Positive electrode active material and battery

Abstract

To provide a positive electrode active material which has a high ratio of nickel in a positive electrode active material and which provides a battery which exhibits an excellent capacity retention rate even under low restraining pressure, and a battery which uses this positive electrode active material.SOLUTION: A positive electrode active material includes nickel as a transition metal, the ratio of nickel is 70 mol% or more of the total transition metals, polycrystalline particles and single crystal particles, and the value of B / A calculated from the amount A (mass%) of lithium hydroxide in the polycrystalline particles and the amount B (mass%) of lithium hydroxide in the single crystal particles is 1.03 to 1.9.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material and a battery.

Background Art

[0002] Lithium transition metal composite oxides, which are composite oxides of lithium and transition metals, are widely used as positive electrode active materials for lithium ion secondary batteries. Among lithium transition metal composite oxides, those containing nickel as a transition metal and having a nickel ratio of 70 mol% or more of the total transition metals have a high energy density and their use as positive electrode active materials is expanding.

[0003] Positive electrode active materials with a high nickel ratio tend to have a large volume change during charge and discharge and are prone to particle cracking. In particular, when the positive electrode active material is in the form of particles having a polycrystalline structure, cracks are likely to occur at the grain boundaries in the polycrystalline structure. Cracking of the particles of the positive electrode active material is a factor in the reduction of the cycle life of the battery. Therefore, suppressing the reduction of the cycle life by combining single crystal particles, which are relatively less likely to crack, with polycrystalline particles of the positive electrode active material has been studied (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] A battery including a positive electrode containing polycrystalline particles and single crystal particles of a positive electrode active material has room for improvement in the capacity retention rate in a state where the nickel ratio of the positive electrode active material is high and the constraint pressure of the battery is low. This disclosure aims to provide a positive electrode active material that can obtain a battery exhibiting excellent capacity retention even when the ratio of nickel in the positive electrode active material is high and the confinement pressure is low, and a battery using this positive electrode active material.

Means for Solving the Problems

[0006] The means for solving the above problems include the following embodiments. <1> It contains nickel as a transition metal, the ratio of nickel is 70 mol% or more of the total transition metals, It contains polycrystalline particles and single crystal particles, A positive electrode active material in which the value of B / A calculated from the lithium hydroxide amount A (mass%) of the polycrystalline particles and the lithium hydroxide amount B (mass%) of the single crystal particles is 1.03 to 1.9. <2> The positive electrode active material according to <1>, wherein the positive electrode active material has a layered structure. <3> The positive electrode active material according to <1> or <2>, wherein the mass ratio of polycrystalline particles to single crystal particles (polycrystalline particles: single crystal particles) is in the range of 60:40 to 90:10. <4> A battery comprising a positive electrode containing the positive electrode active material according to any one of <1> to <3>. <5> The battery according to <4>, wherein the confinement pressure is less than 500 kPa.

Advantages of the Invention

[0007] According to an embodiment of the present disclosure, there are provided a positive electrode active material that can obtain a battery exhibiting excellent capacity retention even when the ratio of nickel in the positive electrode active material is high and the confinement pressure is low, and a battery using this positive electrode active material.

Modes for Carrying Out the Invention

[0008] In the present disclosure, the numerical range indicated by using "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described step by step in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other step-by-step descriptions. In the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the amount of each component means the total amount of a plurality of substances corresponding to each component, unless otherwise specified, when there are a plurality of substances corresponding to each component.

[0009] <Positive electrode active material> The positive electrode active material of the present disclosure is including nickel as a transition metal, the ratio of nickel being 70 mol% or more of the total transition metals, including polycrystalline particles and single crystal particles, The value of B / A calculated from the lithium hydroxide amount A (mass%) of the polycrystalline particles and the lithium hydroxide amount B (mass%) of the single crystal particles is 1.03 to 1.9.

[0010] As shown in the examples described later, a battery obtained by using a positive electrode active material in which the positive electrode active material includes polycrystalline particles and single crystal particles and the value of B / A is in the range of 1.03 to 1.9 exhibits an excellent capacity retention rate compared to a battery obtained by using a positive electrode active material that does not include either polycrystalline particles or single crystal particles, or a positive electrode active material in which the value of B / A is outside the range of 1.03 to 1.9. The reason is speculated as follows, for example. However, the present disclosure is not limited by the following speculation.

[0011] The single-crystalline particles of the positive electrode active material are isolated by repeated expansion and contraction accompanying charge and discharge of the battery, and the conductive path is easily cut off. In the positive electrode active material of the present disclosure, since the amount of lithium hydroxide present on the surface of the single-crystalline particles is relatively large (that is, the value of B / A is 1.03 or more), a relatively large amount of binder and conductive assistant are arranged around the single-crystalline particles in the positive electrode. As a result, the disconnection of the conductive path due to the isolation of the single-crystalline particles is less likely to occur, and the decrease in the capacity retention rate of the battery is suppressed. On the other hand, since the value of B / A is 1.9 or less, a sufficient amount of binder and conductive assistant are also arranged around the polycrystalline particles, and the decrease in the capacity retention rate of the battery is suppressed.

[0012] In the present disclosure, the "polycrystalline particles" are particles having a polycrystalline structure that is an aggregate of single crystals. The polycrystalline particles are, for example, in the state of secondary particles formed by aggregation of primary particles of single crystals. The number of primary particles constituting one secondary particle may be, for example, 10 or more, 50 or more, or 100 or more. The number of primary particles constituting one secondary particle may be, for example, 10,000 or less. The average particle diameter of the polycrystalline particles can be selected, for example, from the range of 5.0 μm to 30.0 μm, or 7.0 μm to 20.0 μm. The average particle diameter of the polycrystalline particles is the particle diameter (D50) when the volume accumulation is 50% in the volume-based particle size distribution. The volume-based particle size distribution is obtained, for example, by the laser diffraction / scattering method.

[0013] In the present disclosure, the "single-crystalline particles" mean particles having the same crystal axis direction at any position. The single-crystalline particles are, for example, in the state of primary particles (single particles) that do not form secondary particles. The average particle diameter of the single-crystalline particles can be selected, for example, from the range of 0.5 μm to 10.0 μm, or 1.0 μm to 5.0 μm. The average particle diameter of the single-crystalline particles is the particle diameter (D50) when the volume accumulation is 50% in the volume-based particle size distribution. The volume-based particle size distribution is obtained, for example, by the laser diffraction / scattering method.

[0014] The cathode active material of the present disclosure contains nickel as a transition metal, and is not particularly limited as long as the ratio of nickel is 70 mol% or more of the total transition metals. The ratio of nickel may be 75 mol% or more, or 80 mol% or more of the total transition metals. The ratio of nickel may be 90 mol% or less, or 85 mol% or less of the total transition metals.

[0015] It is more preferable that the cathode active material contains nickel as a transition metal and at least one selected from cobalt and manganese, and it is even more preferable to contain nickel, cobalt and manganese (NCM, nickel cobalt manganese oxide).

[0016] The cathode active material may consist only of lithium, oxygen, and transition metals selected from Ni, Co, and Mn, or may contain elements other than these (hereinafter also referred to as other elements). When the cathode active material contains other elements, the proportion may be 10 mol% or less, 5 mol% or less, or 1 mol% or less of the entire cathode active material. When the cathode active material contains other elements, the proportion may be 0.001 mol% or more, 0.01 mol% or more, or 0.1 mol% or more of the entire cathode active material.

[0017] The cathode active material is preferably a composite oxide (lithium transition metal composite oxide) containing lithium and one or more transition metals. The cathode active material preferably has a layered structure. The layered structure may be, for example, a crystal structure in which a transition metal layer composed of an octahedral structure composed of transition metal atoms and oxygen atoms and a lithium layer are alternately arranged.

[0018] The cathode active material may be a compound having a composition represented by the following formula (1). Li 1-a Ni x Me 1-x O 2 (1) In formula (1), a satisfies the relationship of -0.3 ≦ a ≦ 0.3, x satisfies the relationship of 0.7 ≦ x ≦ 1.0, Me represents at least one selected from the group consisting of Co, Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, Si, V, Cr, and Ge.

[0019] In the cathode active material of the present disclosure, the mixing ratio of polycrystalline particles and single crystal particles is not particularly limited. From the viewpoint of the balance of the characteristics of the cathode active material, the mass ratio of polycrystalline particles to single crystal particles (polycrystalline particles: single crystal particles) may be selected from the range of 60:40 to 90:10, or 65:35 to 85:15. The cathode active material may or may not contain particles that do not correspond to polycrystalline particles or single crystal particles (such as primary particles with a particle diameter of less than 0.5 μm). The proportion of particles that do not correspond to polycrystalline particles or single crystal particles is preferably 0 mass% to 10 mass% of the entire cathode active material.

[0020] The amount of lithium hydroxide A in the polycrystalline particles and the amount of lithium hydroxide B in the single crystal particles can be independently selected from the range of, for example, 0.05 mass% to 1.0 mass%, or 0.1 mass% to 6.0 mass%. Since the cathode active material of the present disclosure contains nickel at a high content rate, it has a high reactivity with moisture, and there is a tendency for a large amount of lithium hydroxide to be generated on the surface of the fired product obtained by firing the raw material of the cathode active material. Therefore, the adjustment of the amount of lithium hydroxide A in the polycrystalline particles and the amount of lithium hydroxide B in the single crystal particles may be performed by removing a part of the lithium hydroxide present on the surface of the fired product of the raw material of the cathode active material. In the present disclosure, the amount of lithium hydroxide A in the polycrystalline particles and the amount of lithium hydroxide B in the single crystal particles are determined by a neutralization titration method. There is no particular limitation as long as the value of B / A calculated from the amount of lithium hydroxide A in the polycrystalline particles and the amount of lithium hydroxide B in the single crystal particles is within the range of 1.03 to 1.9. From the viewpoint of maintaining the capacity retention rate of the battery well, the value of B / A may be 1.05 or more, 1.08 or more, or 1.1 or more. From the perspective of maintaining a good capacity retention rate of the battery, the value of B / A may be 1.85 or less, 1.8 or less, or 1.7 or less.

[0021] The raw material of the positive electrode active material is not particularly limited and can be selected from known raw materials. For example, the raw material of the positive electrode active material may be a mixture containing a compound containing a transition metal (such as hydroxide, carbonate, etc.) and a compound containing lithium (such as hydroxide, carbonate, etc.).

[0022] <Battery> The battery of the present disclosure includes a positive electrode containing the above-described positive electrode active material. As shown in the examples described below, the battery of the present disclosure exhibits an excellent capacity retention rate even in a state of low constraint pressure. Reducing the constraint pressure of the battery is effective, for example, as a measure to reduce the amount of electrolyte discharged outside the negative electrode when the negative electrode expands during charging of the battery. The constraint pressure of the battery may be less than 500 kPa, 400 kPa or less, 300 kPa or less, or 200 kPa or less. The constraint pressure of the battery may be 100 kPa or more, or 150 kPa or more.

[0023] The positive electrode included in the battery includes, for example, a current collector and a positive electrode layer disposed on the current collector, and the positive electrode layer contains the positive electrode active material of the present disclosure. The positive electrode layer may be disposed on one side or both sides of the current collector.

[0024] Examples of the material constituting the current collector of the positive electrode include aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector include foil, mesh, etc.

[0025] The positive electrode layer is disposed on the current collector, for example, by coating a slurry-like positive electrode material on one side or both sides of the current collector. If necessary, a pressure treatment may be performed to adjust the density of the positive electrode layer. The thickness of the positive electrode layer is not particularly limited and can be selected, for example, from the range of 10 μm to 100 μm.

[0026] The positive electrode material may be in the form of a mixture containing components other than the positive electrode active material, such as a conductive assistant and a binder. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

[0027] Specific examples of the conductive assistant include carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), carbon nanotubes, and graphite. The conductive material contained in the positive electrode material may be a single type or two or more types.

[0028] Specific examples of the binder include polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, polymethacrylate, and the like. The binder contained in the positive electrode material may be a single type or two or more types.

[0029] The battery of the present disclosure includes, for example, a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes, for example, a current collector and a negative electrode layer disposed on the current collector and containing a negative electrode active material. Examples of the type of the negative electrode active material include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon, silicon, metallic lithium, lithium alloy, and lithium titanate (LTO). Examples of the material constituting the current collector of the negative electrode include copper, copper alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector of the negative electrode include foil, mesh, etc.

[0030] The electrolyte may be either liquid or solid. As the liquid electrolyte (electrolyte solution), a known electrolyte such as LiPF 6 dissolved in an organic solvent can be used without particular limitation. Specific examples of the organic solvent include cyclic or chain carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The solvent may be a mixture of two or more solvents, or may be a mixture containing a cyclic carbonate and a chain carbonate. The solvent may contain additives such as vinylene carbonate (VC). As the solid electrolyte, known solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, and halide solid electrolytes can be used without particular limitation.

[0031] The lithium-ion secondary battery may include a separator disposed between the positive electrode and the negative electrode. Examples of the separator include non-woven fabrics, cloths, and microporous films mainly composed of polyolefins such as polyethylene and polypropylene.

Examples

[0032] Hereinafter, the present disclosure will be described in more detail with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0033] <Examples 1 to 3> (1) Preparation of positive electrode Polycrystalline particles (average particle diameter: 10.3 μm) and single crystal particles (average particle diameter: 3.5 μm) of a positive electrode active material containing nickel (80 mol%), cobalt (10 mol%), and manganese (10 mol%) as transition metals were prepared respectively. The amount of lithium hydroxide present on the surfaces of the polycrystalline particles and the single crystal particles was adjusted by changing the particle washing conditions. The amounts of lithium hydroxide A for the polycrystalline particles and B for the single crystal particles were determined by neutralization titration. Specifically, 100 mL of pure water was added to 10 g of the measurement sample, stirred for 1 minute, followed by suction filtration, and titration was performed on the obtained filtrate. 1 mol / L hydrochloric acid was used as the measurement reagent, and F-72 manufactured by HORIBA was used as the pH meter. The amount of lithium hydroxide (g) obtained from the titration result was divided by the amount of the measurement sample (10 g) to calculate the amount of lithium hydroxide A or lithium hydroxide B (mass%).

[0034] The polycrystalline particles and single-crystalline particles of the positive electrode active material were mixed at the ratios shown in Table 1 to obtain a positive electrode active material mixture. The positive electrode active material mixture (97.8 parts by mass), carbon nanotubes (0.8 parts by mass), and polyvinylidene fluoride (1.4 parts by mass) were mixed, and the viscosity was adjusted with a solvent to obtain a slurry-like positive electrode composite material. The positive electrode composite material was applied onto an aluminum foil (thickness: 30 μm) with a doctor blade so that the areal density on one side was 34 mg / cm 2 and dried at 100°C for 10 minutes to form a positive electrode layer. Thereafter, press treatment was performed so that the density of the positive electrode layer became 3.3 g / cm 3 .

[0035] (2) Fabrication of negative electrode Artificial graphite particles (96 parts by mass) with an average particle diameter of 22 μm, styrene-butadiene rubber (3 parts by mass), and carboxymethyl cellulose (1 part by mass) were mixed, and the viscosity was adjusted with a solvent to obtain a slurry-like negative electrode composite material. The negative electrode composite material was applied onto a copper foil (thickness: 15 μm) with a doctor blade so that the areal density on one side was 23 mg / cm 2 (i.e., the ratio of the positive electrode capacity to the negative electrode capacity was 1.1) and dried at 100°C for 10 minutes to form a negative electrode layer. Thereafter, press treatment was performed so that the density of the negative electrode layer became 1.25 g / cm 3 .

[0036] (3) Fabrication of battery The positive electrode and the negative electrode were laminated with a separator (three-layer structure of PP / PE / PP, thickness: 16 μm) interposed therebetween to fabricate an electrode body. A laminated battery was fabricated using this electrode body and an electrolytic solution. As the electrolytic solution, a solution in which 1.1 M of LiPF 6 was dissolved in a mixed solvent of EC (30% by volume), DMC (40% by volume), and EMC (30% by volume) was used. The restraint pressure of the battery was 100 kPa.

[0037] (4) Evaluation of battery performance The activation treatment of the battery was carried out by the constant current-constant voltage method according to the following procedure. Specifically, constant current charging was performed at a current value of 0.1C up to 4.25V, then constant voltage charging was performed until the constant voltage charging time reached 3 hours, and then discharging was performed at a current value of 0.1C up to 3.0V by the constant current method. (Measurement of capacity retention rate) The following charge and discharge were carried out 200 cycles, and the capacity retention rate (%) after 200 cycles with respect to the initial (0 cycle) capacity was calculated. In an environment of 60°C, charging was performed up to 4.25V at a current value of 0.3C by the constant current-constant voltage method, and discharging was performed up to 3.0V at a current value of 0.5C by the constant current-constant voltage method. (Measurement of resistance) The resistance of the cell was calculated by the following charge and discharge, and the resistance at the initial (0 cycle) and the resistance after 200 cycles were compared. The adjustment was made to 3.71V at a current value of 0.3C by the constant current-constant voltage method, and the resistance value was calculated from the cell voltage after discharging for 10 seconds at a current value of 1C.

[0038] <Comparative Examples 1 to 5> Batteries of Comparative Examples 1 to 5 were prepared and evaluated in the same manner as in the Examples, except that the positive electrode active material did not contain either polycrystalline particles or single crystal particles, or the value of B / A was outside the range of 1.03 to 1.9. The results are shown in Table 1.

[0039] <Reference Examples 1 to 3> Batteries of Comparative Examples 1 to 3 were prepared and evaluated in the same manner as in the Examples, except that the nickel ratio of the positive electrode active material was 60 mol%, the positive electrode active material did not contain either polycrystalline particles or single crystal particles, or the value of B / A was outside the range of 1.03 to 1.9. The results are shown in Table 1.

[0040] <Reference Examples 4, 5> Batteries of Comparative Examples 4 and 5 were prepared and evaluated in the same manner as in the Examples, except that the restraint pressure of the battery was 500 kPa and the positive electrode active material did not contain either polycrystalline particles or single crystal particles. The results are shown in Table 1.

[0041]

Table 1

[0042] As shown in Table 1, in Examples 1 to 3 where the nickel ratio of the positive electrode active material is 70 mol% or more of the total transition metals, the positive electrode active material contains polycrystalline particles or single-crystalline particles, and the value of B / A is in the range of 1.03 to 1.9, the batteries produced show excellent capacity retention rates compared to the batteries produced in Comparative Examples 1 to 5 where the positive electrode active material does not contain either polycrystalline particles or single-crystalline particles, or the value of B / A is outside the range of 1.03 to 1.9.

[0043] As shown in the results of Reference Examples 1 to 3, when the nickel ratio of the positive electrode active material is 60 mol%, there is no difference in the capacity retention rate of the battery between the case where the positive electrode active material does not contain either polycrystalline particles or single-crystalline particles and the case where the positive electrode active material contains polycrystalline particles and single-crystalline particles. This result suggests that the effect of applying the positive electrode active material of the present disclosure is remarkable when the nickel ratio of the positive electrode active material is 70 mol% or more.

[0044] As shown in the results of Reference Examples 4 and 5, when the restraint pressure of the battery is 500 kPa, excellent capacity retention rates are shown even when the positive electrode active material does not contain either polycrystalline particles or single-crystalline particles. This result suggests that the effect of applying the positive electrode active material of the present disclosure is remarkable when the restraint pressure of the battery is less than 500 kPa.

Claims

1. The transition metal is nickel, and the ratio of nickel is 70 mol % or more of the total transition metals; Contains polycrystalline particles and single crystal particles, A positive electrode active material, in which the value of B / A calculated from the amount A (mass %) of lithium hydroxide in the polycrystalline particles and the amount B (mass %) of lithium hydroxide in the single crystal particles is 1.03 to 1.

9.

2. The positive electrode active material according to claim 1 , wherein the positive electrode active material has a layered structure.

3. 2. The positive electrode active material according to claim 1, wherein a mass ratio of the polycrystalline particles to the single crystal particles (polycrystalline particles:single crystal particles) is within a range of 60:40 to 90:

10.

4. A battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 3.

5. 5. The battery of claim 4, wherein the confining pressure is less than 500 kPa.

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