Positive Electrode for Lithium-Ion Secondary Battery and Lithium-Ion Secondary Battery

Abstract

To provide a positive electrode for a lithium-ion secondary battery that has a single positive electrode mixture layer, achieves both high safety and high energy density, and can dramatically improve manufacturing efficiency as compared with a positive electrode in which a dual-layered positive electrode mixture layer is formed in a positive electrode current collector.SOLUTION: A positive electrode for a lithium ion secondary battery includes a positive electrode mixture layer formed on one or both surfaces of a positive electrode current collector. A positive electrode active material is a mixture of a first positive electrode active material which is a lithium composite oxide having a layered structure, and a second positive electrode active material which is a polyanionic compound having an olivine structure. When the average secondary particle size of the first positive electrode active material is represented by r1 and the average secondary particle size of the second positive electrode active material is represented r2, r1>r2 is satisfied. When the area occupancy rate of the second positive electrode active material on the surface of the positive electrode mixture layer is represented by α :in percentage expression, the weight ratio of the first positive electrode active material and the second positive electrode active material is represented by 100-β:β(0<β<100), and the relationship between α and β is represented by γ=β / α, γ is equal to 0.33 or more and 0.84 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Currently, lithium-ion secondary batteries are widely used as small power sources for devices such as mobile phones and laptops, as well as large power sources for devices such as electric vehicles. As demand for lithium-ion secondary batteries increases, there is a demand for even higher energy density. Furthermore, lithium-ion secondary batteries have the risk of generating heat, so high safety is also required in addition to high energy density.

[0003] For the positive electrode material of lithium-ion secondary batteries, lithium composite oxides with a layered structure, such as LiCoO2 and LiNiO2, which operate at high potential, are used from the viewpoint of high energy density. On the other hand, polyanion compounds containing olivine-type lithium, such as LiFePO4, which has high thermal stability, are used from the viewpoint of high safety.

[0004] For this reason, Patent Document 1 discloses a positive electrode having a positive electrode composite layer containing a positive electrode active material that is a mixture of a lithium composite oxide having a layered structure with high energy density and a polyanion compound with high thermal stability.

[0005] Patent Document 2 discloses a positive electrode having a two-layer structure in which a first positive electrode composite layer and a second positive electrode composite layer containing different positive electrode active materials are formed on a positive electrode current collector (e.g., aluminum foil). In such a positive electrode, if the first positive electrode composite layer contains a lithium composite oxide having a layered structure with high energy density as the positive electrode active material, and the second positive electrode composite layer contains a polyanion compound with high thermal stability as the positive electrode active material, it becomes possible to achieve both high safety and high energy density. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6607388 [Patent Document 2] Special Publication No. 2020-512669 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in Patent Document 1, the layered lithium composite oxide, which is one of the positive electrode active materials contained in the positive electrode composite layer, has a high energy density, but at high temperatures, due to the influence of a phase change caused by oxygen desorption at high potential, there is a possibility of heat generation occurring due to the reaction between oxygen and the nonaqueous electrolyte. This poses a safety issue. The other positive electrode active material, a polyanionic compound, is highly safe but has a problem of low energy density. As a result, a positive electrode having a positive electrode composite layer containing a positive electrode active material that is a mixture of two types of positive electrode active materials has a trade-off between high safety and high energy density, making it difficult to achieve both high safety and high energy density.

[0008] Furthermore, the positive electrode of Patent Document 2 has manufacturing difficulties because the first and second positive electrode composite layers each contain different positive electrode active materials and binders. Specifically, in forming the bilayer positive electrode composite layer, the first and second positive electrode composite layers are formed by applying a slurry containing a positive electrode active material and a binder to a positive electrode current collector and drying it. Therefore, forming the first and second positive electrode composite layers (bilayer positive electrode composite layers) each containing a positive electrode active material and a binder takes twice as long as forming a single layer (single layer) of positive electrode composite layers. Furthermore, the positive electrode composite layers typically incorporated into lithium-ion secondary batteries are thin, measuring several tens of micrometers, making it extremely difficult to evenly coat the two-layer positive electrode composite layers on the positive electrode current collector. Therefore, positive electrodes with bilayer positive electrode composite layers have poor productivity.

[0009] The present invention provides a positive electrode for a lithium ion secondary battery, which has a single positive electrode composite layer containing, as the positive electrode active material, a mixture of a first positive electrode active material that is a lithium composite oxide having a layered structure and a second positive electrode active material that is a polyanionic compound having an olivine structure, and which achieves both high safety and high energy density and further enables a dramatic improvement in production efficiency compared to a positive electrode in which a two-layered positive electrode composite layer is formed on a positive electrode current collector. [Means for solving the problem]

[0010] According to the present invention, there is provided a positive electrode for a lithium-ion secondary battery, comprising a positive electrode current collector and a positive electrode composite layer formed on one or both surfaces of the positive electrode current collector. The positive electrode composite layer includes a positive electrode active material, a conductive additive, and a binder. The positive electrode active material is a mixture of a first positive electrode active material, which is a lithium composite oxide having a layered structure, and a second positive electrode active material, which is a polyanionic compound having an olivine structure. The average secondary particle diameter of the first positive electrode active material is r1, and the average secondary particle diameter of the second positive electrode active material is r2, satisfying the relationship r1 > r2. When the area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer is α, expressed as a percentage, the weight ratio of the first positive electrode active material to the second positive electrode active material is 100-β:β (0 < β < 100), and the relationship between α and β is γ = β / α, γ is 0.33 to 0.84. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a positive electrode for a lithium ion secondary battery that can achieve both high safety and high energy density and that can be produced more efficiently than a positive electrode having a two-layer structure positive electrode composite layer. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view showing an example of a lithium ion secondary battery according to the second embodiment. [Figure 2] FIG. 2 is a cross-sectional view of the lithium ion secondary battery taken along line II-II of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to the embodiments, and such modifications and improvements can also be included in the present invention.

[0014] First Embodiment The positive electrode for a lithium secondary battery according to the first embodiment includes a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a conductive additive, and a binder.

[0015] The material constituting the positive electrode current collector is not particularly limited, but is preferably a metal. Specific examples include aluminum, nickel, stainless steel, titanium, and other alloys. Among these, aluminum is preferred because of its high electronic conductivity and battery operating potential.

[0016] The positive electrode active material contained in the positive electrode mixture layer is a mixture of a first positive electrode active material which is a composite lithium oxide having a layered structure and a second positive electrode active material which is a polyanionic compound having an olivine structure.

[0017] The first positive electrode active material, a composite lithium oxide having a layered structure, for example, a layered rock salt structure, is represented by the general formula Li a The preferred composite lithium oxides are represented by the general formula LiCo x Ni y Mn z It is expressed as O2(x+y+z=1).

[0018] The second positive electrode active material, a polyanionic compound having an olivine structure, is represented by the general formula Li bThe polyanionic compound is represented by M2PO4, where M2 is at least one transition metal selected from the group consisting of Fe and Mn, and b is 0.9≦b≦1.1. A preferred polyanionic compound is LiFe q Mn r PO4(q+r=1).

[0019] When the average secondary particle diameter of the first positive electrode active material is r1 and the average secondary particle diameter of the second positive electrode active material is r2, the relationship r1>r2 is satisfied. By satisfying this relationship between the average secondary particle diameters r1 and r2, it becomes possible for a large amount of the second positive electrode active material to be present on the surface of the positive electrode mixture layer (the surface opposite to the surface in contact with the positive electrode current collector).

[0020] In the relationship r1>r2, the average secondary particle diameter (r1) of the first positive electrode active material is preferably 9 μm or more and 15 μm or less. When the average secondary particle diameter (r1) of the first positive electrode active material is 9 μm or more and 15 μm or less, the average secondary particle diameter (r2) of the second positive electrode active material is more preferably 2 μm or more and 8 μm or less. By specifying the average secondary particle diameters r1 and r2 of the first and second positive electrode active materials in this way, it becomes possible to have a larger amount of the second positive electrode active material present on the surface of the positive electrode mixture layer (the surface opposite to the surface in contact with the positive electrode current collector). A more preferable average secondary particle diameter (r1) of the first positive electrode active material is The average secondary particle diameter (r2) of the second positive electrode active material is more preferably 10 μm or more and 12 μm or less.

[0021] In the first embodiment, the "average secondary particle diameter" of the positive electrode active material refers to a value measured by the following method (laser diffraction scattering method). Specifically, a laser diffraction particle size distribution analyzer (manufactured by Horiba, Ltd., model number: LA-950) is used to prepare a dispersion by adding 0.1 g of secondary particles of the positive electrode active material to 50 ml of a 0.2 mass % aqueous solution of sodium hexametaphosphate to disperse the secondary particles. The particle size distribution of this dispersion is measured, and a volume-based cumulative particle size distribution curve is obtained. In the obtained cumulative particle size distribution curve, the particle diameter (D50) value as viewed from the fine particle side at 50% accumulation is taken as the average secondary particle diameter of the positive electrode active material.

[0022] In the first embodiment, when the area occupancy rate of the second positive electrode active material on the surface of the positive electrode mixture layer is α in percentage, the weight ratio of the first positive electrode active material to the second positive electrode active material is 100-β:β (0<β<100), and the relationship between α and β is γ=β / α, γ is 0.33 or more and 0.84 or less. Here, when the area of ​​the first positive electrode active material on the surface of the positive electrode mixture layer is a1 and the area of ​​the second positive electrode active material on the surface of the positive electrode mixture layer is a2, the area occupancy rate α (in percentage) of the second positive electrode active material on the surface of the positive electrode mixture layer is expressed by the following formula: α=[a2 / (a1+a2)]×100

[0023] The surface condition of the positive electrode composite layer (area occupancy of the second positive electrode active material) can be confirmed by scanning electron microscope (SEM, EDS) observation. The average secondary particle diameters of the first and second positive electrode active materials can be measured by SEM observation. Furthermore, elemental mapping of the first and second positive electrode active materials on the surface of the positive electrode composite layer can be performed by EDS observation. By using image analysis software to select areas on the surface of the positive electrode composite layer where the contrast of the second positive electrode active material element (e.g., Fe) is high, the area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer (α: expressed as a percentage) can be calculated.

[0024] In the above equation γ=β / α, the change in α is mainly governed by the value of β, but is increased by a factor other than the value of β, namely, the above-mentioned r1>r2. The change in γ depends more on the value of β than on the value of α.

[0025] If γ is less than 0.33, the proportion of the first positive electrode active material in the positive electrode composite layer increases, enabling a high energy density, but the area occupancy of the second positive electrode active material on the positive electrode composite layer surface decreases, making it impossible to ensure safety. On the other hand, if γ exceeds 0.84, the area occupancy of the second positive electrode active material on the positive electrode composite layer surface increases, enabling safety to be ensured, but the proportion of the first positive electrode active material in the positive electrode composite layer decreases, impairing the high energy density. A more preferable γ is 0.33 or more and 0.64 or less.

[0026] The conductive additive contained in the positive electrode mixture layer can be, for example, one or a mixture of two or more selected from the group consisting of conductive carbon powder such as graphite or carbon black, carbon nanotubes, carbon nanofibers, and graphene.

[0027] The binder contained in the positive electrode composite layer can be, for example, one or a mixture of two or more selected from the group consisting of polyethylene, polypropylene, ethylene propylene terpolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, and polyacrylonitrile.

[0028] The positive electrode according to the first embodiment can be produced by the following method. First, a first positive electrode active material, a lithium composite oxide with a layered structure and high energy density, and a second positive electrode active material made of a thermally stable polyanionic compound are prepared. A conductive additive and a binder are added to the first and second positive electrode active materials, and the resulting mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide to prepare a positive electrode slurry. This positive electrode slurry is then applied to one or both surfaces of a positive electrode current collector and dried to form a positive electrode composite layer. If necessary, the positive electrode composite layer is pressed to produce the positive electrode.

[0029] During the drying process of the cathode production, when the average secondary particle diameters of the first and second positive electrode active materials are r1 and r2, respectively, and the average secondary particle diameters of the first and second positive electrode active materials are selected so that the relationship r1 > r2 is satisfied, the second positive electrode active material in the positive electrode mixture layer, which has an average secondary particle diameter smaller than that of the first positive electrode active material, migrates toward the surface opposite to the surface in contact with the positive electrode current collector. This migration causes more of the second positive electrode active material to be present on the surface of the positive electrode mixture layer. As a result, the area occupancy of the second positive electrode active material on the surface of the positive electrode mixture layer, i.e., the surface in contact with the nonaqueous electrolyte, generally depends on the composition ratio of the second positive electrode active material in the positive electrode mixture layer. Increasing the composition ratio increases the area occupancy ratio of the second positive electrode active material, while decreasing the composition ratio decreases the area occupancy ratio of the second positive electrode active material. On the other hand, by selecting the average secondary particle diameters of the first and second positive electrode active materials so as to satisfy the relationship r1>r2, it becomes possible to increase the area occupancy rate of the second positive electrode active material on the surface of the positive electrode mixture layer due to a factor other than the blending ratio of the second positive electrode active material in the positive electrode mixture layer, namely, the action of migration of the second positive electrode active material.

[0030] Therefore, by the above-mentioned method, it is possible to obtain a positive electrode in which the relationship r1>r2 between the average secondary particle diameters (r1, r2) of the first and second positive electrode active materials is satisfied and the γ(α / β) is 0.33 or more and 0.84 or less.

[0031] According to the first embodiment described above, the average secondary particle diameter (r1) of the first positive electrode active material and the average secondary particle diameter (r2) of the second positive electrode active material satisfy the relationship r1 > r2. Furthermore, when the area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer is α (expressed as a percentage), the weight ratio of the first positive electrode active material to the second positive electrode active material is 100-β:β (0 < β < 100), and the relationship between α and β is γ = β / α, γ is specified to be 0.33 to 0.84. This increases the proportion of the first positive electrode active material in the positive electrode composite layer, thereby ensuring high energy density. At the same time, the area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer, i.e., the surface in contact with the nonaqueous electrolyte, is increased, thereby reducing contact between the first positive electrode active material, a lithium composite oxide with a layered structure having high energy density, and the nonaqueous electrolyte, ensuring safety. Therefore, it is possible to provide a positive electrode for a lithium ion secondary battery that achieves both high energy density and safety.

[0032] In particular, by setting the average secondary particle diameter of the first positive electrode active material to 9 μm or more and 15 μm or less and the average secondary particle diameter of the second positive electrode active material to be smaller than the average secondary particle diameter of the first positive electrode active material, for example, 2 μm or more and 8 μm or less, it becomes possible to further increase the area occupation ratio of the second positive electrode active material on the surface of the positive electrode composite layer.

[0033] Furthermore, according to the first embodiment, by providing a single positive electrode composite layer containing first and second positive electrode active materials, a conductive additive, and a binder, it is possible to dramatically improve production efficiency compared to a positive electrode having a two-layer structure of the positive electrode composite layer as disclosed in Patent Document 2 mentioned above.

[0034] <Second embodiment> The lithium ion secondary battery according to the second embodiment includes the above-described positive electrode for lithium ion secondary batteries, a negative electrode capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte solution. The lithium ion secondary battery further includes a separator interposed between the positive electrode and the negative electrode.

[0035] The negative electrode includes a negative electrode current collector and a negative electrode layer containing a negative electrode active material formed on one or both surfaces of the negative electrode current collector.

[0036] The material for the negative electrode current collector is not particularly limited, but is preferably a metal. Specific examples include copper, aluminum, nickel, stainless steel, titanium, and other alloys. Among these, copper is preferred from the viewpoints of electronic conductivity and battery operating potential.

[0037] The negative electrode active material is not particularly limited and can be selected from metallic lithium, lithium alloys, carbon materials, conversion-type negative electrodes, etc., but carbon materials are preferable from the viewpoint of cost. Examples of carbon materials include artificial graphite, natural graphite, mesocarbon microbeads (abbreviated as MCMB), hard carbon, and soft carbon. Among them, artificial graphite and natural graphite are preferred because they have higher capacity.

[0038] When the negative electrode active material is metallic lithium or a lithium alloy, a foil of the metallic lithium or lithium alloy is attached to one or both surfaces of a negative electrode current collector to form a negative electrode layer, thereby producing a negative electrode.

[0039] On the other hand, when the negative electrode active material is a material other than metallic lithium or a lithium alloy, such as a carbon material, the negative electrode active material is dispersed in a solvent together with a binder and, if necessary, a conductive additive to prepare a negative electrode slurry. The negative electrode slurry is applied to one or both surfaces of a negative electrode current collector, dried, and, if necessary, pressed with a roller or the like to form a negative electrode layer containing the negative electrode active material and the binder, thereby producing a negative electrode.

[0040] The binder and conductive additive in the negative electrode layer may be the same as the binder and conductive additive in the positive electrode mixture layer described in the first embodiment.

[0041] The non-aqueous electrolyte contains a non-aqueous solvent with a lithium salt dissolved therein. The lithium salt may be, for example, one or a mixture of two or more selected from the group consisting of LiBF, LiPF, Li(FSO)N, and Li(CFSO)N.

[0042] The nonaqueous solvent is not particularly limited, but examples include one or a mixture of two or more solvents selected from the group consisting of dimethyl carbonate (DEC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, and benzonitrile. DMC, DEC, DPC, EMC, EC, and PC are particularly preferred. Among these, EC is preferred because it can form a good coating on the negative electrode active material.

[0043] The nonaqueous electrolyte preferably further contains an additive other than the lithium salt for the purpose of forming a high-quality coating on the surface of the negative electrode active material through reductive decomposition during charge and discharge. The additive is not particularly limited, but examples include vinylene carbonate, fluoroethylene carbonate, 1,3,2-dioxathiolane-2,2-dioxide (MMDS), 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide, tris(trimethylsilyl) phosphite, 1-propene-1,3-sultone, and Li2PO2F2. These additives may be used alone or in combination.

[0044] The separator may be, for example, a porous sheet made of polymer or fiber, or a nonwoven fabric separator, etc. The separator may also be one in which a ceramic layer, which is a heat-resistant insulating layer, is laminated on a porous substrate.

[0045] The positive electrode, negative electrode, separator, and nonaqueous electrolyte are housed in an exterior body. The exterior body is not particularly limited, but examples thereof include a bag-shaped exterior body having a laminate film, a coin-shaped metal can, a cylindrical metal can, and a rectangular metal can.

[0046] The structure of a lithium ion secondary battery according to the second embodiment will be described below with reference to the drawings, taking a stacked lithium ion secondary battery as an example. Fig. 1 is a perspective view showing an example of a stacked lithium ion secondary battery, and Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1.

[0047] The laminated lithium-ion secondary battery 1 includes a bag-shaped exterior body 2 made of laminate film. A flat electrode element 3 is housed within the exterior body 2. The laminate film has a structure in which, for example, multiple (e.g., two) plastic films are laminated together with a metal foil such as aluminum foil sandwiched between them. One of the two plastic films is a heat-sealable resin film. The exterior body 2 includes two laminate films stacked together with the heat-sealable resin films facing each other, with the electrode element 3 interposed between these laminate films, and the two laminate film portions around the electrode element 3 sealed together by heat sealing, thereby hermetically housing the electrode element 3.

[0048] As shown in FIG. 2, the electrode element 3 has a structure in which a positive electrode 4, a negative electrode 5, and a separator 6 interposed between the positive electrode 4 and the negative electrode 5 are stacked in multiple layers, with the negative electrode 5 being the outermost layer. The positive electrode 4 is composed of a positive electrode current collector 42 and positive electrode composite layers 41, 41 formed on both sides of the current collector 42. The negative electrode 5 positioned in the outermost layer is composed of a negative electrode current collector 52 and a negative electrode layer 51 made of metallic lithium formed on the surface of the current collector 52 facing the separator 6. The negative electrodes 5 positioned between the positive electrodes 4, excluding the negative electrode 5 positioned in the outermost layer, are composed of a negative electrode current collector 52 and a negative electrode layers 51, 51 made of metallic lithium formed on both sides of the current collector 52.

[0049] The positive electrode 4 has a positive electrode lead 43 in which a positive electrode current collector 42 extends from, for example, the right side surface of the positive electrode mixture layer 41. The positive electrode leads 43 are bundled at their tip ends within the exterior body 2 and joined to each other. One end of the positive electrode terminal 7 is joined to the joint of the positive electrode lead 43, and the other end extends to the outside through the sealing part of the exterior body 2. The negative electrode 5 has a negative electrode lead 53 in which a negative electrode current collector 52 extends from, for example, the left side surface of the negative electrode layer 51. The negative electrode leads 53 are bundled at their tip ends within the exterior body 2 and joined to each other. One end of the negative electrode terminal 8 is joined to the joint of the negative electrode lead 53, and the other end extends to the outside through the sealing part of the exterior body 2.

[0050] As described above, according to the second embodiment, by including the positive electrode of the first embodiment described above, it is possible to provide a lithium ion secondary battery that achieves both high safety and high energy density. [Example]

[0051] The present invention will be described in more detail below by way of examples, but is not limited to these examples.

[0052] Example 1 The first positive electrode active material is LiCo having an average secondary particle diameter of 10 μm. 0.2 Ni 0.5 Mn 0.3 O2 (hereafter abbreviated as NCM) and the second positive electrode active material, LiMn 0.5 Fe 0.5The positive electrode active material mixture was mixed with acetylene black (AB) and graphene (LMFP) at a weight ratio of 70:30. The conductive additives, acetylene black (AB) and graphene, and the binder, polyvinylidene fluoride (PVDF), were mixed at a weight ratio of 90:3:3:4 (mixed positive electrode active material:acetylene black:graphene:PVDF). The mixture was then dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil positive electrode current collector, dried, and pressed using a roll press to produce a positive electrode with a positive electrode composite layer with a density of 2.5 g / cc.

[0053] The electrochemical properties were evaluated using a 2032-type coin-type lithium secondary battery (hereinafter abbreviated as coin-type cell) having the structure described below.

[0054] A 300 μm thick lithium metal foil was attached to the surface of a 100 μm thick stainless steel foil negative electrode current collector to form a negative electrode layer, thereby preparing a negative electrode.

[0055] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a lithium salt at a ratio of 1.3 mol / L in a mixed solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3.

[0056] A coin-type cell was produced using the obtained positive electrode, negative electrode, electrolyte, and a separator made of a microporous polyolefin film in an argon atmosphere with a dew point of −50° C. or lower.

[0057] Example 2 A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio (NCM:LMFP) of the first positive electrode active material (NCM) to the second positive electrode active material (LMFP) was set to 80:20, as in Example 1. A coin-type cell was then manufactured using the positive electrode in the same manner as in Example 1.

[0058] Example 3 A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio (NCM:LMFP) of the first positive electrode active material (NCM) to the second positive electrode active material (LMFP) was set to 90:10, as in Example 1. A coin-type cell was then manufactured using the positive electrode in the same manner as in Example 1.

[0059] (Comparative Example 1) A positive electrode was prepared in the same manner as in Example 1, except that the weight ratio (NCM:LMFP) of the first positive electrode active material (NCM) to the second positive electrode active material (LMFP) was set to 60:40, as in Example 1. A coin-type cell was then manufactured using the positive electrode in the same manner as in Example 1.

[0060] (Comparative Example 2) A positive electrode was produced in the same manner as in Example 1, except that the weight ratio (NCM:LMFP) of the first positive electrode active material (NCM) to the second positive electrode active material (LMFP) was set to 90:10, the average secondary particle diameter of the first positive electrode active material was set to 5 μm, and the average secondary particle diameter of the second positive electrode active material was set to 7 μm.Furthermore, a coin-type cell was produced using the positive electrode in the same manner as in Example 1.

[0061] (Comparative Example 3) The first positive electrode active material is LiCo with an average particle size of 10 μm. 0.2 Ni 0.5 Mn 0.3 A positive electrode was prepared in the same manner as in Example 1 except that O2(NCM) was used, and a coin-type cell was produced in the same manner as in Example 1 using the positive electrode.

[0062] The positive electrodes incorporated into the coin-type cells of Examples 1-3 and Comparative Examples 1-3 were observed using an EDS-equipped scanning electron microscope (manufactured by JEOL Ltd., product name: JSM-IT100) to confirm the surface condition. That is, based on the results of SEM images and EDS element mapping, the area occupancy of the LMFP (second positive electrode active material) on the surface of the positive electrode composite layer (the surface opposite to the surface in contact with the positive electrode current collector) was measured. The results are shown in Table 1 below.

[0063] The area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer was represented by α, the weight ratio of the first positive electrode active material to the second positive electrode active material was 100-β:β (0<β<100), and the relationship between α and β was γ=β / α. Based on this area occupancy and the blending ratio of the second positive electrode active material, the value of parameter γ was calculated. The results are shown in Table 1 below.

[0064] The positive electrode used in Comparative Example 3 has a positive electrode active material in the positive electrode mixture layer of LiCo having an average particle size of 10 μm, which is the first positive electrode active material. 0.2 Ni 0.5 Mn 0.3 Since only O2(NCM) is present, the area occupancy rate of the second positive electrode active material (LMFP) and the value of the parameter γ are both zero.

[0065] <Coin cell evaluation test> The coin-type cells of Examples 1-3 and Comparative Examples 1-3 were each transferred to a thermostatic chamber set at 25°C, and an initial activation process was carried out for 5 cycles. The charge / discharge conditions were a constant current and constant voltage of 0.1C, 4.2V, and 0.05C cutoff current for charge, and a constant current of 0.1C and 2.75V cutoff voltage for discharge.

[0066] The mass energy densities (Wh / kg) of the first and second positive electrode active materials were calculated from the obtained charge / discharge curves. The method for calculating the mass energy densities is described below. Energy density (Wh / kg) = Average discharge voltage (V) × discharge capacity (Ah) / mass of positive electrode active material (kg) The results are shown in Table 1.

[0067] <Assembly and evaluation test of lithium-ion secondary batteries> Lithium ion secondary batteries were fabricated to confirm the safety of the positive electrodes of Examples 1-3 and Comparative Examples 1-3. Note that the negative electrodes used in these tests used graphite as the active material of the negative electrodes instead of metallic lithium in order to eliminate the effect of lithium melting at a high temperature of 200°C.

[0068] That is, ten positive electrodes fabricated in the same manner as in Example 1 were prepared and cut to a predetermined size. Furthermore, graphite, which is a negative electrode active material, acetylene black, which is a conductive additive, and polyvinylidene fluoride (PVDF), which is a binder, were mixed in a weight ratio (graphite:acetylene black:PVDF) of 97:1.5:1.5. The mixture was then dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry. The negative electrode slurry was then applied to the surface of a copper foil, which was a negative electrode current collector, dried, and pressed using a roll press to prepare a negative electrode having a negative electrode composite layer with a density of 1.2 g / cc. Eleven of the resulting negative electrodes were prepared and each was cut to a predetermined size. Next, ten bag-shaped separators were prepared, and the ten positive electrodes cut to a predetermined size were housed in each bag-shaped separator. The positive electrode and the negative electrode were then alternately stacked in a manner that the negative electrodes were positioned in the lowermost and uppermost layers, respectively, to obtain a laminated electrode assembly. In the resulting laminated electrode assembly, terminal tabs were connected by ultrasonic welding to the positive electrode composite layer-free portions of the positive electrodes and the negative electrode composite layer-free portions of the negative electrodes, respectively.

[0069] Next, a bag-shaped exterior body made of two laminate films was prepared. Each laminate film had a structure in which aluminum foil was sandwiched between a heat-sealable resin film and a polyethylene terephthalate film. One laminate film had a recess formed, for example, by drawing, while the other laminate film had a flat shape. A tab-attached laminated electrode group was placed in the recess of one laminate film so that the terminal tab protruded outward. Then, the flat laminate film was placed on top of one laminate film so that the heat-sealable resin films were in contact with each other, and the three peripheral edges were heat-sealed to seal the tab-attached laminated electrode group within the exterior body. Next, a nonaqueous electrolyte was injected through the remaining unsealed edge of the exterior body, and the cell was vacuum-sealed to assemble a sealed lithium-ion secondary battery (5 Ah-class laminated cell). The non-aqueous electrolyte used was a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3, in which LiPF6 was dissolved as a lithium salt at a ratio of 1.3 mol / L.

[0070] In Examples 2 and 3 and Comparative Examples 1 to 3, 5 Ah class laminate cells were also assembled by the same method as above, except that the respective positive electrodes were used.

[0071] A heating limit test was conducted using the 5 Ah class laminate cells of Examples 1-3 and Comparative Examples 1-3. In the heating limit test, the lithium ion secondary batteries (Examples 1-3 and Comparative Examples 1-3) adjusted to an upper limit voltage of 4.2 V and an SOC of 100% were heated to 200°C at a heating rate of 10°C / min and held at that temperature for 3 hours. Safety was judged as follows: the presence or absence of ignition in the heating limit test was judged as ×, and the absence or presence of ignition was judged as ◯. The results are shown in Table 1 below.

[0072] When the first positive electrode active material (a lithium composite oxide having a layered structure, such as NCM) is subjected to an electric potential, the decomposition reaction of the electrolyte proceeds gradually. When the first positive electrode active material comes into contact with the non-aqueous electrolyte, oxygen is generated due to the decomposition of the non-aqueous electrolyte. Furthermore, the decomposition of the non-aqueous electrolyte is accelerated at high temperatures. Oxygen generation at high temperatures may lead to fire. This makes it impossible to ensure safety. Therefore, oxygen generation can be effectively suppressed by providing a large amount of a second positive electrode active material (a polyanionic compound with high thermal stability, such as LMFP) on the surface of the positive electrode mixture layer opposite the surface that contacts the positive electrode current collector.

[0073] [Table 1]

[0074] As is clear from Table 1, when the positive electrode composite layer contains a positive electrode active material that is a mixture of first and second positive electrode active materials, and when the average secondary particle diameter of the first positive electrode active material is r1 and the average secondary particle diameter of the second positive electrode active material is r2, the relationship r1>r2 is satisfied, and when the area occupancy of the second positive electrode active material on the surface of the positive electrode composite layer is α expressed as a percentage, the weight ratio of the first positive electrode active material to the second positive electrode active material is 100-β:β (0<β<100), and the relationship between α and β is γ=β / α, it can be seen that all of the coin-type cells of Examples 1-3 equipped with positive electrodes having a parameter γ value of 0.33 or more and 0.84 or less can ensure high safety and high energy density.

[0075] In contrast, the coin-type cell of Comparative Example 1, in which the positive electrode composite layer contains a positive electrode active material that is a mixture of first and second positive electrode active materials, and where the average secondary particle diameters of the first and second positive electrode active materials are r1 and r2, respectively, satisfies the relationship r1>r2, but is equipped with a positive electrode in which the parameter γ value is 0.98, exceeding the upper limit of 0.33 or more and 0.84 or less, can ensure high safety but cannot ensure high energy density.

[0076] Further, when the positive electrode composite layer contains a positive electrode active material that is a mixture of first and second positive electrode active materials, and the average secondary particle diameters of the first and second positive electrode active materials are r1 and r2, respectively, the relationship is r1 < r2, and the coin-type cell of Comparative Example 2 having a positive electrode with a γ value of the parameter exceeding the upper limit value (0.84) of the above range being 0.91 can ensure a high energy density, but it can be seen that the safety is impaired.

[0077] The coin-type cell of Comparative Example 3 having a positive electrode with a positive electrode composite layer containing a positive electrode active material composed of NCM with an average secondary particle diameter of 10 μm as the first positive electrode active material can ensure a high energy density, but the safety is further impaired compared to the coin-type cell of Comparative Example 3. This is because the surface of the positive electrode composite layer in contact with the non-aqueous electrolyte is a lithium composite oxide (for example, NCM) having a layered structure. When it comes into contact with the non-aqueous electrolyte, oxygen is generated due to the decomposition of the non-aqueous electrolyte, and there is a risk of ignition at high temperatures (200 °C or higher).

Explanation of Reference Numerals

[0078] 1... Lithium-ion secondary battery, 2... Outer package, 3... Electrode element, 4... Positive electrode, 5... Negative electrode, 41... Positive electrode composite layer, 42... Positive electrode current collector, 43... Positive electrode lead, 51... Negative electrode layer, 52... Negative electrode current collector, 53... Negative electrode lead, 6... Separator, 7... Positive electrode terminal, 8... Negative electrode terminal

Claims

1. A positive electrode for a lithium ion secondary battery comprising a positive electrode current collector and a positive electrode mixture layer formed on one or both surfaces of the positive electrode current collector, the positive electrode mixture layer includes a positive electrode active material, a conductive additive, and a binder, the positive electrode active material is a mixture of a first positive electrode active material that is a lithium composite oxide having a layered structure and a second positive electrode active material that is a polyanionic compound having an olivine structure, When an average secondary particle diameter of the first positive electrode active material is r1 and an average secondary particle diameter of the second positive electrode active material is r2, a relationship of r1>r2 is satisfied, a positive electrode for a lithium ion secondary battery, wherein α is an area occupancy rate of the second positive electrode active material on the surface of the positive electrode mixture layer expressed as a percentage, γ is a weight ratio of the first positive electrode active material to the second positive electrode active material of 100-β:β (0<β<100), and γ is a relationship between α and β of 0.33 or more and 0.84 or less.

2. The lithium composite oxide having a layered structure, which is the first positive electrode active material, is represented by the general formula Li a M1O 2 2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein M1 is at least one transition metal selected from the group consisting of Co, Ni, Mn, and Al, and a is 0.9≦a≦1.

1.

3. The polyanionic compound having an olivine structure, which is the second positive electrode active material, is represented by the general formula Li ｂ M2PO ４ 3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein M2 is at least one transition metal selected from the group consisting of Fe and Mn, and b is 0.9≦b≦1.

1.

4. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein an average secondary particle diameter (r1) of the first positive electrode active material is 9 μm or more and 15 μm or less, and an average secondary particle diameter (r2) of the second positive electrode active material is smaller than r1.

5. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, a negative electrode capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte solution.

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