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

Abstract

The purpose of the present disclosure is to provide a positive electrode active material which enables the achievement of a nonaqueous electrolyte secondary battery that has good cycle characteristics. A positive electrode active material for nonaqueous electrolyte secondary batteries according to one embodiment of the present disclosure contains large particles and small particles of a lithium transition metal composite oxide; the large particles are secondary particles that have a volume-based median diameter of from 10 to 25 μm, each of said secondary particles being an aggregate of primary particles that have a diameter of 1 μm or less; and the small particles contain primary particles that are not aggregated, while having a diameter of from 1 to 5 μm.

Description

[Technical Field]

[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. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, which are charged and discharged by transferring lithium ions between a positive electrode and a negative electrode via a non-aqueous electrolyte, are used as power sources for portable electronic devices and the like. In recent years, the applications of non-aqueous electrolyte secondary batteries have expanded to applications requiring relatively high capacity, such as on-board use and power storage, and there is a demand for even higher capacity. Patent Document 1 discloses a method for increasing the capacity of batteries by using a positive electrode active material that is a mixture of large particles and small particles to improve the packing density of the positive electrode mixture layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-306546 Summary of the Invention [Problem to be solved by the invention]

[0004] In non-aqueous electrolyte secondary batteries, repeated charge and discharge cycles can lead to a decrease in battery capacity due to side reactions such as electrolyte decomposition and elution of transition metals from the positive electrode active material. Small-particle positive electrode active materials, in particular, have a large specific surface area, making them prone to side reactions. Therefore, even if battery capacity can be increased by using a positive electrode active material that combines large and small particles, side reactions of the small particles can sometimes degrade cycle characteristics. The technology disclosed in Patent Document 1 does not take into consideration the prevention of deterioration in cycle characteristics, and there is still room for improvement.

[0005] Therefore, an object of the present disclosure is to provide a positive electrode active material that enables the realization of a non-aqueous electrolyte secondary battery with good cycle characteristics. [Means for solving the problem]

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure comprises large particles and small particles of a lithium transition metal composite oxide, the large particles being secondary particles having a volume-based median diameter of 10 μm to 25 μm formed by agglomeration of primary particles having a diameter of 1 μm or less, and the small particles comprising non-agglomerated primary particles having a diameter of 1 μm to 5 μm.

[0007] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a positive electrode containing the above-described positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0008] According to the positive electrode active material of one aspect of the present disclosure, a nonaqueous electrolyte secondary battery having high capacity and good cycle characteristics can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery as an example of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] 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 the 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. Furthermore, the exterior body is not limited to a cylindrical shape, and may be, for example, a prismatic or coin-shaped body, or a pouch-shaped body made of a laminate sheet including a metal layer and a resin layer.

[0011] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 that is one example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are 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 be given with the sealing body 16 side referred to as "top" and the bottom side of the exterior body 15 referred to as "bottom."

[0012] The upper open end of the exterior body 15 is closed 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 passes outside the insulating plate 18, extends toward the bottom side of the exterior body 15, and is welded to the inner surface of the bottom of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal.

[0013] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0014] 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 break, 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 break, and gas may be released from the opening 26a of the cap 26.

[0015] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10, particularly the positive electrode active material contained in the positive electrode 11, will be described in detail below.

[0016] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 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 layer. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc., onto the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode current collector 30.

[0017] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 31 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 carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0018] The positive electrode mixture layer 31 contains, as a positive electrode active material, large particles and small particles of a lithium transition metal composite oxide. The compositions of the large particles and the small particles are not particularly limited as long as they are a lithium transition metal composite oxide, and may be substantially the same. In addition, the positive electrode mixture layer 31 may contain a positive electrode active material other than the large particles and the small particles of the lithium transition metal composite oxide described later, as long as the object of the present disclosure is not impaired. However, in the present embodiment, it is assumed that only the large particles and the small particles of the lithium transition metal composite oxide are included as the positive electrode active material.

[0019] The lithium transition metal composite oxide has the general formula Li

[0023] , Ni x Co y M z O 2-b (where 0 < a ≤ 1.2, 0.3 ≤ x ≤ 0.95, 0 ≤ y ≤ 0.3, 0 ≤ z ≤ 0.5, 0 ≤ b < 0.05, x + y + z = 1, and M contains at least one element selected from Mn, Zr, Mo, W, Nb, Al, Cr, V, Ce, Ti, Fe, Si, Ga, In).

[0020] A may satisfy 0 < a ≤ 1.2, which represents the ratio of Li to the total molar number of metal elements excluding Li in the lithium transition metal compound. When a exceeds 1.2, more Li compounds are added compared to the case where a satisfies the above range, so it may not be economical from the perspective of manufacturing cost.

[0021] X, which represents the ratio of Ni to the total molar number of metal elements excluding Li in the lithium transition metal compound, may satisfy 0.3 ≤ x ≤ 0.95, and may satisfy 0.5 ≤ x ≤ 0.95 in order to increase the capacity of the battery and add other metal elements.

[0022] Y, which represents the ratio of Co to the total molar number of metal elements excluding Li in the lithium transition metal compound, may satisfy 0 ≤ y ≤ 0.3. Co is an optional component. Also, since Co is expensive, y ≤ 0.2 may be satisfied from the perspective of reducing manufacturing cost.

[0023] The ratio z, which indicates the ratio of M (M includes at least one element selected from Al, Mn, Ti, Mo, W, and Mg) to the total number of moles of metal elements excluding Li in the lithium transition metal compound, may satisfy the relationship 0≦z≦0.5. M is an optional component. M may be Al or Mn.

[0024] The large particles of the lithium transition metal composite oxide are secondary particles formed by aggregation of primary particles with a diameter of 1 μm or less, with a volume-based median diameter (hereinafter referred to as D50) of 10 μm to 25 μm. By making the diameter of the primary particles relatively small, 1 μm or less, and making the D50 relatively large, 10 μm to 25 μm, it is possible to ensure a rapid electrode reaction of the large particles while also ensuring spaces between the large particles for the small particles to fill.

[0025] The diameter of primary particles can be determined by analyzing SEM images of cross sections observed with a scanning electron microscope (SEM). For example, a positive electrode is embedded in resin, and a cross section of the positive electrode mixture layer is prepared by cross-section polishing (CP) or other processing, and this cross section is photographed by SEM. Alternatively, large particle powder is embedded in resin, and a cross section of the large particles is prepared by CP or other processing, and this cross section is photographed by SEM. Then, 30 primary particles are randomly selected from the SEM image of this cross section. The grain boundaries of the selected 30 primary particles are observed to identify the outer shapes of the primary particles, and the long diameter (maximum long diameter) of each of the 30 primary particles is calculated, and the average of these is taken as the diameter of the primary particles. The diameter of primary particles in small particles, described below, can be measured in a similar manner.

[0026] The volume-based median diameter (D50) of secondary particles refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of large particles can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0027] The small particles of the lithium transition metal composite oxide include non-aggregated primary particles with a diameter of 1 μm to 5 μm. By using small particles in this range in combination with the above-mentioned large particles, the packing density of the positive electrode active material in the positive electrode mixture layer 31 can be increased, thereby achieving high battery capacity and good cycle characteristics. The above-mentioned small particles include non-aggregated primary particles despite their small diameter, which can suppress elution of metals such as Ni, Co, Mn, and Al and reaction with the electrolyte due to charge / discharge cycles. The D50 of the small particles is smaller than that of the large particles, and is preferably, for example, 3 μm to 7 μm.

[0028] The proportion of non-agglomerated primary particles in the small particles is preferably 50% by mass or more, more preferably 60% by mass or more, and particularly preferably 70% by mass or more. If this proportion is 50% by mass or more, the effect of mixing the large and small particles can be more reliably achieved. Small particles made of non-agglomerated primary particles are less likely to experience the phenomenon known as particle cracking, in which secondary particles crack during charge-discharge cycles. Therefore, side reactions with the electrolyte or interruptions in the conductive path at the particle cracked portions are less likely to occur. Therefore, the greater the proportion of non-agglomerated primary particles in the small particles, the greater the effect of mixing the large and small particles.

[0029] The proportion of non-aggregated primary particles in the small particles can be determined by analyzing an SEM image of a cross section of the positive electrode mixture layer or a cross section of the small particles. Specifically, the proportion of non-aggregated primary particles in the small particles can be calculated from the ratio of the area of ​​the aggregated small particles to the area of ​​the non-aggregated small particles in the SEM image of the cross section.

[0030] In the positive electrode mixture layer, the ratio of large particles to small particles, in mass ratio, is preferably 60:40 to 95:5, more preferably 70:30 to 95:5, and particularly preferably 70:30 to 80:20. Within this range, the compressibility of the mixture of large particles and small particles can be increased to achieve a high capacity while also achieving good cycle characteristics.

[0031] The large particles and small particles can be produced by a production method including the following steps. (1) A step of heat-treating a hydroxide containing at least Ni obtained by a coprecipitation method to obtain a composite oxide. (2) A step of mixing the composite oxide with a lithium compound and calcining the mixed particles to obtain a lithium transition metal composite oxide.

[0032] In step (1), for example, an alkaline solution such as sodium hydroxide is added dropwise to a stirred solution of a metal salt containing Ni and an optional metal element (e.g., Co) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a transition metal hydroxide containing Ni and the optional metal element. The transition metal hydroxide is then heat-treated to obtain a transition metal oxide containing Ni and the optional metal element. The heat treatment temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C. The diameter of the primary particles and the D50 of the secondary particles can be adjusted by adjusting the co-precipitation and heat treatment conditions. For example, when producing large particles, extending the reaction time during co-precipitation can increase the secondary particle diameter without changing the diameter of the primary particles. When producing small particles, increasing the pH can make the interior of the primary particles coarse, thereby increasing the diameter of the primary particles after step (2).

[0033] In step (2), the composite oxide obtained in step (1) is mixed with a lithium compound, and then the mixture is calcined. The calcination conditions are, for example, a temperature of 700 to 1000°C in an oxygen atmosphere. When small particles are to be produced, a relatively high calcination temperature can be used to change the interior of the primary particles from a sparse state to a dense state while increasing the diameter of the primary particles. In step (2), a lithium compound is generally used in excess of the stoichiometric ratio of the desired product. Examples of lithium compounds that can be used include lithium carbonate, lithium hydroxide, lithium nitrate, and lithium sulfate. Among these, lithium carbonate and lithium hydroxide are preferred. After calcination, the lithium transition metal composite oxide may be crushed and classified to adjust the D50 to a predetermined range, or the resulting lithium transition metal composite oxide may be washed with water.

[0034] [Negative electrode] The negative electrode 12 has a negative electrode current collector 40 and negative electrode mixture layers 41 formed on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper or a copper alloy, a film having such a metal disposed on the surface layer, etc. can be used. The negative electrode mixture layer 41 contains a negative electrode active material and a binder. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. onto the negative electrode current collector 40, drying the coating film, and then rolling to form the negative electrode mixture layers 41 on both surfaces of the negative electrode current collector 40.

[0035] The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium ions, and generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flaky graphite, massive graphite, and earthy graphite, artificial massive graphite, and artificial graphite such as graphitized mesophase carbon microbeads. Further, as the negative electrode active material, a metal that alloys with Li such as Si or Sn, a metal compound containing Si or Sn, a lithium titanium composite oxide, etc. may be used. For example, a Si-containing material represented by SiO x (0.5 ≦ x ≦ 1.6), or Li 2y SiO (2+y) A Si-containing material in which fine particles of Si are dispersed in a lithium silicate phase represented by (0 < y < 2) etc. may be used in combination with graphite.

[0036] For the binder contained in the negative electrode mixture layer 41, similar to the case of the positive electrode 11, a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, etc. may be used, but preferably styrene-butadiene rubber (SBR) is used. Further, the negative electrode mixture layer 41 may contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. The negative electrode mixture layer 41 contains, for example, SBR and CMC or its salt.

[0037] [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 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.

[0038] [Non-aqueous electrolyte] The non-aqueous electrolyte contains 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 in 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).

[0039] 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.

[0040] Examples of the above ethers include cyclic ethers such as 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, crown ether, etc., and chain ethers such as 1,2 - dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl 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, tetraethylene glycol dimethyl ether, etc.

[0041] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), 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, borate salts such as Li2B4O7, Li(B(C2O4)F2), etc., LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1Examples of the lithium salt include imide salts such as LiPF6 (LiPF2) (where l and m are integers of 0 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferably used from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1 L of the non-aqueous solvent. [Example]

[0042] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples.

[0043] Example 1 [Synthesis of large particle A] While stirring a solution of metal salts containing Ni, Co, and Mn, sodium hydroxide is added dropwise to adjust the pH to the alkaline side. 0.8 Co 0.1 Mn 0.1 (OH)2 is precipitated (co-precipitated) and then heat-treated to form Ni 0.8 Co 0.1 Mn 0.1 O2 was obtained. Then, LiOH and Ni 0.8 Co 0.1 Mn 0.1 O2 was mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.1. This mixture was then fired to obtain large particles A. The obtained large particles A were secondary particles formed by aggregation of primary particles, and the diameter of the primary particles was 0.5 μm, and the D50 of the secondary particles was 17 μm. The composition of large particles A was calculated by ICP emission spectroscopy (using an ICP emission spectrometer iCAP6300 manufactured by Thermo Fisher Scientific), and the result was Li 1.05 Ni 0.8 Co 0.1 Mn 0.1 It was O2.

[0044] [Synthesis of small particles] While stirring a solution of metal salts containing Ni, Co, and Mn, sodium hydroxide was added dropwise to adjust the pH to a more alkaline side than that used for synthesizing large particles A.0.8 Co 0.1 Mn 0.1 (OH)2 is precipitated (co-precipitated) and then heat-treated to form Ni 0.8 Co 0.1 Mn 0.1 O2 was obtained. Then, LiOH and Ni 0.8 Co 0.1 Mn 0.1 O2 was mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.1. This mixture was then fired at a temperature higher than that used to synthesize large particles A, thereby obtaining small particles. The obtained small particles contained non-aggregated primary particles, and the diameter of the primary particles was 5 μm. The proportion of non-aggregated primary particles in the small particles was 70 mass %, and the D50 of the small particles was 5 μm. The composition of the small particles was calculated by ICP emission spectroscopy (using an ICP emission spectrometer iCAP6300 manufactured by Thermo Fisher Scientific), and the results showed that Li 1.05 Ni 0.8 Co 0.1 Mn 0.1 It was O2.

[0045] [Preparation of positive electrode] The positive electrode active material was a mixture of large particles A and small particles in a 50:50 mass ratio. The positive electrode active material was mixed in an amount of 95 parts by mass, acetylene black in 2.5 parts by mass, and polyvinylidene fluoride in 2.5 parts by mass, and this was mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. The slurry was applied to both sides of a positive electrode current collector made of aluminum foil by a doctor blade method, and the coating was dried and then rolled with a rolling roller to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector.

[0046] [Preparation of negative electrode] A negative electrode mixture slurry was prepared by mixing 98 parts by mass of artificial graphite, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC), and then mixing this with water. The slurry was applied to both sides of a negative electrode current collector made of copper foil by a doctor blade method, and the coating was dried and then rolled with a pressure roller to produce a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode current collector.

[0047] [Preparation of non-aqueous electrolyte] Lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 2:8. Furthermore, vinylene carbonate (VC) was dissolved in the mixed solvent at a concentration of 2.0 mass % to prepare an electrolyte solution (non-aqueous electrolyte).

[0048] [Secondary battery production] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, followed by radial press forming to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a nonaqueous electrolyte secondary battery with a design capacity of 800 mAh.

[0049] [Evaluation of tap density] The tap density was evaluated as the compressibility of the positive electrode active material consisting of a mixture of large particles A and small particles. The tap density was measured by placing a connecting filler frame (approximately 50 mL) on top of a measuring cylinder of a fixed volume (50 mL), introducing 60 g of negative electrode active material, tapping 300 times, removing the filler frame, and calculating the mass of the negative electrode active material in the measuring cylinder volume. Specifically, the tap density of the positive electrode active material was measured using a TVP-1A (fixed volume type) manufactured by Tsutsui Scientific Instruments Co., Ltd.

[0050] [Capacity retention rate evaluation] The secondary battery was charged at a constant current of 1 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 0.05 C at 4.2 V. It was then discharged at a constant current of 1 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 300 times. The discharge capacity at the first cycle and the discharge capacity at the 300th cycle were determined, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = 300th cycle discharge capacity ÷ 1st cycle discharge capacity × 100

[0051] <Example 2> In preparing the positive electrode, a positive electrode active material and a secondary battery were prepared and evaluated in the same manner as in Example 1, except that a mixture of large particles A and small particles in a mass ratio of 60:40 was used as the positive electrode active material.

[0052] Example 3 In preparing the positive electrode, a positive electrode active material and a secondary battery were prepared and evaluated in the same manner as in Example 1, except that a mixture of large particles A and small particles in a mass ratio of 70:30 was used as the positive electrode active material.

[0053] Example 4 In preparing the positive electrode, a positive electrode active material and a secondary battery were prepared and evaluated in the same manner as in Example 1, except that a mixture of large particles A and small particles in a mass ratio of 80:20 was used as the positive electrode active material.

[0054] <Example 5> In preparing the positive electrode, a positive electrode active material and a secondary battery were prepared and evaluated in the same manner as in Example 1, except that a mixture of large particles A and small particles in a mass ratio of 90:10 was used as the positive electrode active material.

[0055] <Comparative Example 1> In synthesizing the large particles, the reaction time in the coprecipitation was shorter than that in synthesizing the large particles A, and large particles B having a primary particle diameter of 0.5 μm and a secondary particle D50 of 12 μm were synthesized. Furthermore, a positive electrode active material and a secondary battery were produced and evaluated in the same manner as in Example 1, except that large particles B were used as the positive electrode active material.

[0056] <Comparative Example 2> In synthesizing the small particles, the heat treatment temperature was lowered while adjusting the pH to be smaller, thereby synthesizing small particles with a primary particle diameter of 0.5 μm and a secondary particle D50 of 5 μm. Furthermore, a positive electrode active material and a secondary battery were produced and evaluated in the same manner as in Example 1, except that a positive electrode active material was used in which the large particles A and the small particles were mixed in a mass ratio of 80:20.

[0057] <Comparative Example 3> In the preparation of the positive electrode, a positive electrode active material and a secondary battery were prepared and evaluated in the same manner as in Example 1, except that small particles were used as the positive electrode active material.

[0058] The evaluation results of the Examples and Comparative Examples are shown in Table 1. The tap density is shown as a relative value when the value of Comparative Example 1 is set to 100. Table 1 also shows the proportions of large particles and small particles.

[0059] [Table 1]

[0060] The secondary batteries of Examples 1 to 5 had higher capacity retention rates than the secondary batteries of Comparative Examples 1 to 3. The positive electrode active materials of Examples 2 to 5 had higher tap densities than Comparative Example 1, which had a size between the large particles A and the small particles. This confirmed that a positive electrode active material in which large particles and small particles are mixed in an appropriate ratio can achieve high capacity while improving cycle characteristics. [Explanation of symbols]

[0061] 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 positive electrode current collector, 31 positive electrode mixture layer, 40 negative electrode current collector, 41 negative electrode mixture layer

Claims

1. large particles and small particles of a lithium transition metal composite oxide, the large particles are secondary particles formed by agglomeration of primary particles having a diameter of 1 μm or less, and have a volume-based median diameter of 10 μm to 25 μm; the small particles comprise non-agglomerated primary particles having a diameter of 1 μm to 5 μm; The positive electrode active material for a non-aqueous electrolyte secondary battery has a mass ratio of the large particles to the small particles of 70:30 to 80:

20.

2. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the small particles have a volume-based median diameter of 3 μm to 7 μm.

3. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the proportion of non-aggregated primary particles in the small particles is 50 mass % or more.

4. The lithium transition metal composite oxide has the general formula Li ａ Ni ｘ Co ｙ M ｚ O ２－ｂ (wherein 0<a≦1.2, 0.3≦x≦0.95, 0≦y≦0.3, 0≦z≦0.5, 0≦b<0.05, x+y+z=1, and M contains at least one element selected from Mn, Zr, Mo, W, Nb, Al, Cr, V, Ce, Ti, Fe, Si, Ga, and In).

5. 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 claims 1 to 4, a negative electrode, and a non-aqueous electrolyte.

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