Method for producing positive electrode active material for non-aqueous electrolyte secondary batteries

Abstract

To provide a nonaqueous electrolyte secondary battery in which both improvement of heat resistance and suppression of deterioration of rate characteristics are achieved.SOLUTION: A positive electrode active material for a nonaqueous electrolyte secondary battery is a positive electrode active material containing a lithium transition metal composite oxide containing 80 mol % or more of Ni with respect to the total number of moles of metal elements excluding Li, in which at least B is present on a particle surface of a composite oxide. When a particle having a particle diameter larger than a 70% particle diameter (D70) on a volume basis is defined as a first particle and a particle having a particle diameter smaller than a 30% particle diameter (D30) on a volume basis is defined as a second particle, a molar fraction of the B with respect to the total number of moles of metal elements excluding Li in the second particle is larger than a molar fraction of the B with respect to the total number of moles of metal elements excluding Li in the first particle.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have attracted attention as high-energy-density cathode active materials. For example, Patent Document 1 discloses a method for coating the particle surface of a lithium transition metal composite oxide with a boric acid compound in order to suppress the generation of gas due to the decomposition of the electrolyte on the surface of the cathode active material in a charged state. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2010-40382 [Overview of the project]

[0004] However, in non-aqueous electrolyte secondary batteries such as lithium-ion batteries, if B is present on the particle surface of lithium transition metal composite oxide in the form of a boric acid compound, the decomposition of the electrolyte is suppressed and the heat resistance of the battery is improved, but the resistance of the battery increases and the rate characteristics deteriorate. The technology disclosed in Patent Document 1 still has room for improvement in achieving both improved heat resistance of the battery and suppression of deterioration in rate characteristics.

[0005] The purpose of this disclosure is to achieve both improved heat resistance and suppression of rate characteristic degradation in a non-aqueous electrolyte secondary battery containing a high-energy-density positive electrode active material.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of this disclosure, is a positive electrode active material comprising a lithium transition metal composite oxide containing 80 mol% or more of Ni relative to the total number of moles of metal elements excluding Li, wherein B is present at least on the particle surface of the composite oxide. When particles with a particle size larger than the 70% particle size (D70) based on volume are defined as first particles, and particles with a particle size smaller than the 30% particle size (D30) based on volume are defined as second particles, the mole fraction of B in the second particle relative to the total number of moles of metal elements excluding Li is greater than the mole fraction of B in the first particle relative to the total number of moles of metal elements excluding Li.

[0007] One embodiment of the present disclosure is a non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0008] According to one aspect of the present disclosure, a positive electrode active material can be provided that achieves both improved heat resistance and suppression of a decrease in rate characteristics in a non-aqueous electrolyte secondary battery. [Brief explanation of the drawing]

[0009] [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]

[0010] The present inventors, after diligent research to solve the above problems, have found that by making the mole fraction of B in the smaller second particle higher than the mole fraction of B in the larger first particle, it is possible to improve the heat resistance of the battery while suppressing the deterioration of the rate characteristics. Here, the first and second particles are secondary particles formed by the aggregation of primary particles of a lithium transition metal composite oxide. High-Ni lithium transition metal composite oxides, which have a high energy density, increase the average valence of Ni during charging, making them prone to side reactions with the electrolyte that decompose the electrolyte and adversely affect the heat resistance of the battery. As described in Patent Document 1, the above side reactions can be suppressed by having B present on the surface of the secondary particles of the lithium transition metal composite oxide in the form of a boric acid compound, but this increases the resistance of the battery, which leads to a problem of deterioration in rate characteristics. Therefore, by adjusting the mole fraction of B in the first particle (large particle size) and the second particle (small particle size), we successfully suppressed the deterioration of rate characteristics by increasing the mole fraction of B in the second particle (which has a larger surface area per unit mass), thereby effectively suppressing side reactions with the electrolyte, and by decreasing the mole fraction of B in the first particle, thereby lowering the overall mole fraction of B present in the lithium transition metal composite oxide.

[0011] The following describes in detail an example of a positive electrode active material for a non-aqueous electrolyte secondary battery according to this disclosure, and an example of an embodiment of a non-aqueous electrolyte secondary battery using the positive electrode active material. In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example, but the outer casing is not limited to a cylindrical outer casing, and may be, for example, a rectangular outer casing, or an outer casing composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators in between.

[0012] Figure 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As illustrated in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 that houses the electrode body 14 and the electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0013] A non-aqueous electrolyte comprises 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 the electrolyte salt. Note that the electrolyte is not limited to a liquid electrolyte; it may also be a solid electrolyte using a gel-like polymer or the like.

[0014] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and the width direction (short direction) than the positive electrode 11. The two separators 13 are formed to be at least slightly larger in dimensions than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0015] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. In the example shown in FIG. 1, the positive electrode lead 20 extends toward the sealing body 17 through the through-hole of the insulating plate 18, and the negative electrode lead 21 extends toward the bottom side of the outer can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27 which is the top plate of the sealing body 17 electrically connected to the internal terminal plate 23 serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0016] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness inside the battery. The outer can 16 is formed with a groove-in portion 22 that supports the sealing body 17, with a part of the side surface portion projecting inward. The groove-in portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the outer can 16 by the groove-in portion 22 and the open end portion of the outer can 16 caulked to the sealing body 17.

[0017] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is interposed between the peripheral edges of each. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 is broken, and gas is discharged from the opening of the cap 27.

[0018] Hereinafter, the positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 will be described in detail, particularly the positive electrode active material constituting the positive electrode 11.

[0019] [Positive Electrode] The positive electrode 11 has a positive electrode core body and a positive electrode composite layer provided on the surface of the positive electrode core body. For the positive electrode core body, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum, a film having the metal disposed on the surface layer, etc. can be used. The positive electrode composite layer includes a positive electrode active material, a binder, and a conductive material, and is preferably provided on both surfaces of the positive electrode core body excluding the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be manufactured, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. on the surface of the positive electrode core body, drying the coating film, and then compressing to form the positive electrode composite layer on both surfaces of the positive electrode core body.

[0020] Examples of the conductive material contained in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode composite layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. These resins may be used in combination with a cellulose derivative such as carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc.

[0021] The positive electrode active material includes particles of a lithium transition metal composite oxide containing 80 mol% or more of Ni with respect to the total molar number of metal elements excluding Li. By setting the Ni content to 80 mol% or more, a high-capacity battery can be obtained. Further, B is present at least on the particle surface of the lithium transition metal composite oxide. Hereinafter, for convenience of explanation, the lithium transition metal composite oxide is referred to as "composite oxide (Z)". The positive electrode active material may have the composite oxide (Z) as the main component and may be substantially composed of only the composite oxide (Z). The positive electrode active material may contain a composite oxide other than the composite oxide (Z) or other compounds within a range not impairing the object of the present disclosure.

[0022] The composite oxide (Z) may contain metal elements other than Li, Ni, and B. Examples of such metal elements include Co, Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, Si, etc. An example of a suitable composite oxide (Z) is one with the general formula Li a Ni b Co c M1 d M2 e B f O g This is a composite oxide represented by the formula (wherein 0.8≦a≦1.2, b≧0.80, c≦0.15, 0.01≦d≦0.12, 0≦e≦0.05, 0.001≦f≦0.020, 1≦g≦2, b+c+d+e+f=1, M1 is at least one element selected from Mn and Al, and M2 is at least one element selected from Groups 4 to 6). That is, the mole fraction of B with respect to the total number of moles of metal elements excluding Li is preferably 0.001 to 0.020, and more preferably 0.005 to 0.015. The mole fraction of metal elements in the entire particle of the composite oxide (Z) is measured by inductively coupled plasma (ICP) emission spectroscopy. From the viewpoint of capacity and heat resistance, M1 is preferably Mn.

[0023] The composite oxide (Z) is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles constituting the secondary particle is, for example, 0.05 μm to 1 μm. The particle size of the primary particle is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM). B is present on the surface of the secondary particles of the composite oxide (Z), and may also be present on the surface of the primary particles inside the secondary particles and at the grain boundaries. Furthermore, a portion of B may also be present inside the primary particles and may form a solid solution with other metal elements contained in the composite oxide (Z).

[0024] The composite oxide (Z) consists of particles with a volume-based median diameter (D50) of, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of the composite oxide (Z) can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0025] The composite oxide (Z) is characterized in that, when particles with a volume-based particle size greater than the 70% particle size (D70) are designated as first particles and particles with a volume-based particle size smaller than the 30% particle size (D30) are designated as second particles, the mole fraction of B in the second particles is higher than the mole fraction of B in the first particles. This makes it possible to achieve both improved heat resistance of the battery and suppression of deterioration in rate characteristics. B is contained in both the first and second particles.

[0026] Here, D70 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 70% of the volume-based particle size distribution. Similarly, D30 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 30% of the volume-based particle size distribution. For example, D70 corresponds to 9 μm to 19 μm, and D30 corresponds to 3 μm to 13 μm. Furthermore, the mole fraction of metal elements present on the particle surface of the composite oxide (Z) is measured by X-ray photoelectron spectroscopy (XPS). By setting the X-ray irradiation spot diameter to 1 mmΦ or larger, several hundred composite oxide (Z) particles are contained within the irradiation spot, allowing for the average measurement of the mole fraction of B on the surface of the composite oxide (Z).

[0027] On the surfaces of the first and second particles, the mole fraction of B relative to the total number of moles of metal elements excluding Li (hereinafter sometimes referred to as "surface coverage of B") can be set to 50% to 98%. Within this range, it is possible to improve the heat resistance of the battery while suppressing a decrease in battery capacity. Here, the surface coverage of B can be calculated by measuring the number of moles of metal elements excluding Li on the secondary particle surface using XPS and obtaining the mole fraction of B relative to the total number of moles of metal elements excluding Li.

[0028] On the surfaces of the first and second particles, B may exist in the form of a boron compound containing Li and B. Boron compounds such as boric acid (H3BO3), boron oxide (B2O3), and lithium borate (LiBO2, Li2B4O7) can be used as B sources. When boric acid or boron oxide is used as a B source, a boron compound containing Li and B may be formed during firing by reaction with Li present on the particle surface or a separately added Li source.

[0029] The boron compound may be formed to cover the entire surface of the secondary particles, or it may be scattered on the surface of the secondary particles. In the case of particulate matter, the particle size of the boron compound is generally smaller than the particle size of the primary particles constituting the composite oxide (Z). The boron compound particles can be confirmed by SEM. It is preferable that the boron compound adheres to a wide area of ​​the surface of the secondary particles constituting the composite oxide (Z), rather than being concentrated in only a part of it.

[0030] Furthermore, the thickness of the boron compound on the surface of the first particle and the surface of the second particle is not particularly limited and can be, for example, 10 nm to 100 nm.

[0031] As described above, in the composite oxide (Z), B may also be present inside the primary particles and may be in solid solution with transition metal elements such as Ni. The mole fraction of B relative to the metal elements in solid solution can be confirmed by energy-dispersive X-ray spectroscopy (EDS) in the cross-section of the primary particles. In the composite oxide (Z), the total number of moles of B in the solid solution state and B in the boron compound state present on the surface is preferably 0.001 to 0.020 relative to the total number of moles of metal elements excluding Li.

[0032] The composite oxide (Z) can be prepared, for example, by the following procedure. (1) Two composite compounds (X1) and (X2) that do not contain two types of Li with different D50s are each calcined with a Li source such as lithium hydroxide to synthesize lithium composite oxides (Y1) and (Y2) with different D50s. An example of a composite compound is a composite oxide or hydroxide containing Ni, Co, and Mn. At this time, one type of lithium composite oxide may be classified to obtain a lithium composite oxide with two types of average particle sizes. Conventional known methods can be applied for classification. The obtained lithium composite oxides (Y1) and (Y2) may also be washed with water. By washing with water, not only the amount of Li present on the surface of the lithium composite oxides (Y1) and (Y2) but also the amount of Li present inside Y1 and Y2 is reduced, and voids are created inside the washed Y1 and Y2. (2) A B source is added to each of the composite oxides (Y1) and (Y2), and B is compounded onto the particle surface. After this, the mixture is calcined to synthesize the composite oxides (Z1) and (Z2). Then, the composite oxides (Z1) and (Z2) are mixed to obtain the composite oxide (Z). An example of a B source is boric acid (H3BO3). A dry particle compounding apparatus (for example, NOB-130 manufactured by Hosokawa Micron Corporation) is used for compounding. At this time, a Li source such as lithium hydroxide may be added along with the B source.

[0033] In step (2) above, by adding more H3BO3 to the composite oxide (Y2) than to the composite oxide (Y1), the mole fraction of B in the composite oxide (Z2) can be made larger than the mole fraction of B in the composite oxide (Z1).

[0034] Furthermore, the firing temperature in step (2) above is, for example, 200°C to 500°C. By adjusting the firing temperature of the composite oxides (Y1) and (Y2), the surface coverage of B and the thickness of the boron compound in the composite oxides (Z1) and (Z2) can be adjusted. By firing Y1 and Y2 together with the B source at a high temperature, composite oxides (Z1) and (Z2) with a low surface coverage of B on the particle surface can be synthesized. By firing Y1 and Y2 together with the B source at a low temperature, composite oxides (Z1) and (Z2) with a high surface coverage of B on the particle surface can be synthesized. High temperature is, for example, 350°C to 500°C, and low temperature is, for example, 200°C to 325°C. Note that the relationship between firing temperature and coverage may change depending on the composition and shape of the lithium transition metal composite oxide, firing time, and firing atmosphere.

[0035] [Negative electrode] The negative electrode 12 comprises a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. The negative electrode core can be made of a metal foil that is stable within the potential range of the negative electrode 12, such as copper, or a film with the metal arranged on its surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core, excluding the portion to which the negative electrode lead 21 is connected. The negative electrode 12 can be manufactured, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form the negative electrode composite layer on both sides of the negative electrode core.

[0036] The negative electrode composite layer contains, as the negative electrode active material, a carbon-based active material that reversibly intercepts and releases lithium ions, for example. Suitable carbon-based active materials include natural graphite such as flake graphite, lumpy graphite, and clay-like graphite, as well as artificial graphite such as lumpy 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, and carbon-based active materials and Si-based active materials may be used in combination.

[0037] As the binder contained in the negative electrode composite layer, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc. can be used as in the case of the positive electrode 11, but it is preferable to use styrene-butadiene rubber (SBR). Further, the negative electrode composite layer preferably further contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among them, it is preferable to use SBR in combination with CMC or its salt, and PAA or its salt.

[0038] [Separator] For the separator 13, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator.

Example

[0039] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0040] <Example 1> [Synthesis of Positive Electrode Active Material] The nickel cobalt manganese composite hydroxide with D50 of 12 μm and composition of Ni 0.85 Co 0.08 Mn 0.07 (OH)2 and the nickel cobalt manganese composite hydroxide with D50 of 8 μm and composition of Ni 0.85 Co 0.08 Mn 0.07 (OH)2 were each calcined at 500 °C to obtain a nickel cobalt manganese composite oxide (X1) with a large average particle size and a nickel cobalt manganese composite oxide (Y1) with a small average particle size.

[0041] Next, lithium hydroxide and nickel-cobalt-manganese composite oxide (X1) with a large average particle size were mixed so that the molar ratio of Li to Ni, Co, and Mn was 1.08:1. This mixture was calcined at 700°C for 8 hours in an oxygen atmosphere, and then pulverized to obtain lithium composite oxide (X2) with a large average particle size. The obtained lithium composite oxide (X2) was not washed with water.

[0042] Next, lithium hydroxide and nickel-cobalt-manganese composite oxide (Y1) with a small average particle size were mixed so that the molar ratio of Li to Ni, Co, and Mn was 1.08:1. This mixture was calcined at 700°C for 8 hours in an oxygen atmosphere, and then pulverized to obtain lithium composite oxide (Y2) with a small average particle size. The obtained lithium composite oxide (Y2) was not washed with water.

[0043] Next, lithium composite oxide (X2) with a large average particle size and boric acid (H3BO3) were dry-mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.005. This mixture was then calcined in air at 300°C for 3 hours and then pulverized to obtain lithium composite oxide (X3) in which B was present on the particle surface.

[0044] Next, lithium composite oxide (Y2) with a small average particle size and H3BO3 were dry-mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.015. This mixture was then calcined in air at 300°C for 3 hours and then pulverized to obtain lithium composite oxide (Y3) in which B was present on the particle surface.

[0045] Next, lithium composite oxides (X3) and (Y3) were mixed in a 1:1 mass ratio to form the positive electrode active material. B present on the particle surface and inside the particles can be quantified by ICP. The presence of B on the particle surface in the form of a boron compound containing Li and B can be confirmed by XRD, XPS, XAFS, etc.

[0046] Analysis of the composition of the positive electrode active material by ICP revealed that Li 1.01 Ni 0.84 Co 0.08 Mn 0.07 B 0.01 The result was O2. Therefore, from the ICP results, the mole fraction of B relative to the total number of moles of metal elements excluding Li (Ni, Co, Mn, B) was 1.0%. Furthermore, analysis of the composition of lithium composite oxides (X3) and (Y3) by ICP revealed that the mole fractions of B relative to the total number of moles of metal elements excluding Li (Ni, Co, Mn, B) were 0.5% and 1.5%, respectively.

[0047] The surface coverage of B was calculated by measuring the number of moles of Ni, Co, Mn, and B on the secondary particle surface using XPS, and then determining the mole fraction of B relative to the total number of moles of Ni, Co, and Mn. The surface coverage of B was 96% for both lithium composite oxides (X3) and (Y3). The particle size distribution of the positive electrode active material was 12 μm for D50, 14 μm for D70, and 10 μm for D30.

[0048] [Fabrication of the positive electrode] The above-mentioned positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solid content mass ratio of 96.3:2.5:1.2. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added, and the mixture was kneaded to prepare a positive electrode slurry. This positive electrode slurry was applied to both sides of a positive electrode core made of aluminum foil. After the coating film was dried, the coating film was rolled using a roller and cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode slurry layer was formed on both sides of the positive electrode core. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode core was exposed.

[0049] [Fabrication of the negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution in a solid content mass ratio of 100:1:1 to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and after the coating film was dried, the coating film was rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. An exposed portion was provided on a part of the negative electrode in which the surface of the negative electrode core was exposed.

[0050] [Preparation of non-aqueous electrolytes] A mixed solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved at a concentration of 1.0 mol / liter in this mixed solvent. Furthermore, vinylene carbonate (VC) was dissolved in the above mixed solvent at a concentration of 2.0% by mass to prepare a non-aqueous electrolyte.

[0051] [Battery construction] Aluminum leads were attached to the exposed portion of the positive electrode, and nickel leads were attached to the exposed portion of the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator, and then press-molded radially to produce a flat, wound electrode body. This electrode body was housed in an outer casing made of aluminum laminate sheet, the non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a non-aqueous electrolyte secondary battery with a design capacity of 650 mAh.

[0052] <Example 2> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that X2 and Y2 were washed with water.

[0053] <Comparative Example 1> X2 and H3BO3 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.010, and Y2 and H3BO3 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.010, except that the mixture was prepared in the same manner as in Example 1.

[0054] <Comparative Example 2> X2 and H3BO3 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.010, and Y2 and H3BO3 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B in H3BO3 were 1:0.010, except that the mixture was prepared in the same manner as in Example 2.

[0055] The rate characteristics and thermal runaway temperature were evaluated for each battery in the examples and comparative examples. The evaluation results are shown in Table 1. Furthermore, Table 1 shows the mole fraction of B relative to the total number of moles of metal elements excluding Li in the first and second particles, and the surface coverage of B.

[0056] [Evaluation of rating characteristics] Each battery in the examples and comparative examples was charged at a constant current of 0.5It at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage until the current value reached 0.02It at 4.2V. After that, the batteries were left for 15 minutes. Next, constant current discharge was performed at 0.05It until the battery voltage reached 2.5V, and the discharge capacity C1 at 0.05It was measured. Next, constant voltage charging was performed at 4.2V until the current value reached 0.02It, and the batteries were left for 15 minutes. After that, constant current discharge was performed at 2It until the battery voltage reached 2.5V, and the discharge capacity C2 at 2It was measured. The rate characteristics were calculated using the following formula.

[0057] Rate characteristic (%) = C2 / C1 × 100 [ARC Exam] The fabricated battery was charged at a constant current of 0.3 It in a 25°C environment until the battery voltage reached 4.2V. Then, it was charged again at a constant voltage of 4.2V until the current value was 0.05 It, bringing it to a charged state. After that, the battery was heated to 130°C in an ARC test apparatus, and the self-heating rate (°C / min) of the battery under adiabatic conditions was measured by observing the battery temperature using a thermocouple attached to the flat surface of the battery. The battery temperature at which the self-heating rate reached 10°C / min was defined as the thermal runaway temperature.

[0058] [Table 1]

[0059] As shown in Table 1, all of the batteries in the examples had higher rate characteristics and thermal runaway temperatures compared to the batteries in the comparative examples. In other words, the batteries in the examples achieved both improved heat resistance and suppression of the deterioration of rate characteristics. In Example 1, since no water washing was performed during the synthesis of the positive electrode active material, a large amount of Li remained on the surface of the lithium composite oxide, resulting in good ion conductivity and higher rate characteristics than in Example 2. [Explanation of Symbols]

[0060] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 outer cans 17 Sealing body 18,19 Insulating board 20 Positive leads 21 Negative lead 22 Grooved section 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Upper valve body 27 caps 28 Gaskets

Claims

1. A method for producing a positive electrode active material comprising a lithium transition metal composite oxide containing 80 mol% or more of Ni relative to the total number of moles of metal elements other than Li, wherein B is present on at least the particle surface of the lithium transition metal composite oxide, The content of B in the positive electrode active material is 0.001 or more and 0.020 or less, when the total number of moles of metal elements excluding Li contained in the lithium transition metal composite oxide is set to 1. When particles with a particle size larger than the 70% volume-based particle size (D70) are defined as the first particle, and particles with a particle size smaller than the 30% volume-based particle size (D30) are defined as the second particle, The step of generating the first particle is: The first step involves mixing a lithium source with a composite oxide that does not contain Li and firing it, The second step involves obtaining a lithium composite oxide by crushing it after calcination and not washing it with water, A third step involves mixing the unwashed lithium composite oxide with a boron source and firing it, A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, including the above.

2. The step of generating the second particle is: The fourth step involves mixing a lithium source with a composite oxide that does not contain Li and firing it, The fifth step involves obtaining a lithium composite oxide by crushing it after calcination and not washing it with water, A sixth step involves mixing the unwashed lithium composite oxide with a boron source and firing it, A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, including the method described above.

3. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the amount of boron source added in the sixth step is greater than the amount of boron source added in the third step.

4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the firing temperature in the sixth step is 200°C to 325°C.

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