Positive electrode, secondary battery, and method for manufacturing positive electrode

Abstract

The present invention improves cycle characteristics. This positive electrode includes a positive electrode active material layer. The positive electrode active material layer includes a first positive electrode active material in which a crystal space group is R-3 m, and a second positive electrode active material in which a crystal space group is Fd-3 m and / or Fm-3 m. The average particle diameter of secondary particles of the first positive electrode active material is 1 μm or more, and the average particle diameter of secondary particles of the second positive electrode active material is 100 nm or less.

Description

Positive electrode, secondary battery, and method for manufacturing the positive electrode

[0001] The present invention relates to a positive electrode, a secondary battery, and a method for producing a positive electrode.

[0002] Patent Document 1 discloses a positive electrode using a sintered body of a positive electrode active material.

[0003] Japanese Unexamined Patent Publication No. 8-180904

[0004] However, in the positive electrode disclosed in Patent Document 1, expansion and contraction of the secondary particles of the positive electrode active material due to charge and discharge reactions may cause the secondary particles of the positive electrode active material to separate from each other, reducing the electrical conductivity of the positive electrode and potentially degrading the cycle characteristics.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to improve cycle characteristics.

[0006] A positive electrode according to one aspect of the present invention includes a positive electrode active material layer. The positive electrode active material layer includes a first positive electrode active material having a crystal space group of R-3m and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m, wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more, and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less.

[0007] A secondary battery according to an aspect of the present invention includes the positive electrode according to an aspect of the present invention, a negative electrode, and an electrolyte.

[0008] A method for manufacturing a positive electrode according to one aspect of the present invention includes a step of producing, by firing, a mixture including a first positive electrode active material having a crystal space group of R-3m and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m, wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less.

[0009] According to the present invention, cycle characteristics can be improved.

[0010] FIG. 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. FIG. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode body according to FIG. 1. FIG. 3 is a schematic cross-sectional view showing a positive electrode according to the first embodiment. FIG. 4 is a diagram showing an SEM image showing a first positive electrode active material according to the first embodiment. FIG. 5 is a diagram showing an SEM image showing a second positive electrode active material according to the first embodiment. FIG. 6 is a cut-out view showing a different example of the secondary battery according to the first embodiment. FIG. 7 is a schematic cross-sectional view taken along line VII-VII in FIG. 6. FIG. 8 is a schematic cross-sectional view showing a positive electrode according to the second embodiment. FIG. 9 is a schematic cross-sectional view showing a positive electrode according to a comparative example.

[0011] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0012] First Embodiment Secondary Battery Fig. 1 is a cross-sectional view showing an example of a secondary battery according to a first embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium-ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

[0013] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the secondary battery 1. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the secondary battery 1. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0014] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0015] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or other processing. The insulating layers of the exterior sheets 30a, 30b are made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are made of a metal plate or foil material such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing, punctures, etc., such as nylon.

[0016] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0017] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the secondary battery according to the first embodiment.

[0018] Fig. 3 is a schematic cross-sectional view showing a positive electrode according to the first embodiment. As shown in Fig. 2 and Fig. 3, the positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the example of Fig. 2, in the positive electrode 210, the positive electrode current collector 211 is stacked between the positive electrode active material layers 212. Note that Fig. 3 is a diagram that schematically shows the positional relationship of the components of the positive electrode 210 according to the first embodiment, and is not a diagram that shows the dimensional ratios of the components of the positive electrode 210.

[0019] The positive electrode current collector 211 is a conductor, and may be, for example, aluminum foil. In the example of Fig. 1, the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0020] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material refers to a material contained in the positive electrode 210 that can absorb and desorb carrier ions of the secondary battery 1 through a charge / discharge reaction. As shown in Fig. 3 , the positive electrode active material layer 212 includes a positive electrode active material bond 213 containing the positive electrode active material.

[0021] The positive electrode active material bond 213 is a bond. In the present disclosure, a bond refers to a structure in which a plurality of secondary particles are partially bonded together. In the present disclosure, a secondary particle refers to an aggregated particle in which a plurality of primary particles are at least partially bonded together. In the first embodiment, in the positive electrode active material bond 213, the secondary particles of the positive electrode active material are partially bonded together by sintering. This can improve the cycle characteristics of the secondary battery 1.

[0022] The fact that the positive electrode active material bond 213 is a bond, i.e., has a structure in which multiple secondary particles are partially bonded, can be determined by observing a cross section of the positive electrode active material layer 212 with a transmission electron microscope (TEM). More specifically, if there are at least two partially bonded secondary particles of the positive electrode active material in a TEM image of the cross section of the positive electrode active material bond 213, it can be said that the positive electrode active material bond 213 is a bond of positive electrode active material, i.e., has a structure in which multiple secondary particles of the positive electrode active material are partially bonded. Note that in the example of FIG. 3 , some secondary particles of the positive electrode active material are not bonded to other secondary particles of the positive electrode active material. This is because the secondary particles of the positive electrode active material are bonded to other secondary particles of the positive electrode active material at positions other than the cross section shown in FIG. 3 .

[0023] The positive electrode active material bond 213 is preferably made of a positive electrode active material. This allows the positive electrode active material bond 213 to absorb and desorb carrier ions. Whether the positive electrode active material bond 213 contains a material other than the positive electrode active material, such as a binder, can be determined by observing the shape of a cross section of the positive electrode active material layer 212 using a scanning electron microscope (SEM) or by observing the composition distribution using SEM-EDX (Energy Dispersive X-ray Spectroscopy). Note that the positive electrode active material bond 213 is not limited to being made of a positive electrode active material, and may further contain, for example, a plasticizer, a binder, a conductive agent, and a dispersant.

[0024] The positive electrode active material combination 213 includes a first positive electrode active material 213 a and a second positive electrode active material 213 b as positive electrode active materials. That is, the positive electrode active material combination 213 has a structure in which secondary particles of the first positive electrode active material 213 a and the second positive electrode active material 213 b are partially bonded together.

[0025] In the present disclosure, when a crystalline space group is represented by a pair of "-" and a number, the pair means the number with a "-" above it. That is, the "-3" in the crystalline space groups R-3m, Fd-3m, and Fm-3m means "3" with a "-" above it, as shown in the "Space Group" column of Table 1 described below.

[0026] The first positive electrode active material 213a is a positive electrode active material whose crystal space group is R-3m. As a result, during the charge / discharge reaction of the first positive electrode active material 213a, an insertion / desorption reaction of carrier ions occurs between layers stacked in the c-axis direction, enabling efficient insertion / desorption reaction of carrier ions in the positive electrode. Meanwhile, due to the insertion / desorption reaction of carrier ions, the volume of the first positive electrode active material 213a changes more significantly in the c-axis direction than in the a-axis direction, resulting in anisotropic expansion and contraction. Therefore, if the positive electrode active material layer 212 contains only the first positive electrode active material 213a and does not contain the second positive electrode active material described below, strain may occur between the secondary particles of the first positive electrode active material 213a due to the charge / discharge reaction, causing cracking and cutting the conduction path of carrier ions, which may result in a decrease in cycle characteristics.

[0027] The second positive electrode active material 213b is a positive electrode active material whose crystal space group is at least one of Fd-3m and Fm-3m. As a result, during the charge / discharge reaction of the second positive electrode active material 213b, the volume of the second positive electrode active material 213b changes isotropically due to the insertion / desorption reaction of carrier ions. Therefore, when the first positive electrode active material 213a anisotropically expands and contracts during the charge / discharge reaction, strain between the secondary particles of the first positive electrode active material 213a is alleviated by the isotropic expansion and contraction of the secondary particles of the second positive electrode active material 213b. As a result, it is possible to suppress disconnection of the conduction path of carrier ions between the secondary particles of the first positive electrode active material 213a, thereby improving cycle characteristics. Furthermore, when the positive electrode active material bond 213 does not contain a binder, the second positive electrode active material 213b connects the secondary particles that make up the first positive electrode active material 213a, thereby ensuring a conduction path for carrier ions between the secondary particles and improving the cycle characteristics.

[0028] The presence of the first positive electrode active material 213a and the second positive electrode active material 213b can be identified by identifying the crystal structure using a TEM or STEM (Scanning Transmission Electron Microscope). A cross section of the secondary particles is identified in a cross section along the thickness direction of the positive electrode 210, and an electron diffraction image of the cross section is obtained. If the arrangement of multiple bright spots in the electron diffraction image matches the arrangement of bright spots in the electron diffraction image of crystals of space group R-3m obtained by simulation, the secondary particles can be determined to be secondary particles of the first positive electrode active material. On the other hand, if the arrangement of multiple bright spots in the electron diffraction image matches the arrangement of bright spots in the electron diffraction image of crystals of at least one of space groups Fd-3m and Fm-3m obtained by simulation, the secondary particles can be determined to be secondary particles of the second positive electrode active material.

[0029] In the first embodiment, the first positive electrode active material 213a is a lithium composite oxide. Examples of the lithium composite oxide used as the first positive electrode active material 213a include LiNiO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２ , Li １．１５ Mn ０．６５ Ni ０．２２ Co ０．１３ O ２ Ni-containing compounds such as LiCoO ２ Examples include:

[0030] In the first embodiment, the second positive electrode active material 213b is a lithium composite oxide. Examples of the lithium composite oxide used as the second positive electrode active material 213b include LiCoO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２ , Li １．１５ Mn ０．６５ Ni ０．２２ Co ０．１３ O ２ Co-containing compounds such as LiNiO ２ , LiMn ２ O ４ Second positive electrode active material 213b may be a lithium composite oxide further containing at least one of Al, Ti, Mg, Zr, B, P, S, C, and Zn.

[0031] The compositions of the first positive electrode active material 213 a and the second positive electrode active material 213 b can be identified by quantitative analysis using EDX mapping of STEM-EDX on a cross section of the positive electrode active material layer 212 .

[0032] The average particle size of the secondary particles of the first positive electrode active material 213a is 1 μm or more. This range ensures that the secondary particles of the first positive electrode active material 213a are sufficiently larger than the secondary particles of the second positive electrode active material 213b, allowing the secondary particles of the second positive electrode active material 213b to easily penetrate between the secondary particles of the first positive electrode active material 213a. This allows the secondary particles of the second positive electrode active material 213b to isotropically expand and contract, thereby alleviating the strain between the secondary particles of the first positive electrode active material 213a when the first positive electrode active material 213a anisotropically expands and contracts during charge and discharge reactions. This prevents the conduction paths of carrier ions between the secondary particles of the first positive electrode active material 213a from being cut, improving cycle characteristics. The average particle size of the first positive electrode active material 213a is preferably 50 μm or less in order to prevent the diffusion length of carrier ions from becoming too long. This allows the battery to maintain its required performance. The average particle size of the secondary particles of the first positive electrode active material 213a is more preferably 10 μm or more and 20 μm or less, and even more preferably 15 μm or more and 20 μm or less, which can improve cycle characteristics and also makes it easier for an electrolyte (described later) to be impregnated between the secondary particles of the first positive electrode active material 213a, thereby facilitating the conduction of carrier ions between the secondary particles of the first positive electrode active material 213a.

[0033] The average particle size of the secondary particles of the second positive electrode active material 213b is 100 nm or less. By setting the average particle size within this range, the secondary particles of the second positive electrode active material 213b are sufficiently smaller than the secondary particles of the first positive electrode active material 213a, making it easier for the secondary particles of the second positive electrode active material 213b to penetrate between the secondary particles of the first positive electrode active material 213a. This allows the secondary particles of the second positive electrode active material 213b to isotropically expand and contract, thereby alleviating strain between the secondary particles of the first positive electrode active material 213a when the first positive electrode active material 213a anisotropically expands and contracts during charge and discharge reactions. This prevents the conduction paths of carrier ions between the secondary particles of the first positive electrode active material 213a from being cut, improving cycle characteristics. From the standpoint of manufacturability, the average particle size of the secondary particles of the second positive electrode active material 213b is preferably 1 nm or more, and more preferably 5 nm or more. The average particle size of the second positive electrode active material 213b is more preferably 50 nm or less, which allows the secondary particles of the second positive electrode active material 213b to more easily enter between the secondary particles of the first positive electrode active material 213a, thereby further improving the cycle characteristics.

[0034] In the present disclosure, the average particle size of the secondary particles of the first positive electrode active material 213a and the second positive electrode active material 213b is defined as the average particle size measured by the following method.

[0035] For the first positive electrode active material 213a, a cross section of the positive electrode 210 is observed using a TEM, and secondary particles of the first positive electrode active material 213a are identified using the method described above. From the identified particles of the first positive electrode active material 213a, 50 secondary particles that do not include a constriction in their cross section caused by bonding with other secondary particles are selected. Then, an SEM image containing the selected 50 secondary particles of the first positive electrode active material 213a is obtained.

[0036] For the second positive electrode active material 213b, a cross section of the positive electrode 210 is observed using a TEM or STEM, and secondary particles of the second positive electrode active material 213b are identified using the method described above. From the identified particles of the second positive electrode active material 213b, 50 secondary particles that do not include necks in their cross sections formed by bonding with other secondary particles are selected. Then, an SEM observation image, TEM observation image, or STEM observation image containing the selected secondary particles of the second positive electrode active material 213b is obtained.

[0037] Then, the area occupied by the cross section of the secondary particle to be measured is extracted from each of the acquired observation images of the first positive electrode active material 213a and the second positive electrode active material 213b. Then, using, for example, image analysis software "A-zo-kun" (manufactured by Asahi Kasei Engineering Corporation), the area of ​​the extracted area is calculated as the cross-sectional area S of the secondary particle to be measured. Then, the particle size R is calculated from the cross-sectional area S of the secondary particle using the following formula (1). That is, the particle size R of the secondary particle is calculated by regarding it as the diameter of a circle with an area equal to the cross-sectional area S of the secondary particle. R = 2(S / π) １／２ …(1)

[0038] By the above procedure, the particle diameters R of 50 secondary particles to be measured can be measured, and the arithmetic mean of the particle diameters R of the secondary particles can be calculated as the average particle diameter of the secondary particles. Note that if there are not 50 particles in one SEM, TEM, or STEM observation image, the particle radii may be determined from multiple observation images.

[0039] FIG. 4 is a diagram showing an SEM image of the first positive electrode active material according to the first embodiment. As shown in FIG. 4, the area occupied by the cross section of the secondary particles of the first positive electrode active material 213a can be defined as the area surrounded by an outline B1. In the SEM image of the area including the secondary particles of the first positive electrode active material 213a, the area occupied by the secondary particles of the first positive electrode active material 213a appears as a light gray area with a white outline, where primary particles are aggregated. Therefore, the area occupied by the cross section of the secondary particles of the first positive electrode active material 213a can be extracted as the area surrounded by an outline B1 connecting the outermost white outlines of the secondary particles of the first positive electrode active material 213a.

[0040] FIG. 5 is a diagram showing an SEM observation image of the second positive electrode active material according to the first embodiment. FIG. 5 is an SEM observation image of region A in FIG. 4. As shown in FIG. 5, the area occupied by the cross section of the secondary particles of the second positive electrode active material 213b can be the area surrounded by outline B2. In an SEM, TEM, or STEM observation image of the area including the secondary particles of the second positive electrode active material 213b, the area occupied by the secondary particles of the second positive electrode active material 213b appears as a dark gray area. Therefore, the area occupied by the cross section of the secondary particles of the second positive electrode active material 213b can be extracted as the area surrounded by outline B2 of the dark gray area occupied by the secondary particles of the second positive electrode active material 213b.

[0041] The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is laminated between the negative electrode active material layers 222.

[0042] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0043] The negative electrode active material layer 222 is a layer containing a negative electrode active material. The negative electrode active material layer 222 is not limited to being made of only a negative electrode active material, and may contain, for example, a conductive agent and a binder.

[0044] The negative electrode active material refers to a reducing agent that can absorb and desorb carrier ions of the secondary battery 1 through charge / discharge reactions, such as a carbon material, a metal, a semimetal, a silicon alloy or compound, or a tin (Sn) alloy or compound.

[0045] The negative electrode active material containing silicon includes, for example, elemental silicon, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) as the second constituent element other than silicon. Examples of silicon compounds that can be used as the negative electrode active material include silicon oxide (SiO ｘ Examples of the negative electrode active material include those containing oxygen (O) or carbon (C), such as silicon carbide (SiC), and may contain the second constituent element described above in addition to silicon. The negative electrode active material may also be doped with Li. ｘ In this case, it is preferable that Li is pre-doped by doping with Li in the negative electrode production process. ｘ The negative electrode active material may be a composite of Si and other materials such as carbon, or a composite of a Si alloy and other materials such as carbon. In this case, the irreversible capacity can be reduced. In addition, it is preferable that the particle surfaces of the negative electrode active material are partially or entirely coated with carbon. This can improve the electronic conductivity of the particle surfaces of the negative electrode active material.

[0046] Examples of carbon materials that can be used as the negative electrode active material include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, examples of materials that can be used as the negative electrode active material include pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are produced by firing a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature and carbonizing it.

[0047] The negative electrode active material is not limited to those listed above, and may contain other negative electrode active materials, such as metals, semimetal alloys or compounds, and tin (Sn) alloys or compounds, which can absorb and release lithium. Examples of metals and semimetals that can be used as negative electrode active materials include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Tin is more preferred because it has a high ability to absorb and release lithium and can achieve a high energy density.

[0048] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0049] The negative electrode active material layer 222 is not limited to containing only a negative electrode active material and a binder. For example, the negative electrode active material layer 222 may further contain a negative electrode conductive agent. The negative electrode conductive agent includes at least one of a carbon material, a metal material, and a conductive polymer compound. Specific examples of carbon materials used as the negative electrode conductive agent include particulate carbon materials such as carbon black, acetylene black, and ketjen black, and fibrous carbon materials such as carbon nanotubes. Examples of carbon nanotubes include single-wall carbon nanotubes (SWCNTs). This improves the electronic conductivity of the particle surfaces of the negative electrode active material. The mass ratio of the negative electrode conductive agent to the negative electrode active material layer 222 is preferably 5% or less, more preferably 2% or less. This improves the paintability of the negative electrode slurry.

[0050] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of Fig. 1 , the shape of the separator 230 is a rectangular sheet when viewed in plan in the thickness direction.

[0051] The separator 230 is preferably made of a material that is electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 230 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 230 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0052] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0053] The electrolyte salt is, for example, lithium perchlorate (LiClO ４ ), lithium hexafluorophosphate (LiPF ６), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0054] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0055] The electrolytic solution may further contain an additive such as a fluorinated carboxylic acid ester, a sulfonic acid ester, a sulfonic acid anhydride, or a carboxylic acid anhydride.

[0056] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to that shown in Fig. 1. Other examples will be described below using the drawings, but the same components as those in Figs. 1 and 2 will be designated by reference numerals and will not be described again.

[0057] Fig. 6 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Fig. 7 is a schematic cross-sectional view taken along line VII-VII in Fig. 6. The secondary battery 1A shown in Figs. 6 and 7 differs from the example shown in Fig. 1 in that an electrode body 200A is wound around a positive electrode lead 21A and a negative electrode lead 22A.

[0058] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 7 , the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0059] 7 , the electrode assembly 200A is a laminate for the charge / discharge reaction of the secondary battery according to the first embodiment. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode current collector 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode assembly 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and the negative electrode current collector 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A are laminated in this order from the outside, i.e., from the protective material 23 side. In the electrode body 200A, no layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0060] As described above, the positive electrode 210 according to the first embodiment includes a positive electrode active material layer 212. The positive electrode active material layer 212 includes a first positive electrode active material 213a having a crystal space group of R-3m and a second positive electrode active material 213b having a crystal space group of at least one of Fd-3m and Fm-3m. The average particle size of the secondary particles of the first positive electrode active material 213a is 1 μm or more. The average particle size of the secondary particles of the second positive electrode active material 213b is 100 nm or less. This allows the secondary particles of the second positive electrode active material 213b to suppress disconnection of the conduction pathway of carrier ions between the first positive electrode active material 213a due to charge / discharge reactions, thereby improving cycle characteristics.

[0061] In a preferred embodiment, the average particle size of the secondary particles of the first positive electrode active material 213a is 10 μm or more and 20 μm or less, which can improve cycle characteristics and also makes it easier for an electrolyte (described later) to be impregnated between the particles of the first positive electrode active material 213a, thereby facilitating the conduction of carrier ions between the secondary particles of the first positive electrode active material 213a.

[0062] In a preferred embodiment, the average particle size of the secondary particles of the second positive electrode active material 213b is 50 nm or less, which makes it easier for the secondary particles of the second positive electrode active material 213b to enter between the secondary particles of the first positive electrode active material 213a, thereby further improving the cycle characteristics.

[0063] The first positive electrode active material 213a may be a lithium composite oxide containing Ni, which also improves the cycle characteristics.

[0064] Second positive electrode active material 213b may be a lithium composite oxide containing Co. In this case as well, the cycle characteristics can be improved.

[0065] The positive electrode active material layer 212 includes a positive electrode active material combination 213, which is a combination including a first positive electrode active material 213a and a second positive electrode active material 213b. This can improve the cycle characteristics of the battery 1.

[0066] As described above, the secondary battery 1 according to the first embodiment includes the cathode 210 according to the first embodiment, the anode 220, and an electrolyte. This allows the secondary particles of the second cathode active material 213b to suppress disconnection of the conduction path of carrier ions between the first cathode active material 213a due to charge / discharge reactions, thereby improving cycle characteristics.

[0067] As described above, the secondary battery 1 according to the first embodiment includes the positive electrode 210 according to the first embodiment, the negative electrode 220, and an electrolyte. This allows the coating layer 214 to improve electrical conductivity between secondary particles of the positive electrode active material, thereby improving cycle characteristics.

[0068] A method for manufacturing the positive electrode 210 according to the first embodiment will be described below. Note that the method for manufacturing the positive electrode 210 described below is an example and is not limited to this. The method for manufacturing the positive electrode 210 according to the first embodiment includes a step of preparing a precursor of a positive electrode active material combination, a step of preparing a positive electrode active material combination, and a step of stacking the positive electrode current collector 211.

[0069] In the step of preparing a precursor of the cathode active material combination, the second cathode active material is pulverized and mixed with the first cathode active material to prepare a precursor of the cathode active material combination. More specifically, powder of the second cathode active material, whose crystal space group is R-3m, is dry-pulverized using a planetary ball mill or the like. This allows the crystal space group to be changed to Fm-3m. The dry-pulverized powder of the second cathode active material is then mixed with powder of the first cathode active material to prepare a precursor of the cathode active material combination.

[0070] In the process of producing the cathode active material bond 213, a material containing the cathode active material is fired to produce the cathode active material bond 213. Specifically, a cathode mixture paste containing secondary particles of the cathode active material is applied to a resin sheet, dried and pressed, and then the resin film is peeled off and fired. Here, the firing temperature is preferably 600°C or less. By setting the firing temperature within this range, the crystal space group of the second cathode active material can be set to Fd-3m or Fm-3m by firing. Furthermore, the firing temperature is more preferably 500°C or less. This allows the cathode active material bond to have excellent hardness for handling while being fired at a low temperature, thereby reducing manufacturing costs. Note that the method of producing the cathode active material bond 213 is not limited to this. For example, the cathode active material bond 213 may be produced by firing a green compact obtained by compressing a mixed powder of raw materials of the cathode active material into a plate shape.

[0071] In the step of laminating the positive electrode current collector 211, the positive electrode current collector 211 is laminated on the prepared positive electrode active material layer 212. By laminating the positive electrode current collector 211 after the step of preparing the positive electrode active material bond 213, the positive electrode 210 can be prepared without melting the positive electrode current collector 211 during firing of the positive electrode active material. Here, the lamination of the positive electrode current collector 211 is performed, for example, by laminating a metal foil used as the positive electrode current collector 211 on the positive electrode active material layer 212, but is not limited thereto, and the positive electrode current collector 211 may be laminated by a method such as sputtering, for example.

[0072] As described above, the method for producing a positive electrode according to the first embodiment includes a step of producing a mixture (a positive electrode active material combination) by firing the mixture (a positive electrode active material combination) containing a first positive electrode active material having a crystal space group of R-3m and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m. The average particle size of the secondary particles of the first positive electrode active material is 1 μm or more, and the average particle size of the secondary particles of the second positive electrode active material is 100 nm or less. This allows the secondary particles of the second positive electrode active material 213b to suppress the disconnection of the conduction path of carrier ions between the first positive electrode active material 213a due to charge / discharge reactions, thereby improving cycle characteristics.

[0073] In a preferred embodiment, in the step of producing the mixture (positive electrode active material bond) by firing, the firing temperature is 500° C. or less. Even in this case, a positive electrode active material bond having a hardness that allows for excellent handling can be produced.

[0074] Second Embodiment Fig. 8 is a schematic cross-sectional view showing a positive electrode according to a second embodiment. As shown in Fig. 8, a positive electrode 210B according to the second embodiment differs from the first embodiment in that it further includes a coating layer 214 made of a conductor. Note that Fig. 8 is a diagram schematically showing the positional relationship of components of a positive electrode 210B according to the second embodiment, and is not a diagram showing the dimensional ratios of the components of the positive electrode 210B.

[0075] In the second embodiment, the positive electrode active material bond 213 has pores 215. In the present disclosure, the pores of the positive electrode active material bond 213 refer to spaces surrounded by secondary particles of the positive electrode active material of the positive electrode active material bond 213. The presence or absence of pores in the positive electrode active material bond 213 can be determined by observing a cross section of the positive electrode active material layer 212 with a TEM. If a region surrounded by secondary particles of the positive electrode active material of the positive electrode active material bond 213 is present in a TEM image of the cross section of the positive electrode active material layer 212, it can be said that the positive electrode active material bond 213 has pores 215.

[0076] The coating layer 214 is a layer that coats at least a portion of the surface of the positive electrode active material bond 213. The coating layer 214 is made of a conductor. In the present disclosure, being made of a conductor means that the electrical resistivity at room temperature (20° C.) is 1.0×10 －６ This means that the positive electrode active material is made of a material with a resistivity of Ω·m or less. This improves the electronic conductivity between the secondary particles of the positive electrode active material, thereby preventing the deterioration of the crystal structure of the positive electrode active material and improving the cycle characteristics.

[0077] The coating layer 214 is made of Ru, RuO ２ , Pt, Pd, Rh, and TiN, which allows the coating layer 214 to be formed satisfactorily by atomic layer deposition (ALD), which will be described later, and also increases the electrical conductivity of the coating layer 214, thereby improving the electronic conductivity between the secondary particles of the positive electrode active material.

[0078] The presence and composition of the coating layer 214 can be measured by TEM-EDX on a cross section of the positive electrode active material layer 212. First, the area containing the positive electrode active material layer 212 is identified by TEM, and EDX mapping of the positive electrode active material layer 212 in that area is obtained, thereby identifying the distribution of the coating layer 214. Here, EDX mapping is performed on elements that are not contained in the positive electrode active material bond 213 but are contained in the coating layer 214, such as Ru, Pt, Pd, Rh, and Ti. This allows the presence of the coating layer 214 to be measured. Furthermore, EDX mapping is performed on other elements, such as O and N, in the same area, and the composition of the coating layer 214 can be determined by comparing this with the area occupied by the coating layer 214.

[0079] 8 , coating layer 214 covers at least a portion of the surface of positive electrode active material bonds 213 within pores 215. This means that the surfaces of a larger number of positive electrode active material secondary particles are coated with coating layer 214, thereby further improving the cycle characteristics.

[0080] 8 , coating layer 214 is preferably in contact with positive electrode current collector 211. This electrically connects the surfaces of the positive electrode active material secondary particles and the surface of positive electrode current collector 211 on the positive electrode active material layer 212 side via coating layer 214, thereby improving electronic conductivity between the positive electrode active material secondary particles and positive electrode current collector 211, suppressing deterioration of the crystal structure of the positive electrode active material, and improving cycle characteristics.

[0081] As shown in Fig. 8 , the coating layer 214 preferably exists continuously from one principal surface to the other principal surface of the positive electrode active material layer 212. In other words, there is a path that passes only through the coating layer 214 from one principal surface to the other principal surface of the positive electrode active material layer 212. In the second embodiment, the one principal surface of the positive electrode active material layer 212 corresponds to the surface of the positive electrode active material layer 212 facing the positive electrode current collector 211, and the other principal surface of the positive electrode active material layer 212 corresponds to the surface of the positive electrode active material layer 212 opposite the positive electrode current collector 211. This allows a conductive path to exist through the coating layer 214 from the surface on the positive electrode active material layer 212 side to the surface on the negative electrode 220 side of the positive electrode 210, thereby improving the electronic conductivity between the positive electrode active material secondary particles and the positive electrode current collector 211, suppressing deterioration of the crystalline structure of the positive electrode active material, and improving cycle characteristics.

[0082] Here, the distribution of the coating layer 214 can be measured by TEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method will be described in detail below. First, EDX mapping of the positive electrode active material layer 212 is obtained by TEM in an area that includes the positive electrode active material layer 212, and the distribution of the coating layer 214 is identified in the same manner as the method described above.

[0083] The coating layer 214 preferably covers 40 area% or more, and more preferably 50 area% or more, of the surface of the positive electrode active material bonded body 213. That is, the area of ​​the coating layer 214 relative to the surface area of ​​the positive electrode active material bonded body 213 is preferably 40% or more, and more preferably 50% or more. Here, the surface of the positive electrode active material bonded body 213 refers to the surface of the secondary particles of the positive electrode active material of the positive electrode active material bonded body 213 that is not bonded to secondary particles of other positive electrode active materials. This allows sufficient electrical conductivity to be imparted to the surface of the positive electrode active material bonded body 213, thereby improving the cycle characteristics. In the following description, the area of ​​the coating layer 214 relative to the surface area of ​​the positive electrode active material bonded body 213 may be referred to as the coverage rate of the coating layer 214.

[0084] Here, the coverage of the coating layer 214 can be measured by TEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method will be described in detail below. First, EDX mapping of the positive electrode active material layer 212 is obtained by TEM in an area including one main surface and the other main surface of the positive electrode active material layer 212, and the distribution of the coating layer 214 is identified in the same manner as the method described above. Then, the coverage can be calculated by dividing the square of the length of the outline of the surface of the coating layer 214 on the side of the positive electrode active material bond 213 in that area by the square of the length of the outline of the surface of the positive electrode active material bond 213.

[0085] The thickness of the coating layer 214 is preferably 20 nm or less. By setting the thickness within this range, it is possible to prevent the ionic conductivity of the positive electrode active material layer 212 from being reduced by the coating layer 214. The thickness of the coating layer 214 is more preferably 0.4 nm or more. By setting the thickness within this range, it is possible to impart sufficient electrical conductivity to the surface of the positive electrode active material bond 213, thereby improving the cycle retention rate.

[0086] Here, the thickness of the coating layer 214 can be measured by TEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method will be described in detail below. First, EDX mapping of the positive electrode active material layer 212 is obtained by TEM in an area including one main surface and the other main surface of the positive electrode active material layer 212, and the distribution of the coating layer 214 is identified in the same manner as the method described above. Then, the arithmetic average of the thicknesses of the coating layer 214 in that area can be calculated as the thickness of the coating layer 214.

[0087] As described above, in the positive electrode 210 according to the second embodiment, the positive electrode active material layer 212 covers at least a portion of the surface of the positive electrode active material bond 213 and further includes the coating layer 214 made of a conductor. This improves the cycle characteristics while also improving the electrical conductivity between the secondary particles of the positive electrode active material due to the coating layer 214, thereby improving the cycle characteristics.

[0088] In a preferred embodiment, the coating layer 214 is made of Ru or RuO. ２ , Pt, Pd, Rh, and TiN, whereby cycle characteristics can be further improved.

[0089] In a desirable embodiment, the coating layer 214 covers 40 area % or more of the surface of the positive electrode active material bond 213. This makes it possible to impart sufficient electrical conductivity to the surface of the positive electrode active material bond 213, thereby further improving the cycle characteristics.

[0090] A method for manufacturing the positive electrode 210 according to the second embodiment will be described below. Note that the method for manufacturing the positive electrode 210 described below is an example and is not limited to this. The method for manufacturing the positive electrode 210 according to the second embodiment differs from the first embodiment in that it further includes a step of forming a coating layer 214. The step of forming the coating layer 214 is performed after the step of preparing the positive electrode active material bond by firing and before the step of stacking the positive electrode current collector 211.

[0091] In the step of forming the coating layer 214, the coating layer 214 made of a conductor is formed on the positive electrode active material bonded body 213 to produce the positive electrode active material layer 212. By performing the step of forming the coating layer 214 after the step of producing the positive electrode active material bonded body 213, the coating layer 214 can be formed on the positive electrode active material bonded body 213 without being burned off by firing. Here, the coating layer 214 is formed by atomic layer deposition (ALD). In the embodiment, the coating layer 214 is formed by thermal ALD using a precursor. The precursor is, for example, an organic compound containing a metal element that constitutes the coating layer 214, such as Ru, Ti, Pt, Pd, or Rh. To form the coating layer 214 containing an oxide or nitride, a cycle of introducing a precursor containing a metal element, performing ALD, and exhausting the gas, and then introducing oxygen gas or nitrogen gas into the ALD apparatus, performing ALD, and exhausting the gas, can be repeated.

[0092] FIG. 9 is a schematic cross-sectional view showing a positive electrode according to a comparative example. When the coating layer 214X is formed by sputtering in the coating layer formation process, the target atoms or molecules travel in a straight line and come into contact with the positive electrode active material binders 213. Therefore, as shown in FIG. 9 , the target atoms or molecules cannot enter the pores 215 of the positive electrode active material binders 213, and therefore the coating layer 214X is not formed in large amounts on the surfaces of the positive electrode active material binders 213 within the pores 215, and instead the coating layer 214X is formed in a concentrated manner on the main surfaces of the positive electrode active material binders 213. In this case, the electrical conductivity between the secondary particles of the positive electrode active material cannot be sufficiently improved, which may result in a deterioration in cycle characteristics. Similarly, when the sol-gel method is used in the process of forming the coating layer, the sol containing the coating material does not penetrate into the pores 215 of the positive electrode active material bond 213, so that the coating layer 214X is not formed much on the surface of the positive electrode active material bond 213 inside the pores 215, and the coating layer 214X is formed concentratedly on the main surface of the positive electrode active material bond 213.

[0093] On the other hand, when the coating layer 214 is formed by ALD in the coating layer formation process, as in the positive electrode manufacturing method according to the second embodiment, the precursor containing the coating element can penetrate as a gas into the pores 215 of the positive electrode active material bonded body 213. Furthermore, in ALD, the coating layer 214 is formed due to the self-regulating properties of the precursor molecules. That is, the formation of a coating layer 214 is suppressed in areas where a coating layer 214 has already been formed, making it easier for the coating layer 214 to form in areas where a coating layer 214 has not yet been formed. This is because the bonding of subsequent precursor molecules is inhibited on the surface where precursor molecules have been bonded by ALD. As a result, the coating layer 214 grows along the surface direction of the positive electrode active material bonded body 213. Therefore, in this case, as shown in FIG. 3 , the coating layer 214 is also formed on the surface of the positive electrode active material bonded body 213 within the pores 215, including those with diameters of 10 μm or less, thereby improving the electrical conductivity between the secondary particles of the positive electrode active material and improving the cycle characteristics.

[0094] As described above, the method for producing a positive electrode according to the second embodiment further includes a step of forming a coating layer made of a conductor on the mixture (positive electrode active material bond 213) by atomic layer deposition. This allows coating layer 214 to be formed on the surfaces inside the pores of positive electrode active material bond 213, thereby improving the electrical conductivity between secondary particles of the positive electrode active material and improving cycle characteristics.

[0095] EXAMPLES Examples will be described below, but the present invention is not limited to these examples.

[0096] Table 1 shows examples and comparative examples. In the "Shape Maintenance" column of Table 1, "S" means that the positive electrode active material bond is strong enough to maintain its shape, and "W" means that the positive electrode active material bond is brittle enough to not maintain its shape. Furthermore, "Average particle size" in Table 1 refers to the average particle size of secondary particles.

[0097]

[0098] Example 1 A positive electrode according to Example 1 was produced by the following method.

[0099] First, as the second positive electrode active material, LiCoO ２ (manufactured by Aldrich) was dry-pulverized in a ball mill (PULVERISETTE 7 Classic Line, manufactured by Fritsch) under the following conditions. During the dry-pulverization, the rotation time was paused for 10 minutes every 6 hours, and then the rotation direction was reversed and rotation was resumed. Pot volume: 45 ml Pot material: zirconia Milling balls: zirconia balls (diameter 5 mm) Milling ball charge amount: 70 g Raw material powder charge amount: 4 g Rotation speed: 400 rpm Rotation time: 48 hours Treatment atmosphere: Ar atmosphere TEM observation of the obtained powder of second positive electrode active material confirmed that the average particle size of the secondary particles was 0.05 μm.

[0100] Then, as the first positive electrode active material, an NCA-based active material (LiNi) having an average particle size of 15 μm was used. ０．８５ Co ０．１０ Al ０．０５ O ２The powder of the second positive electrode active material prepared above was added to a powder of the first positive electrode active material in an amount of 1 mass % relative to the mass of the first positive electrode active material, and the mixture was mixed in a particle compositer (Nobilta (registered trademark) Mini, manufactured by Hosokawa Micron Corporation) at a rotation speed of 1000 rpm for 3 minutes to prepare a precursor of the positive electrode active material combination.

[0101] The prepared cathode active material bond precursor was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a cathode mixture slurry, and then the prepared cathode mixture slurry was uniformly applied to a polyethylene terephthalate (PET) sheet. The obtained coating was dried at 90 ° C for 30 minutes, and then the sheet was punched and pressed into a circle with a diameter of 16.5 mm. The sheet was then peeled off, sandwiched between zirconia setters, and baked at 500 ° C in an oxygen atmosphere for 1.5 hours. This produced a sheet-like cathode active material bond. The prepared cathode active material bond was picked up with tweezers and confirmed to be able to maintain its outer shape. Furthermore, the prepared cathode active material bond was observed with a TEM, and it was confirmed that the crystal space groups of the first cathode active material and the second cathode active material were R-3m and Fd-3m, respectively.

[0102] Thereafter, a carbon-coated aluminum foil (manufactured by MTI) was laminated on the positive electrode active material combined body on which the coating layer was formed, to produce the positive electrode according to Example 1. A positive electrode lead was attached to the exposed portion of the positive electrode current collector layer of the produced positive electrode.

[0103] <Charge / Discharge Test> A half cell was prepared using the positive electrode according to Example 1, and a charge / discharge test was performed. The counter electrode for the half cell was metallic lithium, and the separator was a 20 μm-thick microporous polyethylene film (F20BHE, manufactured by Toray Industries, Inc.). The electrolyte for the half cell was a mixture of ethyl carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FE) in a mass ratio of 30:60:10, and LiPF as an electrolyte salt. ６ The half cell was fabricated by laminating a separator and a counter electrode in this order on the positive electrode of Example 1, and then impregnating the separator with 14 μL of the prepared electrolyte solution.

[0104] In the charge-discharge test, the fabricated half cell was subjected to 20 cycles of charge-discharge under the following conditions: Charging method: CCCV, Charge rate: 0.1 C, Charge control voltage: 4.25 V, Charge cut-off current: 0.01 C, Discharging method: CC, Discharge rate: 0.1 C, Discharge cut-off voltage: 2.0 V.

[0105] Example 2 In Example 2, the second positive electrode active material was an NCA-based active material (LiNi ０．８５ Co ０．１０ Al ０．０５ O ２ ) except for fabricating a positive electrode by changing the composition of the positive electrode active material combination to the composition of the first positive electrode active material, a positive electrode was fabricated in the same manner as the positive electrode of Example 1, and measurements and tests were carried out. As a result of observing the fabricated positive electrode active material combination by TEM, the crystal space groups of the first positive electrode active material and the second positive electrode active material were R-3m and Fm-3m, respectively.

[0106] Example 3 In Example 3, the first positive electrode active material was LiCoO ２ A positive electrode was fabricated in the same manner as the positive electrode of Example 1, except that the positive electrode was fabricated by changing the composition to the above, and measurements and tests were carried out. As a result of observing the fabricated positive electrode active material combination with a TEM, the crystal space groups of the first positive electrode active material and the second positive electrode active material were R-3m and Fd-3m, respectively.

[0107] Comparative Example 1 In Comparative Example 1, except that the second positive electrode active material was not used, an attempt was made to fabricate a positive electrode in the same manner as the positive electrode of Example 1. However, when the fabricated positive electrode active material combination was picked up with tweezers, it was brittle and crumbled, and the outer shape could not be maintained, so fabrication and testing of a positive electrode were not possible.

[0108] In Comparative Example 2, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the second positive electrode active material was not used and the firing temperature during preparation of the positive electrode active material combined body was set to a temperature higher than 500° C. Measurements and tests were then carried out. As a result of observing the prepared positive electrode active material combined body with a TEM, the crystal space group of the first positive electrode active material was found to be R-3m.

[0109] As shown in Table 1, in Examples 1 to 3 in which the second positive electrode active material was used, the shape of the positive electrode active material combination could be maintained compared to Comparative Example 1 in which the second positive electrode active material was not used. This shows that by including the second positive electrode active material, a positive electrode active material having a hardness that allows it to be handled even at a firing temperature of 500°C can be produced.

[0110] As shown in Table 1, in Examples 1 to 3 in which the second positive electrode active material was used, the discharge capacity retention rate was improved compared to Comparative Example 2 in which the second positive electrode active material was not used. This shows that the inclusion of the second positive electrode active material can improve the cycle characteristics.

[0111] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0112] For example, the positive electrode current collector 211 and the negative electrode current collector 221 may not be provided. That is, the positive electrode 210 and the negative electrode 220 may be the positive electrode active material layer 212 and the negative electrode active material layer 222, respectively.

[0113] The present invention may also be related to the following aspects.

[0114] (1) A positive electrode comprising a positive electrode active material layer, the positive electrode active material layer including: a first positive electrode active material having a crystal space group of R-3m; and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m, wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more, and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less. (2) The positive electrode according to (1), wherein the average particle size of secondary particles of the first positive electrode active material is 10 μm or more and 20 μm or less. (3) The positive electrode according to (1) or (2), wherein the average particle size of secondary particles of the second positive electrode active material is 50 nm or less. (4) The positive electrode according to any one of (1) to (3), wherein the first positive electrode active material is a lithium composite oxide containing Ni. (5) The positive electrode according to any one of (1) to (4), wherein the second positive electrode active material is a lithium composite oxide containing Co. (6) The positive electrode according to any one of (1) to (5), wherein the positive electrode active material layer includes a positive electrode active material combination that is a combination including the first positive electrode active material and the second positive electrode active material. (7) The positive electrode according to (6), wherein the positive electrode active material layer further includes a coating layer made of a conductor and coating at least a portion of the surface of the positive electrode active material combination. (8) The coating layer is made of a material selected from the group consisting of Ru, RuO ２, Pt, Pd, Rh, and TiN. (9) The positive electrode according to (7) or (8), wherein the coating layer covers 40 area % or more of the surface of the positive electrode active material bond. (10) A secondary battery comprising the positive electrode according to any one of (1) to (9), a negative electrode, and an electrolyte. (11) A method for producing a positive electrode, comprising a step of firing a mixture containing a first positive electrode active material having a crystal space group of R-3m and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m, wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more, and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less. (12) The method for producing a positive electrode according to (11), wherein the firing temperature in the step of firing the mixture is 500° C. or less. (13) The method for producing a positive electrode according to (11) or (12), further comprising a step of forming a coating layer made of a conductor on the mixture by atomic layer deposition.

[0115] REFERENCE SIGNS LIST 1, 1A Secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Depression 32 Adhesive material 200, 200A Electrode body 210, 210A, 210B, 210X Positive electrode 211, 211A Positive electrode current collector 212, 212A Positive electrode active material layer 213 Positive electrode active material bond 213a First positive electrode active material 213b Second positive electrode active material 214, 214X Coating layer 215 Pore 220, 220A Negative electrode 221, 221A Negative electrode current collector 222, 222A Negative electrode active material layer 230, 230A Separator

Claims

1. A positive electrode comprising a positive electrode active material layer, the positive electrode active material layer including: a first positive electrode active material having a crystal space group of R-3m; and a second positive electrode active material having a crystal space group of at least one of Fd-3m and Fm-3m, wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more, and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less.

2. The positive electrode according to claim 1, wherein the average particle size of the secondary particles of the first positive electrode active material is 10 μm or more and 20 μm or less.

3. The positive electrode according to claim 1 or 2, wherein the average particle size of the secondary particles of the second positive electrode active material is 50 nm or less.

4. The positive electrode according to any one of claims 1 to 3, wherein the first positive electrode active material is a lithium composite oxide containing Ni.

5. The positive electrode according to any one of claims 1 to 4, wherein the second positive electrode active material is a lithium composite oxide containing Co.

6. The positive electrode according to any one of claims 1 to 5, wherein the positive electrode active material layer includes a positive electrode active material combination that is a combination including the first positive electrode active material and the second positive electrode active material.

7. The positive electrode according to claim 6, wherein the positive electrode active material layer further includes a coating layer made of a conductor, the coating layer covering at least a portion of the surface of the positive electrode active material bond.

8. The coating layer is made of Ru, RuO ２ 8. The positive electrode according to claim 7, comprising at least one selected from the group consisting of Pt, Pd, Rh and TiN.

9. The positive electrode according to claim 7 or 8, wherein the coating layer covers 40% or more by area of ​​the surface of the positive electrode active material combination.

10. A secondary battery comprising the positive electrode according to any one of claims 1 to 9, a negative electrode, and an electrolyte.

11. A method for manufacturing a positive electrode, comprising: a step of producing by firing a mixture containing a first positive electrode active material whose crystal space group is R-3m and a second positive electrode active material whose crystal space group is at least one of Fd-3m and Fm-3m; wherein the average particle size of secondary particles of the first positive electrode active material is 1 μm or more; and the average particle size of secondary particles of the second positive electrode active material is 100 nm or less.

12. The method for producing a positive electrode according to claim 11, wherein the step of producing the positive electrode by firing the mixture is carried out at a firing temperature of 500°C or less.

13. The method for producing a positive electrode according to claim 11 or 12, further comprising the step of forming a coating layer made of a conductor on the mixture by atomic layer deposition.

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