Manufacturing method for positive electrode active material for nonaqueous electrolyte secondary battery

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] The present 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 been attracting attention as positive electrode active materials with high energy density. For example, Patent Document 1 discloses a method of depositing a boric acid compound on the particle surface of a lithium transition metal composite oxide in order to suppress gas generation due to electrolyte decomposition on the surface of a positive electrode active material in a charged state. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-40382 Summary of the Invention

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

[0005] An object of the present disclosure is to achieve both improved heat resistance and suppression of deterioration in rate characteristics 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 embodiment of the present disclosure is a positive electrode active material that includes 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, and in which B is present at least on the particle surfaces of the composite oxide. When particles having a particle size larger than the 70% particle size (D70) on a volume basis are defined as first particles and particles having a particle size smaller than the 30% particle size (D30) on a volume basis are defined as second particles, the molar fraction of B relative to the total number of moles of metal elements excluding Li in the second particles is larger than the molar fraction of B relative to the total number of moles of metal elements excluding Li in the first particles.

[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, a negative electrode, and a non-aqueous electrolyte.

[0008] According to the positive electrode active material of one aspect of the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery that achieves both improved heat resistance and suppression of deterioration in rate characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0010] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors discovered that by increasing the molar fraction of B in the second particles having a smaller particle size compared to the molar fraction of B in the first particles having a larger particle size, it is possible to improve the heat resistance of a battery while suppressing a deterioration in rate characteristics. Here, the first and second particles are secondary particles formed by aggregation of primary particles of a lithium transition metal composite oxide. A lithium transition metal composite oxide having a high Ni content and high energy density has a high average Ni valence during charging, which easily decomposes the electrolyte through a side reaction with the electrolyte, adversely affecting the heat resistance of the battery. As described in Patent Document 1, the side reaction can be suppressed by adding B to the surface of the secondary particles of the lithium transition metal composite oxide in the form of a boric acid compound, etc., but this increases the resistance of the battery, resulting in a problem of a deterioration in rate characteristics. Therefore, by adjusting the molar fraction of B in the first particles with a large particle size and the second particles with a small particle size, and increasing the molar fraction of B in the second particles with a large surface area per unit mass, side reactions with the electrolyte were effectively suppressed, while by decreasing the molar fraction of B in the first particles, the molar fraction of B present in the entire lithium transition metal composite oxide was reduced, thereby successfully suppressing a decrease in rate characteristics.

[0011] An example of an embodiment of a positive electrode active material for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom will be exemplified. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes 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 spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0013] 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 include esters, ethers, nitriles, amides, 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. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel polymer or the like.

[0014] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 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 disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface 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 airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved 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 top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0017] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0019] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. The positive electrode core can be a foil of a metal such as aluminum 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 composite layer contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0020] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. 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).

[0021] The positive electrode active material contains particles of 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. By making the Ni content 80 mol% or more, a high-capacity battery can be obtained. Furthermore, B is present at least on the particle surface of the lithium transition metal composite oxide. Hereinafter, for convenience of explanation, this lithium transition metal composite oxide will be referred to as "composite oxide (Z)". The positive electrode active material may contain the composite oxide (Z) as a main component and may be composed essentially of the composite oxide (Z) alone. The positive electrode active material may also contain a composite oxide other than the composite oxide (Z) or other compounds, as long as the object of the present disclosure is not impaired.

[0022] The composite oxide (Z) may contain a metal element other than Li, Ni, and B. Examples of the metal element include Co, Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, and Si. An example of a suitable composite oxide (Z) is a compound represented by the general formula Li a Ni b Co c M1 d M2 e B f O g (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 molar fraction of B relative 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 molar fraction of metal elements in the entire particle of the complex oxide (Z) is measured by inductively coupled plasma (ICP) atomic emission spectrometry. From the viewpoints 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 a plurality of 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 a particle image observed with a scanning electron microscope (SEM). B is present on the surface of the secondary particle of the composite oxide (Z), and may also be present on the surface of the primary particle inside the secondary particle or at the grain boundary. Furthermore, a portion of B may also be present inside the primary particle and form a solid solution with other metal elements contained in the composite oxide (Z).

[0024] The composite oxide (Z) is a particle having 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 in the volume-based particle size distribution is 50%, 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., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0025] The composite oxide (Z) is characterized in that, when particles having a volumetric particle size larger than the 70% particle size (D70) are defined as first particles and particles having a volumetric particle size smaller than the 30% particle size (D30) are defined as second particles, the molar fraction of B in the second particles is higher than the molar fraction of B in the first particles. This makes it possible to achieve both improved heat resistance and suppressed deterioration of rate characteristics of the battery. B is contained in both the first particles and the second particles.

[0026] Here, D70 refers to the particle size at which the cumulative frequency of particle size distribution based on volume is 70% from the smallest particle size. Similarly, D30 refers to the particle size at which the cumulative frequency of particle size distribution based on volume is 30% from the smallest particle size. For example, D70 is 9 μm to 19 μm, and D30 is 3 μm to 13 μm. The molar 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 more, several hundred particles of the composite oxide (Z) are contained within the irradiation spot, and the molar fraction of B on the surface of the composite oxide (Z) can be measured on average.

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

[0028] On the surfaces of the first and second particles, B may be present in the form of a boron compound containing Li and B. As the B source, boron compounds such as boric acid (H3BO3), boron oxide (BO), and lithium borate (LiBO2, Li2BO7) are used. When boric acid or boron oxide is used as the B source, it may react with Li present on the particle surface or a separately added Li source during firing to produce a boron compound containing Li and B.

[0029] The boron compound may be formed so as to cover the entire surface of the secondary particles, or may be scattered on the surface of the secondary particles. In the case of particulate form, the particle size of the boron compound is generally smaller than the particle size of the primary particles constituting the composite oxide (Z). Note that the particles of the boron compound can be confirmed by SEM. It is preferable that the boron compound is attached over a wide area, without being unevenly distributed on a part of the surface of the secondary particles constituting the composite oxide (Z).

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

[0031] As described above, in the composite oxide (Z), B is present inside the primary particles and may be dissolved in a solid solution with a transition metal element such as Ni. The molar fraction of B relative to the dissolved metal element can be determined by energy dispersive X-ray spectroscopy (EDS) on the cross section of the primary particle. In the composite oxide (Z), the total number of moles of dissolved B and boron compound B 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 types of Li-free composite compounds (X1) and (X2) with different D50s are calcined after adding a Li source such as lithium hydroxide to each compound to synthesize lithium composite oxides (Y1) and (Y2) with different D50s. Examples of the composite compounds include composite oxides or hydroxides containing Ni, Co, and Mn. In this process, one type of lithium composite oxide may be classified to obtain two types of lithium composite oxides with different average particle sizes. A conventionally known method can be used for classification. The resulting lithium composite oxides (Y1) and (Y2) may also be washed with water. Washing reduces 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, resulting in the formation of voids inside the washed Y1 and Y2. (2) A B source is added to each of the composite oxides (Y1) and (Y2), and B is composited onto the particle surface. After calcination, composite oxides (Z1) and (Z2) are synthesized. The composite oxides (Z1) and (Z2) are then mixed to obtain composite oxide (Z). An example of a B source is boric acid (H3BO3). A dry particle compositer (e.g., NOB-130, manufactured by Hosokawa Micron Corporation) is used for the composite. At this time, a Li source, such as lithium hydroxide, may be added together with the B source.

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

[0034] The calcination temperature in step (2) is, for example, 200°C to 500°C. By adjusting the calcination 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 calcining Y1 and Y2 together with a B source at a high temperature, composite oxides (Z1) and (Z2) having a low surface coverage of B on the particle surface can be synthesized. By calcining Y1 and Y2 together with a B source at a low temperature, composite oxides (Z1) and (Z2) having a high surface coverage of B on the particle surface can be synthesized. A high temperature is, for example, 350°C to 500°C, and a low temperature is, for example, 200°C to 325°C. The relationship between the calcination temperature and the coverage may vary depending on the composition and shape of the lithium transition metal composite oxide, the calcination time, the calcination atmosphere, and other factors.

[0035] [Negative electrode] The negative electrode 12 has 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 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 composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the portion to which the negative electrode lead 21 is connected. The negative electrode 12 can be produced, 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 a negative electrode composite layer on both sides of the negative electrode core.

[0036] The negative electrode mixture layer contains, as the negative electrode active material, 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 amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0037] The binder contained in the negative electrode mixture layer may be, as in the case of the positive electrode 11, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but is preferably 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.

[0038] [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 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] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0040] Example 1 [Synthesis of positive electrode active material] Obtained by coprecipitation, D50 is 12 μm and composition is Ni 0.85 Co 0.08 Mn 0.07 (OH)2 nickel cobalt manganese composite hydroxide and D50 8μm and composition Ni 0.85 Co 0.08 Mn 0.07 The nickel-cobalt-manganese composite hydroxides of (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 a nickel-cobalt-manganese composite oxide (X1) having a large average particle size were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.08:1. This mixture was fired at 700°C in an oxygen atmosphere for 8 hours and then pulverized to obtain a lithium composite oxide (X2) having 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) having a small average particle size were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.08:1. This mixture was fired at 700°C in an oxygen atmosphere for 8 hours and then pulverized to obtain lithium composite oxide (Y2) having a small average particle size. The obtained lithium composite oxide (Y2) was not washed with water.

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

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

[0045] Next, the lithium composite oxides (X3) and (Y3) were mixed in a 1:1 mass ratio to form the positive electrode active material. The B present on the particle surface and inside the particles can be quantified using ICP. The presence of B on the particle surface in the form of a boron compound containing Li and B can be confirmed using 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 O2. Therefore, from the results of ICP, the molar fraction of B relative to the total number of moles of metal elements excluding Li (Ni, Co, Mn, B) was 1.0%. Furthermore, as a result of analyzing the compositions of lithium composite oxides (X3) and (Y3) by ICP, the molar 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 calculating the molar 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 D50 12 μm, D70 14 μm, and D30 10 μm.

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

[0049] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution at a solids mass ratio of 100:1:1 to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, exposing the surface of the negative electrode core.

[0050] [Preparation of non-aqueous electrolyte] Lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. Furthermore, vinylene carbonate (VC) was dissolved in the mixed solvent at a concentration of 2.0 mass% to prepare a nonaqueous electrolyte.

[0051] [Battery construction] 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 650 mAh.

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

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

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

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

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

[0057] Rate characteristic (%) = C2 / C1 x 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 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It, bringing the battery into a charged state. The battery was then heated to 130°C in an ARC test device, and the battery temperature was observed using a thermocouple attached to the flat surface of the battery to measure the self-heating rate (°C / min) of the battery in an adiabatic environment. The battery temperature at which the self-heating rate of the battery reached 10°C / min was defined as the thermal runaway temperature.

[0058] [Table 1]

[0059] As shown in Table 1, all of the batteries of the Examples had higher rate characteristics and thermal runaway temperatures than the batteries of the Comparative Examples. In other words, it can be seen that the batteries of the Examples achieved both improved heat resistance and suppressed deterioration in rate characteristics. In Example 1, since no water washing was performed during synthesis of the positive electrode active material, a large amount of Li remained on the surface of the lithium composite oxide, resulting in good ionic 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 can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket

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 excluding Li, and B being present at least on the particle surfaces of the lithium transition metal composite oxide, comprising: 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 1, When particles having a particle size larger than the 70% particle size (D70) on a volume basis are defined as first particles, and particles having a particle size smaller than the 30% particle size (D30) on a volume basis are defined as second particles, The step of generating the first particles includes: a first step of mixing a lithium source and a composite oxide not containing Li and firing the mixture; a second step of obtaining a lithium composite oxide by pulverizing the calcined product without washing it with water; a third step of mixing the unwashed lithium composite oxide with a boron source and calcining the mixture; The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery includes the steps of:

2. The step of generating second particles includes: a fourth step of mixing a lithium source and a composite oxide not containing Li and firing the mixture; A fifth step of obtaining a lithium composite oxide by pulverizing the calcined product without washing it with water; a sixth step of mixing the unwashed lithium composite oxide with a boron source and calcining the mixture; The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 , comprising:

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

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

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