Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

Provided is a positive electrode active material for a non-aqueous electrolyte secondary battery that can improve the capacity and durability of the non-aqueous electrolyte secondary battery. The positive electrode active material contained in a non-aqueous electrolyte secondary battery comprises a lithium transition metal composite oxide containing at least 70 mol% of Ni and Mn with respect to the total molar amount of metal elements other than Li, the lithium transition metal composite oxide is composed of single particles, the average particle size of the single particles is 0.65 μm-4 μm, and the crystallite size of the single particles is 380 Å-750 Å.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] Lithium-transition metal composite oxides are used as positive electrode active materials in lithium-ion secondary batteries, which are non-aqueous electrolyte secondary batteries. In recent years, efforts have been made to make lithium-transition metal composite oxides into single particles in order to improve durability. For example, Patent Document 1 describes a lithium-transition metal composite oxide having an average particle size of 2 μm to 20 μm and a BET specific surface area of ​​0.15 m. ２ / g to 1.9m ２ Patent Document 2 discloses single particles of an NCM-based lithium transition metal composite oxide (Ni content of 0.3≦Ni≦0.6) having an average particle size of 3 μm to 8 μm and a crystallite size of 1100 Å to 2000 Å.

[0003] Special table publication No. 2018-532236 Publication Patent No. 6850949

[0004] As a result of intensive studies by the present inventors, it was found that even a lithium transition metal composite oxide in a single particle form may have poor durability. Furthermore, after further studies, the present inventors found that a single particle may not achieve both high battery capacity and durability. Patent Documents 1 and 2 do not consider achieving both high capacity and high durability, and there is still room for improvement.

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

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide containing 70 mol % or more of Ni and Mn relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide being composed of single particles, the single particles having an average particle size of 0.65 μm to 4 μm, and the single particles having a crystallite size of 380 Å to 750 Å.

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by including a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0008] According to the positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, a non-aqueous electrolyte secondary battery with high capacity and improved durability can be provided.

[0009] Fig. 1 is an axial cross-sectional view of a non-aqueous electrolyte secondary battery that is an example of an embodiment; Fig. 2 is a schematic cross-sectional view of a test cell produced in an example and a comparative example; Fig. 3 is an SEM image of a positive electrode active material according to Example C1 before and after crushing; Fig. 4 is an SEM image of a positive electrode active material according to Example C2 before and after crushing; Fig. 5 is an SEM image of a positive electrode active material according to Example C3 before and after crushing; Fig. 6 is an SEM image of a positive electrode active material according to Comparative Example C4 before and after crushing; Fig. 7 is an SEM image of a positive electrode active material according to Comparative Example C5 before and after crushing.

[0010] In recent years, with the widespread use of non-aqueous electrolyte secondary batteries in automotive and power storage applications, there has been an increasing demand for non-aqueous electrolyte secondary batteries with high capacity and excellent durability. Furthermore, there is a demand for cost reduction in non-aqueous electrolyte secondary batteries, and a positive electrode active material containing relatively inexpensive Ni and Mn as its main components is preferred. To improve durability, the creation of single-particle lithium transition metal composite oxides as positive electrode active materials has been investigated, but the properties of the single particles remain largely unknown.

[0011] As a result of intensive research to solve the above problems, the present inventors have found that it is possible to achieve both high capacity and high durability by having single particles of a lithium transition metal composite oxide containing Ni and Mn as main components, with a predetermined average particle size and crystallite size.

[0012] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode body is housed in a cylindrical exterior body will be exemplified. However, the electrode body is not limited to a wound type, and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. The exterior body is not limited to a cylindrical shape, and may be, for example, a prismatic or coin-shaped body. Furthermore, the exterior body may be a pouch-type body made of a laminate sheet including a metal layer and a resin layer. Furthermore, in this specification, the expression "numerical value (A) to numerical value (B)" means greater than or equal to numerical value (A) and less than or equal to numerical value (B).

[0013] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. The secondary battery 10 shown in Fig. 1 includes an electrode assembly 14 and a non-aqueous electrolyte (not shown) housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. For ease of explanation, the following description will refer to the sealing body 16 side as the "top" and the bottom side of the exterior body 15 as the "bottom."

[0014] The upper end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 to the bottom side of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that when the negative electrode lead 20 is provided at the outer end of the winding, the negative electrode lead 20 passes outside the insulating plate 18, extends to the bottom side of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0015] The exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the internal sealing of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16, formed, for example, by pressing the side surface from the outside. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 on its upper surface via the gasket 27.

[0016] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may rupture, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may rupture, allowing gas to be released from the opening 26a of the cap 26.

[0017] The positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte that constitute the electrode assembly 14 will be described in detail below, with the positive electrode 11 being particularly described below.

[0018] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, a binder, and the like. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm on one side of the positive electrode current collector. The positive electrode 11 is produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of the positive electrode current collector, drying the coating, and then rolling it to form a positive electrode mixture layer on both sides of the positive electrode current collector.

[0019] Examples of the conductive agent contained in the positive electrode mixture layer include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphene, and graphite. These may be used alone or in combination of two or more. The content of the conductive agent is, for example, 0.1% by mass to 5.0% by mass relative to 100 parts by mass of the positive electrode active material.

[0020] Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO). The binder content is, for example, 0.1% to 5.0% by mass relative to 100 parts by mass of the positive electrode active material.

[0021] The positive electrode active material contained in the positive electrode mixture layer includes a lithium transition metal composite oxide. The lithium transition metal composite oxide is composed of single particles. The positive electrode active material may include secondary particles formed by agglomeration of single particles in addition to the single particles. This increases the charge density of the positive electrode active material in the positive electrode mixture layer, thereby increasing the capacity of the secondary battery 10. The secondary particles formed by agglomeration of single particles are formed by agglomeration of, for example, 2 to 1000 single particles. The positive electrode active material may also include general primary particles that are not single particles, and secondary particles formed by agglomeration of these primary particles. In addition to the lithium transition metal composite oxide, the positive electrode active material may also include LiF, Li ２ S and the like may be contained.

[0022] The proportion of the single particles in the positive electrode active material is preferably 10% or more, more preferably 80% or more, particularly preferably 90% or more, and may be substantially 100% by mass. The lithium transition metal composite oxide may contain secondary particles formed by aggregation of more than 100 primary particles.

[0023] The average particle size of the single particles is 0.65 μm to 4 μm. In this specification, the average particle size refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the positive electrode active material can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium. If the particle size of the single particles is too large, the battery capacity will decrease and the charge / discharge efficiency will deteriorate. If the particle size of the single particles is too small, secondary aggregation will occur, which will cause battery degradation.

[0024] The BET specific surface area of ​​a single particle is, for example, 0.5 m ２ / g to 4m ２ / g. Secondary particles have voids within them, so even if the particle diameter is large, the specific surface area is relatively large. On the other hand, single particles have no voids within them, so the larger the particle diameter, the smaller the BET specific surface area. The particle shapes of secondary particles and single particles vary depending on the production conditions, so the BET specific surface area also varies. The BET specific surface area can be measured using a Tristar II 3020 (manufactured by Shimadzu Corporation) under the following conditions. Number of measurement points: 11 points (P / P0: 0.05-0.3) Warm Free Space: Measured Equilibration Interval: 5s Analysis Adsorptive: N2 Analysis Bath Temp.: 77.3K (liquefied nitrogen temperature) Cold Free Space: Measured Low Pressure Done: None Analysis method: BET multi-point method

[0025] The BET specific surface area of ​​the lithium transition metal composite oxide is defined as A (m ２ / g) and the average particle size of the lithium transition metal composite oxide is B (μm), the product AB of A and B preferably satisfies 1.5≦AB≦6. This significantly improves the battery capacity and durability of the secondary battery 10. Secondary particles formed by agglomeration of single particles with small particle sizes may have grain boundary cracks, resulting in reduced charge-discharge cycle characteristics. Single particles with large particle sizes may reduce battery capacity. Furthermore, if the BET specific surface area of ​​the single particles is small, the contact area with the non-aqueous electrolyte may be small, resulting in a reduced battery capacity and poor load characteristics. If the BET specific surface area of ​​the single particles is large, many side reactions, such as gas generation, may occur in the positive electrode, resulting in reduced charge-discharge cycle characteristics. Therefore, by having the product of the average particle size (D50) and the BET specific surface area within the above range, a secondary battery 10 with high capacity, high durability, and suppressed gas generation can be obtained.

[0026] The crystallite size of the single particles is 380 Å to 750 Å. When the single particles have the above average particle size and crystallite size, a secondary battery 10 with high capacity and improved durability can be obtained. Here, the crystallite size is calculated from the half-width of the diffraction peak of the (104) plane in the X-ray diffraction pattern by the Scherrer equation shown below. In the formula below, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is 0.9. s = Kλ / B cos θ

[0027] The X-ray diffraction pattern was obtained by powder X-ray diffraction using a powder X-ray diffractometer (Rigaku Corporation, trade name "RINT-TTR", Cu-Kα source) under the following conditions: Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: divided pseudo-Voigt function Constraint conditions: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the Ni content of each element) ICSD No.: 98-009-4814

[0028] The lithium transition metal composite oxide contains 70 mol % or more of Ni and Mn relative to the total molar amount of metal elements excluding Li. This allows a lithium transition metal composite oxide with a high capacity to be obtained at a relatively low cost. Note that the lithium transition metal composite oxide may be composed only of Ni and Mn.

[0029] Ni is preferably contained in the largest amount among the metal elements other than Li constituting the lithium transition metal composite oxide. The Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more, more preferably 70 mol% or more, based on the total molar amount of the metal elements excluding Li. The upper limit of the Ni content may be 95 mol%, but is preferably 90 mol%.

[0030] Among the metal elements other than Li that constitute the lithium transition metal composite oxide, Mn is preferably contained in the second largest amount after Ni. Mn can stabilize the crystal structure of the lithium transition metal composite oxide. The Mn content in the lithium transition metal composite oxide is, for example, 5 mol % to 50 mol % relative to the total molar amount of the metal elements excluding Li.

[0031] The use of single particles allows a high capacity retention rate to be maintained even at a high charging potential. In particular, a lithium transition metal composite oxide having a Ni content of 80% or less and a high Mn content in its composition can obtain a high capacity by increasing the charging potential, and therefore single particles that are resistant to high potentials are required.

[0032] A surface modification layer containing a boron compound may be formed on the surface of the single particle. This improves charge / discharge efficiency. It is presumed that the boron compound suppresses decomposition of the electrolyte and promotes the exchange of Li ions between the non-aqueous electrolyte and the positive electrode active material on the surface of the lithium transition metal composite oxide. The boron compound is a compound containing B (boron). Examples of the boron compound include boron oxide, boron fluoride, boron chloride, and boron sulfide. The boron compound is preferably boron oxide. Examples of the boron oxide include boric acid (H ３ BO ３ ), boron oxide (B ２ O ３ ), lithium borate (LiBO２ , LiB ３ O ５ , Li ２ B ４ O ７ The boron compound present on the surface of the lithium transition metal composite oxide can be confirmed by low-energy SEM, TEM-EDX, or the like.

[0033] The thickness of the surface modification layer is, for example, 1 nm to 100 nm. The amount of the boron compound in the surface modification layer is, for example, 0.1 mol % to 7 mol % relative to the total molar amount of metal elements excluding Li in a single particle. The atomic concentration of each element can be measured by X-ray photoelectron spectroscopy (XPS).

[0034] The lithium transition metal composite oxide may further contain at least one metal element selected from the group consisting of Ca, Sr, W, and S. These metal elements may be contained in the lithium transition metal composite oxide, but are preferably present on the surface of the lithium transition metal composite oxide. This can suppress side reactions between the lithium transition metal composite oxide and the electrolyte, thereby suppressing battery degradation. These metal elements may be contained in the surface modification layer together with B. The positive electrode active material may contain these metal elements in an amount of, for example, 0.01 mol % to 5 mol % relative to the total amount of Ni and Mn.

[0035] Next, an example of a method for producing the positive electrode active material according to this embodiment will be described. The method for producing the positive electrode active material includes, for example, a synthesis step, a washing step, a drying step, and a crushing step.

[0036] In the synthesis step, a metal hydroxide containing 70 mol % or more of Ni and Mn is mixed with a Li compound and the mixture is fired to obtain a lithium transition metal composite oxide.

[0037] The metal hydroxide can be obtained, for example, by adding dropwise an alkaline solution such as sodium hydroxide to a stirred solution of a metal salt containing Ni, Mn, and an optional metal element (such as Fe), adjusting the pH to the alkaline side (for example, 8.5 to 12.5), and causing precipitation (co-precipitation). Instead of the metal hydroxide, a metal oxide obtained by heat-treating a metal hydroxide may also be used. The smaller the particle size of the metal hydroxide, the easier it is for primary particles to grow, so a particle size of 7 μm or less is preferred.

[0038] Examples of Li compounds include Li ２ CO ３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ Examples of the metal hydroxide include LiO, LiH, and LiF. The mixing ratio of the metal hydroxide and the Li compound is preferably set so that the molar ratio of metal elements other than Li to Li is in the range of 1:0.98 to 1:1.1, in order to easily adjust each of the above parameters to the ranges specified above. When mixing the metal hydroxide and the Li compound, a Ca compound, a Sr compound, a W compound, and the like may be added. Examples of the Ca compound include CaO and Ca(OH). ２ , and CaCO ３ Examples of Sr compounds include SrO and Sr(OH). ２ , and SrCO ３ Examples of W compounds include WO ３ , Li ２ WO ４ , Li ４ WO ５ , and Li ６ W ２ O ９ Examples include:

[0039] The mixture of metal hydroxide and Li compound, etc., is fired, for example, in an oxygen atmosphere (flowing gas with an oxygen concentration of 80% or more). The firing conditions may include a temperature rise rate of more than 1.0°C / min and less than 5.5°C / min from 450°C to 680°C, and a maximum temperature of 850°C to 1100°C. The temperature rise rate from above 680°C to the maximum temperature may be, for example, 0.1°C / min to 3.5°C / min. The maximum temperature may be maintained for 1 hour to 30 hours. This firing process may be a multi-stage firing, and multiple first and second temperature rise rates may be set for each temperature range as long as they are within the above-specified range. The particle size of the single particles can be adjusted by adjusting the firing conditions. For example, the particle size of the single particles can be increased by increasing the maximum temperature.

[0040] In the washing step, the lithium transition metal composite oxide obtained in the synthesis step is washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be performed using known methods and conditions. The washing and dehydration should be performed within a range that does not cause lithium to leach out of the lithium transition metal composite oxide and deteriorate the battery characteristics. Ca compounds, Sr compounds, W compounds, S compounds, P compounds, etc. may also be added to the cake-like composition.

[0041] In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powdery composition. The drying step may be performed in a vacuum atmosphere. The drying conditions are, for example, 150°C to 400°C and 0.5 hours to 15 hours.

[0042] The powder composition obtained in the drying step can be crushed to obtain single particles. A jet mill or the like can be used for crushing. Crushing using a jet mill can be performed using, for example, a PJM-80 (manufactured by Nippon Pneumatic Co., Ltd.) under the following conditions: Compressed air consumption: 0.5 Nm ３ / min Supply gas pressure: 0.53 MPa Processing capacity: 2000 g / hour

[0043] The obtained single particles were treated with boric acid (H ３ BO ３A surface modification layer containing the boron compound may be formed on the surface of the single particle by adding a boron-containing compound such as Li(II) or the like and heating the mixture to 200° C. to 400° C. The amount of the boron-containing compound added is, for example, 0.1 mol % to 7 mol % relative to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide.

[0044] [Negative Electrode] The negative electrode 12 has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer is preferably formed on both sides of the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode mixture layer contains, for example, a negative electrode active material, a binder, and the like. The negative electrode 12 is produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0045] The negative electrode 12 may contain boron. Some of the boron present on the surface of the positive electrode active material may migrate from the positive electrode 11 to the negative electrode 12. Even if a metal element such as Ni is precipitated on the negative electrode surface, the coexistence of B can suppress battery degradation. The amount of boron contained in the negative electrode is preferably 50 μg or more, more preferably 400 μg or more and 1200 μg or less, per 1 g of positive electrode active material. For example, 35% or more of the boron added to the positive electrode is precipitated on the negative electrode, and 55% or less remains in the positive electrode.

[0046] The negative electrode active material contained in the negative electrode mixture layer includes, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and earthy graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, a Si-based active material composed of at least one of Si and a Si-containing compound may be used as the negative electrode active material, or a carbon-based active material and a Si-based active material may be used in combination.

[0047] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof. The negative electrode mixture layer may also contain a conductive agent.

[0048] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin, or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

[0049] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0050] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0051] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0052] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４, LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ ) F ４ ), LiPF ６－ｘ (C ｎ F ２ｎ＋１ ) ｘ (1<x<6, n is 1 or 2), LiB １０ Cl １０ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ ), borates such as LiN(SO ２ CF ３ ) ２ , LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２ ) {l and m are integers of 0 or more}. The lithium salt may be used alone or in combination of two or more. Among these, LiPF ６ The concentration of the lithium salt is preferably, for example, 0.8 to 1.8 mol per liter of the non-aqueous solvent.

[0053] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0054] [Preparation of Positive Electrode Active Material] <Positive Electrode Active Material A1> LiOH and Ni having an average particle size of 6 μm obtained by coprecipitation method ０．５ Mn ０．５ (OH) ２The powders were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.1:1 to obtain a mixture. This mixture was fired under an oxygen stream with an oxygen concentration of 90% or more (flow rate of 0.15 to 0.2 L / min per 1 L of furnace volume) from room temperature to 650°C for 5 hours, then fired to 1000°C for 2 hours, and held for 9 hours to obtain a lithium transition metal composite oxide. Excess lithium from this lithium transition metal composite oxide was removed by washing with water, and the resulting mixture was dried to obtain secondary particles in which single particles were aggregated. Furthermore, these secondary particles were crushed using a jet mill to obtain positive electrode active material A1.

[0055] <Positive Electrode Active Materials A2 to A5> Positive electrode active materials A2 to A5 were obtained in the same manner as positive electrode active material A1, except that the maximum temperature reached was changed as shown in Tables 1 and 3.

[0056] <Positive electrode active materials B1 to B4> The composition of the hydroxide to be mixed is Ni ０．６ Mn ０．４ (OH) ２ and the maximum temperature reached was changed as shown in Tables 1 and 3, and the same procedures as for the positive electrode active material A1 were carried out to obtain positive electrode active materials B1 to B4.

[0057] <Positive electrode active materials C1 to C5> In the preparation of the positive electrode active materials, the composition of the hydroxides to be mixed is Ni ０．７ Mn ０．３ (OH) ２Cathode active materials C1 to C5 were obtained in the same manner as cathode active material A1, except that the temperature was changed to 0.015°C and the maximum temperature was changed as shown in Tables 1 and 3. SEM images of cathode active materials C1 to C5 before and after crushing are shown in Figures 3 to 7. Before crushing in Figures 3, 4, and 6, all of the primary particles formed secondary particles in the form of aggregates. After crushing in Figures 3 and 4, the shape of single particles can be confirmed, whereas after crushing in Figure 6, the shape of single particles cannot be confirmed. Thus, even if the shapes of secondary particles in the SEM images are substantially the same, some are single particles and others are not. Furthermore, even if some aggregates remain after crushing into single particles as shown in Figure 3, the effect of single particles is exhibited as long as the average particle size of the single particles is 0.65 μm to 4 μm and the crystallite size of the single particles is 380 Å to 750 Å. However, particles that are barely crushed, as shown in Figure 6, are not single particles and do not exhibit the effect of single particles. On the other hand, some particles are in the form of single particles even before crushing, as shown in Figures 5 and 7. However, if the average particle size or crystallite size of the particles is outside the above range, as in the example shown in Figure 7, the battery characteristics are not sufficiently improved.

[0058] <Positive electrode active materials D1 to D3> In the preparation of the positive electrode active materials, the composition of the hydroxides to be mixed is Ni ０．７５ Mn ０．２５ (OH) ２ and the maximum temperature reached was changed as shown in Tables 1 and 3, and the same procedures as for the positive electrode active material A1 were carried out to obtain positive electrode active materials D1 to D3.

[0059] <Positive electrode active material E1> The positive electrode active material A1 was added with boric acid (H ３ BO ３ The amount of boric acid added was 2 mol % relative to the total molar amount of metal elements excluding Li in the single particle.

[0060] <Positive Electrode Active Materials E2 to E4> Positive electrode active materials E2 to E4 were obtained in the same manner as positive electrode active material E1, except that the maximum temperature reached was changed as shown in Tables 2 and 4.

[0061] <Positive electrode active materials F1 to F4> The composition of the hydroxide to be mixed is Ni ０．６ Mn ０．４ (OH) ２ and the maximum temperature reached was changed as shown in Tables 2 and 4, and the same procedures as for the positive electrode active material E1 were carried out to obtain positive electrode active materials F1 to F4.

[0062] <Positive electrode active materials G1 to G4> In the preparation of the positive electrode active materials, the composition of the hydroxides to be mixed was changed to Ni ０．７ Mn ０．３ (OH) ２ and the maximum temperature reached was changed as shown in Tables 2 and 4, and the same procedures as for the positive electrode active material E1 were carried out to obtain positive electrode active materials G1 to G4.

[0063] <Positive electrode active materials H1 and H2> In the preparation of the positive electrode active material, the composition of the hydroxide to be mixed is Ni ０．７５ Mn ０．２５ (OH) ２ and the maximum temperature reached was changed as shown in Tables 2 and 4, and the same procedures as for the positive electrode active material E1 were carried out to obtain positive electrode active materials H1 and H2.

[0064] [Preparation of Test Cell] The test cell shown in Figure 2 was prepared by the following procedure. First, the above-mentioned positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride (binder) were mixed in a weight ratio of 80:10:10, and then slurried using N-methyl-2-pyrrolidone. Next, this slurry was applied to an aluminum foil current collector, which served as a positive electrode current collector, and vacuum dried at 110°C to prepare a working electrode 30 (positive electrode).

[0065] In dry air with a dew point of −50° C. or lower, an electrode group, consisting of the working electrode 30 with electrode tabs 38 attached, the counter electrode 31 (negative electrode), and the reference electrode 32, with a separator 34 interposed between them, was housed in an outer casing 35. An electrolyte solution 36 was then poured into the outer casing 35, and the outer casing 35 was sealed to produce a test cell, which is a nonaqueous electrolyte secondary battery. This resulted in test cells A1 to H4, each corresponding to a positive electrode active material A1 to H4. Details of each component are as follows: Counter electrode: lithium metal; Reference electrode: lithium metal; Separator: polyethylene separator; Nonaqueous electrolyte: LiPF as an electrolyte salt, in a nonaqueous solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. ６ was dissolved to a concentration of 1.0 mol / l.

[0066] [Evaluation 1 of Charge Capacity, Discharge Capacity, and Charge / Discharge Efficiency] At an ambient temperature of 25° C., the test cell was charged to 4.5 V (lithium reference) at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.5 V. It was then discharged to 2.5 V at a constant current of 0.1 C. The charge capacity and discharge capacity at this time were measured, and the discharge capacity was divided by the charge capacity to calculate the charge / discharge efficiency.

[0067] [Durability Evaluation 1] At an ambient temperature of 25°C, the test cell was charged to 4.5 V at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.5 V. It was then discharged to 2.5 V at a constant current of 0.1 C. This charge / discharge cycle was counted as one cycle, and measurements were taken under the same conditions as above for the 11th, 21st, and 31st cycles. For the remaining cycles, the charge / discharge cycles were performed in the same manner as above, except that the constant current during discharge was set to 0.2 C. The capacity retention rate was calculated using the following formula: Capacity retention rate = (discharge capacity at 31st cycle / discharge capacity at 1st cycle) x 100

[0068] Tables 1 and 2 show the evaluation results of Examples and Comparative Examples. Table 1 shows the results for test cells A1 to D2, and Table 2 shows the results for test cells E1 to H2. The results of test cells containing single particles with an average particle size of 0.65 μm to 4 μm and a crystallite size of 380 Å to 750 Å were designated Examples, and the results of test cells other than those described above were designated Comparative Examples. For example, the result of test cell F1 was designated Example F1-1.

[0069]

[0070]

[0071] [Evaluation 2 of Charge Capacity, Discharge Capacity, and Charge / Discharge Efficiency] At an ambient temperature of 25°C, the test cell was charged to 4.7 V (lithium reference) at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.7 V. It was then discharged to 2.5 V at a constant current of 0.1 C. The charge capacity and discharge capacity at this time were measured, and the discharge capacity was divided by the charge capacity to calculate the charge / discharge efficiency.

[0072] [Durability Evaluation 2] At an ambient temperature of 25°C, the test cell was charged to 4.7 V at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.7 V. It was then discharged to 2.5 V at a constant current of 0.1 C. This charge / discharge cycle was counted as one cycle, and measurements were taken under the same conditions as above for the 11th, 21st, and 31st cycles. For the remaining cycles, the charge / discharge cycles were performed in the same manner as above, except that the constant current during discharge was set to 0.2 C. The capacity retention rate was calculated using the following formula: Capacity retention rate = (discharge capacity at 31st cycle / discharge capacity at 1st cycle) × 100

[0073] Tables 3 and 4 show the evaluation results of Examples and Comparative Examples. Table 3 shows the results for test cells A1 to D3, and Table 4 shows the results for test cells E1 to H2. The results of test cells containing single particles with an average particle size of 0.65 μm to 4 μm and a crystallite size of 380 Å to 750 Å were designated Examples, and the results of test cells other than those described above were designated Comparative Examples. For example, the result of test cell E1 was designated Example E1-2.

[0074]

[0075]

[0076] In Tables 1 and 2, the test cells of the examples achieve both high charge capacity and high capacity retention. On the other hand, in Tables 1 and 2, the test cells of the comparative examples are inferior to the examples in either charge capacity or capacity retention. This shows that high capacity and high durability can be achieved by the single particles having a predetermined average particle size and crystallite size. Furthermore, as shown in Tables 3 and 4, while the single particles themselves can improve battery characteristics, the boron compound protects the positive electrode surface, which further improves battery capacity, charge / discharge efficiency, and capacity retention, as well as suppresses gas generation.

[0077] The present disclosure will be further described by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide containing 70 mol % or more of Ni and Mn relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide being composed of single particles, the single particles having an average particle size of 0.65 μm to 4 μm, and the single particles having a crystallite size of 380 Å to 750 Å. Configuration 2: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide containing 70 mol % or more of Ni and Mn relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide being composed of single particles, the single particles having an average particle size of 0.65 μm to 4 μm, and the single particles having a crystallite size of 380 Å to 750 Å. ２ / g) and the average particle size of the lithium transition metal composite oxide is B (μm), the product AB of A and B satisfies 1.5≦AB≦6. Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein a surface modification layer containing a boron compound is formed on the surface of the single particles. Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the lithium transition metal composite oxide further contains at least one metal element selected from the group consisting of Ca, Sr, W, S, and P. Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, further comprising secondary particles formed by aggregation of the single particles in addition to the single particles. Configuration 6: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the single particles account for 10 mass % or more of the total amount of the positive electrode active material for a non-aqueous electrolyte secondary battery. Configuration 7: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of configurations 1 to 6, a negative electrode, and a non-aqueous electrolyte.

[0078] REFERENCE SIGNS LIST 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Exterior body, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Grooved portion, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Opening, 27 Gasket, 30 Working electrode (positive electrode), 31 Counter electrode (negative electrode), 32 Reference electrode, 34 Separator, 35 Exterior body, 36 Electrolyte, 38 Electrode tab

Claims

1. The lithium transition metal composite oxide contains 70 mol % or more of Ni and Mn relative to the total molar amount of metal elements excluding Li, The lithium transition metal composite oxide is composed of single particles, The average particle size of the single particle is 0.65 μm to 4 μm, The positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the single particle has a crystallite size of 380 Å to 750 Å.

2. The BET specific surface area of ​​the lithium transition metal composite oxide is A (m ２ / g) and the average particle size of the lithium transition metal composite oxide is B (μm), 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a product AB of A and B satisfies 1.5≦AB≦6.

3. 2 . The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 , wherein a surface modification layer containing a boron compound is formed on a surface of the single particle.

4. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains at least one element selected from the group consisting of Ca, Sr, W, S, and P.

5. 2 . The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 , further comprising, in addition to the single particles, secondary particles formed by aggregation of the single particles.

6. 2 . The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 , wherein the single particles are contained in an amount of 10 mass % or more based on a total amount of the positive electrode active material for a non-aqueous electrolyte secondary battery.

7. 7. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, a negative electrode, and a non-aqueous electrolyte.