Positive electrode active material for non-aqueous electrolyte secondary batteries

Abstract

This non-aqueous electrolyte secondary battery positive electrode active material is characterized by including a lithium-containing complex oxide composed of secondary particles formed by flocculation of primary particles, the lithium-containing complex oxide including Ni, Mn, and an additive element M, the additive element M including at least any one element selected from Nb, Mo, Ta, and W, the variation coefficient (standard deviation / arithmetic mean) of the concentration of the at least one element of the Nb, the Mo, the Ta, and the W when the secondary particle cross-section is subjected to EPMA analysis being 0.7 or less, and the standard deviation of the primary particle size being less than 0.05.

Description

Technical Field

[0001] The present invention relates to a cathode active material for a non-aqueous electrolyte secondary battery.

Background Art

[0002] In recent years, as a secondary battery with high output and high energy density, a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, which performs charge and discharge by moving lithium ions between the positive electrode and the negative electrode, has been widely used.

[0003] For example, in Patent Document 1, a composition represented by the general formula (A): Li 1+u Ni x Co y Mn z M t O2 (where -0.05 ≦ u ≦ 0.50, x + y + z + t = 1, 0.3 ≦ x ≦ 0.90, 0 ≦ y ≦ 0.5, 0.0 ≦ z ≦ 0.5, 0 < t ≦ 0.05, and M is one or more additive elements selected from Mg, Al, Si, Ca, Ti, Si, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), and a cathode active material composed of lithium transition metal-containing composite oxide particles having a layered rock salt-type crystal structure are disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] An object of the present disclosure is to provide a cathode active material for a non-aqueous electrolyte secondary battery that can improve the initial capacity of the non-aqueous electrolyte secondary battery.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized in that it is composed of secondary particles formed by the aggregation of primary particles, and comprises a lithium-containing composite oxide containing at least one selected from the group consisting of Ni, Co, Mn, and Al, and an additive element M, wherein the additive element M contains at least one of Nb, Mo, Ta, and W, and the coefficient of variation (standard deviation / arithmetic mean) of the concentration of at least one of Nb, Mo, Ta, and W when the cross-section of the secondary particles is analyzed by EPMA is 0.7 or less, and the standard deviation of the primary particle size is less than 0.05.

[0007] According to one aspect of this disclosure, it is possible to provide a positive electrode active material for a non-aqueous electrolyte secondary battery that can improve the initial capacity of the non-aqueous electrolyte secondary battery. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. [Modes for carrying out the invention]

[0009] An example of an embodiment will be described in detail below. The drawings referenced in the description of the embodiment are schematic representations, and the dimensional ratios of the components depicted in the drawings may differ from those of the actual objects.

[0010] Figure 1 is a schematic cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. The non-aqueous electrolyte secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, a non-aqueous electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Examples of battery cases 15 include metal cases such as cylindrical, rectangular, coin-shaped, and button-shaped cases, and resin cases formed by laminating resin sheets (so-called laminated type).

[0011] Non-aqueous electrolytes include, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of lithium salts such as LiPF6 are used as electrolyte salts. Note that the electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte using a gel-like polymer or the like.

[0012] The case body 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and its upper surface supports the sealing body 17.

[0013] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the non-aqueous electrolyte secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0014] In the non-aqueous electrolyte secondary battery 10 shown in Figure 1, the positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 attached to the negative electrode 12 extends outside the insulating plate 19 towards the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the case body 16, and the case body 16 becomes the negative electrode terminal.

[0015] The positive electrode 11, negative electrode 12, and separator 13 are described in detail below.

[0016] [Positive electrode] The positive electrode 11 comprises a positive electrode current collector and a positive electrode composite layer provided on the positive electrode current collector. The positive electrode composite layer contains a positive electrode active material. The positive electrode composite layer may also contain a conductive material, a binder, etc. The positive electrode 11 can be obtained, for example, by applying and drying a positive electrode composite slurry containing a positive electrode active material, a conductive material, and a binder, etc., onto the positive electrode current collector to form a positive electrode composite layer on the positive electrode current collector, and then rolling the positive electrode composite layer. The positive electrode composite layer may be provided on one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0017] For the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode, such as aluminum or an aluminum alloy, or a film having such a metal disposed on the surface layer can be used. The positive electrode current collector has a thickness of, for example, about 10 μm to 100 μm.

[0018] The positive electrode active material is composed of secondary particles in which primary particles are aggregated, and contains a lithium-containing composite oxide including at least one selected from the group consisting of Ni, Co, Mn, and Al, and an additive element M.

[0019] The proportion of Ni in the lithium-containing composite oxide is, for example, preferably in the range of 50 mol% or more and 95 mol% or less, more preferably in the range of 65 mol% or more and 95 mol% or less, based on the total molar amount of metal elements excluding Li, in terms of improving the high capacity of the battery and suppressing the degradation of charge-discharge cycle characteristics.

[0020] The proportion of Mn in the lithium-containing composite oxide is, for example, preferably in the range of 0 mol% or more and 50 mol% or less, more preferably in the range of 0 mol% or more and 40 mol% or less, based on the total molar amount of metal elements excluding Li, in terms of improving the high capacity of the battery and suppressing the degradation of charge-discharge cycle characteristics.

[0021] The proportion of Co in the lithium-containing composite oxide is, for example, preferably in the range of 0 mol% or more and 20 mol% or less, more preferably in the range of 0 mol% or more and 10 mol% or less, based on the total molar amount of metal elements excluding Li, in terms of improving the high capacity of the battery and suppressing the degradation of charge-discharge cycle characteristics.

[0022] The proportion of Al in the lithium-containing composite oxide is, for example, preferably in the range of 0 mol% or more and 20 mol% or less, more preferably in the range of 0 mol% or more and 10 mol% or less, based on the total molar amount of metal elements excluding Li, in terms of improving the high capacity of the battery and suppressing the degradation of charge-discharge cycle characteristics.

[0023] The additive element M contained in the lithium-containing composite oxide includes at least one of Nb, Mo, Ta, and W. The additive element M may also include elements other than Nb, Mo, Ta, and W. Examples of elements other than Nb, Mo, Ta, and W include Zr, B, Mg, Sc, Y, Ti, Fe, Cu, Zn, Cr, Pb, Sn, Na, K, Ba, Sr, Ca, Si, V, and Hf. Here, the additive element M may exist on the particle surface in the form of the additive element M alone or in a compound containing the additive element M, or it may be incorporated into the crystal structure of the lithium-containing composite oxide and contained inside the particle.

[0024] The proportion of additive element M in the lithium-containing composite oxide is preferably in the range of 0.01 mol% to 10 mol%, and more preferably 0.01 mol% to 5 mol%, relative to the total molar amount of metal elements excluding Li, for example, in terms of increasing the battery capacity and suppressing the deterioration of charge-discharge cycle characteristics.

[0025] The composition of lithium-containing composite oxides can be confirmed by methods such as inductively coupled plasma emission spectrometry and Rietveld analysis of parameters obtained by powder X-ray diffraction.

[0026] In lithium-containing composite oxides, in order to improve the initial capacity of non-aqueous electrolyte secondary batteries, the coefficient of variation of the concentration of at least one of Nb, Mo, Ta, and W when the cross-section of the secondary particle is analyzed by EPMA must be 0.7 or less, preferably 0.6 or less, and more preferably 0.4 or less. This coefficient of variation is an indicator of the distribution of Nb, Mo, Ta, and W present within the secondary particle. A lower value of the coefficient of variation indicates that the distribution of Nb, Mo, Ta, and W within the secondary particle is less biased and more uniformly dispersed.

[0027] The coefficients of variation of the concentrations of the additive elements Nb, Mo, Ta, and W are determined as follows. First, the positive electrode 11 or positive electrode active material is embedded in the resin, and the cross-section of the secondary particles is exposed by processing such as cross-section polishing (CP). Then, five secondary particles are randomly selected from an arbitrarily selected analysis area (a 40 μm × 30 μm area) within the processed cross-section, and concentration mapping of Nb, Mo, Ta, or W is performed on the cross-sections of the five selected secondary particles by EPMA (electron beam microanalyzer) analysis. The conditions for EPMA analysis are an acceleration voltage of 10 kV, irradiation current of 30 nA, beam diameter of <1 μm, step size of 0.16 μm, number of measurement points of 256 × 192 points, analysis area of ​​40 μm × 30 μm, and integration time of 50 milliseconds. Then, the arithmetic mean (μ) and standard deviation (σ) of the concentrations of Nb, Mo, Ta, or W are determined from the concentration mapping of Nb, Mo, Ta, or W obtained by EPMA analysis. Then, the coefficient of variation (CV) of the concentration of Nb, Mo, Ta, or W is determined using the formula: CV = σ / μ.

[0028] Furthermore, in lithium-containing composite oxides, the standard deviation of the primary particle size must be less than 0.05, and preferably 0.04, in order to improve the initial capacity of non-aqueous electrolyte secondary batteries.

[0029] The average size of primary particles in lithium-containing composite oxides is 0.07 μm, which can, for example, improve the initial capacity of non-aqueous electrolyte secondary batteries. 2 Preferably, the following: 0.06 μm 2 The following is more preferable:

[0030] The average size and standard deviation of primary particles in lithium-containing composite oxides are determined as follows: First, the positive electrode 11 or positive electrode active material is embedded in resin, and the cross-section of the secondary particles is exposed by cross-section polishing (CP) or the like, and this cross-section is photographed with a scanning electron microscope (SEM). Then, 30 primary particles are randomly selected from this cross-sectional SEM image. The grain boundaries of the selected 30 primary particles are observed to determine the shape of the primary particles, and the area of ​​each of the 30 primary particles is measured. This area is taken as the size of the primary particle, and the average size and standard deviation of the primary particles are determined.

[0031] The average particle size of secondary particles in lithium-containing composite oxides is preferably 3 μm to 25 μm, and more preferably 7 μm to 20 μm, in that it can further improve the initial capacity of non-aqueous electrolyte secondary batteries. The average particle size of secondary particles in lithium-containing composite oxides is the volume-average particle size measured by laser diffraction, and is the median diameter at which the integrated volume value in the particle size distribution is 50%. The average particle size of secondary particles in lithium-containing composite oxides can be measured by laser diffraction using, for example, Microtrac-Bell MT3000II.

[0032] The crystallite size of the lithium-containing composite oxide is preferably 300 Å to 600 Å, and more preferably 350 Å to 550 Å, in that it can further improve the initial capacity of a non-aqueous electrolyte secondary battery.

[0033] The crystallite size of lithium-containing composite oxides is calculated by analyzing the X-ray diffraction pattern obtained by X-ray diffraction using the whole powder pattern resolution method (hereinafter referred to as the "WPPD method").

[0034] The analysis procedure using the WPPD method is as follows: Step 1: Launch the software (TOPAS) and load the measurement data. Step 2: Set the Emission Profile. (Select Cu tube and Bragg Brentano focused optical system.) Step 3: Set the background. (Use Legendre's polynomial as the profiling function, with 8 to 20 terms.) Step 4: Configure the instrument. (Use Fundamental Parameters and enter slitting conditions, filament length, and sample length.) Step 5: Set Corrections. (Use Sample displacement. If the sample packing density in the sample holder is low, also use Absorption. In this case, Absorption is fixed to the linear absorption coefficient of the sample being measured.) Step 6: Set the crystal structure. (Set the space group to R3-m. Use the lattice constant, crystallite size, and lattice strain. Set the profile broadening due to the crystallite size and lattice strain to the Lorentz function.) Step 7: Perform the calculation. (Refine background, sample displacement, diffraction intensity, lattice constant, crystallite size, and lattice strain; the Le-ball formula is used for the calculation.) Step 8: If the standard deviation of the crystallite size is 6% or less of the refined value, the analysis is complete. If it is greater than 6%, proceed to Step 9. Step 9: Set the profile broadening due to lattice strain to a Gaussian function. (The crystallite diameter remains a Lorentz function.) Step 10: Perform the calculation. (Refine background, sample displacement, diffraction intensity, lattice constant, crystallite size, and lattice strain.) Step 11: If the standard deviation of the crystallite size is 6% or less of the refined value, the analysis is complete. If it is greater than 6%, the analysis is not possible.

[0035] An example of a method for producing the lithium-containing composite oxide used in this embodiment is described below.

[0036] The method for producing the positive electrode active material of this embodiment comprises, for example, a first step of obtaining a transition metal-containing compound comprising at least one selected from the group consisting of Ni, Mn, Co, and Al, and at least one of Nb, Mo, Ta, and W, and a second step of calcining a mixture containing the transition metal-containing compound obtained in the first step and a lithium compound to obtain a lithium-containing composite oxide.

[0037] The transition metal-containing compounds in the first step can be obtained by precipitation using solutions containing each metal element, or by mechanical milling such as a ball mill. Existing methods such as those utilizing differences in solubility, sol-gel methods, and spray drying can be used for the precipitation step.

[0038] Examples of lithium compounds used in the second step include lithium carbonate and lithium hydroxide.

[0039] The firing temperature of the mixture in the second step is, for example, 600 to 1000°C. The firing atmosphere may be air, oxygen, or the like.

[0040] As described above, by compounding at least one of Nb, Mo, Ta, and W in the first step, at least one of Nb, Mo, Ta, and W can be dispersed throughout the secondary particles of the lithium-containing composite oxide. This makes it easy to control the coefficient of variation of the concentration of at least one of Nb, Mo, Ta, and W to 0.7 or less, and the standard deviation of the primary particle size to less than 0.05. Furthermore, the coefficient of variation can also be controlled, for example, by adjusting the firing temperature and firing time in the second step. In addition, the primary particle size of the lithium-containing composite oxide can also be controlled by adjusting the firing temperature and firing time in the second step, or by adjusting the amount of Nb, Mo, Ta, and W added.

[0041] Examples of conductive materials include carbon black (CB), acetylene black (AB), Ketjenblack, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0042] Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), cellulose derivatives such as polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, carboxymethylcellulose (CMC) or its salts, and polyethylene oxide (PEO). These may be used individually or in combination of two or more types.

[0043] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector, such as a metal foil, and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode composite layer includes, for example, a negative electrode active material and a binder.

[0044] The negative electrode 12 is obtained, for example, by applying and drying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto the negative electrode current collector to form a negative electrode mixture layer on the negative electrode current collector, and then rolling the negative electrode mixture layer. The negative electrode mixture layer may be provided on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0045] The negative electrode active material is not particularly limited as long as it is a material capable of intercalating and releasing lithium ions. Examples include lithium alloys such as metallic lithium, lithium-aluminum alloys, lithium-lead alloys, lithium-silicon alloys, and lithium-tin alloys; carbon materials such as graphite, coke, and calcined organic materials; and metal oxides such as SnO2, SnO, and TiO2. These can be used individually or in combination of two or more.

[0046] The binder can be, for example, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but examples include styrene-butadiene rubber (SBR), CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0047] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include olefin resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used. [Examples]

[0048] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0049] <Example 1> [Fabrication of positive electrode active material] A solution of NiSO4, MnSO4, and ammonium metatungstate mixed in a molar ratio of 80:20:0.2 was granulated by spray drying, and then calcined in air at 400°C for 4 hours to obtain a metal element-containing compound containing Ni, Mn, and W. The obtained metal element-containing compound and a lithium compound were mixed so that the molar ratio of the total amount of Ni and Mn to Li was 1:1.11. This mixture was calcined in oxygen at 790°C for 10 hours, and then pulverized to obtain a lithium-containing composite oxide.

[0050] Elemental analysis of the obtained lithium-containing composite oxide was performed by ICP emission spectrometry, and the molar ratios of Ni, Mn, and W to the total metal elements excluding lithium were 80, 20, and 0.2, respectively. This lithium-containing composite oxide was used as the positive electrode active material in Example 1.

[0051] [Fabrication of the positive electrode] A positive electrode slurry was prepared by mixing 98 parts by mass of positive electrode active material, 1 part by mass of acetylene black (AB) as a conductive material, and 1 part by mass of polyvinylidene fluoride as a binder, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). Next, this positive electrode slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after the coating film was dried, it was rolled using a rolling roller. In this way, a positive electrode was produced in which positive electrode slurry layers were formed on both sides of the positive electrode current collector.

[0052] From the obtained cathode, a cross-section of the cathode composite layer was prepared using the cross-section polishing (CP) process described above. Then, EPMA analysis was performed on the secondary particle cross-section of the cathode active material under the conditions described above, and the coefficient of variation of the W concentration (standard deviation of W concentration / arithmetic mean of W concentration) was calculated to be 0.046. Furthermore, the cross-section of the cathode composite layer was observed using SEM, and the primary particle size of the cathode active material was measured under the conditions described above. As a result, the average primary particle size of the cathode active material was found to be 0.05 μm. 2 The standard deviation of the primary particle size was 0.04.

[0053] Furthermore, the average particle size of the secondary particles of the positive electrode active material was 17 μm, and the crystallite size of the positive electrode active material was 470 Å. The measurement conditions for these physical properties were as described above.

[0054] [Fabrication of the negative electrode] A negative electrode slurry was prepared by mixing 98 parts by mass of graphite, 1 part by mass of carboxymethylcellulose (CMC), and 1 part by mass of styrene-butadiene rubber (SBR). Next, the negative electrode slurry was applied to both sides of a negative electrode current collector made of copper foil, and after the coating was dried, it was rolled using a rolling roller. In this way, a negative electrode was produced in which negative electrode slurry layers were formed on both sides of the negative electrode current collector.

[0055] [Preparation of Non-aqueous Electrolyte] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 20:75:5. LiPF6 was dissolved in the mixed solvent at a concentration of 1.3 mol / L to prepare a non-aqueous electrolyte.

[0056] [Fabrication of Non-aqueous Electrolyte Secondary Battery] The above positive electrode and negative electrode were each cut into a predetermined size, electrode tabs were attached, and a wound electrode body was fabricated by winding through a separator. Next, the electrode body was housed in an aluminum laminate film, the above non-aqueous electrolyte was injected, and it was sealed. This was used as the non-aqueous electrolyte secondary battery of Example 1.

[0057] [Example 2] A lithium-containing composite oxide was prepared in the same manner as in Example 1, except that ammonium metatungstate was changed to ammonium niobium oxalate, and the molar ratio of the total amount of Ni and Mn to Nb was changed to 100:0.25.

[0058] When elemental analysis of the obtained lithium-containing composite oxide was performed by ICP emission spectrometry, the molar ratios of each of the elements Ni, Mn, and Nb to the total metal elements excluding lithium were 80, 20, and 0.25, respectively. This lithium-containing composite oxide was used as the positive electrode active material of Example 2.

[0059] Then, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode active material of Example 2 was used.

[0060] In the same manner as in Example 1, a cross-section of the positive electrode mixture layer was prepared. Then, EPMA analysis was performed on the cross-section of the secondary particles of the positive electrode active material, and as a result of obtaining the coefficient of variation of the Nb concentration (standard deviation of the Nb concentration / arithmetic mean of the Nb concentration), it was 0.392. Further, as a result of observing the cross-section of the positive electrode mixture layer by SEM, the average primary particle size of the positive electrode active material was 0.06 μm 2The standard deviation of the primary particle size was 0.04.

[0061] Furthermore, the average particle size of the secondary particles of the positive electrode active material was 17 μm, and the crystallite size of the positive electrode active material was 510 Å.

[0062] <Comparative Example 1> A nickel-manganese composite oxide, lithium hydroxide, and tungsten oxide powder were mixed. The molar ratio of lithium, the total amount of nickel and manganese, and tungsten in this mixture was 1.11:1:0.002. This mixture was calcined in oxygen at 790°C for 10 hours to obtain a lithium-containing composite oxide.

[0063] Elemental analysis of the obtained lithium-containing composite oxide was performed by ICP emission spectrometry, and the molar ratios of Ni, Mn, and W to the total metal elements excluding lithium were 80, 20, and 0.2, respectively. This lithium-containing composite oxide was used as the positive electrode active material in Comparative Example 1.

[0064] Then, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode active material of Comparative Example 1 was used.

[0065] A cross-section of the positive electrode composite layer was prepared in the same manner as in Example 1. EPMA analysis was then performed on the cross-section of the secondary particles of the positive electrode active material, and the coefficient of variation of the W concentration (standard deviation of W concentration / arithmetic mean of W concentration) was calculated to be 0.726. Furthermore, SEM observation of the cross-section of the positive electrode composite layer revealed that the average primary particle size of the positive electrode active material was 0.07 μm. 2 The standard deviation of the primary particle size was 0.05.

[0066] Furthermore, the average particle size of the secondary particles of the positive electrode active material was 17 μm, and the crystallite size of the positive electrode active material was 520 Å.

[0067] <Comparative Example 2> A nickel-manganese composite oxide, lithium hydroxide, and niobium oxide powder were mixed. The molar ratio of lithium, the combined amount of nickel and manganese, and niobium in this mixture was 1.11:1:0.0025. This mixture was calcined in air at 790°C for 10 hours to obtain a lithium-containing composite oxide.

[0068] Elemental analysis of the obtained lithium-containing composite oxide was performed by ICP emission spectrometry, and the molar ratios of Ni, Mn, and Nb to the total metal elements excluding lithium were 80, 20, and 0.25, respectively. This lithium-containing composite oxide was used as the positive electrode active material in Comparative Example 2.

[0069] Then, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode active material of Comparative Example 2 was used.

[0070] A cross-section of the positive electrode composite layer was prepared in the same manner as in Example 1. EPMA analysis was performed on the cross-section of the secondary particles of the positive electrode active material, and the coefficient of variation of the Nb concentration (standard deviation of Nb concentration / arithmetic mean of Nb concentration) was calculated to be 1.018. Furthermore, SEM observation of the cross-section of the positive electrode composite layer revealed that the average primary particle size of the positive electrode active material was 0.07 μm. 2 The standard deviation of the primary particle size was 0.05.

[0071] Furthermore, the average particle size of the secondary particles of the positive electrode active material was 17 μm, and the crystallite size of the positive electrode active material was 520 Å.

[0072] [Measurement of initial battery capacity] The batteries of each example and comparative example were charged at a constant current of 0.2C in a 25°C environment until the cell voltage reached 4.5V, and then charged at a constant voltage of 4.5V until the current value reached 0.02C. Next, they were discharged at a constant current of 0.2C until the cell voltage reached 2.5V. The discharge capacity of this first cycle was defined as the initial capacity of the battery.

[0073] [Charge-discharge cycle test] Furthermore, for each example and comparative example, 25 charge-discharge cycles were performed under the above charge-discharge conditions, and the capacity retention rate during the charge-discharge cycle was determined using the following formula. Capacity retention rate = (Discharge capacity at 25th cycle / Discharge capacity at 1st cycle) × 100

[0074] Table 1 summarizes the results for the initial capacity, initial efficiency (discharge capacity / charge capacity in the first cycle) × 100), and capacity retention rate of the batteries in each example and comparative example.

[0075] [Table 1]

[0076] Comparing Example 1, where the additive element for the lithium-containing composite oxide is W, with Comparative Example 1, Example 1, where the coefficient of variation (standard deviation / arithmetic mean) of the W concentration when the secondary particle cross-section of the lithium-containing composite oxide was analyzed by EPMA was 0.7 or less, and the standard deviation of the primary particle size of the lithium-containing composite oxide was less than 0.05, showed improved initial battery capacity compared to Comparative Example 1, where the coefficient of variation of the W concentration and the standard deviation of the primary particle size did not meet the above ranges. Similarly, comparing Example 2, where the additive element for the lithium-containing composite oxide is Nb, with Comparative Example 2, Example 2, where the coefficient of variation (standard deviation / arithmetic mean) of the Nb concentration when the secondary particle cross-section of the lithium-containing composite oxide was analyzed by EPMA was 0.7 or less, and the standard deviation of the primary particle size of the lithium-containing composite oxide was less than 0.05, showed improved initial battery capacity compared to Comparative Example 2, where the coefficient of variation of the Nb concentration and the standard deviation of the primary particle size did not meet the above ranges. Furthermore, Examples 1 and 2 also showed improved capacity retention rates during charge-discharge cycles compared to Comparative Examples 1 and 2. [Explanation of Symbols]

[0077] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. It comprises a lithium-containing composite oxide composed of secondary particles formed by the aggregation of primary particles, and containing at least one metallic element selected from the group consisting of Ni, Co, Mn, and Al, and an additive element M. The aforementioned additive element M includes at least one of Nb, Mo, Ta, and W. In the lithium-containing composite oxide, the proportions of Ni, Co, Mn, Al, and additive element M relative to the total molar amount of metal elements excluding Li are as follows: the proportion of Ni is 50 mol% or more and 95 mol% or less; the proportion of Mn is 0 mol% or more and 50 mol% or less; the proportion of Co is 0 mol% or more and 20 mol% or less; the proportion of Al is 0 mol% or more and 20 mol% or less; and the proportion of additive element M is 0.01 mol% or more and 10 mol% or less. When the secondary particle cross-section is analyzed by EPMA, the coefficient of variation (standard deviation / arithmetic mean) of the concentration of at least one of Nb, Mo, Ta, and W is 0.7 or less. A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the standard deviation of the primary particle size is less than 0.

05.

2. The lithium-containing composite oxide comprises Ni, Mn, and the additive element M, as described in Claim 1, for a positive electrode active material for a non-aqueous electrolyte secondary battery.

3. The average size of the primary particles is 0.07 μm. ２ The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, which is as follows:

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the average particle diameter of the secondary particles is 3 μm or more and 25 μm or less.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the crystallite size of the lithium-containing composite oxide determined by X-ray diffraction is 300 Å or more and 600 Å or less.

Images