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

Abstract

In the present invention, a positive electrode active material for a non-aqueous electrolyte secondary battery comprises: first lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or greater, D50 is greater than 8 μm but not greater than 30 μm, and (D90–D10) / D50 is 0.7–1.5; and second lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or greater, D50 is greater than 1 μm but not greater than 6 μm, and D90–D10 is 6 μm or less. The first and / or second lithium transition metal composite oxide particles include(s) at least one element selected from the group consisting of Ca, Sr, B, Ti, and Nb.

Description

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

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

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, the positive electrode active material has a significant effect on battery performance, such as input / output characteristics, capacity, cycle characteristics, and thermal stability, and therefore much research has been conducted on the positive electrode active material. Generally, lithium transition metal composite oxides containing transition metal elements such as Ni, Co, and Mn are used as the positive electrode active material. The types and amounts of elements contained in the lithium transition metal composite oxide, as well as the crystalline structure of the composite oxide, significantly affect battery performance, and even slight changes in these physical properties may prevent the desired performance from being achieved.

[0003] For example, Patent Documents 1 and 2 propose positive electrode active materials for non-aqueous electrolyte secondary batteries containing two types of lithium transition metal composite oxides with different particle size distributions, mainly for the purpose of increasing the capacity of the battery.

[0004] JP 2021-120937 A Patent No. 6273581

[0005] Meanwhile, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are required to have high capacity as well as high safety, and it is desirable that the batteries do not generate heat or other abnormalities even when placed in a high-temperature environment. An object of the present disclosure is to provide a positive electrode active material that can realize a non-aqueous electrolyte secondary battery with excellent thermal stability. Note that the positive electrode active materials disclosed in the above patent documents still have a large room for improvement in terms of the thermal stability of the battery.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure comprises first lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the 50% volumetric particle diameter D50 is more than 8 μm and 30 μm or less, and (90% volumetric particle diameter D90 - 10% volumetric particle diameter D10) / D50 is 0.7 or more and 1.5 or less, and second lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the D50 is more than 1 μm and 6 μm or less, and the D90 - D10 is 6 μm or less, and at least one of the first and second lithium transition metal composite oxide particles contains at least one element selected from the group consisting of Ca, Sr, B, Ti, and Nb.

[0007] The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure includes the steps of: preparing first lithium transition metal composite oxide particles, in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the 50% volumetric particle diameter D50 is more than 8 μm and 30 μm or less, and (90% volumetric particle diameter D90 - 10% volumetric particle diameter D10) / D50 is 0.7 or more and 1.5 or less; preparing second lithium transition metal composite oxide particles, in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the D50 is more than 1 μm and 6 μm or less, and D90 - D10 is 6 μm or less; and mixing the first and second lithium transition metal composite oxide particles.

[0008] A non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0009] The positive electrode active material according to the present disclosure can realize a nonaqueous electrolyte secondary battery with excellent thermal stability.

[0010] 1 is a longitudinal cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0011] As a result of extensive research into improving the thermal stability of non-aqueous electrolyte secondary batteries, the inventors have found that by using first and second lithium transition metal composite oxide particles having the above-mentioned specific composition and particle size distribution as the positive electrode active material, the thermal runaway temperature of the battery when placed in a high-temperature environment is specifically increased. Although the detailed mechanism by which this effect occurs is not clear, the finding that the particle size distribution of a positive electrode active material having a specific composition significantly contributes to the thermal stability of the battery is extremely important in achieving further high performance of non-aqueous electrolyte secondary batteries.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0013] In the embodiment described below, a nonaqueous electrolyte secondary battery 10 is exemplified, which is a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom, but the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include, for example, a prismatic battery having a prismatic outer can, a coin-shaped battery having a coin-shaped outer can, and a pouch-shaped battery having an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown 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 nonaqueous electrolyte. The nonaqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. 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 with a bottom and an open end in the axial direction. 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.

[0015] The positive electrode 11, negative electrode 12, and separator 13 that constitute 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 length direction (longitudinal direction) and width direction (transverse direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged 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.

[0016] 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.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior 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 exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0018] 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.

[0019] 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 that constitutes the positive electrode 11, will be described in detail below.

[0020] [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface layer. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0021] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0022] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the positive electrode mixture layer.

[0023] The positive electrode active material includes first lithium transition metal composite oxide particles (hereinafter sometimes referred to as "first composite oxide particles") which are large particles, and second lithium transition metal composite oxide particles (hereinafter sometimes referred to as "second composite oxide particles") which are small particles. The first and second composite oxide particles are particles of a lithium nickel composite oxide in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, and at least one of the particles includes at least one element selected from the group consisting of Ca, Sr, B, Ti, and Nb (hereinafter sometimes collectively referred to as "element M").

[0024] As will be described in detail later, the first composite oxide particles have a volume-based 50% particle diameter (D50) of more than 8 μm and not more than 30 μm. The second composite oxide particles have a D50 of more than 1 μm and not more than 6 μm. Furthermore, the first composite oxide particles have a (volume-based 90% particle diameter (D90) - volume-based 10% particle diameter (D10)) / D50 of 0.7 or more and 1.5 or less, and the second composite oxide particles have a D90 - D10 of 6 μm or less. The composite oxide particles may be particles containing a large amount of internal voids, but from the viewpoint of improving the energy density of the battery, they preferably have a solid structure with a low internal void ratio.

[0025] The first and second composite oxide particles preferably contain at least one element selected from the group consisting of Co, Mn, and Al as an element other than Li, Ni, and element M. Among these, Co and Mn are preferred. The positive electrode active material preferably contains substantially only the first and second composite oxide particles, but may also contain a third composite oxide particle or other compound other than the two types described above, as long as the object of the present disclosure is not impaired. The third composite oxide particles may be, for example, a composite oxide that does not contain Ni.

[0026] The first and second composite oxide particles may have, for example, substantially the same composition, or may have different compositions as long as they satisfy the above-mentioned composition. The content of the elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0027] The first and second composite oxide particles have, for example, a layered rock salt structure. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of increasing capacity and stability of the crystal structure.

[0028] Although the mixing ratio of the first and second composite oxide particles is not limited, it is preferable that the content of the first composite oxide particles is equal to or greater than the content of the second composite oxide particles. In this case, it is possible to achieve, for example, high capacity and high durability while ensuring excellent thermal stability of the battery. As described above, the positive electrode active material may contain composite oxide particles other than the first and second composite oxide particles. However, the content of the first and second composite oxide particles is preferably 90 mass% or more of the total mass of the positive electrode active material, and may be substantially 100 mass%.

[0029] A preferred example of the content of the first composite oxide particles is 50% by mass to 95% by mass, or 60% by mass to 90% by mass, or 65% by mass to 85% by mass, or 70% by mass to 80% by mass, of the total mass of the positive electrode active material. A preferred example of the content of the second composite oxide particles is 5% by mass to 50% by mass, or 10% by mass to 40% by mass, or 15% by mass to 35% by mass, or 20% by mass to 30% by mass, of the total mass of the positive electrode active material.

[0030] From the viewpoint of increasing the capacity of the battery, the first and second composite oxide particles preferably have a molar ratio (a) of Ni relative to the total molar amount of elements excluding Li and O of 75 mol% or more. The molar ratio (a) of Ni in the first composite oxide particles and the molar ratio (a) of Ni in the second composite oxide particles may be the same or different. However, when the molar ratios (a) of Ni are different from each other, it is preferable that the molar ratio (a) of Ni in the first composite oxide particles is higher than the molar ratio (a) of Ni in the second composite oxide particles. In this case, the effect of improving the thermal stability of the battery becomes more significant.

[0031] The molar ratio (a) of Ni in the first composite oxide particles is more preferably 75 mol% to 99 mol%, particularly preferably 80 mol% to 95 mol%, and may be 85 mol% to 95 mol%. The molar ratio (a) of Ni in the second composite oxide particles is more preferably 75 mol% to 95 mol%, particularly preferably 75 mol% to 90 mol%, and may be 75 mol% to 85 mol%. When the molar ratio (a) of Ni in each composite oxide particle is within this range, it is easy to achieve both high capacity and thermal stability.

[0032] When the first composite oxide particles contain Co and Mn, the molar ratios of Co (b) and Mn (c) to the total molar amount of elements excluding Li and O are, for example, 1 mol% to 20 mol%, or 2 mol% to 15 mol%, or 3 mol% to 10 mol%, or 4 mol% to 6 mol%. When the second composite oxide particles contain Co and Mn, the molar ratios of Co (b) and Mn (c) to the total molar amount of elements excluding Li and O are, for example, 2 mol% to 30 mol%, or 3 mol% to 25 mol%, or 5 mol% to 20 mol%, or 7 mol% to 15 mol%, respectively.

[0033] As described above, at least one of the first and second composite oxide particles contains element M. When the composite oxide particles contain at least Ni and element M and have a specific particle size distribution, the thermal stability of the battery is specifically improved. Although element M is effective even in very small amounts, the lower limit of the molar ratio (d) of element M to the total molar amount of elements excluding Li and O is preferably 0.01 mol%, more preferably 0.03 mol%, and particularly preferably 0.05 mol%. The upper limit of the molar ratio (d) of element M is, for example, preferably 1.5 mol%, more preferably 1 mol%, and particularly preferably 0.7 mol%. Even if element M is added in an amount exceeding 1.5 mol%, the effect of improving thermal stability is limited, and problems such as increased resistance and decreased charge capacity may occur.

[0034] An example of a suitable range for the molar ratio (d) of element M is 0.01 mol% to 1.5 mol%, or 0.01 mol% to 1 mol%, or 0.01 mol% to 0.7 mol%, or 0.03 mol% to 1.5 mol%, or 0.03 mol% to 1 mol%, or 0.03 mol% to 0.7 mol%, or 0.05 mol% to 1.5 mol%, or 0.05 mol% to 1 mol%, or 0.05 mol% to 0.7 mol%. When the composite oxide particles contain multiple types of elements M, it is preferable that the total molar ratio thereof is within this range.

[0035] The preferred molar ratio (d) varies somewhat depending on the type of element M. When the composite oxide particles contain Ca, the preferred molar ratio of Ca to the total molar amount of elements excluding Li and O is, for example, 0.1 mol% to 0.5 mol%, or 0.15 mol% to 0.35 mol%. When the composite oxide particles contain Sr, the preferred molar ratio of Sr to the total molar amount of elements excluding Li and O is, for example, 0.05 mol% to 0.3 mol%, or 0.07 mol% to 0.15 mol%.

[0036] At least one of the first and second composite oxide particles preferably contains, as the element M, at least one element selected from the group consisting of Ca and Sr. In this case, the effect of improving the thermal stability of the battery becomes more pronounced. At least one of the first and second composite oxide particles contains, as the element M, Ca or Sr, and may further contain at least one element selected from the group consisting of B, Ti, and Nb, but particularly preferably contains Ca and Sr. At least one of the first and second composite oxide particles contains, for example, 0.1 mol% to 0.5 mol% of Ca and 0.05 mol% to 0.3 mol% of Sr, relative to the total molar amount of elements excluding Li and O.

[0037] At least one of the first and second composite oxide particles may further contain at least one element selected from the group consisting of W, Mo, and Zr. At least one of the first and second composite oxide particles contains, for example, at least one element selected from the group consisting of Ca and Sr and at least one element selected from the group consisting of W, Mo, and Zr. When the composite oxide particles contain at least one element selected from the group consisting of W, Mo, and Zr, the total molar ratio of these elements is preferably 0.05 mol% to 1.5 mol%, more preferably 0.1 mol% to 1 mol%, based on the total molar amount of elements excluding Li and O.

[0038] Both the first and second composite oxide particles may contain the element M, but when only one of them contains the element M, it is preferable that the first composite oxide particles contain the element M. The first composite oxide particles preferably contain at least one element selected from the group consisting of Ca and Sr as the element M, and may further contain at least one element selected from the group consisting of other elements M (B, Ti, Nb), W, Mo, and Zr. It is particularly preferable that the first composite oxide particles contain Ca and Sr.

[0039] The first and second composite oxide particles may be, for example, a compound represented by the general formula Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ M2 ｅ M3 ｆ O ２－ｙ (wherein 0.95≦x≦1.05, 0.75≦a≦0.95, 0≦b≦0.15, 0≦c≦0.25, 0≦d≦0.015, 0≦e≦0.015, 0≦f≦0.03, 0≦y<0.05, a+b+c+d+e+f=1, M2 is at least one element selected from the group consisting of W, Mo, and Zr, and M3 is an element other than Li, Ni, Co, Mn, element M, element M2, and O). In the first composite oxide particles, it is preferable that the molar ratio (d) of element M in the above general formula is 0<d≦0.015.

[0040] The first composite oxide particles are preferably secondary particles formed by agglomeration of a large number of primary particles. The second composite oxide particles may be secondary particles formed by agglomeration of a large number of primary particles, but are preferably single primary particles or secondary particles formed by agglomeration of 2 to 5 primary particles. When the first composite oxide particles are secondary particles formed by agglomeration of a large number (e.g., 100 or more) of primary particles and the second composite oxide particles are secondary particles formed by agglomeration of a single primary particle or 2 to 5 primary particles, the thermal runaway temperature of the battery increases, for example, when placed in a high-temperature environment.

[0041] The first composite oxide particles are, for example, polycrystalline particles formed by the aggregation of a large number of primary particles having an average particle size of 0.3 μm or less. The primary particles constituting one particle of the composite oxide are adhered to each other with a strength that prevents them from breaking apart even when a strong force is applied, such as during the preparation of a positive electrode mixture slurry. The second composite oxide particles are, for example, also called non-aggregated particles or single particles, and are single-crystal primary particles having no internal grain boundaries. Note that the second composite oxide particles may contain five or fewer primary particles. Whether the composite oxide particles are polycrystalline or single-crystal particles can be confirmed, for example, by analyzing electron beam diffraction images using a transmission electron microscope (TEM).

[0042] Ca and Sr are present at least at the interfaces between primary particles inside the secondary particles of the composite oxide, and are present, for example, at a higher density on the surface than inside the primary particles. The Ca and Sr present on the surfaces of the primary particles can be confirmed by TEM-EDX and STEM-EDX. Ca and Sr are present, for example, in a state of being evenly dispersed at the interfaces between the primary particles. Other elements M (B, Ti, Nb), W, Mo, and Zr may be present at the interfaces between primary particles inside the secondary particles of the composite oxide, similar to Ca and Sr, or may form solid solutions with other metal elements such as Ni. The amount of each element in solid solution can be confirmed by ICP-AES or energy dispersive X-ray spectroscopy (EDS).

[0043] When the composite oxide particles are secondary particles formed by agglomeration of a large number of primary particles, the average particle size of the primary particles constituting the secondary particles is preferably 0.3 μm or less, more preferably 0.02 μm or more and 0.2 μm or less. The particle size of the primary particles is determined by imaging the cross section of the particle with a scanning electron microscope (SEM) and analyzing the SEM image. For example, the positive electrode or composite oxide particles are embedded in a resin, a cross section is prepared by cross-section polishing (CP) processing, and this cross section is imaged with an SEM. 50 primary particles are randomly selected from the SEM image, and the grain boundaries are observed. The area of ​​each of the 50 primary particles is determined, and the diameter of the circle corresponding to that area is calculated, and the average value is used as the average particle size.

[0044] As described above, the first composite oxide particles have a D50 of more than 8 μm and less than 30 μm, and (D90 - D10) / D50 is 0.7 or more and less than 1.5. The second composite oxide particles have a D50 of more than 1 μm and less than 6 μm, and D90 - D10 is 6 μm or less. The positive electrode active material of this embodiment contains large particles with a broad particle size distribution and small particles with a distinctive particle size distribution. By using the first and second composite oxide particles having the above-mentioned specific composition and particle size distribution as the positive electrode active material, the thermal runaway temperature of the battery is increased, and excellent thermal stability is obtained.

[0045] D50 means the particle size at which the cumulative frequency is 50% from the smallest particle size in a volume-based particle size distribution, and is generally called the median diameter. D10 means the particle size at which the cumulative frequency is 10% from the smallest particle size in a volume-based particle size distribution, and D90 means the particle size at which the cumulative frequency is 90% from the smallest particle size in a volume-based particle size distribution. The value of (D90-D10) / D50 is used as an index showing the width of the particle size distribution. Generally, the larger this value, the broader the particle size distribution, and the smaller this value, the sharper the particle size distribution.

[0046] The particle size distribution of the first and second composite oxide particles is measured using a laser diffraction particle size distribution analyzer with water as a dispersion medium. The particle size distribution of the composite oxide particles in the examples described below was measured using an MT3000II manufactured by Microtrac-Bell Corporation.

[0047] The D50 of the first composite oxide particles is preferably more than 8 μm and not more than 25 μm, more preferably more than 8 μm and not more than 20 μm, and particularly preferably more than 8 μm and not more than 15 μm. If the D50 of the first composite oxide particles is within this range, the effect of improving the thermal stability of the battery becomes more significant. Note that if the D50 is outside the range of more than 8 μm and not more than 30 μm, the effect of improving the thermal stability cannot be obtained. The D50 of the first composite oxide particles may be, for example, from 8.5 μm to 15 μm, or from 8.8 μm to 12.5 μm.

[0048] The (D90-D10) / D50 of the first composite oxide particles is preferably 0.8 or more and 1.5 or less, more preferably 0.9 or more and 1.5 or less, and particularly preferably 1 or more and 1.5 or less. When the (D90-D10) / D50 of the first composite oxide particles is within this range, the effect of improving the thermal stability of the battery becomes more significant. Note that when the (D90-D10) / D50 is outside the range of 0.8 or more and 1.5 or less, the effect of improving the thermal stability cannot be obtained. The (D90-D10) / D50 of the first composite oxide particles may be, for example, 1 or more and 1.4 or less, or 1.1 or more and 1.3 or less.

[0049] The D50 of the second composite oxide particles is preferably more than 1 μm and not more than 5 μm, more preferably more than 1 μm and not more than 4.5 μm, and particularly preferably more than 1 μm and not more than 4 μm. If the D50 of the second composite oxide particles is within this range, the effect of improving the thermal stability of the battery becomes more significant. Note that if the D50 is outside the range of more than 1 μm and not more than 6 μm, the effect of improving the thermal stability cannot be obtained. The D50 of the second composite oxide particles may be, for example, 1 μm or more and 3.5 μm or less, or 1.5 μm or more and 3 μm or less.

[0050] The D90-D10 of the second composite oxide particles is preferably 0.5 μm or more and 5.5 μm or less, more preferably 0.8 μm or more and 5 μm or less, and particularly preferably 1 μm or more and 4.5 μm or less. When the D90-D10 of the second composite oxide particles is within this range, the effect of improving the thermal stability of the battery becomes more significant. Note that when the D90-D10 exceeds 6 μm, the effect of improving the thermal stability cannot be obtained. The D90-D10 of the second composite oxide particles may be, for example, 1 μm or more and 3.5 μm or less, or 1.5 μm or more and 3 μm or less.

[0051] The volumetric particle size distribution of the positive electrode active material of this embodiment includes a first peak in the particle size range of more than 8 μm and not more than 30 μm, and a second peak in the particle size range of more than 1 μm and not more than 6 μm. Although the particle size distribution may have three or more peaks, the particle size distribution preferably has substantially two peaks. The particle size distribution of the positive electrode active material obtained by determining the area of ​​the composite oxide particles from an SEM image of the positive electrode cross section and calculating the diameter of a circle corresponding to that area also has the first and second peaks.

[0052] In the volumetric particle size distribution of the positive electrode active material of this embodiment, the maximum value of the first peak is preferably equal to or less than the maximum value of the second peak, and is preferably smaller than the maximum value of the second peak. In the particle size distribution obtained from an SEM image of the positive electrode cross section, the maximum value of the first peak is also equal to or less than the maximum value of the second peak. Even when three or more peaks exist in the particle size distribution, it is preferable that the maximum value of the second peak be the largest. When determining the particle size distribution of the positive electrode active material from SEM image analysis of the positive electrode cross section, it is preferable to evaluate the particle size distribution excluding particles whose particle shape is deformed due to cracks or the like.

[0053] A positive electrode active material having the above configuration can be manufactured, for example, through the following steps: (1) preparing first composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, D50 is more than 8 μm and 30 μm or less, and (D90 - D10) / D50 is 0.7 or more and 1.5 or less; (2) preparing second composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, D50 is more than 1 μm and 6 μm or less, and D90 - D10 is 6 μm or less; and (3) mixing the first and second composite oxide particles.

[0054] The process for producing a positive electrode active material may further include a step of washing the first and second composite oxide particles, or a mixture thereof. In the washing step, the fired product obtained in the firing step of the composite oxide particles is washed with a solvent such as water or alcohol to reduce impurities such as LiOH, and the washed fired product is then heated and dried. If necessary, the fired product is pulverized, classified, or the like to adjust the D50 of the composite oxide particles to a desired range. An example of a suitable drying temperature is 150°C to 250°C, but drying may be performed at a temperature below 100°C. The drying process may be performed under vacuum, atmospheric pressure, or pressure. The drying atmosphere may be air, air from which oxygen, nitrogen, or carbon dioxide has been removed, or a mixture thereof. An example of the drying process time is 1 hour to 5 hours.

[0055] The first and second composite oxide particles can be synthesized, for example, by mixing and firing a composite oxide containing at least Ni and preferably further containing a metal element such as Co or Mn, a material containing Li, and a material containing the element M. The composite oxide containing Ni and the like can be obtained by precipitating (co-precipitating) a composite hydroxide containing Ni and the like, and then heat-treating the composite hydroxide. The composite hydroxide can be synthesized, for example, by adding an alkaline solution such as sodium hydroxide dropwise to a solution of a metal salt containing Ni and the like while stirring it, and adjusting the pH to the alkaline side (for example, 8.5 to 12.5).

[0056] The particle size of the composite hydroxide tends to be smaller as the pH during synthesis increases. The particle size of the composite hydroxide can also be adjusted by controlling the amount of metal salt solution added; for example, the particle size tends to increase as the amount of solution increases. The particle size distribution of the first and second composite oxide particles can be adjusted, for example, by controlling the particle size of the precursor composite hydroxide. The heat treatment temperature is not particularly limited, but is, for example, in the range of 250°C to 600°C. The particle size distribution of the first and second composite oxide particles also varies depending on the elements added during synthesis of the composite hydroxide and the elements added during the calcination, and can also be adjusted by controlling the type and amount of the added elements.

[0057] An example of a material containing Li is Li ２ CO３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ Examples of such elements include O, LiH, and LiF. The composite oxide containing Ni or the like and the material containing Li are preferably mixed in such a manner that the molar ratio of the total amount of elements excluding Li and O in the oxide to the Li in the Li raw material is 1:0.98 to 1:1.12.

[0058] An example of a material containing element M is Ca(OH). ２ , CaHPO ４ , Ca(H ２ P.O. ４ ) ２ , Ca ３ (P.O. ４ ) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ) ２、 CaCl ２ , CaAlO ４ , Sr(OH) ２ , Sr(OH) ２ ・8H ２ O, SrO, SrCO ３ , SrSO ４ , Sr(NO ３ ) ２ Examples of M2 raw materials include Nb ２ O ５ , Nb ２ O ５ ・nH ２ O, TiO ２ , Ti(OH) ４ , B(OH) ３ Furthermore, a material containing element M2 may be further added. An example of a material containing element M2 is WO ３ , Li ２ WO ４ , ZrO ２ , Zr(OH) ４ , MoO ３ , Li ２ MoO４ etc.

[0059] The calcination step of the mixture may include a multi-stage calcination step. The calcination of the mixture is carried out in an oxygen atmosphere, and the oxygen concentration is set to, for example, 85% or more. For example, when synthesizing polycrystalline composite oxide particles, calcination is carried out at a temperature of 650°C or more and less than 800°C, and when synthesizing single-crystalline composite oxide particles, calcination is carried out at a temperature of 800°C or more and 900°C or less, followed by calcination at a temperature of 650°C or more and less than 800°C.

[0060] The firing step is carried out by charging the mixture into a firing furnace. The firing step may include multiple temperature-raising steps with different heating rates. For example, the temperature may be raised from room temperature to a first firing temperature at a rate of 1.0°C / min to 5.5°C / min (first temperature-raising step), and then raised from the first firing temperature to a second firing temperature at a rate of 0.1°C / min to 3.5°C / min, which is slower than the rate in the first temperature-raising step (second temperature-raising step). The maximum temperature reached in the firing step may be maintained for a predetermined time (e.g., 1 hour to 10 hours).

[0061] The first and second composite oxide particles can be synthesized by the method described in the Examples below, but the first composite oxide particles are synthesized, for example, at a lower firing temperature than when the second composite oxide particles are synthesized. In the case of the second composite oxide particles (single-crystal composite oxide particles), they are fired at a high temperature for a long time to promote crystal growth. Furthermore, when synthesizing the first composite oxide particles, an element that suppresses crystal growth may be added.

[0062] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be fabricated, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. Metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may be composed only of a negative electrode core, with metallic lithium being deposited on the core surface during battery charging.

[0063] The negative electrode active material is not particularly limited as long as it reversibly absorbs and releases lithium ions, and typically, carbon materials such as graphite are used. Furthermore, elements that alloy with Li, such as Si and Sn, or materials containing such elements, may also be used as the negative electrode active material. Among these, silicon-containing materials containing Si are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

[0064] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. Examples of the silicon-containing material functioning as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0065] As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. A single binder may be used, or multiple binders may be used in combination. The negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. These function as thickeners in the negative electrode mixture slurry. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[0066] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0067] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0068] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0069] The electrolyte solution includes, for example, 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).

[0070] 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 (EP).

[0071] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0072] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0073] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0074] Examples of unsaturated cyclic carbonates include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0075] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0076] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0077] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0078] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0079] Example 1 Synthesis of First Lithium Transition Metal Composite Oxide Particles A1 A composite hydroxide containing Ni, Co, and Mn was synthesized by a coprecipitation method, and then heat-treated at 400°C to form Ni. ０．９ Co ０．０５ Mn ０．０５A composite oxide represented by formula (I) was obtained. The composite oxide was mixed with lithium hydroxide, calcium hydroxide, and strontium hydroxide in a predetermined mass ratio. This mixture was fired at 750°C for 3 hours. The fired product was pulverized, washed with water, and then vacuum-dried at 170°C to obtain first lithium transition metal composite oxide particles A1.

[0080] The obtained composite oxide particles A1 were analyzed by ICP-AES, and the results showed that the Ni content was 89.7 mol%, the Co content was 4.98 mol%, the Mn content was 4.98 mol%, the Ca content was 0.25 mol%, and the Sr content was 0.1 mol%, relative to the total amount of elements excluding Li and O. Since the contents of Ca and Sr were small compared to the contents of Ni, Co, and Mn, the molar ratio of Ni, Co, and Mn in Table 1 is 90:5:5 (the same applies to the following Examples, Comparative Examples, and Reference Examples).

[0081] The volume-based particle size distribution of the composite oxide particles A1 measured using an MT3000II manufactured by Microtrac-Bell Corporation with water as the dispersion medium was D50 of 9.5 μm, D90 of 15.4 μm, D10 of 4.0 μm, and (D90-D10) / D50 of 1.2. SEM images confirmed that the composite oxide particles A1 were secondary particles formed by aggregation of primary particles, and TEM-EDX confirmed that Ca and Sr were present on the surfaces of the primary particles.

[0082] [Synthesis of Second Lithium Transition Metal Composite Oxide Particles B1] A composite hydroxide containing Ni, Co, and Mn is synthesized by a coprecipitation method, and then heat-treated at 400°C to form Ni. ０．８ Co ０．１ Mn ０．１ A composite oxide (D50 = 2 μm) represented by formula (I) was obtained. The composite oxide and lithium hydroxide were mixed in a predetermined mass ratio, and the mixture was fired at 850°C for 6 hours, and then further fired at 750°C for 3 hours. The fired product was pulverized, washed with water, and then vacuum-dried at 170°C to obtain second lithium transition metal composite oxide particles B1.

[0083] The volume-based particle size distribution of the composite oxide particles B1 measured using an MT3000II manufactured by Microtrac-Bell Co., Ltd. with water as a dispersion medium was found to be D50 of 2.2 μm, D90 of 4.1 μm, D10 of 0.9 μm, and D90-D10 of 3.2 μm. The composite oxide particles B1 were single-crystal particles substantially composed of a single primary particle.

[0084] [Preparation of Positive Electrode] The positive electrode active material was a mixture of the first and second lithium transition metal composite oxides in a mass ratio of 75:25. This positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The positive electrode mixture 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 having a positive electrode mixture layer formed on both sides of the positive electrode core. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

[0085] The particle size distribution of the positive electrode active material had a first peak near 9.5 μm and a second peak near 1.2 μm, and the maximum value of the first peak was smaller than the maximum value of the second peak. Similarly, in the following examples, the particle size distribution of the positive electrode active material included a first peak in the particle size range of more than 8 μm to 30 μm and a second peak in the particle size range of more than 1 μm to 6 μm, and the maximum value of the first peak was smaller than the maximum value of the second peak. Similar results were obtained from the particle size distribution obtained by analyzing SEM images of the positive electrode cross section.

[0086] [Fabrication of Negative Electrode] SiO was used as the negative electrode active material. ｘA silicon-containing material represented by (0<x≦2) and natural graphite were mixed in a mass ratio of 3:97. A carbon coating was formed on the particle surfaces of the silicon-containing material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and a dispersion of styrene-butadiene rubber (SBR) were mixed in a solids mass ratio of 100:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. The negative electrode mixture 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, resulting in 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 in part of the negative electrode, exposing the surface of the negative electrode core.

[0087] [Preparation of Non-Aqueous Electrolyte] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25° C.). ６ was dissolved in the solution at a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte solution.

[0088] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] 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 to prepare a wound electrode assembly. This electrode assembly was placed in a cylindrical outer can with a bottom, and the non-aqueous electrolyte solution was poured into it. The opening of the outer can was then sealed with a sealer to obtain a test cell.

[0089] Example 2 Synthesis of second lithium-transition metal composite oxide particles B2 In the synthesis method of the composite oxide particles B1 of Example 1, Ni ０．８ Co ０．１ Mn ０．１ Composite oxide particles B2 were synthesized in the same manner as above, except that the D50 of the composite oxide represented by O was set to 4 μm.

[0090] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the composite oxide particles B2 were used as the positive electrode active material instead of the composite oxide particles B1.

[0091] Comparative Example 1 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0092] Comparative Example 2 [Synthesis of second lithium-transition metal composite oxide particles B3] In the synthesis method of the composite oxide particles B1 of Example 1, Ni ０．８ Co ０．１ Mn ０．１ Composite oxide particles B2 were synthesized in the same manner as above, except that the D50 of the composite oxide represented by O was set to 7.2 μm.

[0093] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the composite oxide particles B3 were used as the positive electrode active material instead of the composite oxide particles B1.

[0094] Comparative Example 3 [Synthesis of first lithium-transition metal composite oxide particles A2] In the synthesis method of the composite oxide particles A1 of Example 1, Ni ０．８ Co ０．１ Mn ０．１ Composite oxide particles A2 were synthesized in the same manner as above except that a composite oxide represented by O was used.

[0095] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the composite oxide particles A2 were used as the positive electrode active material instead of the composite oxide particles A1.

[0096] The thermal runaway temperature of each test cell in the Examples and Comparative Examples was evaluated by the following method. A lower thermal runaway temperature indicates higher thermal stability of the test cell. The evaluation results are shown in Table 1, along with the composition and physical properties of the positive electrode active material. The thermal runaway temperatures of the test cells in the following Examples, Comparative Examples, and Reference Examples were also evaluated in the same manner, and the results are shown in Tables 2 to 4, along with the composition and physical properties of the positive electrode active material.

[0097] [Evaluation of Thermal Runaway Temperature] Using a runaway reaction measurement device (Accelerated Rate Calorimeter: ARC, manufactured by Thermal Hazard Technology), the thermal runaway temperature of the test cell was measured under the following conditions: Measurement start temperature: 100°C, Holding temperature: 20 minutes, Heat generation detection temperature: 0.02°C / min, Temperature rise rate: 5°C, Battery voltage: 4.2V, Charge state. The charged test cell was placed in an electric furnace and heated, and the set temperature was held for 20 minutes to measure the temperature of the cell surface. If a temperature rise equal to or greater than the heat generation detection temperature was not detected, the temperature was raised to the next set temperature 5°C higher and measurement was performed again. The battery temperature when the self-heating rate of the battery reached 10°C / min was defined as the thermal runaway temperature.

[0098]

[0099] As shown in Table 1, it is understood that the test cells of Examples 1 and 2 all have higher thermal runaway temperatures and are more excellent in thermal stability than the test cells of Comparative Examples 1 to 3. When the positive electrode active material does not contain second composite oxide particles (Comparative Example 1), when the D50 and D90-D10 of the second composite oxide particles exceed 6 μm (Comparative Example 2), and when the (D90-D10) / D50 of the first composite oxide particles is less than 0.7 (Comparative Example 3), the thermal runaway temperatures are lower than when the positive electrode active materials of Examples 1 and 2 are used.

[0100] Example 3 Synthesis of second lithium transition metal composite oxide particles B4 A composite hydroxide containing Ni, Co, and Mn was synthesized by a coprecipitation method, and then heat-treated at 400°C to obtain Ni. ０．８ Co ０．１ Mn ０．１ A composite oxide (D50 = 2 μm) represented by formula (I) was obtained. This composite oxide was mixed with lithium hydroxide, calcium hydroxide, strontium hydroxide, and zirconium oxide in a predetermined mass ratio, and the mixture was fired at 750°C for 3 hours. The fired product was pulverized, washed with water, and then vacuum dried at 170°C to obtain second lithium transition metal composite oxide particles B4. Note that SEM images confirmed that the composite oxide particles B4 were secondary particles formed by the aggregation of a large number (100 or more) of primary particles.

[0101] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the composite oxide particles B4 were used as the positive electrode active material instead of the composite oxide particles B1.

[0102] Comparative Example 4 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 3, except that the composite oxide particles A2 were used as the positive electrode active material instead of the composite oxide particles A1.

[0103]

[0104] As shown in Table 2, the test cell using the positive electrode active material of Example 3 had a higher thermal runaway temperature and better thermal stability than the test cell of Comparative Example 4 using a positive electrode active material containing first composite oxide particles in which (D90-D10) / D50 was less than 0.7.

[0105] Example 4 [Synthesis of first lithium-transition metal composite oxide particles A3] In the synthesis method of the composite oxide particles A1 of Example 1, Ni ０．９ Co ０．０５ Mn ０．０５ Instead of the composite oxide represented by O, Ni ０．８ Co ０．１ Mn ０．１ Composite oxide particles A3 were synthesized in the same manner as above except that a composite oxide represented by O was used.

[0106] [Synthesis of Second Lithium Transition Metal Composite Oxide Particles B5] Second lithium transition metal composite oxide particles B5 were synthesized in the same manner as in the synthesis of composite oxide particles B1 of Example 1.

[0107] A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that, as the positive electrode active material, composite oxide particles A3 were used instead of composite oxide particles A1, and composite oxide particles B5 were used instead of composite oxide particles B1.

[0108] Comparative Example 5 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 4, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0109] Example 5 [Synthesis of first lithium-transition metal composite oxide particles A4] In the synthesis method of the composite oxide particles A1 of Example 1, Ni ０．９Co ０．０５ Mn ０．０５ Instead of the composite oxide represented by O, Ni ０．５ Co ０．２ Mn ０．３ Composite oxide particles A3 were synthesized in the same manner as above except that a composite oxide represented by O was used.

[0110] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 4, except that the composite oxide particles A4 were used as the positive electrode active material instead of the composite oxide particles A1.

[0111] Comparative Example 6 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 5, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0112]

[0113] As shown in Table 3, the test cells using the positive electrode active materials of Examples 4 and 5 had higher thermal runaway temperatures and better thermal stability than the corresponding test cells of Comparative Examples 5 and 6 using positive electrode active materials that did not contain the second composite oxide particles.

[0114] Example 6 Synthesis of First Lithium Transition Metal Composite Oxide Particles A5 First lithium transition metal composite oxide particles A5 were synthesized in the same manner as in Example 1 for the composite oxide particles A1.

[0115] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 4, except that the composite oxide particles A5 were used as the positive electrode active material instead of the composite oxide particles A3.

[0116] Comparative Example 7 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0117] Example 7 Synthesis of first lithium transition metal composite oxide particles A6 First lithium transition metal composite oxide particles A6 were obtained in the same manner as in Example 6, except that the amount of calcium hydroxide added was reduced to set the Ca content to 0.07 mol %.

[0118] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A6 were used as the positive electrode active material instead of composite oxide particles A5.

[0119] Comparative Example 8 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 7, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0120] Example 8 Synthesis of first lithium transition metal composite oxide particles A7 First lithium transition metal composite oxide particles A7 were obtained in the same manner as in Example 6, except that the amount of strontium hydroxide added was reduced to set the Sr content to 0.03 mol %.

[0121] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A7 were used as the positive electrode active material instead of composite oxide particles A5.

[0122] Comparative Example 9 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 8, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0123] Example 9 Synthesis of First Lithium Transition Metal Composite Oxide Particles A8 First lithium transition metal composite oxide particles A8 were obtained in the same manner as in Example 6, except that strontium hydroxide was not added.

[0124] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A8 were used as the positive electrode active material instead of composite oxide particles A5.

[0125] Comparative Example 10 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 9, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0126] Example 10 Synthesis of First Lithium Transition Metal Composite Oxide Particles A9 First lithium transition metal composite oxide particles A9 were obtained in the same manner as in Example 6, except that calcium hydroxide was not added.

[0127] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A9 were used as the positive electrode active material instead of composite oxide particles A5.

[0128] Comparative Example 11 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 10, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0129] Example 11 Synthesis of first lithium transition metal composite oxide particles A10 First lithium transition metal composite oxide particles A10 were obtained in the same manner as in Example 6, except that boron oxide (B content: 0.5 mol %) was added instead of strontium hydroxide.

[0130] A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A10 were used as the positive electrode active material instead of the composite oxide particles A5.

[0131] Comparative Example 12 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 11, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0132] Example 12 Synthesis of first lithium transition metal composite oxide particles A11 First lithium transition metal composite oxide particles A11 were obtained in the same manner as in Example 6, except that titanium oxide (Ti content: 0.5 mol %) was added instead of strontium hydroxide.

[0133] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A11 were used as the positive electrode active material instead of composite oxide particles A5.

[0134] Comparative Example 13 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 12, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0135] Example 13 Synthesis of first lithium transition metal composite oxide particles A12 First lithium transition metal composite oxide particles A12 were obtained in the same manner as in Example 6, except that niobium oxide (Nb content: 0.5 mol %) was added instead of strontium hydroxide.

[0136] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A12 were used as the positive electrode active material instead of the composite oxide particles A5.

[0137] Comparative Example 14 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 13, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0138] Example 14 Synthesis of first lithium transition metal composite oxide particles A13 First lithium transition metal composite oxide particles A13 were obtained in the same manner as in Example 6, except that tungsten oxide (W content: 0.5 mol %) was added instead of strontium hydroxide.

[0139] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A13 were used as the positive electrode active material instead of the composite oxide particles A5.

[0140] Comparative Example 15 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 14, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0141] Example 15 Synthesis of first lithium transition metal composite oxide particles A14 First lithium transition metal composite oxide particles A14 were obtained in the same manner as in Example 6, except that molybdenum oxide (Mo content: 0.5 mol %) was added instead of strontium hydroxide.

[0142] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A14 were used as the positive electrode active material instead of the composite oxide particles A5.

[0143] Comparative Example 16 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 15, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0144] Example 16 Synthesis of first lithium transition metal composite oxide particles A15 First lithium transition metal composite oxide particles A15 were obtained in the same manner as in Example 6, except that zirconium oxide (Zr content: 0.3 mol%) was added instead of strontium hydroxide.

[0145] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A15 were used as the positive electrode active material instead of the composite oxide particles A5.

[0146] Comparative Example 17 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 16, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0147] Example 17 Synthesis of first lithium transition metal composite oxide particles A16 First lithium transition metal composite oxide particles A16 were obtained in the same manner as in Example 6, except that boron oxide (B content: 0.5 mol%) was added instead of calcium hydroxide and strontium hydroxide.

[0148] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles A16 were used as the positive electrode active material instead of the composite oxide particles A5.

[0149] Comparative Example 18 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 17, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0150] Example 18 Synthesis of first lithium transition metal composite oxide particles A17 First lithium transition metal composite oxide particles A17 were obtained in the same manner as in Example 6, except that titanium oxide (B content: 0.5 mol%) was added instead of calcium hydroxide and strontium hydroxide.

[0151] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A17 were used as the positive electrode active material instead of composite oxide particles A5.

[0152] Comparative Example 19 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 18, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0153] Example 19 Synthesis of first lithium transition metal composite oxide particles A18 First lithium transition metal composite oxide particles A18 were obtained in the same manner as in Example 6, except that niobium oxide (Nb content: 0.5 mol%) was added instead of calcium hydroxide and strontium hydroxide.

[0154] A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that composite oxide particles A18 were used as the positive electrode active material instead of composite oxide particles A5.

[0155] Comparative Example 20 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 19, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0156] Reference Example 1 Synthesis of first lithium transition metal composite oxide particles X1 First lithium transition metal composite oxide particles X1 were obtained in the same manner as in Example 6, except that aluminum hydroxide (Al content: 0.5 mol%) was added instead of calcium hydroxide and strontium hydroxide.

[0157] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles X1 were used as the positive electrode active material instead of the composite oxide particles A5.

[0158] Reference Example 2 A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Reference Example 1, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0159] Reference Example 3 Synthesis of First Lithium Transition Metal Composite Oxide Particles X2 First lithium transition metal composite oxide particles X2 were obtained in the same manner as in Example 6, except that calcium hydroxide and strontium hydroxide were not added.

[0160] A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 6, except that the composite oxide particles X2 were used as the positive electrode active material instead of the composite oxide particles A5.

[0161] Reference Example 4 A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Reference Example 3, except that the second lithium transition metal composite oxide particles were not used as the positive electrode active material.

[0162]

[0163] As shown in Table 4, all of the test cells of the examples had higher thermal runaway temperatures and better thermal stability than the test cells of the corresponding comparative examples. In particular, when at least one of Ca and Sr was added as element M (Examples 6 to 16), and particularly when both Ca and Sr were added (Examples 6 to 8), the effect of improving thermal stability was significant. As is clear from the reference examples, even if the particle size distributions of the two types of composite oxide particles satisfied the same conditions as those of the positive electrode active materials of the examples, the effect of improving thermal stability was not obtained in the absence of element M.

[0164] The present disclosure is further described by the following embodiments: Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: first lithium transition metal composite oxide particles, in which the molar ratio (a) of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the 50% volumetric particle diameter D50 is more than 8 μm and 30 μm or less, and (90% volumetric particle diameter D90 - 10% volumetric particle diameter D10) / D50 is 0.7 or more and 1.5 or less, and second lithium transition metal composite oxide particles, in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the D50 is more than 1 μm and 6 μm or less, and the D90 - D10 is 6 μm or less, and at least one of the first and second lithium transition metal composite oxide particles contains at least one element selected from the group consisting of Ca, Sr, B, Ti, and Nb. Configuration 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the first and second lithium transition metal composite oxide particles have a molar ratio of Ni to the total molar amount of elements excluding Li and O of 75 mol % or more. Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein at least one of the first and second lithium transition metal composite oxide particles contains Ca and Sr. Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 3, wherein at least one of the first and second lithium transition metal composite oxide particles contains 0.1 mol % to 0.5 mol % of Ca and 0.05 mol % to 0.3 mol % of Sr, relative to the total molar amount of elements excluding Li and O. Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein at least one of the first and second lithium transition metal composite oxide particles further contains at least one element selected from the group consisting of W, Mo, and Zr. Aspect 6: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 5, wherein the second lithium transition metal composite oxide particles are single primary particles or secondary particles formed by aggregation of 2 to 5 primary particles. Aspect 7: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 6, wherein the volumetric particle size distribution includes a first peak existing in a particle size range of more than 8 μm to 30 μm and a second peak existing in a particle size range of more than 1 μm to 6 μm.Aspect 8: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 7, wherein the maximum value of the first peak is equal to or less than the maximum value of the second peak. Aspect 9: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of Aspects 1 to 8, a negative electrode, and a non-aqueous electrolyte. Aspect 10: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the steps of: preparing first lithium transition metal composite oxide particles, in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the 50% volumetric particle diameter D50 is more than 8 μm and 30 μm or less, and (90% volumetric particle diameter D90 - 10% volumetric particle diameter D10) / D50 is 0.7 or more and 1.5 or less; preparing second lithium transition metal composite oxide particles, in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol % or more, the D50 is more than 1 μm and 6 μm or less, and D90 - D10 is 6 μm or less; and mixing the first and second lithium transition metal composite oxide particles. Aspect 11: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 10, further comprising the step of washing the first and second lithium transition metal composite oxide particles or a mixture thereof.

[0165] REFERENCE SIGNS LIST 10 non-aqueous electrolyte 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 electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket

Claims

1. The molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, the 50% particle size D50 based on volume is more than 8 μm and 30 μm or less, and (90% particle size D90 based on volume - 10% particle size D10 based on volume) / D50 is 0.7 or more and 1.5 or less for the first lithium transition metal composite oxide particles; the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, D50 is more than 1 μm and 6 μm or less, and D90 - D10 is 6 μm or less for the second lithium transition metal composite oxide particles; and the positive electrode active material for a non-aqueous electrolyte secondary battery contains at least one of the first and the second lithium transition metal composite oxide particles, and at least one of the first and the second lithium transition metal composite oxide particles contains at least one selected from the group consisting of Ca, Sr, B, Ti, and Nb.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the molar ratio of Ni to the total molar amount of elements excluding Li and O in the first and the second lithium transition metal composite oxide particles is 75 mol% or more.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein at least one of the first and the second lithium transition metal composite oxide particles contains Ca and Sr.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 3, wherein at least one of the first and the second lithium transition metal composite oxide particles contains 0.1 mol% or more and 0.5 mol% or less of Ca and 0.05 mol% or more and 0.3 mol% or less of Sr with respect to the total molar amount of elements excluding Li and O.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein at least one of the first and the second lithium transition metal composite oxide particles further contains at least one selected from the group consisting of W, Mo, and Zr.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the second lithium transition metal composite oxide particles are single primary particles or secondary particles formed by aggregation of 2 or more and 5 or less primary particles.

7. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the particle size distribution based on volume includes a first peak existing in a particle size range of more than 8 μm and 30 μm or less and a second peak existing in a particle size range of more than 1 μm and 6 μm or less.

8. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 7, wherein the maximum value of the first peak is equal to or less than the maximum value of the second peak.

9. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 8, a negative electrode, and a non-aqueous electrolyte.

10. A step of preparing first lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, the 50% particle size D50 on a volume basis exceeds 8 μm and is 30 μm or less, and (90% particle size D90 on a volume basis - 10% particle size D10 on a volume basis) / D50 is 0.7 or more and 1.5 or less; a step of preparing second lithium transition metal composite oxide particles in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 50 mol% or more, D50 exceeds 1 μm and is 6 μm or less, and D90 - D10 is 6 μm or less; and a step of mixing the first and second lithium transition metal composite oxide particles, the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

11. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 10, further comprising a step of washing the first and second lithium transition metal composite oxide particles or a mixture thereof.

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