 minutes or less. Then, the resistance (DCIR) is measured. The resistance (DCIR) is measured by discharging for 1 second at a current of 2 mA / g or more and 10 mA / g or less, typically 6 mA / g, per weight of the positive electrode active material. The resistance (DCIR) can be calculated by dividing the difference between the open circuit voltage before discharge and the voltage during discharge by the current (e.g., 6 mA / g).

[0339] When the open circuit voltage before discharge (after charge) is 4.50 V or more and 4.60 V or less, the resistance DCIR of a coin cell having the positive electrode active material of one embodiment of the present invention is preferably 40 Ω or less, and more preferably 25 Ω or less. Note that a series of measurements of the resistance DCIR may be performed in the same temperature environment as the charge-discharge cycle test or the rate-specific discharge capacity test, for example, in a 25°C environment or a 45°C environment.

[0340] <<Powder Resistivity Measurement>> The positive electrode active material particles 20 according to one embodiment of the present invention have 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 particles 20 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 is small, it has an O3' type and / or monoclinic O1(15) type crystal structure.

[0341] A feature of the positive electrode active material particles 20 according to one aspect of the present invention is that the volume resistivity of the powder of the positive electrode active material particles 20 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ It is preferable that the resistance is Ω cm or less, and 5.0 × 10 ８ Ω・cm or more 1.5×10 ９ It is more preferable that the resistivity is Ω·cm or less.

[0342] The positive electrode active material particles 20 having the above volume resistivity have a stable crystal structure even at high voltages, and can be used as an indicator that the surface layer portion 20a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been well formed.

[0343] A method for measuring the volume resistivity of the powder of the positive electrode active material particles 20 according to one embodiment of the present invention will be described.

[0344] The volume resistivity of a powder is preferably measured using an instrument with a resistance measurement terminal and a mechanism for applying pressure to the powder to be measured. The resistance measurement terminal preferably has four terminals (also referred to as four-point probes). For example, the MCP-PD51 manufactured by Mitsubishi Chemical Analytech Co., Ltd. can be used as a measurement device having a resistance measurement terminal and a mechanism for applying pressure to the powder (sample) to be measured. The resistance measurement device can be the Loresta-GP low resistance meter or the Hiresta-GP high resistance meter. The Loresta-GP can be used to measure low resistance samples, and the Hiresta-GP can be used to measure high resistance samples. The measurement environment is preferably a stable environment such as a dry room. A dry room environment, for example, with a temperature of 25°C and a dew point of -40°C or lower, is preferable. When performing measurements in a humid environment, the electrical resistance may decrease due to the influence of moisture in the air, potentially preventing the actual physical property values ​​from being obtained.

[0345] 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.

[0346] 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 16 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.

[0347] When the above-described measurement is performed, the volume resistivity of the powder of the positive electrode active material particles 20 according to one embodiment of the present invention is 1.0×10 when measured under a pressure of 64 MPa. ８ Ω・cm or more 1.0×10 １０ When the capacitance is 5.0×10 Ω cm or less, favorable cycle characteristics are exhibited in a charge-discharge cycle test under high charge voltage conditions. ８ Ω・cm or more 1.5×10 ９ When the electrical resistance is Ω·cm or less, more preferable cycle characteristics are exhibited in a charge / discharge cycle test under high voltage conditions.

[0348] Unless otherwise specified in the present specification, the volume resistivity measured as above is the volume resistivity of the powder.

[0349] <Nail Penetration Test> Next, the nail penetration test will be described. In the nail penetration test, a secondary battery is fully charged and a nail having a predetermined diameter selected from 2 mm to 20 mm is inserted into the secondary battery at a predetermined speed. Full charge refers to a state in which the charge rate, expressed as State Of Charge (hereinafter referred to as SOC), is 100%. In this embodiment, a nail penetration test device will first be described.

[0350] FIG. 14A shows a side view of a nail penetration test device 1000. The nail penetration test device 1000 includes a stage 1001, a drive unit 1002, a nail 1003, a voltage measuring device 1015, a temperature measuring device 1016, and a control unit 1018. The drive unit 1002 includes a drive mechanism 1012 that moves the nail 1003 in the direction of the arrow in the figure, and the drive mechanism 1012 operates to cause the nail 1003 to penetrate a secondary battery 1004 placed on the stage 1001. At this time, the secondary battery 1004 is fully charged, and this operation is called a nail penetration operation. Note that the dashed line shown in FIG. 14A indicates a recess in the stage 1001 that is provided to accommodate the nail 1003 that has penetrated the secondary battery 1004 during the nail penetration operation.

[0351] The voltage measuring device 1015 transmits information about the voltage of the secondary battery during the nail penetration operation to the control unit 1018. Specifically, the amount of voltage change and the like are transmitted to the control unit 1018. Furthermore, the temperature measuring device 1016 transmits information about the temperature during the nail penetration operation to the control unit 1018. When controlling the operating conditions of the nail 1003, the control unit 1018 can transmit a control signal to the drive unit 1002.

[0352] FIG. 14B is a perspective view illustrating the vicinity of the upper portion of the stage 1001 of the nail penetration test device 1000. The secondary battery 1004 placed on the stage 1001 is electrically connected to wires 1005a and 1005b. The wires 1005a and 1005b belong to a voltage measuring device 1015, and the wires 1005a and 1005b are electrically connected to the positive and negative electrode tabs of the secondary battery 1004, respectively, to measure the voltage of the secondary battery 1004. The voltage of the secondary battery 1004 is simply referred to as voltage, voltage value between the positive and negative electrodes, battery voltage, cell voltage, or open-circuit voltage. When a temperature sensor is used as the temperature measuring device 1016, the temperature sensor is disposed so as to contact the surface of the exterior body of the secondary battery 1004.

[0353] 14B shows an example in which a first temperature sensor 1006a and a second temperature sensor 1006b are arranged on the secondary battery 1004, and a third temperature sensor 1006c is further provided on the nail 1003 shown in FIG. 14A, but one or three or more temperature sensors may be arranged on the secondary battery 1004. In the secondary battery 1004, the first temperature sensor 1006a is arranged on the side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is arranged on the side where the wiring 1005a and the wiring 1005b are arranged. Arranging two or more temperature sensors in this manner is preferable because the other temperature sensors can be used even if one temperature sensor becomes unusable due to expansion of the exterior body or the like.

[0354] Furthermore, there is a welded region on the side where the wiring 1005a and the wiring 1005b are arranged, but the exterior body is folded back on the side where the wiring 1005a and the wiring 1005b are not arranged, so there is no adhesive region. Therefore, even if the exterior body expands, the expansion is suppressed on the side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is more unlikely to peel off than the first temperature sensor 1006a, which is preferable.

[0355] The dashed ellipse in FIG. 14B represents the area where the nail 1003 penetrates the secondary battery 1004 during the nail penetration operation. The first temperature sensor 1006a and the second temperature sensor 1006b provided on the secondary battery 1004 are preferably provided equidistant from the area where the nail 1003 penetrates. Typically, the first temperature sensor 1006a and the second temperature sensor 1006b are provided within 5 cm, preferably within 2 cm, of the area where the nail 1003 penetrates. By providing the sensors in this manner, it is possible to grasp temperature changes in the area where the nail 1003 penetrates and its vicinity, which is preferable. Here, "vicinity" refers to an area within 1 cm of the penetration area. When two or more temperature sensors are provided, it is preferable to start the nail penetration operation after confirming that the difference in temperature indicated by the temperature sensors is within ±5°C, preferably within ±2°C.

[0356] To prevent a secondary battery from igniting or from experiencing thermal runaway in a nail penetration test, it is preferable to suppress the temperature rise of the secondary battery and to have stable properties at high temperatures for the negative electrode, positive electrode, and / or electrolyte. An example of a configuration that suppresses the temperature rise is a structure of a positive electrode active material that slows the rate of current flow. Specifically, the rate of current flow can be slowed by a configuration in which an additive element such as magnesium is segregated in the surface layer of the positive electrode active material.

[0357] <Characteristics of Secondary Battery in Nail Penetration Test> When AC impedance measurement is performed on a secondary battery in a fully charged state, it is preferable that the internal resistance has an AC impedance value of 100 mΩ or less, preferably less than 90 mΩ, at a frequency of 1 kHz. A secondary battery having such an internal resistance can be said to be highly safe. When fully charged, the charge capacity of the secondary battery is preferably 2000 mAh or more, preferably 2400 mAh or more. Since a fully charged secondary battery may discharge over time, it is recommended to perform AC impedance measurement within 24 hours, preferably within 12 hours, and more preferably within 6 hours after being fully charged.

[0358] This embodiment can be used in combination with other embodiments.

[0359] Embodiment 3 In this embodiment, a method for manufacturing a positive electrode according to one embodiment of the present invention will be described with reference to Fig. 15 and Fig. 16. In this embodiment, a method for manufacturing a positive electrode using a fibrous conductive material will be described.

[0360] <<Positive Electrode Fabrication Method 1>> <Step S11> In step S11 shown in Fig. 15, a fibrous conductive material 1 is prepared. CNT is used as the fibrous conductive material 1. Furthermore, a drying process may be performed to remove moisture from the CNT. The drying process includes natural drying, and it is preferable to heat the CNT at a temperature of 100°C or higher and 200°C or lower, more preferably 150°C or higher and 200°C or lower. Furthermore, the drying time in this step is preferably 12 hours or longer, and more preferably 24 hours or longer.

[0361] In step S11, an organic solvent 2 and a dispersant 3 are further prepared. A representative example of the organic solvent 2 is NMP. A representative example of the dispersant 3 is PVP. When CNT is used as the fibrous conductive material 1, when CNT:PVP:NMP=x:y:(100-x-y) (weight ratio) (0<(x+y)<100), it is preferable that 0.1<x<5. If the proportion of CNT is too high, the aggregation does not break up even when PVP is added, and a uniform dispersion cannot be obtained. It is also preferable that 0<y<1. If the proportion of PVP is too high, the proportion of the binder will be reduced, which may reduce the binding strength of the electrode.

[0362] <Step S12> In step S12 shown in Figure 15, ultrasonic treatment is performed to disperse the fibrous conductive material 1. When CNTs are used as the fibrous conductive material 1, ultrasonic treatment can peel off the interfaces between aggregated CNTs. A frequency of 20 kHz is preferable as the ultrasonic treatment condition. However, the specific conditions for the ultrasonic treatment are not limited in any way. Adding a dispersant 3 in this step is preferable because it suppresses re-aggregation of the CNTs and allows them to be properly dispersed in the organic solvent 2.

[0363] <Step S13> Through step S12, the dispersion liquid X1 in which the fibrous conductive material 1 is well dispersed can be obtained.

[0364] 15 , a positive electrode active material 21 is prepared. The positive electrode active material particles of one embodiment of the present invention may be used as the positive electrode active material 21. In step S15, a binder 4 and an organic solvent 5 are also prepared. The binder 4 can be selected from those described in the above embodiment. The organic solvent 5 is NMP.

[0365] <Step S16> In step S16 shown in Figure 15, the dispersion liquid X1, the positive electrode active material 21, the binder 4, and the organic solvent 5 are mixed. In this step, the above materials are placed in a polypropylene container (PP container) and mixed using a rotation / revolution mixer. After that, it is preferable to add a kneading operation using a spatula. It is also preferable to further repeat the kneading operation using the rotation / revolution mixer and the spatula. By mixing in this manner, it is possible to suppress the disintegration of the fibrous conductive material 1 while maintaining the dispersibility of the fibrous conductive material 1.

[0366] 15, dispersion X2 (having the same composition as dispersion X1) obtained through steps S11 and S12 is further added. This makes it easier to adjust the viscosity of the target material during mixing in step S15. Furthermore, since dispersion X2, which has good dispersibility of the fibrous conductive material 1, is newly added, this is preferable because the dispersibility of the fibrous conductive material 1 is less likely to be impaired.

[0367] 15, stirring is performed, and if necessary, the viscosity of the dispersion is adjusted in step S19. The viscosity can be adjusted by evaporating the organic solvent 5.

[0368] 15, positive electrode slurry 30 is obtained. The viscosity of positive electrode slurry 30 can be confirmed using the solid content ratio of positive electrode slurry 30. The solid content ratio of positive electrode slurry 30 is preferably 50% or more and 85% or less, and more preferably 60% or more and 80% or less.

[0369] In this way, a positive electrode slurry can be obtained in which the fibrous conductive material is well dispersed. In the positive electrode slurry, the fibrous conductive material, typically CNT, is prevented from being crushed, thereby increasing the strength of the positive electrode.

[0370] <<Positive Electrode Fabrication Method 2>> <Step S17> Steps S11 to S16 shown in Fig. 16 are the same as steps S11 to S16 shown in Fig. 15. In step S17 of positive electrode fabrication method 2, a positive electrode active material 21b different from the previously added positive electrode active material 21 is prepared. The different positive electrode active material 21b has a different particle diameter.

[0371] 15, step S19 is the same as step S19 shown in FIG. 15, and positive electrode slurry 30b is obtained in step S20. The viscosity of positive electrode slurry 30b can be confirmed using the solid content ratio of positive electrode slurry 30b. The solid content of positive electrode slurry 30b is preferably 55% to 85%, and more preferably 65% ​​to 80%.

[0372] In this way, it is possible to obtain a positive electrode slurry in which the fibrous conductive material 1 has good dispersibility and in which the fibrous conductive material 1 actively wraps around the positive electrode active material 21. In the positive electrode slurry, the fibrous conductive material 1, typically CNT, is prevented from being crushed, so that a positive electrode having sufficient electrode strength can be obtained.

[0373] This embodiment can be used in combination with other embodiments.

[0374] Embodiment 4 In this embodiment, a method for manufacturing a positive electrode active material 21 including a plurality of positive electrode active material particles according to one embodiment of the present invention will be described with reference to FIGS.

[0375] <<Method 1 for Producing Cathode Active Material>> <Step S110> In step S110 shown in FIG. 17 , a lithium source (Li source in the figure) and a cobalt source (Co source in the figure) are prepared as starting materials. The lithium source is preferably a lithium-containing compound, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source is preferably highly pure, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher. The cobalt source is preferably a cobalt-containing compound, for example, a cobalt oxide such as tricobalt tetroxide or cobalt hydroxide. The cobalt source is preferably highly pure, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.995%) or higher. The use of high-purity materials allows for the control of impurities in the cathode active material. As a result, the decrease in discharge capacity during charge-discharge cycles can be suppressed. Furthermore, the reliability of the secondary battery is improved.

[0376] <Step S112> In step S112 shown in FIG. 17, the lithium source and the cobalt source are mixed while being pulverized. This step produces a mixed material containing the lithium source and the cobalt source. The pulverization and mixing method can be either a dry method or a wet method. The wet method is superior to the dry method in that it allows for finer pulverization. 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 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. The water content of dehydrated acetone is reduced to 10 ppm or less. By mixing the lithium source and the cobalt source with dehydrated acetone and pulverizing the mixture, the inclusion of impurities in the mixed material can be reduced.

[0377] A ball mill, a bead mill, or the like can be used as a means for grinding and mixing. A bead mill is a means using a stirring mechanism. A ball mill is a means for rotating a pot containing media. It is preferable to use aluminum oxide balls or zirconium oxide balls as the media. Typical zirconium oxide balls are made of a material that emits little impurities, so there is little contamination from the media. Furthermore, in order to suppress contamination from the media, it is preferable to set the peripheral speed of the ball mill pot to 100 mm / s or more and 2000 mm / s or less.

[0378] <Step S113> In step S113 shown in FIG. 17, the mixed material is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably at approximately 950°C. If the temperature is too low, the lithium source and the cobalt source may not be sufficiently melted. On the other hand, if the temperature is too high, lithium may sublimate from the lithium source and be reduced, and cobalt may be excessively reduced. For example, if cobalt is reduced from trivalent to divalent, oxygen defects may be induced. Such a high temperature is undesirable for the method of producing a positive electrode active material.

[0379] If the heating time is too short, lithium cobalt oxide may not be synthesized, which is undesirable, but if it is too long, productivity is thought to decrease. Therefore, the heating time is preferably 1 hour or more and 50 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0380] 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.

[0381] For heating the composite oxide, the atmosphere preferably contains oxygen. Furthermore, it is preferable that the atmosphere has little water, and typically, the heating atmosphere is preferably dry air. For example, there is a method of continuously introducing dry air into the furnace. In this case, it is preferable that the flow rate of the dry air is 5 L / min or more and 20 L / min or less. The method of continuously introducing the atmospheric gas into the furnace and the gas flowing through the furnace as described above is called flow. In addition, as a method of preventing flow, a method can be used in which the furnace is depressurized and then filled with the atmospheric gas (which can also be called purging) to prevent the gas from entering or leaving the furnace. For example, it is preferable to depressurize the furnace until the differential pressure gauge on the furnace reaches -970 hPa, and then fill the furnace with the atmospheric gas until it reaches 50 hPa. In the case of purging, dry air can also be used as the atmospheric gas.

[0382] 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.

[0383] A rotary kiln or a roller hearth kiln may be used as the furnace for heating. Heating in a rotary kiln can be performed while stirring, whether in a continuous or batch system.

[0384] The container for containing the mixed materials during heating is preferably a crucible or a setter (also called a sheath). The purity of the container is 3N (99.9%) or higher, preferably 4N (99.99%) or higher. Placing a lid on the container before heating can prevent unnecessary volatilization of the materials. Aluminum oxide, zirconium oxide, or mullite-cordierite may also be used as the material for the crucible and setter.

[0385] Furthermore, it is preferable to use a container that has been used multiple times rather than a new one. In this specification, a new container refers to one that has undergone the process of adding lithium and mixed materials and heating two or fewer times. A container that has been used multiple times refers to one that has undergone the process of adding mixed materials and heating three or more times. This is because using a new container may cause some of the materials to be absorbed into the container during heating and / or some of the materials to adhere to the container. If some of the materials are lost, there is a greater concern that a high-quality positive electrode active material will not be obtained. On the other hand, a container that has been used multiple times is less susceptible to the above-mentioned risk, and the decrease in discharge capacity during charge-discharge cycles may be suppressed.

[0386] After heating, the mixture may be crushed and sieved as necessary. When recovering the heated material, the mixed material may be transferred from the container to a mortar and then recovered. It is preferable to use an aluminum oxide mortar.

[0387] <Step S114> Through the above steps, lithium cobalt oxide can be synthesized as shown in step S114 in Fig. 17. It is preferable that the particle diameter or median diameter (D50) of the lithium cobalt oxide is 10 µm or less, because this suppresses a decrease in discharge capacity during charge-discharge cycles.

[0388] Although an example of producing lithium cobalt oxide by a solid phase method as in steps S110 to S114 has been shown, lithium cobalt oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0389] Alternatively, pre-synthesized lithium cobalt oxide may be used in step S114. The particle size or median diameter (D50) of the pre-synthesized lithium cobalt oxide is set to 10 μm or less. In this case, steps S110 to S113 can be omitted.

[0390] It is preferable that the pre-synthesized lithium cobalt oxide that can be used in step S114 has a concentration of elements other than the main component within a certain range. Note that the main component elements of lithium cobalt oxide refer to lithium, oxygen, and cobalt, and the elements other than the main component refer to elements other than lithium, oxygen, and cobalt. Elements that fall under the category of additive elements, which will be described later, can be considered elements other than the main component.

[0391] The concentrations of the main components and elements other than the main components will now be described. For example, when lithium cobalt oxide is analyzed using GD-MS, the concentration of each element (unit: weight parts per million (wt ppm)) can be obtained. Tables 1 to 3 show the concentration of each element for four types of lithium cobalt oxide (material Sm-1, material Sm-2, material Sm-3, and material Sm-4). In the tables, "Matrix" refers to the main component, "Binder" refers to the auxiliary electrode, "Source" refers to the influence of components of the measurement device, "<" refers to a value below the detection limit, "≦" refers to the presence of interfering elements but below the numerical value, and "~" refers to the presence of variation or the presence of some interfering elements but a semi-quantitative value.

[0392]

[0393]

[0394]

[0395] From the above table, it is possible to read the concentration range of each element contained in the lithium cobalt oxide (material Sm-1, material Sm-2, material Sm-3, material Sm-4). For example, when material Sm-1 is used as the lithium cobalt oxide used in step S114, it can be read from the above table that the magnesium concentration is preferably 40 wt ppm or less and the titanium concentration is preferably 40 wt ppm or less.

[0396] <Step S115> Next, in step S115 shown in Fig. 17, the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S115 may be referred to as initial heating. Alternatively, because this heating is performed before step S120 described below, it may be referred to as preheating or pretreatment. The container and / or lid used in this step are the same as those used in step S113. Although the following effects are expected from the initial heating, the initial heating is not essential for obtaining a positive electrode active material that is one embodiment of the present invention.

[0397] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer 20a of the lithium cobalt oxide. This is also expected to have the effect of improving the crystallinity of the inner portion 20b. Furthermore, impurities may be mixed into the lithium source and / or cobalt source prepared in step S111, etc. The initial heating can reduce the amount of impurities in the lithium cobalt oxide completed in step S114.

[0398] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. A smooth surface means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a smooth surface means that there is little foreign matter adhering to the surface. Foreign matter is thought to be the cause of irregularities, so it is preferable that it does not adhere to the surface.

[0399] For this initial heating, it is not necessary to prepare a lithium source, a source of an additional element, or a material that functions as a flux.

[0400] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S113. In addition to the heating conditions, the heating temperature in this step is preferably lower than the temperature in step S113 in order to maintain the crystalline structure of lithium cobalt oxide. Furthermore, the heating time in this step is preferably shorter than the time in step S113 in order to maintain the crystalline structure of lithium cobalt oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0401] The effect of increasing the crystallinity of the inner portion 20b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the difference in shrinkage of the lithium cobalt oxide produced in step S113.

[0402] The heating in step S113 may cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference may induce a shrinkage difference. The temperature difference may cause a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. The internal stress difference is also referred to as strain, and this energy is sometimes called strain energy. The internal stress is removed by the initial heating in step S115; in other words, the strain energy may be homogenized by the initial heating in step S115. Homogenizing the strain energy relieves the strain in the lithium cobalt oxide. This may result in a smoother surface for the lithium cobalt oxide. This is also referred to as an improved surface. In other words, step S115 is preferable because it relieves the shrinkage difference that occurred in the lithium cobalt oxide and smooths the surface of the lithium cobalt oxide.

[0403] Furthermore, the difference in shrinkage may cause microscopic deviations, such as crystal deviations, in the lithium cobalt oxide. It is preferable to carry out this step in order to reduce such deviations. This step makes it possible to equalize the deviations in the composite oxide. Equalizing the deviations may result in a smoother surface for the lithium cobalt oxide. This is also referred to as crystalline grain alignment. In other words, step S115 is preferable because it alleviates deviations, such as deviations of crystals, that have occurred in the lithium cobalt oxide, and smooths the surface of the lithium cobalt oxide.

[0404] When lithium cobalt oxide with a smooth surface is used as the positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced and cracks in the positive electrode active material can be prevented.

[0405] Note that pre-synthesized lithium cobalt oxide may be used in step S114. In this case, steps S111 to S113 can be omitted. By performing step S115 on pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0406] <Step S120> As shown in FIG. 17 , an additive element is added to lithium cobalt oxide in step S120. Specifically, a compound containing the additive element (referred to as the additive element source) is prepared. In the manufacturing method 1 for a positive electrode active material described in this embodiment, the additive element is added in multiple steps. Therefore, in the flow shown in FIG. 17 , the additive element prepared first is designated A1, the additive element prepared second is designated A2, and the additive element prepared third is designated A3. The compound containing the additive element A1 (A1 source) is preferably a compound different from the compound containing the additive element A2 (A2 source), and further, the A2 source is preferably a compound different from the compound containing the additive element A3 (A3 source). Step S120 for adding the additive element A1 will be described in detail using FIG. 19A .

[0407] 19A, an Al source to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0408] The additional element A1 can be one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0409] When magnesium is selected as the additive element, the A1 source can be called a magnesium source (Mg source in the figure). A magnesium compound can be used as the magnesium source, and representative examples include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. A plurality of the above-mentioned magnesium sources may also be used.

[0410] When fluorine is selected as the additive element, the Al source can be called a fluorine source (F source in the figure). As the fluorine source, a fluorine compound can be used, for example, lithium fluoride (LiF), magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), 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 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.

[0411] 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 S121 is lithium carbonate.

[0412] The fluorine source is preferably a gas, and 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 F), nitrogen trifluoride (NF３ ) or the like may be used to mix the gas into the heated atmosphere. Also, a plurality of the above-mentioned fluorine sources may be used as the gas.

[0413] In the manufacturing method 1 described with reference to FIG. 19A, magnesium and fluorine are used as the additive element A1, and lithium fluoride (LiF) is prepared as the fluorine source and magnesium fluoride (MgF ２ ) is prepared. Magnesium fluoride is called the magnesium source.

[0414] <Step S122> Next, in step S122 shown in FIG. 19A, the magnesium source and the fluorine source are crushed and mixed. This step can be performed under the crushing and mixing conditions selected from those described in step S112. Lithium fluoride and magnesium fluoride are mixed in a mixture of 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 too excessive, which may deteriorate the charge-discharge cycle characteristics. Therefore, the molar ratio of lithium fluoride and 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.

[0415] <Step S123> In step S123 shown in Fig. 19A, the pulverized and mixed materials are collected to obtain the Al source. If necessary, sieving may be performed during collection. The Al source shown in step S123 contains multiple starting materials and can be called a mixture.

[0416] The particle size or median diameter (D50) of the mixture is preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. Even when a single material is used as the Al source, the particle size or median diameter (D50) is preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less.

[0417] Such a finely powdered mixture (including the case where only one kind of additive element is included) is preferable because when it is mixed with lithium cobalt oxide in a later step, the mixture can be easily adhered uniformly to the surfaces of the lithium cobalt oxide particles, which facilitates uniform diffusion or distribution of the additive element in the surface layer portion of the positive electrode active material after heating.

[0418] 17 , lithium cobalt oxide and an Al source are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦3), and more preferably Co:Mg=100:y (0.3≦y≦1).

[0419] The mixing in step S131 can be performed using a compounding process using mechanical energy. The compounding process involves applying mechanical energy (energy related to impact, compression, or shear) to a powder layer using a rotor or the like to cause a mechanochemical reaction and create a new material. The rotor rotation speed in the compounding process is preferably 2000 rpm or more and 4000 rpm or less, and the processing time is preferably 5 minutes or more and 30 minutes or less. Furthermore, since heat may be generated by the rotation, the compounding treatment device may be equipped with a water-cooling function.

[0420] A ball mill, a bead mill, or the like can be used for the mixing in step S131. It is preferable to use milder conditions than those in step S112 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 shorter mixing time than those in step S112. It can also be said that dry mixing provides milder conditions than wet mixing.

[0421] 17, the mixed materials are collected to obtain a mixture 901. When collecting the materials, sieving may be performed as necessary.

[0422] 17, the mixture 901 is heated under heating conditions selected from those described in step S113.

[0423] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S133 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is preferably a temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S133 is preferably 650° C. or higher.

[0424] Of course, if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 901, the reaction will proceed more easily. ２ When LiF and MgF ２ Since the eutectic point of LiF is around 742°C, it is preferable to set the heating temperature in step S133 to 742°C or higher. 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. For example, when LiF and MgF are used as additive element sources, ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, it is preferable that the heating temperature in step S133 be 742°C or higher.

[0425] Also, LiCoO ２ :LiF:MgF ２ The mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 showed an initial melting temperature T ｉｍ is 779 ° C, the melting peak temperature Tｐｍ is 815°C, and the melting end temperature T ｅｍ The heating temperature was 826° C. Therefore, it is preferable to set the heating temperature to 826° C. or higher.

[0426] The heating temperature is set to be lower than the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, there is a concern that a small amount of lithium cobalt oxide may melt. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source, etc., within an appropriate range. If the temperature is too high, fluoride decreases due to sublimation. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, the heating temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, even more preferably 900°C or lower, and even more preferably 850°C or lower. When the sublimation of lithium fluoride is suppressed, the surface layer of the positive electrode active material can have appropriate concentrations of fluorine and lithium. Furthermore, if the surface layer contains sufficient lithium, a heterophase (MgTiO ３ Another advantage is that it makes it harder for problems like these to occur.

[0427] Taking these factors into consideration, the heating temperature in step S133 is preferably 650°C to 1130°C, more preferably 650°C to 1000°C, even more preferably 650°C to 950°C, and even more preferably 650°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 826°C to 1100°C, preferably 826°C to 1130°C, more preferably 826°C to 1000°C, even more preferably 826°C to 950°C, and even more preferably 826°C to 900°C. The heating temperature in step S133 is preferably lower than that in step S113.

[0428] In manufacturing method 1, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be set to, for example, 742°C or higher and 950°C or lower, improving productivity. However, since LiF has a lower specific gravity in a gaseous state than oxygen, LiF may sublimate upon heating, and if it sublimes, the amount of LiF in mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the sublimation of LiF. Even if LiF is not used as the fluorine source, Li on the surface of the lithium cobalt oxide may react with F in the fluorine source to produce LiF, which may then sublimate. Therefore, even if a fluorine source other than LiF is used, it is still necessary to suppress sublimation.

[0429] Therefore, it is preferable to heat the mixture 901 in an atmosphere containing LiF by placing a lid on the container containing the mixture, for example, so that sublimation of LiF in the mixture 901 can be suppressed.

[0430] The furnace should be kept in an oxygen-containing atmosphere during heating. If there is a lack of oxygen, cobalt and other elements may be reduced, and the lithium cobaltate may no longer be able to maintain its layered rock salt crystal structure. For example, the pressure inside the furnace may be set to exceed atmospheric pressure.

[0431] In this step, it is preferable to heat the mixture 901 so that the particles do not stick together. If the particles of the mixture 901 stick together during heating, this may block the path for the added element (e.g., fluorine) to diffuse, which may worsen the distribution of the added element (e.g., magnesium and fluorine) in the surface layer. Furthermore, the sticking of the particles may reduce the contact area with oxygen.

[0432] When heating using a rotary kiln, it is preferable to heat by controlling the flow rate of the oxygen-containing atmosphere in the furnace. Furthermore, after the atmosphere is first purged and the oxygen atmosphere is introduced into the furnace, the atmosphere is not flowed. When heating using a roller hearth kiln, for example, the mixture 901 can be heated in an atmosphere containing LiF by placing a lid on a container containing the mixture 901.

[0433] A higher heating temperature is preferred because the reaction proceeds more easily, the heating time is shorter, and productivity is high. The heating time is preferably 2 hours or more, and more preferably 5 hours to 20 hours.

[0434] A supplementary note on heating conditions follows. The heating time varies depending on conditions such as the heating temperature, the particle size or median diameter (D50) of the lithium cobalt oxide in step S133, and the composition. When the particle size or median diameter (D50) of the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when the particle size or median diameter (D50) is large. When the particle size or median diameter (D50) of the lithium cobalt oxide in step S133 of FIG. 17 is about 7 μm, the heating temperature is preferably 650° C. or higher and 950° C. or lower, and more preferably 750° C. or higher and 950° C. or lower. The heating time is preferably 5 hours or higher and 20 hours or lower, and more preferably 8 hours or higher and 12 hours or lower.

[0435] 17, the heated material is recovered to obtain a composite oxide 902. In this step, after recovery, the material may be crushed as necessary.

[0436] <Step S140> In step S140 shown in FIG. 17, an A2 source is prepared. The additive element described in step S121 can be used as the additive element A2, and it is preferable to select an element different from the additive element A1 as the additive element A2. In manufacturing method 1, nickel and aluminum are used as the additive element A2. A nickel compound can be used as the nickel source, and typically nickel oxide, nickel hydroxide, etc. can be used. An aluminum compound can be used as the aluminum source, and typically aluminum oxide, aluminum hydroxide, etc. can be used. Step S140, in which the additive element A2 is added, will be described in detail using FIG. 19B.

[0437] <Steps S141 to S143> In step S141 of FIG. 19B , a nickel source (Ni source in the figure) and an aluminum source (Al source in the figure) are prepared. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of nickel atoms Ni in the A2 source is preferably Co:Ni = 100:z1 (0.2≦z1≦0.7), more preferably Co:Ni = 100:z1 (0.4≦z1≦0.6). The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of aluminum atoms Al in the A2 source is preferably Co:Al = 100:z2 (0.1≦z2≦0.5), more preferably Co:Al = 100:z2 (0.15≦z2≦0.3). Furthermore, in this step, the aluminum concentration is preferably lower than the nickel concentration.

[0438] Next, in step S142, the nickel source and the aluminum source are each pulverized. The pulverization conditions can refer to the conditions in step S122. Then, in step S143, the pulverized materials are mixed to obtain the A2 source.

[0439] 17, the composite oxide 902 and the A2 source are mixed. The mixing conditions can be found in the description of step S131.

[0440] 17, the mixed materials are collected to obtain a mixture 903. When collecting the materials, sieving may be performed.

[0441] <Step S153> Next, in step S153 shown in FIG. 17, the mixture 903 is heated. For heating conditions, the description of step S133 can be referred to. Note that in this step as well, it is preferable to place a lid on the container that holds the mixture. Furthermore, a supplementary note will be made regarding the heating time in this step. A shorter heating time is preferable as it increases productivity. The heating time in this step can be shorter than that in step S133. Specifically, it is preferably one hour or more, and more preferably one hour to five hours.

[0442] <Step S154> In step S154 shown in FIG. 17 , the heated material is recovered to obtain positive electrode active material 21. In this step, the recovered material may be crushed as needed. Sieving may be performed during recovery. The positive electrode active material of one embodiment of the present invention can be produced by production method 1 as described above. Production method 1 is a highly productive method, and can provide a production method that reduces costs.

[0443] <<Production Method 2 for Positive Electrode Active Material>> A production method 2 for positive electrode active material 21, which is one embodiment of the present invention and differs from production method 1, will be described with reference to Fig. 18. Production method 2 differs from production method 1 mainly in the number of times the additive element is added, and for other descriptions, the description of production method 1 can be referred to.

[0444] The steps up to step S154 are carried out in the same manner as in manufacturing method 1. However, the heat treatment in step S115 may not be carried out.

[0445] <Step S160> In step S160 shown in Fig. 18, an A3 source is prepared. The additive element described in step S121 can be used as the additive element A3, and it is preferable to select an element different from the additive elements A1 and A2 as the additive element A3. In manufacturing method 1, titanium is used as the additive element A3. A titanium-containing compound (titanium compound) is used as the titanium source, and examples of titanium compounds include titanium oxide and lithium titanate. Examples of titanium oxide include TiO ２ There is a compound represented by the formula: Lithium titanate has Li ２ TiO ３ , LiTiO ２ One or more compounds selected from the above-mentioned compounds can be used as the titanium source. Step S160 of adding the additional element A3 will be described in detail with reference to FIG. 19C.

[0446] 19C , a titanium source (Ti source in the figure) is prepared. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of titanium atoms Ti in the A3 source is preferably Co:Ti=100:z3 (0.01≦z3≦0.5), and more preferably Co:Ti=100:z3 (0.05≦z3≦0.25).

[0447] Next, in step S162, the titanium source is pulverized. The pulverization conditions can refer to the conditions in step S122. Then, in step S163, the pulverized material is recovered to obtain the A3 source.

[0448] 16, the composite oxide 904 and the A3 source are mixed. The mixing conditions can be found in the description of step S131.

[0449] 18, the mixed materials are collected to obtain a mixture 905. When collecting the materials, sieving may be performed.

[0450] <Step S173> In step S173 shown in Figure 18, the mixture 905 is heated. For heating conditions, the description of step S133 can be referred to. Note that in this step as well, it is preferable to place a lid on the container that holds the mixture. Furthermore, a supplementary note will be made regarding the heating time in this step. A shorter heating time is preferable as it increases productivity. The heating time in this step can be shorter than that in step S133. Specifically, it is preferably one hour or more, and more preferably one hour to five hours.

[0451] 18 , the heated material is recovered to obtain positive electrode active material 21. Sieving may be performed during recovery. By the above-described production method 1, a positive electrode active material according to one embodiment of the present invention can be produced. Production method 1 is a highly productive method, and can provide a production method at low cost.

[0452] 20A and 20B , a method 3 for producing a cathode active material 21, which is one embodiment of the present invention and differs from the methods 1 and 2, will be described. The method 3 differs from the method 1 mainly in the number of times that the additive element is added, and for other details, the description of the method 1 can be referred to.

[0453] In manufacturing method 2, magnesium and fluorine were added as the additional element A1, and nickel and aluminum were added as the additional element A2 at different times, but magnesium, fluorine, nickel, and aluminum may also be added in the same step. A method of adding these in the same step as the A1+A2 source in step S120a of Figures 20A and 20B is shown. This manufacturing method can also be said to be highly productive because it can reduce the mixing and heating steps, and can provide a manufacturing method that reduces costs.

[0454] <<Production Method 4 of Cathode Active Material>> Production Method 4 of cathode active material 21, which is an embodiment of the present invention and differs from Production Methods 1 to 3, will be described. In Production Method 4 of cathode active material, in step S110, an additive element source (A0 source) is prepared in addition to a lithium source and a cobalt source. In Production Method 4, magnesium, fluorine, nickel, and aluminum are added in the same step, but of these, the magnesium source and fluorine source are prepared as the A0 source in the same step as the lithium source and cobalt source. This production method can also be said to be highly productive because it can reduce the mixing and heating steps, and can provide a production method that reduces costs.

[0455] Alternatively, a lithium cobalt oxide containing magnesium and fluorine may be used, in which the A0 source is added in the same step as the lithium source and the cobalt source, and in this case, steps S111 to S113 can be omitted.

[0456] <<Production Method 5 of Cathode Active Material>> A description will be given of Production Method 4 of cathode active material 21, which is one embodiment of the present invention and differs from Production Methods 1 to 4. Production Method 5 is a method in which an Al source is added, and then an aluminum source, a nickel source, a titanium source, and the like are added as an A2 source and an A3 source. This production method can also be said to be a highly productive method because it can reduce the mixing and heating steps, and can provide a production method that reduces costs.

[0457] A titanium compound is used as the A3 source, and the order of adding the A3 source after first adding at least the A1 source, or adding the A1 source and the A2 source before first adding the A3 source, is a preferred step in order to suppress a decrease in discharge capacity during charge-discharge cycles.

[0458] The manufacturing methods 1 to 5 can be combined as appropriate.

[0459] This embodiment mode can be used in combination with other embodiments.

[0460] Embodiment Mode 5 In this embodiment mode, an example of a secondary battery will be described.

[0461] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 21A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 21B is an external view, and Fig. 21C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices. In this specification and the like, coin-type secondary batteries include button-type secondary batteries.

[0462] 21A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding. Therefore, FIGS. 21A and 21B are not completely corresponding views.

[0463] 21A shows how the positive electrode 304, negative electrode 307, spacer 342, and washer 332 are stacked and sealed with the negative electrode can 302 and positive electrode can 301. Note that the electrolyte and separator described in the above embodiment are not shown in FIG. 21A. The spacer 342 and washer 332 are used to protect the interior or fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 342 or washer 332 is made of stainless steel or an insulating material.

[0464] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0465] FIG. 21B is a perspective view of the completed coin-type secondary battery 300.

[0466] 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, may be 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 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.

[0467] It is preferable that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each have an active material layer formed on only one surface.

[0468] As shown in FIG. 21C , the positive electrode can 301 is placed downward, and the positive electrode 304, the negative electrode 307, and the negative electrode can 302 are stacked in this order, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303 to produce a coin-type secondary battery 300.

[0469] By using the secondary battery of the present invention for the coin-type secondary battery 300, the secondary battery becomes highly safe.

[0470] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 22A. As shown in Fig. 22A, a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0471] 22B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 22B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0472] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with an electrolyte layer 605 sandwiched therebetween. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. Inside the battery can 602, the wound battery element, in which the positive electrode, the negative electrode, and the separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) of one embodiment of the present invention is injected into the battery can 602 in which the battery element is provided.

[0473] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector. While the secondary battery 616 shown in Figures 22A to 22D has a cylinder whose height is greater than its diameter, this is not limiting. A secondary battery whose diameter is greater than its height may also be used. This configuration, for example, can reduce the size of the secondary battery.

[0474] 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 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 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 ceramic materials, etc. can be used.

[0475] 22C shows an example of a power storage system 615. The power storage system 615 has multiple secondary batteries 616 and is sometimes called a battery pack. The positive electrodes of each secondary battery 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 each secondary battery are electrically connected to the control circuit 620 via wiring 626. A protection circuit or the like that prevents overcharging or overdischarging can be used as the control circuit 620.

[0476] 22D 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.

[0477] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0478] 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.

[0479] 22D , 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.

[0480] By using the secondary battery of the present invention for the cylindrical secondary battery 616, a highly safe secondary battery can be obtained.

[0481] [Another Structural Example of Secondary Battery] Structural examples of secondary batteries will be described with reference to FIGS. 23 and 24. FIG.

[0482] A secondary battery 913 shown in FIG. 23A has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolyte according to one embodiment of the present invention inside the housing 930. The terminal 952 contacts 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 in FIG. 23A , for convenience, the housing 930 is shown separated, 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.

[0483] 23B, the housing 930 shown in Fig. 23A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 23B 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.

[0484] The housing 930a can be made of a laminate of a metal material and a resin material. In particular, by forming an organic resin, which is a resin material, on the surface on which the antenna is formed, the electric field caused by the secondary battery 913 can be suppressed. Note that if the electric field is not significantly blocked by the housing 930a, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material or a laminate of a metal material and a resin material.

[0485] 23C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and an electrolyte layer 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the electrolyte layer 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the electrolyte layer 933 may be stacked.

[0486] 24A to 24C may be used as a secondary battery 913 having a wound body 950a. The wound body 950a shown in Fig. 24A has a negative electrode 931, a positive electrode 932, and an electrolyte layer 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.

[0487] The electrolyte layer 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.

[0488] 24B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0489] 24C , wound body 950a is covered with housing 930 to form secondary battery 913. It is preferable to provide housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of housing 930 reaches a predetermined internal pressure, and can prevent the secondary battery from exploding.

[0490] As shown in Fig. 24B, 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 charge / discharge capacity. For other elements of the secondary battery 913 shown in Figs. 24A and 24B, the descriptions of the secondary battery 913 shown in Figs. 23A to 23C can be referenced.

[0491] By using the secondary battery of the present invention for the secondary battery 913 having a wound body, a highly safe secondary battery can be obtained.

[0492] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0493] Sixth Embodiment In this embodiment, an example of application to an electric vehicle (EV) will be described with reference to FIGS. 25A to 25C.

[0494] 25A, 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. By using the secondary battery of the present invention for the first batteries 1301a and 1301b, a highly safe secondary battery can be obtained.

[0495] 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.

[0496] The internal structure of the first battery 1301a may be a wound type or a stacked type. The first battery 1301a may be an all-solid-state battery. Using an all-solid-state battery for the first battery 1301a can increase capacity, improve safety, and reduce size and weight.

[0497] 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 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 battery pack.

[0498] In addition, in a secondary battery for vehicle use, in order to cut off power from multiple secondary batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0499] 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.

[0500] 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.

[0501] The first battery 1301a will be described with reference to FIG. 25B.

[0502] FIG. 25B shows an example in which nine prismatic secondary batteries 1300 are combined into one 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 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 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.

[0503] 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 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).

[0504] 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 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, and magnesium) may be used as the oxide. In particular, the In-M-Zn oxide that can be used as the 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 oxide. A CAAC-OS is an oxide semiconductor having multiple crystalline regions, each with its c-axis aligned in a specific direction. The specific direction refers to the thickness direction of the CAAC-OS film, the normal direction to the surface where the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. A crystalline region is a region with periodic atomic arrangement. If the atomic arrangement is considered as a lattice arrangement, a crystalline region is also a region with a uniform lattice arrangement. Furthermore, a CAAC-OS has a region where multiple crystalline regions are connected in the AB-plane direction, and the region may have distortion. Note that distortion refers to a portion where the lattice arrangement changes between a region with a uniform lattice arrangement and a region with another uniform lattice arrangement in a region where multiple crystalline regions are connected. In other words, a CAAC-OS is an oxide semiconductor with a c-axis aligned and no clear orientation in the AB-plane direction.

[0505] Furthermore, since the control circuit unit 1320 can be used in low-temperature environments, it is preferable to use a transistor using an oxide semiconductor. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. A transistor using an oxide semiconductor for the semiconductor layer has a wider operating ambient temperature range than single-crystal Si, from −40° C. to 150° C., and its characteristics change less when the secondary battery is heated than that of a single-crystal Si transistor. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement 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 does not become sufficiently large. The control circuit unit 1320 can improve safety.

[0506] 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.

[0507] A micro-short circuit is a type of internal short circuit that occurs within a secondary battery. One of the causes of a micro-short circuit is said to be local current concentration in parts of the positive electrode and negative electrode due to uneven distribution of the positive electrode active material caused by multiple charge and discharge cycles, or the generation of by-products due to side reactions, which causes a micro-short circuit.

[0508] 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.

[0509] FIG. 25C shows an example of a block diagram of the battery pack 1415 shown in FIG. 25B.

[0510] 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 external current and the upper limit of the output current. The range between the lower and upper voltage limits of the secondary battery 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 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).

[0511] The switch unit 1324 can be configured by combining an n-channel transistor and a p-channel transistor. The switch unit 1324 is not limited to a switch having a Si transistor using single crystal silicon, and may be, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaO ｘThe switch portion 1324 may be formed using a power transistor having gallium oxide (x is a real number greater than 0). Furthermore, a memory element using an OS transistor can be freely arranged by stacking it on a circuit using a Si transistor, and thus integration can be easily achieved. By stacking the control circuit portion 1320 using an OS transistor on the switch portion 1324 and integrating them, it is possible to form it into a single chip, thereby enabling miniaturization.

[0512] The first batteries 1301a and 1301b primarily supply power to 42V (high-voltage) in-vehicle devices, while the second battery 1311 supplies power to 14V (low-voltage) in-vehicle devices. Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Using a secondary battery as the second battery 1311 offers the advantage of being maintenance-free, but over extended use, e.g., three years or more, there is a risk of abnormalities occurring that cannot be detected at the time of manufacture. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, the motor may not be able to start even if the first batteries 1301a and 1301b have remaining capacity. If the second battery 1311 is a lead-acid battery, the first battery supplies power to the second battery, and the second battery is charged to maintain a full charge state at all times, preventing the motor from being unable to operate as described above.

[0513] In this embodiment, an example in which secondary batteries are used for both the first battery 1301 a and the second battery 1311 is shown, but a lead-acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. By using the secondary battery of the present invention as the above-mentioned secondary battery, a highly safe secondary battery can be obtained.

[0514] 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 from the motor controller 1303 and 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.

[0515] 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.

[0516] Although not shown, when an external charger is connected, the charger's outlet or the charger's 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 the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. 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 includes a microcomputer. The ECU uses a CPU or a GPU.

[0517] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0518] 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.

[0519] Furthermore, by installing secondary batteries in vehicles, next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Secondary batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0520] 26A to 26D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 26A is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor and 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 the above embodiment is installed in one or more locations. By using the secondary battery of the present invention as a secondary battery installed in a vehicle, a highly safe secondary battery can be obtained.

[0521] 26A includes a battery pack 2200, which includes a battery module to which a plurality of secondary batteries are connected. The battery pack 2200 preferably further includes a charge control device electrically connected to the battery module.

[0522] Furthermore, the automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging can be performed using a predetermined charging method and connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging facility is preferably a charging station installed in a commercial facility, or may be a household power source. For example, plug-in technology can be used to charge an electric 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.

[0523] Although not shown, a power receiving device can be mounted on a vehicle and can be charged by receiving power contactlessly from a ground-based power transmitting device. 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, solar cells can be installed on the exterior of the vehicle, and the secondary battery can be charged while the vehicle is stopped and moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0524] Figure 26B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The 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. Other than the number of secondary batteries in the battery pack 2201, the battery pack 2201 has the same functions as those shown in Figure 26B, and therefore a description thereof will be omitted. By using a secondary battery of the present invention as the secondary battery of the battery pack 2201, a highly safe secondary battery can be obtained.

[0525] Figure 26C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of the transport vehicle 2003 has, for example, 100 or more secondary batteries with a nominal voltage of 3.0 V to 5.0 V connected in series to produce a maximum voltage of 600 V. Furthermore, except for the number of secondary batteries constituting the battery module of the battery pack 2202, the battery module has the same functions as that shown in Figure 25B, and therefore a description thereof will be omitted. By using the secondary battery of the present invention as the secondary battery in the module, a highly safe secondary battery can be obtained.

[0526] Fig. 26D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 26D has wheels for takeoff and landing, it can also be said to be part of a transportation vehicle, and has a battery pack 2203 that includes a battery module formed by connecting multiple secondary batteries and includes the battery module and a charge control device.

[0527] The battery module of the aircraft 2004 is, for example, eight 4 V secondary batteries connected in series, with a maximum voltage of 32 V. Other than the number of secondary batteries constituting the battery module of the battery pack 2203, the battery module has the same functions as those shown in Fig. 25B, and therefore a description thereof will be omitted.

[0528] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0529] Embodiment 7 In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle such as a motorcycle or a bicycle will be described.

[0530] 27A illustrates an example of an electric bicycle using the secondary battery of one embodiment of the present invention. The secondary battery of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 27A. The secondary battery of one embodiment of the present invention may include a protection circuit.

[0531] 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. 27B . The power storage device 8702 includes a plurality of secondary batteries 8701 of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. Use of the secondary battery of the present invention as the secondary battery 8701 enables the secondary battery to be highly safe.

[0532] The power storage device 8702 also includes a control circuit 8704 capable of controlling charging or detecting an abnormality of the secondary battery. The control circuit 8704 is electrically connected to the positive and negative electrodes of the secondary battery 8701. This can greatly contribute to preventing accidents such as fires caused by secondary batteries.

[0533] 27C illustrates an example of a two-wheeled vehicle using a secondary battery of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 27C 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. Use of the secondary battery of the present invention as the secondary battery makes it possible to provide a highly safe secondary battery.

[0534] 27C can store a power storage device 8602 in an under-seat storage space 8604. The power storage device 8602 can be stored in the under-seat storage space 8604 even if the under-seat storage space 8604 is small.

[0535] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0536] Embodiment 8 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.

[0537] 28A shows an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, operation buttons 2...

Claims

A positive electrode is provided. The positive electrode has positive electrode active material particles and a fibrous conductive material, The positive electrode active material particles include lithium cobalt oxide, the positive electrode active material particles have a surface layer portion having a rock salt type crystal structure and an interior portion having a layered rock salt type crystal structure, the surface layer portion contains cobalt, nickel, magnesium and fluorine, the inner portion comprises cobalt and aluminum; a secondary battery, wherein, in a top view or a cross-sectional view of the positive electrode, the fibrous conductive material has a region that substantially entirely surrounds the positive electrode active material particles. 　 In claim 1, the fibrous conductive material has at least a region that wraps around the edge surfaces of the positive electrode active material particles. 　 A positive electrode is provided. The positive electrode includes first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material; the second positive electrode active material particles have lithium cobalt oxide; the second positive electrode active material particles have a surface layer portion having a rock salt type crystal structure and an interior portion having a layered rock salt type crystal structure, the surface layer portion contains cobalt, nickel, magnesium and fluorine, the inner portion comprises cobalt and aluminum; a secondary battery, wherein, in a top view of the positive electrode or a cross-sectional view of the positive electrode, the fibrous conductive material has a region that substantially entirely surrounds the second positive electrode active material particles. 　 A positive electrode is provided. The positive electrode includes first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material; the second positive electrode active material particles have lithium cobalt oxide; the second positive electrode active material particles have a surface layer portion having a rock salt type crystal structure and an interior portion having a layered rock salt type crystal structure, the surface layer portion contains cobalt, nickel, titanium, magnesium, and fluorine; the inner portion comprises cobalt and aluminum; a secondary battery, wherein, in a top view of the positive electrode or a cross-sectional view of the positive electrode, the fibrous conductive material has a region that substantially entirely surrounds the second positive electrode active material particles. 　 A positive electrode is provided. The positive electrode includes first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material; the first positive electrode active material particles have lithium cobalt oxide; the first positive electrode active material particles have a surface layer portion having a rock salt type crystal structure and an interior portion having a layered rock salt type crystal structure, the surface layer portion contains cobalt, nickel, magnesium and fluorine, the inner portion comprises cobalt and aluminum; a secondary battery, wherein, in a top view of the positive electrode or a cross-sectional view of the positive electrode, the fibrous conductive material has a region that substantially entirely surrounds the first positive electrode active material particles. 　 A positive electrode is provided. The positive electrode includes first positive electrode active material particles having a particle diameter of less than 10 μm, second positive electrode active material particles having a particle diameter of 10 μm or more and 50 μm or less, and a fibrous conductive material; the first positive electrode active material particles have lithium cobalt oxide; the first positive electrode active material particles have a surface layer portion having a rock salt type crystal structure and an interior portion having a layered rock salt type crystal structure, the surface layer portion contains cobalt, nickel, titanium, magnesium, and fluorine; the inner portion comprises cobalt and aluminum; a secondary battery, wherein, in a top view of the positive electrode or a cross-sectional view of the positive electrode, the fibrous conductive material has a region that substantially entirely surrounds the first positive electrode active material particles. 　 In any one of claims 1, 3 to 6, The secondary battery, wherein the fibrous conductive material comprises carbon fiber. 　 In any one of claims 1, 3 to 6, The secondary battery, wherein the fibrous conductive material comprises carbon nanotubes.