Positive electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

The purpose of the present invention is to provide a positive electrode capable of achieving both high energy density and long life of a lithium ion secondary battery. This positive electrode for a lithium ion secondary battery is provided with a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector, and the positive electrode active material layer includes a positive electrode first active material layer laminated on the positive electrode current collector and a positive electrode second active material layer laminated on the positive electrode first active material layer. The positive electrode first active material layer contains a positive electrode first active material mainly composed of a lithium-containing composite oxide containing Li and Ni, and the positive electrode second active material layer contains a positive electrode second active material mainly composed of a lithium-containing composite oxide containing Li and Ni. The molar fraction of Ni in the positive electrode second active material in the positive electrode second active material layer is less than the molar fraction of Ni in the positive electrode first active material in the positive electrode first active material layer.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery including the positive electrode. Background Art

[0002] Conventionally, lithium ion secondary batteries having a positive electrode including a plurality of active material layers are known (see, for example, Patent Documents 1 and 2). Prior art literature Patent Literature

[0003] Patent Document 1: International Publication No. 2021 / 095818 Patent Document 2: International Publication No. 2020 / 179149 Summary of the invention Problem that the invention aims to solve

[0004] There is a demand for longer life of lithium ion secondary batteries incorporating a positive electrode including an active material layer developed for the purpose of increasing the energy density of lithium ion secondary batteries. Technical means of solving problems

[0005] The positive electrode for a lithium ion secondary battery of the present invention comprises: a positive electrode collector, a positive electrode active material layer stacked on the above-mentioned positive electrode collector, the above-mentioned positive electrode active material layer comprises: a positive electrode first active material layer stacked on the above-mentioned positive electrode collector, and a positive electrode second active material layer stacked on the above-mentioned positive electrode first active material layer, the above-mentioned positive electrode first active material layer comprises a positive electrode first active material with a lithium-containing composite oxide containing Li and Ni as a main component, the above-mentioned positive electrode second active material layer comprises a positive electrode second active material with a lithium-containing composite oxide containing Li and Ni as a main component, and the molar fraction of Ni in the above-mentioned positive electrode second active material of the above-mentioned positive electrode second active material layer is less than the molar fraction of Ni in the above-mentioned positive electrode first active material of the above-mentioned positive electrode first active material layer.

[0006] The lithium ion secondary battery of the present invention is a lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is a positive electrode for a lithium ion secondary battery of the present invention. Effects of the Invention

[0007] According to the lithium ion secondary battery including the positive electrode for lithium ion secondary battery of the present invention, it is possible to achieve both high energy density and long life. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a schematic perspective view showing a battery 1 including a positive electrode 110 according to an example of the embodiment. Figure 2 Yes means Figure 1The schematic perspective view of the charging and discharging body 100 of the battery 1 is shown. Figure 3 The positive electrode 110, the negative electrode 120 and the separator 130 are positioned at different positions to indicate the inclusion of Figure 2 The diagram is a schematic perspective view of the positive electrode tab 111 b and the negative electrode tab 121 b of the charge / discharge body 100 shown. Figure 4 Yes means Figure 3 A schematic side view of the charging and discharging body 100 is shown. Figure 5 This is a schematic enlarged view of a portion of a die head and a backing roll used in manufacturing a positive electrode. Figure 6 It is a schematic cross-sectional view of a positive electrode and a negative electrode of a separatorless battery including a positive electrode according to another example of the embodiment. Figure 7 This is a schematic enlarged cross-sectional view of the interface between the positive electrode second active material layer and the positive electrode electron insulating layer of the positive electrode active material layer of another example of the embodiment and its vicinity. DETAILED DESCRIPTION

[0009] The various embodiments of the present invention are described with reference to the accompanying drawings. In order to facilitate understanding of the various embodiments, the sizes and ratios of the components are sometimes exaggerated in the various drawings. In the various drawings, the same symbols are given to the same components. In the various drawings, arrows are used to indicate the lateral width direction X (X-axis direction), depth direction Y (Y-axis direction), and height direction Z (Z-axis direction) of the battery 1 and the components of the battery 1. However, in the various drawings, the lateral width direction X, the depth direction Y, and the height direction Z represent relative directional relationships. That is, for example, when the battery 1 is rotated 180 degrees and the upper surface and the lower surface are reversed, or when the battery 1 is rotated 90 degrees and the upper surface is configured as a side surface, the lateral width direction X, the depth direction Y, and the height direction Z of the battery 1 change.

[0010] Hereinafter, the “positive electrode for lithium ion secondary battery” may be simply referred to as the “positive electrode.” The “lithium ion secondary battery” may be simply referred to as the “battery.”

[0011] (Configuration of a Battery Having a Positive Electrode According to an Example of Embodiment) Reference Figures 1 to 4 The configuration of a battery including a positive electrode according to an example of the embodiment will be described.

[0012] The battery 1 including the positive electrode 110 of one embodiment is a lithium ion secondary battery, such as Figure 1 As shown, the device includes a charge / discharge body 100 for charging and discharging electric power, a container 200 for accommodating the charge / discharge body 100 , and an external terminal 300 connected to the charge / discharge body 100 and mounted on the container 200 .

[0013] like Figures 2 to 4 As shown in FIG. 1 , the charge-discharge body 100 includes a positive electrode 110, a negative electrode 120, and a separator 130. The charge-discharge body 100 is contained in a container 200, and an electrolyte solution in which a supporting salt (electrolyte) is dissolved is impregnated in the separator 130. Figure 2 and Figure 3 As shown, the charge-discharge body 100 is formed by winding a long positive electrode 110 and a long negative electrode 120 with a long separator 130 interposed therebetween. The charge-discharge body 100 is formed into a rectangular parallelepiped shape with rounded ends in a state where the constituent members are wound.

[0014] The positive electrode 110 is a positive electrode for a lithium ion secondary battery, such as Figure 3 and Figure 4 As shown, the positive electrode 110 includes a positive electrode current collector 111 and a positive electrode active material layer 112 stacked on the positive electrode current collector 111. The positive electrode active material layer 112 includes a positive electrode first active material layer 113 stacked on the positive electrode current collector 111 and a positive electrode second active material layer 114 stacked on the positive electrode first active material layer 113. That is, the positive electrode 110 includes a plurality of active material layers.

[0015] The positive electrode current collector 111 is formed in a long strip shape extending along the horizontal width direction X. Figure 3 and Figure 4 As shown in FIG. 1 , the positive electrode current collector 111 includes a current collecting portion 111 a and a positive electrode tab 111 b. The current collecting portion 111 a is long in the horizontal width direction X and is formed in a foil shape. Figure 3 and Figure 4 As shown, the positive electrode tab 111b protrudes from the side edge 111c along the long side direction of the current collecting portion 111a in the short side direction (upward in the height direction Z) of the current collecting portion 111a. The positive electrode tab 111b is formed integrally with the current collecting portion 111a. For example, one positive electrode tab 111b is formed on the current collecting portion 111a. The current collecting portion 111a is formed of, for example, aluminum or an aluminum alloy, for example, aluminum foil having a plate-like (sheet-like) shape.

[0016] like Figure 4 As shown, the first positive electrode active material layer 113 is bonded to the current collecting portion 111a of the positive electrode current collector 111. The first positive electrode active material layer 113 may also be formed on both sides of the current collecting portion 111a. For example, the first positive electrode active material layer 113 faces the entire region of the current collecting portion 111a along the short side direction (height direction Z). Figure 4 As shown, the positive electrode second active material layer 114 is bonded to the positive electrode first active material layer 113 .

[0017] The first positive electrode active material layer 113 includes large-diameter solid particles 113f and small-diameter solid particles 113s having a smaller median diameter (average diameter) than the large-diameter solid particles 113f as the first positive electrode active material. The first positive electrode active material layer 113 generally corresponds to the positive electrode active material layer used in electric vehicles (BEV: Battery Electric Vehicle).

[0018] The large-diameter solid particles 113f are, for example, solid particles having a particle diameter of 8 μm or more and 20 μm or less. The small-diameter solid particles 113s are, for example, solid particles having a particle diameter of 2 μm or more and 6 μm or less.

[0019] The bulk density of the large-diameter solid particles 113f is, for example, 2.5 g / cm 3 Above 3.5g / cm 3 In addition, the bulk density of the small-diameter solid particles 113s is, for example, 1.4 g / cm 3 Above 2.4g / cm 3 Here, the bulk density of solid particles refers to the density when a container of a certain volume is filled with powder of solid particles and the internal volume is taken as the volume.

[0020] The large-diameter solid particles 113 f and the small-diameter solid particles 113 s have the same composition, which is a ternary lithium-containing composite oxide represented by the following general formula. Li 1+X M A O 2 (1) (Where X satisfies -0.15≤X≤0.15, M A represents an element group containing at least one element selected from the group consisting of Mn and Al, Ni, and Co) In addition, the solid particles 113 s having a small particle size are preferably single crystals because the single crystal solid particles 113 s contribute to extending the life of the battery 1 .

[0021] In addition to the first positive electrode active material, the first positive electrode active material layer 113 also includes, for example, a conductive aid 113c, a binder 113b, and the like. As for the material of the conductive aid 113c of the first positive electrode active material layer 113, carbon nanotubes are particularly preferred. This is because the electronic conductivity of the first positive electrode active material layer 113 can be improved. In addition, since the diffusion path of lithium ions can be fully ensured, in addition to improving the cycle durability of the battery 1 during repeated charge and discharge, the storage durability of lithium ions, and the charging rate characteristics, the energy density can also be improved. Furthermore, the content of the first positive electrode active material can be increased instead of reducing the content of the conductive aid in the first positive electrode active material layer 113.

[0022] The weight of the first positive electrode active material (the total weight of the large-diameter solid particles 113f and the small-diameter solid particles 113s) is preferably, for example, 94% by weight or more and 99% by weight or less relative to the total weight of the first positive electrode active material layer 113. The weight of the large-diameter solid particles 113f is preferably, for example, 60% by weight or more and 90% by weight or less in the first positive electrode active material layer 113.

[0023] Hollow particles 114s (positive electrode second active material) are included as the positive electrode second active material in the positive electrode second active material layer 114. The positive electrode second active material layer 114 generally corresponds to a positive electrode active material layer used in a hybrid electric vehicle (HEV: Hybrid Electric Vehicle).

[0024] The hollow particles 114s are, for example, hollow particles having a particle diameter of 3 μm or more and 7 μm or less. The bulk density of the hollow particles 114s is, for example, 1.2 g / cm 3 Above 2.0g / cm 3 Here, the bulk density of hollow particles refers to the density when a container of a certain volume is filled with the powder of the hollow particles and the internal volume is taken as the bulk density.

[0025] The hollow particles 114 s (positive electrode second active material) are a ternary lithium-containing composite oxide represented by the following general formula. Li 1+P M B O 2 (2) (Where P satisfies -0.15≤X≤0.15, M B represents an element group containing at least one element selected from the group consisting of Mn and Al, Ni, and Co)

[0026] The positive electrode second active material layer 114 further includes a conductive auxiliary agent 114 c and a binder 114 b in addition to the positive electrode second active material.

[0027] The ratio of the weight of the positive electrode second active material to the total weight of the positive electrode second active material layer 114 is preferably, for example, 85 wt % or more and 96 wt % or less.

[0028] In the positive electrode active material layer 112 , the molar fraction of Ni in the hollow particles 114 s (positive electrode second active material) of the positive electrode second active material layer 114 is lower than the molar fraction of Ni in the large-diameter solid particles 113 f (positive electrode first active material) of the positive electrode first active material layer 113 .

[0029] In the positive electrode active material layer 112, the median particle size (average particle size) of the positive electrode second active material (hollow particles 114s) of the positive electrode second active material layer 114 is smaller than the median particle size (average particle size) of the positive electrode first active material (including both solid particles 113f with large particle size and solid particles 113f with large particle size) of the positive electrode first active material layer 113.

[0030] The median particle size (average particle size) of the positive electrode first active material (including both large-diameter solid particles 113f and large-diameter solid particles 113f) of the positive electrode first active material layer 113 is, for example, in the range of 5.6 μm to 18.6 μm. The median particle size (average particle size) of the positive electrode second active material (hollow particles 114s) of the positive electrode second active material layer 114 is, for example, in the range of 3.0 μm to 8.0 μm.

[0031] The negative electrode 120 is a negative electrode for a lithium ion secondary battery, such as Figure 3 and Figure 4 As shown, the negative electrode current collector 121 and the negative electrode active material layer 122 stacked on the negative electrode current collector 121 are included.

[0032] The negative electrode current collector 121 is formed in a long strip shape extending along the horizontal width direction X. Figure 3 and Figure 4 As shown in FIG. 1 , the negative electrode current collector 121 includes a current collecting portion 121 a and a negative electrode tab 121 b. The current collecting portion 121 a is long in the horizontal width direction X and is formed in a foil shape. Figure 4 As shown, the collector portion 121a of the negative electrode 120 is longer in width along the short side direction (height direction Z) than the collector portion 111a of the positive electrode 110. Both ends of the collector portion 111a of the positive electrode 110 along the short side direction (the upper end to the lower end in the height direction Z) are located within the range of the collector portion 121a of the negative electrode 120 along the short side direction (the upper end to the lower end in the height direction Z) through the separator 130. Figure 3 and Figure 4 As shown, the negative electrode tab 121b protrudes from the side edge 121c along the long side direction of the collector portion 121a in the short side direction (above the height direction Z) of the collector portion 121a. The negative electrode tab 121b protrudes in the same direction (above the height direction Z) as the positive electrode tab 111b of the positive electrode 110 when stacked with the positive electrode 110 via the separator 130. The negative electrode tab 121b is separated from the positive electrode tab 111b of the positive electrode 110 in the horizontal width direction X when stacked with the positive electrode 110 via the separator 130. The negative electrode tab 121b is formed integrally with the collector portion 121a. For example, one negative electrode tab 121b is formed on the collector portion 121a. The collector portion 121a is formed of, for example, copper or a copper alloy.

[0033] like Figure 4As shown, the negative electrode active material layer 122 is bonded to the current collecting portion 121a of the negative electrode current collector 121. The negative electrode active material layer 122 may be formed on both sides of the current collecting portion 121a. For example, the negative electrode active material layer 122 faces the entire region of the current collecting portion 121a along the width direction (height direction Z).

[0034] The negative electrode active material layer 122 contains a negative electrode active material. The negative electrode active material is not particularly limited as long as it can be a material that can insert and release lithium ions. For example, carbon materials such as natural graphite, artificial graphite, difficult graphitizable carbon (hard carbon), easy graphitizable carbon (soft carbon), and graphite coated with amorphous carbon can be cited.

[0035] In addition to the negative electrode active material, the negative electrode active material layer 122 further includes, for example, a conductive agent, a binder, etc. As the material of the conductive agent of the negative electrode active material layer 122, for example, the same material as the material of the conductive agent 113c of the positive electrode first active material layer 113 is used. As the material of the binder of the negative electrode active material layer 122, for example, the same material as the material of the binder 113b of the positive electrode first active material layer 113 is used.

[0036] like Figure 3 and Figure 4 As shown in FIG. 1 , the separator 130 has the functions of insulating the positive electrode 110 and the negative electrode 120, preventing a short circuit between the positive electrode 110 and the negative electrode 120, and retaining the non-aqueous electrolyte. The separator 130 allows lithium ions to pass through the electrolyte. The separator 130 is formed in a long strip shape. Figure 4 As shown, the width of the separator 130 along the short side direction (height direction Z) is longer than the width of the collector portion 111a of the positive electrode 110 and the collector portion 121a of the negative electrode 120 along the short side direction. Both ends of the collector portion 111a of the positive electrode 110 along the short side direction (the upper end to the lower end in the height direction Z) are located within the range of the separator 130 along the short side direction (the upper end to the lower end in the height direction Z), and both ends of the collector portion 121a of the negative electrode 120 along the short side direction (the upper end to the lower end in the height direction Z) are located within the range of the separator 130 along the short side direction (the upper end to the lower end in the height direction Z). The separator 130 is made of a porous material. As the separator 130 , a porous sheet made of a resin such as polyethylene (PE: PolyEthylene), polypropylene (PP: PolyPropylene), polyester, cellulose, polyamide, or a laminated sheet thereof (for example, a sheet consisting of three layers of PP / PE / PP) is used.

[0037] A layer containing an inorganic material (such as aluminum oxide particles) and a binder may be provided on one or both sides of the separator 130. Thus, even when the battery 1 is used in an abnormal state (for example, when the temperature of the lithium-ion secondary battery rises to 160° C. or above due to overcharging or crushing), the separator 130 can be prevented from melting and the insulation function can be maintained. Therefore, the safety of the battery 1 is improved.

[0038] The electrolyte is impregnated in the separator 130 and is in contact with the positive electrode 110 and the negative electrode 120. The electrolyte contains an organic solvent, a supporting salt (electrolyte), and may also contain additives. As an organic solvent, for example, carbonates are used. As a supporting salt, for example, a lithium salt is used. As an additive, for example, vinylene carbonate or fluoroethylene carbonate is used.

[0039] like Figure 1 As shown, the container 200 contains the charge-discharge body 100. The container 200 includes a case 201 and a lid 202. The lid 202 is joined to the opening of the case 201 and seals the charge-discharge body 100 together with the case 201. The charge-discharge body 100 sealed by the case 201 and the lid 202 is filled with an electrolyte.

[0040] like Figure 1 As shown, the external terminal 300 includes a positive terminal 301 and a negative terminal 302. The positive terminal 301 and the negative terminal 302 relay the input and output of power between the charge-discharge body 100 and the external device. In addition, when a plurality of batteries 1 are used to form an assembled battery, the positive terminal 301 of an adjacent battery and the negative terminal 302 of another adjacent battery are joined via a busbar. The positive terminal 301 is indirectly or directly joined to the positive electrode tab 111b via the positive electrode collector plate. The negative terminal 302 is indirectly or directly joined to the negative electrode tab 121b via the negative electrode collector plate. The positive terminal 301 and the negative terminal 302 are mounted on the cover 202.

[0041] (Method for Manufacturing Battery Having Positive Electrode According to One Example of Embodiment) A battery including the positive electrode of one example of the embodiment can be manufactured using a technique known in the technical field of the present invention, except for the method for manufacturing the positive electrode.

[0042] The positive electrode 110 of an example of an embodiment can be manufactured, for example, as follows. First, prepare the materials contained in the positive electrode first active material layer 113 (for example, a positive electrode active material, a conductive aid, a binder, etc.). The material can be in powder form. Then, mix the materials and disperse the resulting mixture in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to obtain a positive electrode first slurry. Next, the positive electrode first slurry is applied to the surface (one side or both sides) of the positive electrode collector 111 by known techniques, dried, and calendered as needed to form the positive electrode first active material layer 113.

[0043] Next, prepare the materials contained in the positive electrode second active material layer 114 (for example, positive electrode active material, conductive aid, binder, etc.). The material can be in powder form. Next, mix the materials, and disperse the obtained mixture in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the positive electrode second slurry. Next, apply the positive electrode second slurry on the surface (one side or both sides) of the positive electrode first active material layer 113 by known techniques, dry it, and perform calendering treatment as needed to form the positive electrode second active material layer 114. The positive electrode 110 is obtained by the above manufacturing method. However, the positive electrode 110 is not limited to being manufactured by the above manufacturing method, and can also be manufactured by other methods.

[0044] (Effects of a battery having a positive electrode according to an example of an embodiment) Reference Figure 4 The effects of a battery including the positive electrode according to an example of the embodiment will be described.

[0045] In the positive electrode 110 of one embodiment, the positive electrode active material layer 112 includes: a positive electrode first active material layer 113 stacked on a positive electrode collector 111, and a positive electrode second active material layer 114 stacked on the positive electrode first active material layer 113. By arranging the positive electrode second active material layer 114 having a lower molar fraction of Ni than the positive electrode first active material layer 113 on the separator 130 side of the positive electrode active material layer 112, it is possible to suppress the degradation of the active material layer due to the formation of an inert layer containing nickel oxide (NiO), etc., thereby improving the durability of the battery 1 and achieving a longer life of the battery 1. On the other hand, since the positive electrode second active material layer 114 having a higher molar fraction of Ni is arranged on the positive electrode collector 111 side of the positive electrode active material layer 112, the energy density of the battery 1 can be sufficiently ensured. Therefore, in the battery 1 having the positive electrode 110 of one embodiment, it is possible to achieve both high energy density and long life.

[0046] Next, the configuration of the positive electrode for lithium ion secondary batteries and the lithium ion secondary batteries including the positive electrode according to the embodiment will be described in further detail.

[0047] 1. Positive electrode for lithium-ion secondary batteries The positive electrode for lithium ion secondary battery of the embodiment comprises: a positive electrode collector, and a positive electrode active material layer stacked on the positive electrode collector, wherein the positive electrode active material layer comprises: a positive electrode first active material layer stacked on the positive electrode collector, and a positive electrode second active material layer stacked on the positive electrode first active material layer.

[0048] (1) Positive electrode first active material layer The first positive electrode active material layer contains the first positive electrode active material. The first positive electrode active material is composed of a lithium-containing composite oxide containing Li and Ni. The lithium-containing composite oxide constituting the first positive electrode active material is not particularly limited as long as it contains Li and Ni, but preferably contains Co in addition to Li and Ni, and particularly preferably contains at least one element selected from the group consisting of Mn and Al in addition to Li, Ni and Co, and particularly preferably a ternary lithium-containing composite oxide represented by the following general formula. Li 1+X M A O 2 (1) (Where X satisfies -0.15≤X≤0.15, M A represents an element group containing at least one element selected from the group consisting of Mn and Al, Ni, and Co) In addition, the value of X in the general formula (1) was determined by ICP analysis.

[0049] The ternary lithium-containing composite oxide represented by the general formula (1) has high thermal stability and high stability under high potential conditions. By using this oxide, the safety and various battery characteristics of the battery 1 can be improved. The first positive electrode active material only needs to be a single crystal in whole or in part. The life characteristics of the first positive electrode active material can be improved by using a single crystal positive electrode active material with high purity and high uniformity. In addition, the cycle durability of the battery 1 during repeated charge and discharge and the storage durability of lithium ions can be improved.

[0050] As the first positive electrode active material layer, there is no particular limitation as long as it contains the first positive electrode active material, but it is preferred that all or part of the first positive electrode active material is formed solid. Compared with the first positive electrode active material formed as a hollow, the solid first positive electrode active material has excellent charging characteristics. Therefore, the energy density of the lithium-ion secondary battery can be improved. As the first positive electrode active material layer, as shown in one example of an embodiment, it is preferred that a material containing solid particles with a large particle size, solid particles with a small particle size whose average particle size (e.g., median particle size (D50)) is smaller than that of solid particles with a large particle size is used as the first positive electrode active material. This is because the filling density of the positive electrode active material in the first positive electrode active material layer can be increased, and the energy density of the battery can be increased.

[0051] When the positive electrode first active material layer contains hollow particles as the positive electrode first active material, it is preferred that the positive electrode first active material contains more solid particles than hollow particles.

[0052] In the first positive electrode active material layer, as the first positive electrode active material, it is preferred that in addition to particles with a large average particle size, particles with an average particle size smaller than the particles with a large average particle size are included. The particles with a small average particle size in the first positive electrode active material are preferably particles with a particle size that is sufficiently small to less than 1 / 2 of the particles with a large average particle size in the first positive electrode active material. High density filling is possible, which helps to obtain high battery performance.

[0053] In addition, the particles with a small average particle size in the first positive electrode active material are preferably larger than the particles with a small average particle size in the second positive electrode active material described later. This is because even if the shapes of the particles with a large average particle size in the first positive electrode active material vary, a highly filled electrode can be formed.

[0054] The particles with a small average particle size in the first positive electrode active material are preferably particles with a small average particle size substantially the same as the particles with a small average particle size in the second positive electrode active material described later. Raw materials and manufacturing management become easy, so it is preferred. The particles with a small average particle size in the second positive electrode active material described later and the particles with a small average particle size in the first positive electrode active material can use particles of the same specifications. In addition, substantially the same means that the average particle size is a difference within 10%. This is because the deviation of the particle size is taken into account.

[0055] The particles with a small average particle size in the first positive electrode active material preferably have an average particle size smaller than that of the second positive electrode active material described below. Since the particles can be arranged in the gaps between various large particles, a battery with a high energy density can be provided.

[0056] The positive electrode first active material layer is not particularly limited as long as it contains the positive electrode first active material. For example, in addition to the positive electrode first active material, it also contains at least one additive selected from the group consisting of a conductive aid and a binder, etc., and it is particularly preferred to also contain a conductive aid, and it is particularly preferred to also contain a conductive aid and a binder.

[0057] As the material of the conductive auxiliary agent of the first active material layer of the positive electrode, a carbon-based material can be used. As the carbon-based material, crystalline carbon, amorphous carbon or a mixture thereof can be used. As examples of crystalline carbon, natural graphite (such as flake graphite), artificial graphite, carbon fiber or a mixture thereof can be cited. As examples of amorphous carbon, carbon black (such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black or a mixture thereof) can be cited. As examples of carbon fibers, carbon nanotubes can be cited.

[0058] The material of the binder for the first active material layer of the positive electrode may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polypropylene fluoride, polychloroprene, butyl rubber, nitrile rubber, styrene-butadiene rubber (SBR), polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylic resins or mixtures thereof.

[0059] The ratio of the weight of the positive electrode first active material to the total weight of the positive electrode first active material layer is preferably, for example, 94 wt % or more and 99 wt % or less.

[0060] The stacking direction of the positive electrode first active material layer (for example, Figure 4 The thickness of one side in the depth direction Y) (for example, Figure 4 The first thickness T1) may be, for example, an average thickness of 5 μm or more and 500 μm or less, for example, an average thickness of 10 μm or more and 300 μm or less.

[0061] (2) Positive electrode second active material layer The positive electrode second active material layer contains the positive electrode second active material. The positive electrode second active material is composed of a lithium-containing composite oxide containing Li and Ni. The lithium-containing composite oxide constituting the positive electrode second active material is not particularly limited as long as it contains Li and Ni, but preferably contains Co in addition to Li and Ni, and particularly preferably contains at least one element selected from the group consisting of Mn and Al in addition to Li, Ni and Co, and particularly preferably a ternary lithium-containing composite oxide represented by the following general formula. Li 1+P M B O 2 (2) (Where P satisfies -0.15≤X≤0.15, M B represents an element group containing at least one element selected from the group consisting of Mn and Al, Ni, and Co) In addition, the P value of the general formula (2) was obtained by ICP analysis.

[0062] As the second positive electrode active material, as an example of the embodiment, it is preferably formed entirely or partially hollow. This is because, compared with the second positive electrode active material formed as a solid, in the second positive electrode active material layer, by impregnating the electrolyte (electrolyte solution) into the voids of the hollow particles of the second positive electrode active material, the diffusion rate of lithium ions is increased, and the resistance of the positive electrode active material layer can be reduced. In addition, the charging characteristics of the lithium ion secondary battery, particularly the rapid charging characteristics, can be improved. Furthermore, the cycle durability during repeated charge and discharge of the lithium ion secondary battery and the storage durability of lithium ions can be improved. In addition, the ionic conductivity becomes higher, and a battery with high capacity and high output can be obtained.

[0063] In addition, when solid particles are included as the second positive electrode active material in the second positive electrode active material layer, as the second positive electrode active material, it is preferable that the hollow particles are more than the solid particles.

[0064] The second positive electrode active material layer only needs to contain the second positive electrode active material and is not particularly limited. For example, in addition to containing the second positive electrode active material, it also contains at least one additive selected from the group consisting of a conductive assistant and a binder, etc. It is particularly preferable to further contain a conductive assistant, and it is especially preferable to further contain a conductive assistant and a binder. The material of the conductive assistant in the second positive electrode active material layer is the same as that of the conductive assistant in the first positive electrode active material layer, so the description is omitted here. The material of the binder in the second positive electrode active material layer is the same as that of the binder in the first positive electrode active material layer, so the description is omitted here.

[0065] The proportion of the weight of the second positive electrode active material relative to the total weight of the second positive electrode active material layer is preferably, for example, 85% by weight or more and 96% by weight or less.

[0066] (3) Positive electrode active material layer In the positive electrode active material layer, the molar fraction of Ni in the second positive electrode active material of the second positive electrode active material layer is less than the molar fraction of Ni in the first positive electrode active material of the first positive electrode active material layer. That is, it satisfies that the molar fraction of Ni in the first positive electrode active material > the molar fraction of Ni in the second positive electrode active material. Specifically, for example, when the first positive electrode active material is composed of a lithium-containing composite oxide represented by the above general formula (1) and the second positive electrode active material is composed of a lithium-containing composite oxide represented by the above general formula (2), the molar fractions of Ni, Co, and at least one element selected from the group consisting of Mn and Al of all the elements represented by the element group represented by M A in the above general formula (1) are set as Y (0 < Y < 1), Z (0 < Z < 1), and 1 - Y - Z respectively, and the M in the above general formula (2) BWhen the mole fraction of Ni, the mole fraction of Co, and the mole fraction of at least one element selected from the group consisting of Mn and Al in all the elements of the represented element group are Q (0 < Q < 1), R (0 < R < 1), and 1 - Q - R, respectively, Y > Q is satisfied. Thus, by disposing a second positive electrode active material layer having a lower mole fraction of Ni than the first positive electrode active material layer on the separator side of the positive electrode active material layer, deterioration of the active material layer caused by the formation of an inert layer containing nickel oxide (NiO) or the like can be suppressed, so that the durability of the battery can be improved and the long life of the battery can be achieved. In addition, nickel oxide is inert in the lithium ion secondary battery and does not contribute to the battery reaction. Here, since the first positive electrode active material layer receives lithium ions via the second positive electrode active material layer, the contact with the electrolyte is relatively less than that of the second positive electrode active material layer. That is, the generation and growth of nickel oxide are less in the first positive electrode active material layer as the high-capacity layer than in the second positive electrode active material layer. Therefore, the cycle durability during repeated charge and discharge of the lithium ion secondary battery and the storage durability of lithium ions can be improved. In addition, the values of Y and Z and the values of Q and R are obtained by ICP analysis.

[0067] As the positive electrode active material layer, it is only necessary to satisfy that the mole fraction of Ni in the first positive electrode active material > the mole fraction of Ni in the second positive electrode active material, and there is no particular limitation. For example, it is preferably satisfied that 50 ≤ the mole fraction of Ni in the first positive electrode active material ≤ 96 and 25 ≤ the mole fraction of Ni in the second positive electrode active material < 50. This is because when the mole fraction of Ni in the first positive electrode active material is above the lower limit of these ranges and the mole fraction of Ni in the second positive electrode active material is below the upper limit of these ranges, on the basis of being able to sufficiently improve the energy density of the battery, deterioration of the active material layer caused by the formation of an inert layer containing nickel oxide (NiO) or the like can be effectively suppressed on the separator side of the positive electrode active material layer, and the durability of the battery can be effectively improved.

[0068] As the positive electrode active material layer, the lithium-containing composite oxide of the positive electrode first active material constituting the positive electrode first active material layer preferably further contains Co, and the lithium-containing composite oxide of the positive electrode second active material constituting the positive electrode second active material layer preferably further contains Co, and the molar fraction of Co in the positive electrode second active material is greater than the molar fraction of Co in the positive electrode first active material. That is, it is preferred to satisfy the molar fraction of Co in the positive electrode first active material < the molar fraction of Co in the positive electrode second active material. This is because the positive electrode second active material layer having a molar fraction of Co greater than that of the positive electrode first active material layer is arranged on the separator side of the positive electrode active material layer, thereby facilitating the diffusion of lithium ions, and in particular, being able to reduce the resistance of the positive electrode active material layer at low temperatures, thereby being able to improve the low temperature output of the battery 1. Furthermore, the positive electrode active material layer can suppress heat generation as the resistance increase is suppressed.

[0069] As the positive electrode active material layer, it is preferred to satisfy the molar fraction of Co in the first positive electrode active material < the molar fraction of Co in the second positive electrode active material, and it is particularly preferred, for example, to satisfy 0 ≤ the molar fraction of Co in the first positive electrode active material ≤ 25 and 25 < the molar fraction of Co in the second positive electrode active material ≤ 40. This is because by making the molar fraction of Co in the second positive electrode active material be above the lower limit of these ranges, the resistance of the positive electrode active material layer at low temperature can be more effectively reduced.

[0070] As the positive electrode active material layer, it is preferred that the average particle size of the positive electrode second active material of the positive electrode second active material layer is smaller than the average particle size of the positive electrode first active material of the positive electrode first active material layer. Specifically, for example, it is preferred that when the average particle size of the positive electrode first active material of the positive electrode first active material layer is set to Mf [μm] measured as the median particle size (D50), and the average particle size of the positive electrode second active material of the positive electrode second active material layer is set to Ms [μm] measured as the median particle size (D50), it is preferred to satisfy Mf>Ms. This is because by configuring the positive electrode second active material layer containing an active material with a small average particle size on the separator side of the positive electrode active material layer, the diffusion path of lithium ions in the thickness direction of the positive electrode active material layer can be shortened, and the resistance of the positive electrode active material layer can be reduced. Specifically, the reaction area of ​​the positive electrode active material per unit volume in the positive electrode active material layer is relatively large compared to the positive electrode first active material layer as the high-capacity layer. Moreover, compared to the positive electrode first active material layer as the high-capacity layer, the positive electrode second active material layer as the high-input-output layer can relatively shorten the diffusion path of lithium ions. Therefore, the charging characteristics of the lithium-ion secondary battery, especially the fast charging characteristics, can be improved. On the other hand, the reaction area of ​​the positive electrode active material per unit volume in the positive electrode active material layer is relatively small compared to the positive electrode second active material layer as the high-input-output layer. Therefore, in the positive electrode first active material layer as the high-capacity layer, the cycle durability of the lithium-ion secondary battery during repeated charge and discharge and the storage durability of lithium ions can be improved. Here, the median diameter (D50) refers to the diameter of particles when the cumulative value is 50% in particle size distribution measurement by laser diffraction scattering particle size distribution measurement.

[0071] The positive electrode active material layer preferably satisfies Mf>Ms, and particularly preferably satisfies, for example, 5.6 μm≤Mf≤18.6 μm and 3.0 μm≤Ms≤8.0 μm.

[0072] As the positive electrode active material layer, it is preferred that the weight of the conductive aid relative to the total weight of the positive electrode second active material layer is greater than the weight of the conductive aid relative to the total weight of the positive electrode first active material layer. This is because by configuring the positive electrode second active material layer with a large weight ratio of the conductive aid on the separator side of the positive electrode active material layer, the diffusion path of lithium ions can be shortened, the electronic conductivity can be improved, and thus the resistance of the positive electrode active material layer can be reduced. It is possible to improve the cycle durability of lithium ion secondary batteries during repeated charge and discharge, the storage durability of lithium ions, and the charging rate characteristics.

[0073] As the positive electrode active material layer, it is preferred that the ratio of the weight of the conductive auxiliary agent to the total weight of the positive electrode second active material layer is greater than the ratio of the weight of the conductive auxiliary agent to the total weight of the positive electrode first active material layer. It is particularly preferred, for example, that the ratio of the weight of the conductive auxiliary agent to the total weight of the positive electrode first active material layer is 0.5 weight % to 2 weight %, and that the ratio of the weight of the conductive auxiliary agent to the total weight of the positive electrode second active material layer is 0.8 weight % to 5 weight %.

[0074] The stacking direction of the positive electrode second active material layer (for example, Figure 4 The thickness of one side in the depth direction Y) (for example, Figure 4 The second thickness T2) may be, for example, an average thickness of 5 μm or more and 500 μm or less, and for example, an average thickness of 10 μm or more and 300 μm or less.

[0075] (4) Manufacturing method As a method for manufacturing a positive electrode for a lithium ion secondary battery according to an embodiment, a method for manufacturing a positive electrode first active material layer and a positive electrode second active material layer by simultaneously coating and forming a positive electrode active material layer may be used. Figure 5 An example of the production method will be described.

[0076] In this manufacturing method, the materials contained in the first active material layer of the positive electrode (for example, a positive electrode active material, a conductive aid, a binder, etc.) are prepared. The materials are mixed, and the obtained mixture is dispersed in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to obtain a first positive electrode slurry. In addition, the materials contained in the second active material layer of the positive electrode (for example, a positive electrode active material, a conductive aid, a binder, etc.) are prepared. The materials are mixed, and the obtained mixture is dispersed in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to obtain a second positive electrode slurry.

[0077] Then, for example, use Figure 5The die head 50 shown in the figure applies the positive electrode first slurry and the positive electrode second slurry on the positive electrode collector 34a at the same time. The die head 50 has an outlet block 57, a three-dimensional gasket 58 and an inlet block 59. A manifold 52 for the positive electrode second slurry and a manifold 51 for the positive electrode first slurry are provided inside the die head 50. The positive electrode second slurry and the positive electrode first slurry are discharged from each manifold 52, 51 simultaneously to the positive electrode collector 34a conveyed along the back roller 56. Thus, the positive electrode second slurry layer 33d and the positive electrode first slurry layer 33b are formed. Next, the solvent contained in the positive electrode first slurry layer 33b and the positive electrode second slurry layer 33d is volatilized by means of a drying furnace or the like, so that the positive electrode first slurry layer 33b and the positive electrode second slurry layer 33d are dried. Thus, a positive electrode first active material layer (not shown) and a positive electrode second active material layer (not shown) are formed on one side of the positive electrode collector 34a. Next, the positive electrode collector 34a, the positive electrode first active material layer and the positive electrode second active material layer are pressed. Specifically, a laminate including the positive electrode collector 34a, the positive electrode first active material layer and the positive electrode second active material layer is clamped with a roller heated to 60 to 120°C to apply pressure. Thereafter, the laminate is cut to a specified width. Thus, a positive electrode is obtained.

[0078] In a battery having a positive electrode manufactured by the above manufacturing method using double-layer simultaneous coating, the interface between the positive electrode first slurry layer 33b and the positive electrode second slurry layer 33d has a concave-convex shape, so the tightness of the positive electrode first slurry layer 33b and the positive electrode second slurry layer 33d is improved, and even if volume changes accompanying charging and discharging occur, the two layers will not peel off, thereby achieving the effect of improving the reliability of the battery.

[0079] In addition, in a battery having a positive electrode manufactured by the above manufacturing method using double-layer simultaneous coating, the interface between the positive electrode first active material layer (positive electrode first slurry layer 33b) and the positive electrode second active material layer (positive electrode second slurry layer 33d) is not pressed by a roller. For example, the interface of the positive electrode second active material layer (positive electrode second slurry layer 33d) on the opposite side of the positive electrode first active material layer (positive electrode first slurry layer 33b) is pressed by a roller. Thus, in the positive electrode first active material layer containing particles with large particle size and particles with small particle size whose average particle size is smaller than that of particles with large particle size as the positive electrode first active material, it is possible to suppress the large particle size particles from being damaged by the roller.

[0080] Furthermore, in a battery having a positive electrode manufactured by the above manufacturing method using double-layer simultaneous coating, the surface of the positive electrode second active material layer (positive electrode second slurry layer 33d) stacked on the positive electrode first active material layer (positive electrode first slurry layer 33b) opposite to the positive electrode first active material layer (positive electrode first slurry layer 33b) is rolled. Thus, the adhesion between the positive electrode first active material layer and the positive electrode second active material layer can be improved, so it is preferred.

[0081] In particular, compared with the surface of the positive electrode second active material layer (positive electrode second slurry layer 33d) facing the roller during roller pressing, the interface between the positive electrode second active material layer (positive electrode second slurry layer 33d) and the positive electrode first active material layer (positive electrode first slurry layer 33b) can be formed in a manner where the bumps and depressions become larger, so the adhesion is also good and stable ion conductivity can be obtained, which is therefore preferred.

[0082] 2. Lithium-ion secondary battery The lithium ion secondary battery of the embodiment is a battery having a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is a positive electrode for the lithium ion secondary battery of the embodiment. The lithium ion secondary battery is not limited to a battery mounted on an electric vehicle and supplying power to a driving motor of the electric vehicle. For example, the lithium ion secondary battery can be applied to a battery mounted on a portable electronic device such as a smart phone (registered trademark) and a battery mounted on a fixed power generation device.

[0083] The lithium ion secondary battery of the embodiment is not particularly limited, and for example, as in one embodiment, includes a charge-discharge body having a positive electrode, a negative electrode, and a separator in which an electrolyte is impregnated. The lithium ion secondary battery of the embodiment preferably includes an electrolyte solution in which the electrolyte is dissolved.

[0084] Furthermore, the lithium ion secondary battery of the embodiment is a battery having a solid electrolyte as an electrolyte, and includes a positive electrode, a negative electrode, a solid electrolyte layer containing a solid electrolyte, and a charge / discharge body in which the solid electrolyte layer is interposed between the positive electrode and the negative electrode.

[0085] A battery with such a solid electrolyte does not need to contain an electrolyte, so it can have high safety. In addition, since the positive electrode second active material layer containing a positive electrode second active material having a smaller average particle size and a larger specific surface area than the positive electrode first active material layer is arranged on the solid electrolyte layer side compared to the positive electrode first active material layer, good ion conduction is achieved. In addition, the interface of the positive electrode second active material layer with the solid electrolyte layer preferably has a greater unevenness in the thickness direction than the interface of the solid electrolyte layer on the opposite side of the positive electrode second active material layer. Due to the high adhesion, it is preferred for lithium ion movement.

[0086] In addition, examples of solid electrolytes include sulfide-based solid electrolytes such as Li 10 G 2 S 12 , Li 6 PS 5 Cl, Li 2 SP 2 S 5Glass, Li 2 S-SiS 2 Glass, Li 2 SP 2 S 5 -GeS 2 Glass, Li 2 SB 2 S 3 Glass, oxide-based solid electrolytes such as Li 7 La 3 Zr 2 O 12 、LiLaTiO 3 、LiTi(PO 4 ) 3 、LiGe(PO 4 ) 3 , and complex hydride solid electrolytes such as LiBH 4 -LiI, LiBH 4 -LiNH 2 , and mixtures of two or more thereof.

[0087] 3. Others A battery including the positive electrode of another embodiment may further include a positive electrode electron insulating layer provided on the positive electrode and a negative electrode electron insulating layer provided on the negative electrode in place of the separator.

[0088] (Configuration of a separator-less battery having a positive electrode according to another example of the embodiment) Below, refer to Figure 6 and Figure 7 The structure of such a separator-less battery will be described.

[0089] like Figure 6 As shown, in a separatorless battery 1 (lithium ion secondary battery) having a positive electrode 34 of another example of an embodiment, the positive electrode 34 comprises: a positive electrode collector 34a, a positive electrode active material layer 34b (positive electrode mixture layer) bonded to both surfaces of the positive electrode collector 34a, and the positive electrode active material layer 34b includes: a positive electrode first active material layer 34b1 bonded to both surfaces of the positive electrode collector 34a, and a positive electrode second active material layer 34b2 bonded to each of the positive electrode first active material layer 34b1. The positive electrode 34 further comprises a positive electrode electronic insulating layer 34d bonded to each of the positive electrode second active material layers 34b2. The negative electrode 32 comprises: a negative electrode collector 32a, a negative electrode active material layer 32b (negative electrode mixture layer) bonded to both surfaces of the negative electrode collector 32a, and a negative electrode electronic insulating layer 32d bonded to each of the negative electrode active material layers 32b.

[0090] A portion (hereinafter referred to as the "positive electrode collector exposed portion") 34c that is not covered by the positive electrode active material layer 34b or the positive electrode electronic insulating layer 34d is provided at one end of the positive electrode collector 34a. The positive electrode collector exposed portion 34c is provided at the end surface of the winding group (not shown) and its vicinity. The positive electrode collector exposed portion 34c is opposite to and electrically connected to the positive electrode side connecting end portion (not shown) of the positive electrode collector plate (not shown). Similarly, a portion (hereinafter referred to as the "negative electrode collector exposed portion") 32c that is not covered by the negative electrode active material layer 32b or the negative electrode electronic insulating layer 32d is provided at one end of the negative electrode collector 32a. The negative electrode collector exposed portion 32c is provided at the end surface of the winding group and its vicinity. The negative electrode collector exposed portion 32c is opposite to and electrically connected to the negative electrode side connecting end portion (not shown) of the negative electrode collector plate (not shown).

[0091] The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d have the function of preventing the short circuit between the positive electrode active material layer 34b and the negative electrode active material layer 32b and the function of conducting ions between the positive electrode active material layer 34b and the negative electrode active material layer 32b. The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can also be a porous layer composed of an electrically insulating (i.e., electronically insulating and ionically insulating) material. The porous layer can retain an electrolyte in its micropores, so that ions can be conducted between the positive electrode active material layer 34b and the negative electrode active material layer 32b with the help of the electrolyte.

[0092] The porous positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can also have the function of buffering the expansion and contraction of the positive electrode active material layer 34b and the negative electrode active material layer 32b accompanying the charging and discharging of the lithium ion secondary battery 100. The expansion and contraction of the negative electrode active material layer 32b accompanying the charging and discharging of the battery 1 is generally greater than the expansion and contraction of the positive electrode active material layer 34b. Therefore, the negative electrode electronic insulating layer 32d may have an average micropore diameter larger than the average micropore diameter of the positive electrode electronic insulating layer 34d, so as to buffer the expansion and contraction of the larger negative electrode active material layer 32b. In addition, in the present application, the average micropore diameter of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d means the average value of the volume-based micropore diameter measured by mercury intrusion.

[0093] The total content of Na and Fe in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be 300 ppm or less based on the weight of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d. The amount of each element contained in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be measured by ICP (Inductive Coupled Plasma) method.

[0094] The positive electrode electronic insulating layer 34d may include positive electrode electronic insulating particles, and the negative electrode electronic insulating layer 32d may include negative electrode electronic insulating particles. Hereinafter, the positive electrode electronic insulating particles and the negative electrode electronic insulating particles are collectively referred to as electronic insulating particles as appropriate. The electronic insulating particles may be electrically insulating particles. As an example of electrically insulating particles, ceramic particles may be cited. The ceramic particles may contain a material selected from aluminum oxide (Al 2 O 3 ), boehmite (Al 2 O 3 hydrate), magnesium oxide (MgO), zirconium oxide (ZrO 2 ), titanium oxide (TiO 2 ), iron oxide, silicon dioxide (SiO 2 ) and barium titanate (BaTiO 2 ) is preferably composed of at least one of the group consisting of aluminum oxide, boehmite, magnesium oxide, zirconium oxide and titanium oxide. The electronic insulating particles may have an average particle size in the range of 0.7 to 1.1 μm. The average particle size of the electronic insulating particles can be obtained by calculating the arithmetic mean of the equivalent diameters of the projected area circles of more than 100 randomly selected electronic insulating particles based on the microscope observation images of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d. The electronic insulating particles may contain at least one of 100 to 200 ppm of Na, 50 to 100 ppm of Fe, or 50 to 100 ppm of Ca based on the weight of the electronic insulating particles.

[0095] The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may further include a binder. The binder may be a binder dispersed or dissolved in an aqueous solvent or a non-aqueous solvent (e.g., N-methyl-2-pyrrolidone (NMP)), and may contain at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), and carboxymethyl cellulose (CMC).

[0096] The positive electrode electron insulating layer 34d and the negative electrode electron insulating layer 32d may further include a dispersant. The dispersant may include at least one selected from the group consisting of carboxylic acid compounds and phosphoric acid compounds.

[0097] The interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b (positive electrode second active material layer 34b2) has a concavoconvex shape, and the concavoconvex height is greater than 2 μm, preferably within the range of 2 to 4 μm. The interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b has a concavoconvex shape, and the concavoconvex height is greater than 2 μm, preferably within the range of 2 to 4 μm. By making the concavoconvex height of the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b and the interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b greater than 2 μm, the adhesion between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b and the adhesion between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b can be improved. This can prevent or reduce the separation of the positive electrode electronic insulating layer 34 d and the negative electrode electronic insulating layer 32 d from the positive electrode active material layer 34 b and the negative electrode active material layer 32 b , respectively, thereby improving the reliability of the lithium ion secondary battery 100 .

[0098] The concavoconvex height of the interface 34e between the positive electrode active material layer 34b (positive electrode second active material layer 34b2) and the positive electrode electronic insulating layer 34d can be controlled, for example, by the particle size of the positive electrode second active material particles (positive electrode second active material) contained in the positive electrode second active material layer 34b2 and the positive electrode electronic insulating particles contained in the positive electrode electronic insulating layer 34d. Figure 7 As shown, when the average particle size of the positive electrode second active material particles 34b2p (positive electrode second active material) contained in the positive electrode second active material layer 34b2 is larger than the average particle size of the positive electrode electronic insulating particles 34dp contained in the positive electrode electronic insulating layer 34d, the positive electrode electronic insulating particles 34dp enter the gaps between the positive electrode second active material particles 34b2p, and the interface 34e between the positive electrode second active material layer 34b2 and the positive electrode electronic insulating layer 34d becomes a concavoconvex shape. For example, by using spherical positive electrode second active material particles 34b2p having an average particle size in the range of 4.5 to 5.5 μm and positive electrode electronic insulating particles 34dp having an average particle size in the range of 0.7 to 1.1 μm, the concavoconvex height of the interface 34e between the positive electrode active material layer 34b (positive electrode second active material layer 34b2p) and the positive electrode electronic insulating layer 34d can be set to 2 μm or more, preferably in the range of 2 to 4 μm.

[0099] The uneven height of the interface 32e between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d can also be controlled by the particle size of the negative electrode active material particles (negative electrode active material) contained in the negative electrode active material layer 32b and the negative electrode electronic insulating particles contained in the negative electrode electronic insulating layer 32d. For example, by using flaky negative electrode active material particles with an average particle size in the range of 9 to 11 μm and negative electrode electronic insulating particles with an average particle size in the range of 0.7 to 1.1 μm, the uneven height of the interface 32e between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d can be set to 2 μm or more, preferably in the range of 2 μm to 4 μm.

[0100] In the present application, the concavo-convex height of the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b (positive electrode second active material layer 34b2), and the interface 32e between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d are measured as follows. A scanning electron microscope (SEM) is used to obtain cross-sectional SEM images of any three locations of the positive electrode 34 or the negative electrode 32, and the distance from any 10 or more points on the interface 34e, 32e to a specified reference plane is measured in each cross-sectional SEM image (for example, the distance from any 10 or more points on the interface 34e, 32e to the surface 34f of the positive electrode electronic insulating layer 34d, and the distance from any 10 or more points on the interface 32f to the surface 32f of the negative electrode electronic insulating layer 32d, that is, the thickness of the positive electrode electronic insulating layer 34d and the thickness of the negative electrode electronic insulating layer 32d at any 10 or more locations). The standard deviation of the obtained distance values ​​is taken as the concavo-convex height of the interface 34e, 32e. In addition, the surface 34f of the positive electrode electronic insulating layer 34d and the surface 32f of the negative electrode electronic insulating layer 32d are opposite to each other and can be sufficiently flat compared to the interfaces 34e and 32e. For example, the surface 34f of the positive electrode electronic insulating layer 34d and the surface 32f of the negative electrode electronic insulating layer 32d can have a concave-convex height of less than one tenth of the concave-convex height of the interfaces 34e and 32e, respectively.

[0101] "The interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b has a concavo-convex shape" can also be expressed as "a positive electrode mixed layer containing a positive electrode active material and an electronic insulating material is provided between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b". Similarly, "the interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b has a concavo-convex shape" can be expressed as "a negative electrode mixed layer containing a negative electrode active material and an electronic insulating material is provided between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b". The thickness of the positive electrode mixed layer is greater than 2 μm, preferably within the range of 2 to 4 μm. The thickness of the negative electrode mixed layer is greater than 2 μm, preferably within the range of 2 to 4 μm. The thickness of the positive electrode mixture layer and the negative electrode mixture layer can be measured in the same manner as the height of the irregularities at the interfaces 34e and 32e between the positive electrode electron insulating layer 34d and the negative electrode electron insulating layer 32d and the positive electrode active material layer 34b and the negative electrode active material layer 32b.

[0102] The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may contact each other. Preferably, the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may contact each other without being fixed to each other. By not fixing the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d to each other, the stress generated by the expansion and contraction of the negative electrode active material layer 32b and the positive electrode active material layer 34b accompanying the charging and discharging of the lithium ion secondary battery 100 can be alleviated, and the dendrites that may cause a short circuit between the positive electrode active material layer 34b and the negative electrode active material layer 32b can be prevented or reduced from penetrating the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d and growing.

[0103] The peel strength of the positive electrode electronic insulating layer 34d relative to the positive electrode active material layer 34b and the peel strength of the negative electrode electronic insulating layer 32d relative to the negative electrode active material layer 32b may be greater than the peel strength of the positive electrode electronic insulating layer 34d relative to the negative electrode electronic insulating layer 32d. The peel strength can be measured, for example, by a 180° tape peel test in accordance with JIS C 0806-3 1999.

[0104] The height of the unevenness of the interface 34e between the positive electrode active material layer 34b (positive electrode second active material layer 34b2) and the positive electrode electronic insulating layer 34d can be controlled by the particle size of the positive electrode active material particles and the positive electrode electronic insulating particles mentioned above, and can also be controlled by the type and viscosity of the solvent of the positive electrode slurry used in the coating formation of the positive electrode active material layer 34b (positive electrode second active material layer 34b2) and the positive electrode electronic insulating material slurry used in the coating formation of the positive electrode active material layer 34b. Similarly, the height of the unevenness of the interface 32e between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d can also be controlled by the type and viscosity of the solvent of the negative electrode slurry used in the coating formation of the negative electrode active material layer 32b and the negative electrode electronic insulating material slurry used in the coating formation of the negative electrode active material layer 32d.

[0105] The average pore size of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be controlled by the particle size of the electronic insulating particles and the pressing pressure during the pressing process. Specifically, the higher the pressing pressure, the smaller the average pore size, and the smaller the particle size of the electronic insulating particles, the smaller the average pore size.

[0106] In the diaphragm-free battery 1 having the positive electrode 34 of the above other examples, since the strength of the electronic insulating layer (positive electrode electronic insulating layer 34d and negative electrode electronic insulating layer 32d) is higher than that of the diaphragm, the effect of improving the safety of the battery can be obtained. In addition, the battery 1 of such other examples, in particular, is equipped with a positive electrode manufactured by the manufacturing method using the above-mentioned two-layer simultaneous coating, or a positive electrode manufactured by the manufacturing method using the three-layer simultaneous coating described later, which can help provide a battery with high energy density and long life.

[0107] (Variation Example) Furthermore, in a modification (lithium ion secondary battery) of the membraneless battery 1 having the positive electrode 34 of the other examples above, the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d are each a layer containing a solid electrolyte (i.e., a material having both electronic insulation and ion conductivity). The battery (lithium ion secondary battery) of this modification does not need to contain an electrolyte, so it can have high safety. In this modification, the electronic insulating particles contained in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be solid electrolyte particles. The solid electrolyte can be well formed by compression molding, so in this case, the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d do not necessarily contain a binder and a dispersant.

[0108] Furthermore, at least one of the first positive electrode active material layer 34b1 and the second positive electrode active material layer 34b2 included in the positive electrode active material layer 34b may contain a solid electrolyte in addition to the active material and an arbitrarily selected binder, conductive aid, and dispersant. Thus, the ion conductivity of at least one of the first positive electrode active material layer 34b1 and the second positive electrode active material layer 34b2 can be improved.

[0109] The negative electrode active material layer 32 b may contain a solid electrolyte in addition to the electrode active material and an optionally selected binder, conductive aid, and dispersant, thereby improving the ion conductivity of the negative electrode active material layer 32 b.

[0110] In the battery 1 of the above modification, it is not necessary to include an electrolyte and the strength of the electronic insulating layer (positive electrode electronic insulating layer 34d and negative electrode electronic insulating layer 32d) is higher than that of the separator, so it can have high safety. In addition, the battery of such a modification, in particular, is equipped with an electrode manufactured by the manufacturing method using the above-mentioned two-layer simultaneous coating, or an electrode manufactured by the manufacturing method using the three-layer simultaneous coating described later, so as to help provide a battery with high energy density and long life.

[0111] (Method for Manufacturing Separatorless Battery Having Positive Electrode According to Other Example of Embodiment) Such a separator-less battery (lithium-ion secondary battery) can be manufactured using a technique known in the technical field of the present invention, in addition to the method for manufacturing the positive electrode of the other example of the embodiment.

[0112] The positive electrode 34 of another embodiment can be manufactured by simultaneously applying the positive electrode first active material layer 34b1 and the positive electrode second active material layer 34b2 of the positive electrode active material layer 34b and the positive electrode electron insulating layer 34d, for example, as described below.

[0113] First, prepare the materials contained in the first positive electrode active material layer 34b1 (e.g., positive electrode active material, conductive aid, binder, etc.). Mix these materials and disperse the resulting mixture in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the first positive electrode slurry. In addition, prepare the materials contained in the second positive electrode active material layer 34b2 (e.g., positive electrode active material, conductive aid, binder, etc.). Mix these materials and disperse the resulting mixture in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the second positive electrode slurry. Further, prepare the materials contained in the positive electrode electronic insulating layer 34d (e.g., positive electrode electronic insulating particles, binder, dispersant, etc.). Mix these materials and disperse the resulting mixture in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain a positive electrode electronic insulating material slurry.

[0114] Next, the first positive electrode slurry, the second positive electrode slurry, and the positive electrode electronic insulating material slurry are simultaneously applied to the positive electrode collector. Thus, the second positive electrode slurry layer, the first positive electrode slurry layer, and the positive electrode electronic insulating material slurry layer are stacked in this order. Next, with the aid of a drying furnace, the solvent contained in the second positive electrode slurry layer, the first positive electrode slurry layer, and the positive electrode electronic insulating material slurry layer is volatilized, and the second positive electrode slurry layer, the first positive electrode slurry layer, and the positive electrode electronic insulating material slurry layer are dried. Thus, the first positive electrode active material layer 34b1, the second positive electrode active material layer 34b2, and the positive electrode electronic insulating layer 34d are stacked in this order on one surface of the positive electrode collector 34a. Next, the positive electrode collector 34a, the first positive electrode active material layer 34b1, the second positive electrode active material layer 34b2, and the positive electrode electronic insulating layer 34d are pressed. Specifically, a stack of the positive electrode current collector 34a, the positive electrode first active material layer 34b1, the positive electrode second active material layer 34b2, and the positive electrode electronic insulating layer 34d stacked in this order is sandwiched by a roller heated to 60 to 120°C to apply pressure. Thereafter, the stack is cut to a predetermined width to obtain a positive electrode.

[0115] In the battery 1 having the positive electrode 34 manufactured by the manufacturing method using the above three-layer simultaneous coating, as Figure 6 The structure of the positive electrode 34 shown has concavoconvex on the interface of each layer, so it is firmly close, so high safety and reliability can be obtained. Furthermore, if this manufacturing method is applied to the lithium ion secondary battery of the above-mentioned modified example, it is not necessary to include an electrolyte, and higher safety and reliability can be obtained.

[0116] In addition, in the battery 1 having the positive electrode 34 manufactured by the manufacturing method using the above three-layer simultaneous coating, the interface 34m on the positive electrode second active material layer 34b2 (positive electrode second slurry layer) side of the positive electrode first active material layer 34b1 (positive electrode first slurry layer) is not pressed by a roller. For example, the surface 34f of the positive electrode electronic insulating layer 34d on the opposite side of the positive electrode second active material layer 34b2 is pressed by a roller. Thus, in the positive electrode first active material layer 34b1 containing large-sized particles and small-sized particles whose average particle size is smaller than that of the large-sized particles as the positive electrode first active material, it is possible to suppress the large-sized particles from being damaged by the roller.

[0117] Furthermore, in the battery 1 having the positive electrode 34 manufactured by the manufacturing method using the above three-layer simultaneous coating, the surface 34f of the positive electrode electronic insulating layer 34d, which is a coating layer stacked on the positive electrode second active material layer 34b2, on the opposite side of the positive electrode second active material layer 34b2, is rolled. This can improve the adhesion of the positive electrode first active material layer 34b1, the positive electrode second active material layer 34b2, and the positive electrode electronic insulating layer 34d (coating layer), and is therefore preferred.

[0118] In particular, it is preferred that the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode second active material layer 34b2 and the interface 34m between the positive electrode second active material layer 34b2 and the positive electrode first active material layer 34b1 be formed in a manner where the unevenness becomes larger than the surface 34f of the positive electrode electronic insulating layer 34d facing the roller during roller pressing, so that the adhesion is also good and stable ion conductivity can be obtained, which is preferred. In particular, it is preferred that, for example, the difference between the unevenness of the interface 34e between the positive electrode electronic insulating layer 34d and the unevenness of the interface 34m between the positive electrode second active material layer 34b2 and the positive electrode first active material layer 34b1 is smaller than the difference between the unevenness of the surface 34f of the positive electrode electronic insulating layer 34d and the unevenness of the interface 34m between the positive electrode second active material layer 34b2 and the positive electrode first active material layer 34b1. Example

[0119] Hereinafter, the present invention will be specifically described with reference to reference examples, but the present invention is not limited to these reference examples.

[0120] [Reference Example 1] Prepared by the general formula Li 1.0 Ni Y Co Z Mn (1-Y-Z) O 2 A lithium-containing composite oxide represented by (Y=0.33 and Z=0.34) was used as the positive electrode active material. In addition, the values ​​of Y and Z in the general formula of the positive electrode active material were obtained by ICP analysis. In addition, acetylene black was prepared as a conductive auxiliary agent and polyvinylidene fluoride (PVdF) was prepared as a binder.

[0121] The positive electrode active material, the conductive aid and the binder were mixed at a weight ratio of 90:5:5. N-methyl-2-pyrrolidone (NMP) was added to the obtained mixture to adjust the viscosity, thereby obtaining a positive electrode slurry.

[0122] Aluminum foil with a thickness of 15 μm was prepared as a positive electrode collector. The positive electrode slurry was applied to both sides of the positive electrode collector by a slit coating method to form a positive electrode slurry layer. Then, the positive electrode slurry layer was dried and pressed. Thus, a positive electrode (positive electrode for lithium ion secondary battery) having a positive electrode active material layer formed on both sides of the positive electrode collector was obtained.

[0123] Natural graphite coated with amorphous carbon was prepared as a negative electrode active material, styrene-butadiene rubber (SBR) was prepared as a binder, and carboxymethyl cellulose (CMC) was prepared as a dispersion material.

[0124] The negative electrode active material, the binder and the dispersant were mixed at a weight ratio of 98:1:1. Ion exchange water was added to the obtained mixture to adjust the viscosity, thereby obtaining a negative electrode slurry.

[0125] A copper foil with a thickness of 10 μm is prepared as a negative electrode collector. A negative electrode slurry is applied to both sides of the negative electrode collector by a slit coating method to form a negative electrode slurry layer. Then, the negative electrode slurry layer is dried and pressed. Thus, a negative electrode (negative electrode for lithium ion secondary battery) having a negative electrode active material layer formed on both sides of the negative electrode collector is obtained.

[0126] A separator, a negative electrode, another separator, and a positive electrode are stacked and wound in this order. Thus, a winding group is prepared. In addition, vinylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:2, and LiPF is dissolved in the resulting mixed solution. 6 Thus, 1.0 mol / L LiPF was obtained as a non-aqueous electrolyte. 6 Solution. A lithium-ion secondary battery is produced using a wound assembly and a non-aqueous electrolyte.

[0127] [Reference Example 2] Prepare and use the formula Li 1.0 Ni Q Co R Mn (1-Q-R) O 2 A lithium ion secondary battery was prepared in the same manner as in Reference Example 1 except that a lithium-containing composite oxide represented by (Q=0.8 and R=0.1) was used as a positive electrode active material to obtain a positive electrode (positive electrode for lithium ion secondary battery). The values ​​of Q and R of the general formula of the positive electrode active material were determined by ICP analysis.

[0128] [evaluate] The life characteristics of the lithium ion secondary batteries of Reference Examples 1 and 2 were evaluated. Specifically, the initial battery capacity and direct current resistance (DCR) of the lithium ion secondary batteries of Reference Examples 1 and 2 were measured, and then the battery capacity and direct current resistance (DCR) after high temperature storage were measured. Then, based on these measurement results, the capacity retention rate and DCR increase rate of the battery before and after high temperature storage were calculated. The following is a specific description.

[0129] (1) Determination of initial battery capacity The lithium-ion secondary battery was charged at a constant current of 0.2 CA until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V. The charging was performed for a total of 2.5 hours. After 30 minutes of rest, the lithium-ion secondary battery was discharged at a constant current of 0.1 CA until the battery voltage became 2.5 V, and the discharge capacity was calculated. The discharge capacity was taken as the initial battery capacity.

[0130] (2) Determination of initial DCR The lithium-ion secondary battery was charged to a battery voltage of 4.2 V. Then, 5% of the battery capacity was discharged. After 2 hours of rest, the open circuit voltage (OCV) of the lithium-ion secondary battery was measured. Similarly, the discharge of 5% of the battery capacity and the measurement of OCV were repeated to obtain the relationship between the charging rate (SOC) and OCV.

[0131] According to the relationship between SOC and OCV, the lithium-ion secondary battery is charged from SOC0% to SOC 50% in a constant current-constant voltage (CC-CV) mode. The charging current in the constant current charge is 0.2CA. Then, the lithium-ion battery is discharged at a constant current of 1CA for 10 seconds, and the voltage drop value caused by the discharge is measured. Furthermore, the same constant current discharge is performed with a discharge current of 2CA and 3CA. The discharge current is plotted on the horizontal axis, the voltage drop value is plotted on the vertical axis, and the slope of the graph is used as the initial DCR.

[0132] (3) High temperature storage According to the relationship between SOC and OCV, the lithium-ion secondary battery was charged from SOC0% to SOC100% in a constant current-constant voltage (CC-CV) mode. Then, the lithium-ion secondary battery was stored in a constant temperature chamber at 80°C for 30 days. The lithium-ion secondary battery was taken out of the constant temperature chamber at 80°C and discharged at a constant current of 1CA until the battery voltage became 2.8V.

[0133] (4) Determination of battery capacity after high temperature storage The battery capacity of the lithium ion secondary battery after high-temperature storage was measured by the same method as the initial battery capacity measurement.

[0134] (5) Determination of DCR after high temperature storage The DCR of the lithium ion secondary battery after high-temperature storage was measured in the same manner as the initial DCR.

[0135] (6) Capacity maintenance rate and DCR increase rate Based on the above measurement results, the battery capacity after high-temperature storage was divided by the initial battery capacity to obtain the capacity retention rate. In addition, the DCR after high-temperature storage was divided by the initial DCR to obtain the DCR increase rate. The capacity retention rate and DCR increase rate of the lithium-ion secondary battery of each reference example are shown in Table 1 below.

[0136] [Table 1] Capacity maintenance rate [%] DCR increase rate [%] Reference Example 1 90 127 Reference Example 2 78 161

[0137] As shown in Table 1, the lithium ion secondary battery of Reference Example 1 having a low molar fraction of Ni in the positive electrode active material has a higher capacity retention rate and a lower DCR increase rate than the lithium ion secondary battery of Reference Example 2 having a high molar fraction of Ni in the positive electrode active material. One of the reasons for the increase in DCR is believed to be the formation of an inert layer. In the positive electrode of a lithium-ion secondary battery, an inert layer is generated on the diaphragm side (away from the collector side). Therefore, by multi-layering the electrode, the Ni content of the layer on the diaphragm side (away from the collector side) is lower than the content of the layer close to the collector side, and the formation of the inert layer of the positive electrode can be suppressed. Thus, a battery can be provided that contains the desired Ni as an electrode as a whole, minimizes the hindrance to ion conduction, has high energy density and a long life.

[0138] The present invention includes the following aspects. [Item 1] A lithium ion secondary battery comprising: A positive electrode comprising a positive electrode current collector, a positive electrode mixture layer disposed on the positive electrode current collector, and a positive electrode electron insulating layer disposed on the positive electrode mixture layer; and A negative electrode comprising a negative electrode current collector, a negative electrode mixture layer provided on the negative electrode current collector, and a negative electrode electron insulating layer provided on the negative electrode mixture layer. The height of the unevenness of the interface between the positive electrode mixture layer and the positive electrode electronic insulating layer is 2 μm or more. The height of the irregularities at the interface between the negative electrode mixture layer and the negative electrode electronic insulating layer is 2 μm or more. [Item 2] The lithium ion secondary battery according to item 1, The positive electrode electronic insulating layer and the negative electrode electronic insulating layer are in contact with each other. [Item 3] The lithium ion secondary battery according to item 1 or 2, The positive electrode electronic insulating layer and the negative electrode electronic insulating layer are not in fixed contact with each other. Explanation of symbols

[0139] 1 battery (lithium ion secondary battery), 100 charging and discharging body, 110 positive electrode (positive electrode for lithium ion secondary battery), 111 positive electrode collector, 111a current collecting portion, 111b positive electrode tab, 111c side edge, 112 positive electrode active material layer, 113 positive electrode first active material layer, 114 positive electrode second active material layer, 120 negative electrode, 121 negative electrode collector, 121a current collecting portion, 121b negative electrode tab, 121c side edge, 122 negative electrode active material layer, 130 separator, 200 container, 201 case, 202 cover, 300 external terminal, 301 positive electrode terminal, 302 negative electrode terminal, X width direction of battery 1, Y depth direction of battery 1, Z height direction of battery 1. All publications, patents and patent applications cited in this specification are incorporated herein by direct reference.

Claims

1. A positive electrode for a lithium ion secondary battery, characterized in that have: a positive electrode current collector; and a positive electrode active material layer stacked on the positive electrode current collector, The positive electrode active material layer includes: a positive electrode first active material layer stacked on the positive electrode current collector, and a positive electrode second active material layer stacked on the positive electrode first active material layer. The positive electrode first active material layer comprises: a positive electrode first active material having a lithium-containing composite oxide containing Li and Ni as a main component, The positive electrode second active material layer comprises: a positive electrode second active material having a lithium-containing composite oxide containing Li and Ni as a main component, The molar fraction of Ni in the positive electrode second active material of the positive electrode second active material layer is smaller than the molar fraction of Ni in the positive electrode first active material of the positive electrode first active material layer.

2. The positive electrode for lithium ion secondary battery according to claim 1, characterized in that The lithium-containing composite oxide of the first positive electrode active material constituting the first positive electrode active material layer further contains Co, and the lithium-containing composite oxide of the second positive electrode active material constituting the second positive electrode active material layer further contains Co, The molar fraction of Co in the second positive electrode active material is greater than the molar fraction of Co in the first positive electrode active material.

3. The positive electrode for lithium ion secondary battery according to claim 1 or 2, characterized in that: The average particle size of the positive electrode second active material in the positive electrode second active material layer is smaller than the average particle size of the positive electrode first active material in the positive electrode first active material layer.

4. The positive electrode for a lithium ion secondary battery according to claim 1 or 2, characterized in that: The first positive electrode active material layer and the second positive electrode active material layer further include a conductive additive, wherein the weight ratio of the conductive additive to the total weight of the second positive electrode active material layer is greater than the weight ratio of the conductive additive to the total weight of the first positive electrode active material layer.

5. The positive electrode for lithium ion secondary battery according to claim 1 or 2, characterized in that: The first positive electrode active material layer includes large-diameter solid particles and small-diameter solid particles having an average particle size smaller than the large-diameter solid particles as the first positive electrode active material, and the second positive electrode active material layer includes hollow particles as the second positive electrode active material.

6. A lithium ion secondary battery, characterized in that: The lithium ion secondary battery is a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is the positive electrode for a lithium ion secondary battery according to claim 1 or 2.

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