Positive electrode active material and non-aqueous electrolyte secondary battery

Abstract

The invention relates to a positive electrode active material and a nonaqueous electrolyte secondary battery. The positive electrode active material includes a small particle group and a large particle group, the small particle group is composed of a plurality of small particles, the large particle group is composed of a plurality of large particles, and the formula 1.0 < = [D1 (90)-D1 (10)] / D1 (50) is satisfied, where D1 (90), D1 (50), and D1 (10) are the particle diameters at which the cumulative particle volume from the small particle diameter side in the volume-based particle size distribution of the small particle group becomes 90%, 50%, and 10% of the total particle volume, respectively. The small particles include single particles, the BET specific surface area of the small particles is 0.6-0.85, the large particles include aggregated particles, the aggregated particles are formed by aggregating a plurality of primary particles, and the ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles is 4.9620 or more.

Description

Technical Field The present disclosure relates to a positive electrode active material and a non-aqueous electrolyte secondary battery including the same. Background Art In Japanese Unexamined Patent Application Publication No. 2020-35625, in order to suppress variations in volume resistivity caused by the size of the particle diameter of positive electrode active material particles for secondary batteries, positive electrode active material particles for secondary batteries having a particle size distribution width with a specified breadth are proposed. In Japanese Unexamined Patent Application Publication No. 2018-125305, a manufacturing method for obtaining a positive electrode active material is proposed, the positive electrode active material including lithium transition metal oxide particles composed of single particles or having a small number of primary particles constituting one secondary particle. Summary of the Invention When the positive electrode active materials described in Patent Documents 1 and 2 are used for the positive electrode of a non-aqueous electrolyte secondary battery, sometimes the output resistance of the non-aqueous electrolyte secondary battery increases and the amount of gas generated during storage increases. An object of the present disclosure is to provide a positive electrode active material and a non-aqueous electrolyte secondary battery that suppress an increase in output resistance and an increase in the amount of gas generated during storage. The present disclosure provides the following positive electrode active material and non-aqueous electrolyte secondary battery.

[0001] A positive electrode active material including a small particle group and a large particle group, the small particle group being composed of a plurality of small particles, the large particle group being composed of a plurality of large particles, when the cumulative particle volumes from the small particle size side in the volume-based particle size distribution of the small particle group are 90%, 50%, and 10% of the total particle volume, the particle diameters are respectively set as D1(90), D1(50), and D1(10), the following formula (1) is satisfied: (1) 1.0 ≤ [D1(90) - D1(10)] / D1(50), The small particles include single particles, the BET specific surface area of the small particles is 0.6 or more and 0.85 or less, the large particles include aggregated particles, the aggregated particles are formed by aggregation of a plurality of primary particles, and the ratio of the c-axis length to the a-axis length of the lattice of the primary particles is 4.9620 or more.

[0002] The positive electrode active material according to [1], wherein the following formula (2) is satisfied: (2) [D1(90) - D1(10)] / D1(50) ≤ 1.4.

[0003] The positive electrode active material according to [1] or [2], wherein D1(50) is 3 μm or more and 6 μm or less.

[0004] The positive electrode active material according to any one of [1] to [3], wherein when the particle size at which the cumulative particle volume from the small particle size side in the particle size distribution based on volume of the large particle group becomes 50% of the total particle volume is defined as D2(50), D2(50) is 14 μm or more.

[0005] A non-aqueous electrolyte secondary battery comprising the positive electrode active material according to any one of [1] to [4]. The above and other objects, features, aspects and advantages of the present invention will become apparent from the following detailed description of the present invention understood in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 It is a schematic diagram showing an example of the lithium ion battery of the present embodiment. Figure 2 It is a schematic diagram showing an example of the electrode body of the present embodiment. Figure 3 It is a conceptual diagram showing the positive electrode of the present embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS The embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments. <Positive Electrode Active Material> The positive electrode active material of the present embodiment is used for a non-aqueous electrolyte secondary battery (hereinafter also referred to as a battery). Details of the battery will be described later. The positive electrode active material includes a small particle group and a large particle group. The small particle group has a smaller D(50) than the large particle group. D(50) refers to the particle size at which the cumulative particle volume from the small particle size side in the particle size distribution based on volume becomes 50% of the total particle volume. The D(50) of the small particle group and the D(50) of the large particle group are also referred to as D1(50) and D2(50), respectively. D1(50) and D2(50) will be described later. In the particle size distribution based on volume of the positive electrode active material, the ranges of the particle size distributions of the small particle group and the large particle group do not overlap with each other. The particle size distribution based on volume of the positive electrode active material may be bimodal. The particle size distribution based on volume is measured according to the measurement method described in the Examples section below. The positive electrode active material may basically be composed of a small particle group and a large particle group. The positive electrode active material may be composed of a small particle group and a large particle group. The positive electrode active material may be a powder. In this specification, the description "substantially composed of..." means that, within the scope not hindering the object of the present disclosure, additional components may be included in addition to the necessary components. For example, components generally assumed in this technical field (such as unavoidable impurities, etc.) may be included as additional components. By making the positive electrode active material consist of a small particle group and a large particle group, it tends to be easy to suppress the generation of storage gas. The positive electrode active material contains a small particle group and a large particle group, for example, in a mass ratio of 4:6 to 6:4 (small particle group: large particle group). From the viewpoint of output resistance and storage gas generation, the positive electrode active material preferably contains a small particle group and a large particle group in a mass ratio of 1:1 (small particle group: large particle group). (Small particle group) The small particle group is composed of a plurality of small particles. The small particle group is an aggregate of small particles. The small particles can have any shape. For example, the small particles can be spherical, cylindrical, blocky, etc. Furthermore, in this specification, the elements expressed in the singular form can also include the plural form unless otherwise specified. For example, "particle" not only refers to "a particle", but also refers to "an aggregate of particles (powder, powder, particle group)". In the volume-based particle size distribution of the small particle group, when the particle sizes at which the cumulative particle volume from the small particle size side becomes 90%, 50%, and 10% of the total particle volume are respectively set as D1(90), D1(50), and D1(10), the following formula (1) is satisfied: (1)1.0≤[D1(90)-D1(10)] / D1(50). The volume-based particle size distribution is measured by the method described in the column of the embodiments described later. [D1(90)-D1(10)] / D1(50) on the right side of formula (1) is sometimes also referred to as the particle size distribution width. When the positive electrode active material satisfies formula (1), it is believed that the proportion of small particles that are beneficial to the diffusion of lithium (Li) in the solid increases, and as a result, it tends to be easy to suppress the increase in output resistance. The particle size [D1(50)] and particle size distribution width of the small particles can be controlled, for example, by selecting the particle size (D50) and particle size distribution width of the transition metal compound that serves as the raw material of the small particles. From the viewpoint of output resistance, the left side of the formula (1) is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.3 or more. The small particle group can further satisfy the following formula (2), for example: (2)[D1(90)-D1(10)] / D1(50)≤1.4. D1(50) may be, for example, 5 μm or less, or 3 μm or more. D1(50) can be controlled by, for example, selecting the particle size (D50) and particle size distribution width of the transition metal compound serving as a raw material for the small particles. The small particles contain single particles (individual particles). The single particle grows into a larger primary particle. By making the small particle a single particle, it tends to be easy to suppress the generation of the storage gas. The single particle can have any shape. The single particle can be, for example, spherical, columnar, massive, etc. The small particle can be formed by one single particle. The small particle can be formed by aggregation of 2 to 10 single particles. The number of single particles contained in the small particle is measured in the SEM (scanning electron microscope) image of the small particle. The magnification of the SEM image is appropriately adjusted according to the size of the particle. The magnification of the SEM image can be, for example, 10,000 times to 30,000 times. Furthermore, in the SEM image of the particle, for example, when two single particles overlap, it may be impossible to confirm the inner (inside) particle. However, in the present embodiment, the number of single particles that can be confirmed by the SEM image is regarded as the number of single particles contained in the small particle. The same applies to the aggregated particles described later. The small particle can be composed of, for example, basically 1 to 10 single particles. The small particle can be composed of 1 to 10 single particles. The small particle can be composed of 1 to 5 single particles. The small particle can be composed of 1 to 3 single particles. The small particle can be composed of 1 single particle. The "single particle" in the present embodiment is a particle whose grain boundary cannot be confirmed visually in the SEM image of the particle, and represents a particle having a first maximum diameter of 0.5 μm or more. The first maximum diameter represents the distance between the two points farthest apart on the contour line of the single particle. In the present embodiment, the "contour line of the particle" can be confirmed in the two-dimensional projection image of the particle, or can also be confirmed in the cross-sectional image of the particle. The contour line of the particle can be confirmed, for example, in the SEM image of the powder, or can also be confirmed in the cross-sectional SEM image of the particle. The single particle can have, for example, a first maximum diameter of 1 μm to 7 μm. The single particle can have, for example, a first maximum diameter of 2 μm to 5 μm. The single particle can have, for example, a first maximum diameter of 2.5 μm to 3.8 μm. The average value of the first maximum diameter can be, for example, 1 μm to 5 μm. The average value of the first maximum diameter is calculated from the first maximum diameters of 100 single particles. 100 single particles are randomly selected. The BET specific surface area of the small particles is 0.6 or more and 0.85 or less. When the BET specific surface area of the small particles including single particles is within the above range, it is easy to suppress the increase in the amount of storage gas accompanying the increase in the proportion of small particles in the positive electrode active material. As a result, it tends to be easy to suppress the decrease in the amount of storage gas while suppressing the increase in the output resistance of the battery. From the viewpoint of the amount of storage gas generated, the BET specific surface area of the small particles is preferably 0.6 or more and 0.8 or less, and more preferably 0.6 or more and 0.7 or less. The BET specific surface area in the present embodiment is measured by the BET multi-point method. The BET specific surface area is measured according to the method described in the column of the following examples. (Large particle group) The large particle group is composed of a plurality of large particles. The large particle group is an aggregate of large particles. When the particle size D2(50) at which the cumulative particle volume from the small particle size side in the volume-based particle size distribution of the large particle group becomes 50% of the total particle volume is set, D2(50) can be, for example, 12 μm or more and, for example, 20 μm or less. D2(50) can be controlled, for example, by selecting the particle size (D50) and the particle size distribution width of the transition metal compound that is the raw material of the large particles. The large particles can have any shape. The large particles can be, for example, spherical, columnar, massive, etc. The large particles contain aggregated particles. The large particles can be, for example, substantially composed of aggregated particles. The large particles can be composed of aggregated particles. The aggregated particles can be formed by the aggregation of 50 or more primary particles. The number of primary particles contained in the aggregated particles is measured in the SEM image of the aggregated particles. The magnification of the SEM image can be, for example, 10,000 times to 30,000 times. The aggregated particles can be formed by the aggregation of 100 or more primary particles. There is no upper limit to the number of primary particles in the aggregated particles. The aggregated particles can be formed by the aggregation of 10,000 or less primary particles. The aggregated particles can be formed by the aggregation of 1,000 or less primary particles. The primary particles can have any shape. The primary particles can be, for example, spherical, columnar, massive, etc. The aggregated particles can have a particle size of, for example, 10 μm or more and 25 μm or less. For the aggregated particles, the particle size represents the distance between the two farthest points on the contour line of the aggregated particles. The particle size of the aggregated particles is measured in the SEM image of the aggregated particles. The aggregated particles can have a particle size of, for example, 12 μm or more and 20 μm or less. The average value of the particle size of the aggregated particles can be, for example, 14 μm or more and 20 μm or less. The average value of the particle size of the aggregated particles is calculated from the particle sizes of 100 aggregated particles. 100 aggregated particles are randomly selected. In the present embodiment, the "primary particles" are particles that cannot visually confirm grain boundaries in the SEM image of the particles, and represent particles having a second maximum diameter of less than 0.5 μm. The primary particles contained in the large particles have a smaller particle size compared to the single particles contained in the small particles. The second maximum diameter represents the distance between the two farthest points on the contour line of the primary particle. The primary particles may have, for example, a second maximum diameter of 0.05 μm to 0.2 μm. When 10 or more primary particles randomly extracted from the SEM image of one aggregated particle have a second maximum diameter of 0.05 μm to 0.2 μm, it is regarded that all of the primary particles contained in the aggregated particle have a second maximum diameter of 0.05 μm to 0.2 μm. The primary particles may have, for example, a second maximum diameter of 0.1 μm to 0.2 μm. The average value of the second maximum diameter may be, for example, 0.1 μm to 0.3 μm. The average value is calculated from the second maximum diameters of 100 primary particles. 100 primary particles are randomly extracted. Furthermore, as long as the aggregated particle contains 50 or more primary particles having a maximum diameter of less than 0.5 μm, it may further contain primary particles having a maximum diameter of 0.5 μm or more. The ratio of the c-axis length to the a-axis length of the lattice of the primary particle (hereinafter also referred to as the c / a ratio) is 4.9620 or more. When the c / a ratio is within the above range, it tends to be easy to suppress the amount of gas generated during storage. It is speculated that this is because when the c / a ratio is large, it is considered that the amount of Li ions at the Li site is relatively small, and Li that does not enter the crystal generates a coating containing a Li compound more on the surface of the positive electrode. Due to the generated coating, it is easy to suppress the side reaction with the electrolyte that causes the generation of gas during storage. The upper limit of the c / a ratio may be, for example, 4.9660 or less, 4.9650 or less, 4.9640 or less, etc. The c / a ratio is measured by the method described in the column of the examples described later. The c / a ratio can be controlled, for example, by adjusting the firing parameters (such as firing time and firing temperature, etc.) of the firing process described later. The small particles (single particles) and large particles (primary particles) of the present embodiment may have any crystal structure. The single particles and the primary particles may each independently have, for example, a layered structure, a spinel structure, an olivine structure, etc. The small particles (single particles) and large particles (primary particles) of this embodiment can each independently have any chemical composition. The small particles (single particles) can have the same chemical composition as the large particles (primary particles). The small particles (single particles) can have a chemical composition different from that of the large particles (primary particles). For example, the single particles and primary particles can each independently contain at least one selected from LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. Among them, the notations such as "(NiCoMn)" in the composition formula like "Li(NiCoMn)O2" indicate that the total of the composition ratios within the parentheses is 1. For example, both the small particles (single particles) and the large particles (primary particles) can contain a layered metal oxide. The layered metal oxide has a layered structure. The layered metal oxide can contain, for example, Ni. The layered metal oxide containing Ni can have a large specific capacity. The single particles (small particles) can contain, for example, a first layered metal oxide. The first layered metal oxide is represented by formula (i): Li a1 Ni 1-b1 M 1 b1 O2 (i). In formula (i), a1 satisfies the relationship of 0.7 ≤ a1 ≤ 1.3. b1 satisfies the relationship of 0 < b1 ≤ 0.5. M 1 represents at least one selected from Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si. The first layered metal oxide is preferably represented by formula (i'): Li a1’ Ni 1-c1-d1 Co c1 Mn d1 M 1’ e1 O2 (i'). In formula (i'), a1' satisfies the relationship of 1.00 ≤ a1' ≤ 1.10. c1 satisfies the relationship of 0.18 ≤ c1 ≤ 0.22. d1 satisfies the relationship of 0.18 ≤ c1 ≤ 0.22. e1 satisfies the relationship of 0 ≤ e1 ≤ 0.10. M 1’represents at least one selected from Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si. The primary particles (large particles) may contain, for example, a second layered metal oxide. The second layered metal oxide is represented by formula (ii): Li a2 Ni 1-b2 M 2 b2 O2 (ii). In formula (ii), a2 satisfies the relationship 0.7 ≤ a2 ≤ 1.3. b2 satisfies the relationship 0 < b2 ≤ 0.5. M 2 represents at least one selected from Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si. The second layered metal oxide is preferably represented by formula (ii'): Li a2’ Ni 1-c2-d2 Co c2 Mn d2 M 2’ e2 O2 (ii'). In formula (ii'), a2' satisfies the relationship 1.00 ≤ a2' ≤ 1.10. c2 satisfies the relationship 0.18 ≤ c2 ≤ 0.30. d2 satisfies the relationship 0.18 ≤ d2 ≤ 0.30. e2 satisfies the relationship 0 ≤ e2 ≤ 0.10. M 2’ represents at least one selected from Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si. The single particles (small particles) and the primary particles (large particles) may independently contain, for example, a selection from LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.55 Co0.2 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, and LiNi 0.6 Co 0.1 Mn 0.3 at least one of O2. (Method for manufacturing a positive electrode active material) The positive electrode active material can be manufactured, for example, by first synthesizing small particles and large particles, and then mixing the synthesized small particles and large particles. The small particles and large particles can be synthesized, for example, through a two-stage firing process. The method for manufacturing the positive electrode active material can include, for example: a first firing process of firing a first mixture containing a lithium compound and a transition metal compound; and a second firing process of firing a second mixture containing the fired product obtained in the first firing process and a transition metal compound. The crystal structures of the small particles and large particles can be arbitrarily controlled by adjusting the firing parameters (such as firing temperature and firing time, etc.) in the first firing process and the second firing process. The transition metal compound used in the synthesis of the small particles and large particles can contain, for example, one or more compounds containing nickel (Ni), cobalt (Co), and manganese (Mn) (hereinafter also referred to as NCM compounds). The NCM compound preferably contains a nickel-cobalt-manganese composite hydroxide. The nickel-cobalt-manganese composite hydroxide can be obtained, for example, by a coprecipitation method or the like. The nickel-cobalt-manganese composite hydroxide can be, for example, a compound represented by the general formula: NixCoyMnz(OH)2 (where x + y + z = 1). By mixing the synthesized small particles and large particles in a mass ratio of 4:6 to 6:4 (small particle group: large particle group), preferably a mass ratio of 1:1, a positive electrode active material can be obtained. <Non-aqueous electrolyte secondary battery> Figure 1 It is a schematic diagram showing an example of the battery of this embodiment. Figure 1 The battery 100 shown can be used in any application. The battery 100 can be used, for example, as a main power source or a power assist power source in an electric vehicle. By connecting a plurality of batteries 100, a battery module or a battery pack can be formed. The battery 100 can be a lithium-ion battery. The battery 100 includes an outer package 90. The outer package 90 is square (flat rectangular parallelepiped shape). However, the square is just an example. The outer package 90 can be, for example, cylindrical, or can be in a bag shape. The outer package 90 can be made of, for example, an aluminum (Al) alloy. The outer package 90 houses the electrode body 50 and an electrolyte (not shown). The electrode body 50 is connected to the positive terminal 91 through the positive electrode current collector member 81. The electrode body 50 is connected to the negative terminal 92 through the negative electrode current collector member 82. Figure 2Schematic diagram showing an example of the electrode body of the present embodiment. The electrode body 50 is a wound type. The electrode body 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. That is, the battery 100 includes the positive electrode 10. The positive electrode 10, the separator 30, and the negative electrode 20 are all strip-shaped sheets. The electrode body 50 may include two separators 30. The electrode body 50 is formed by laminating the positive electrode 10, the separator 30, and the negative electrode 20 in sequence and winding them into a spiral shape. The electrode body 50 is formed into a flat shape after winding. Furthermore, the wound type is an example. The electrode body 50 may be, for example, a stacked type. (Positive electrode) The positive electrode 10 includes a positive electrode active material layer 12 and a positive electrode substrate 11. The positive electrode active material layer 12 is formed on the surface of the positive electrode substrate 11. The positive electrode active material layer 12 may be directly formed on the surface of the positive electrode substrate 11. For example, an interlayer (intervening layer, not shown) may be formed between the positive electrode active material layer 12 and the positive electrode substrate 11. In the present embodiment, when the interlayer is formed, it is also regarded as the positive electrode active material layer 12 being formed on the surface of the positive electrode substrate 11. The interlayer may have a thickness smaller than that of the positive electrode active material layer 12. The interlayer may include, for example, a conductive material, an insulating material, etc. The positive electrode active material layer 12 may be formed only on one side of the positive electrode substrate 11. The positive electrode active material layer 12 may be formed on both the front and back surfaces of the positive electrode substrate 11. The positive electrode active material layer 12 may have a thickness of, for example, 10 μm to 200 μm. The positive electrode active material layer 12 may have a high density. The positive electrode active material layer 12 may have a density of, for example, 3.5 g / cm 3 The above density, may have a density of 3.6 g / cm 3 The above density, may have a density of 3.7 g / cm 3 The above density. The upper limit of the density is arbitrary. The positive electrode active material layer 12 may have a density of, for example, 3.8 g / cm 3 The following density. Figure 3 Conceptual diagram showing the positive electrode of the present embodiment. The positive electrode active material layer 12 contains the above-mentioned positive electrode active material. The positive electrode active material contains small particles 1 and large particles 2. The small particles 1 contain single particles. The large particles 2 contain aggregated particles. The small particles 1 and the large particles 2 are densely packed. For example, in an aggregate of small particles 1 that expand in a sea-like shape, the large particles 2 are distributed in an island-like manner. In addition to the positive electrode active material, the positive electrode active material layer 12 may further contain a conductive material (not shown), an adhesive (not shown), etc. The conductive material may contain any component. The conductive material may contain, for example, acetylene black, etc. The compounding amount of the conductive material relative to 100 parts by mass of the positive electrode active material may be, for example, 0.1 part by mass to 10 parts by mass. The adhesive may contain any component. The adhesive may contain, for example, polyvinylidene fluoride (PVdF), etc. The compounding amount of the adhesive relative to 100 parts by mass of the positive electrode active material may be, for example, 0.1 part by mass to 10 parts by mass. The positive electrode substrate 11 is a conductive sheet. The positive electrode substrate 11 may have a thickness of, for example, 10 μm to 30 μm. The positive electrode substrate 11 may have a thickness of, for example, 10 μm to 20 μm. The positive electrode substrate 11 may have a thickness of, for example, 10 μm to 15 μm. The positive electrode substrate 11 may include, for example, an Al alloy foil, a pure Al foil, etc. The positive electrode substrate 11 may be substantially composed of an Al alloy foil. The positive electrode substrate 11 may be composed of an Al alloy foil. The positive electrode 10 can be manufactured by coating a positive electrode paste on the surface of the positive electrode substrate 11 to form a positive electrode active material layer 12, then calendering the positive electrode active material layer 12 and the positive electrode substrate 11 to manufacture a master, and then cutting it to a specified planar size according to the specifications of the battery 100. The positive electrode paste is prepared by mixing a positive electrode active material and additional components. (Negative electrode) The negative electrode 20 includes a negative electrode active material layer 22 and a negative electrode substrate 21. The negative electrode substrate 21 may include, for example, a copper foil, etc. The negative electrode active material layer 22 is formed on the surface of the negative electrode substrate 21. The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material may contain any components. The negative electrode active material powder may include, for example, at least one selected from graphite, soft carbon, hard carbon, Si, SiO, Si-based alloys, Sn, SnO, Sn-based alloys, and Li4Ti5O 12 among others. The negative electrode active material layer 22 may further include a binder, etc., in addition to the negative electrode active material powder. The binder may include, for example, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), etc. The negative electrode 20 can be manufactured by coating a negative electrode paste on the surface of the negative electrode substrate 21 to form a negative electrode active material layer 22, then calendering the negative electrode active material layer 22 and the negative electrode substrate 21 to manufacture a master, and then cutting it to a specified planar size according to the specifications of the battery 100. The negative electrode paste is prepared by mixing a negative electrode active material and other components. (Separator) At least a part of the separator (diaphragm) 30 is disposed between the positive electrode 10 and the negative electrode 20. The separator 30 separates the positive electrode 10 from the negative electrode 20. The separator 30 is porous. The separator 30 allows the electrolyte to pass through. The separator 30 is electrically insulating. The separator 30 may be made of, for example, polyolefin. Further, when the electrolyte is solid, sometimes the electrolyte also functions as a separator. (Electrolyte) The electrolyte comprises at least one selected from liquid electrolytes (electrolyte solutions, ionic liquids), gel electrolytes, and solid electrolytes. As an example in this embodiment, the electrolyte solution will be described. The electrolyte solution comprises a solvent and a supporting electrolyte. The electrolyte solution may further comprise optional additives. The supporting electrolyte is dissolved in the solvent. The solvent is aprotic. The solvent may comprise, for example, at least one selected from ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The supporting electrolyte is dissolved in the solvent. The supporting electrolyte may comprise any components. The supporting electrolyte may comprise, for example, at least one selected from LiPF6, LiBF4, and LiN(FSO2)2. Hereinafter, the present invention will be described in more detail by way of examples. Examples [Evaluation of output resistance] For the test cells fabricated in the examples and comparative examples, the ambient temperature of the battery was cooled to 0 °C, and a constant current charge was carried out at a current density of 0.2 mA / cm 2 until it reached 3.44 V, and then a constant voltage charge was carried out at 3.49 V until the current density became 0.04 mA / cm 2 , and then a constant current discharge was carried out for 10 seconds at a current value of 0.2 mA / cm 2 . The voltage at the moment after 10 seconds was measured. The measurements were carried out respectively at current densities of 0.4, 0.6, 0.8, 1.0, and 1.2 mA / cm 2 , and the 10-second DC resistance was calculated from the slope of the current value and the voltage value. [Evaluation of the amount of gas generated during storage] The amount of gas in the battery cell in the above discharge state was set as the amount of gas before storage. Then, at a temperature of 25 °C, a constant current charge was carried out at a current density of 0.2 mA / cm 2 until it reached 4.25 V, and then a constant voltage charge was carried out at 4.25 V until the current density became 0.04 mA / cm 2 , and then it was stored in a constant temperature bath at 60 °C for 30 days. Then, a constant current discharge was carried out at a current density of 0.2 mA / cm 2 until it reached 3.0 V, and then the amount of gas was measured to obtain the amount of gas after storage. The amount of gas generated during storage was determined by subtracting the amount of gas before storage from the amount of gas after storage. [Measurement of BET specific surface area] The BET specific surface area of the small particles was measured using a fully automatic specific surface area measuring device (Macsorb HM model-1208 manufactured by MOUNTECH Co., Ltd.). [Measurement of volume-based particle size distribution] The volume-based particle size distributions D1(90), D1(50), D1(10) of small particles (small particle groups) and the volume-based particle size distribution D2(50) of large particles (large particle groups) were measured using a laser diffraction particle size analyzer (Mastersizer-3000 manufactured by Malvern Panalytical). [Measurement of c / a ratio] For the c / a ratio of large particles, a measuring device (a fully automatic multi-purpose X-ray diffractometer (SmartLab manufactured by Rigaku)) was used. The powder sample of large particles was filled into a special sample holder, and the crystal peak data measured by the reflection method under the following measurement conditions were subjected to Rietveld analysis to calculate the a-axis length and c-axis length in the crystal (crystal system: trigonal system, space group: R-3m), and the c / a ratio was obtained. (Measurement conditions) Tube ball (target element): Cu Tube voltage: 45 kV Tube current: 200 mA <Example 1> (Synthesis of large particles) LiOH and a transition metal compound represented by Ni 0.55 Co 0.20 Mn 0.25 (OH)2 were mixed to obtain a first mixture. The first mixture was fired to obtain a fired product, and then a second mixture containing the obtained fired product and the transition metal compound was fired to synthesize large particles. D2(50) of the large particles (large particle groups) was 15.8 μm, and the c / a ratio was 4.9620. (Synthesis of small particles) LiOH and a transition metal compound represented by Ni 0.60 Co 0.20 Mn 0.20 (OH)2 were mixed to obtain a third mixture. The third mixture was fired to obtain a fired product, and then a fourth mixture containing the obtained fired product and the transition metal compound was fired to synthesize small particles. D1(50) of the small particles (small particle groups) was 4.0 μm, the particle size distribution width [[D1(90)-D1(10)] / D1(50)] was 1.367, and the BET specific surface area was 0.68 m 2 / g. (Preparation of positive electrode active material) The obtained large particles and small particles were mixed at a mass ratio of 1:1 to prepare a positive electrode active material. (Fabrication of positive electrode plate) The prepared positive electrode active material was 97.5 parts by mass, carbon black as the conductive material was 1.5 parts by mass, and polyvinylidene fluoride (PVdF) as the binder material was 1.0 part by mass. Then, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode slurry. The slurry was coated on a current collector composed of aluminum foil, and the coating film was dried to form a positive electrode composite layer. Then, using a compression roller, it was compressed so that the positive electrode composite density became 3.55 g / cm 3 and cut into a specified size, and an aluminum electrode tab was installed to fabricate a positive electrode plate. (Fabrication of negative electrode plate) Graphite negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder were weighed so that their mass ratios were 98:1:1, and they were dispersed in water to prepare a negative electrode slurry. The negative electrode composite slurry was coated on a current collector composed of copper foil to form a negative electrode composite layer. Then, it was dried, rolled with a rolling roller to a specified thickness, cut into a specified size, and a nickel electrode tab was installed to fabricate a negative electrode plate. (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:30:40. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent to a concentration of 1.15 mol / L. Further, vinylene carbonate (VC) was added at an addition rate of 1.0% by mass based on the total mass of the mixed solvent to prepare a non-aqueous electrolyte. (Fabrication of test battery cell) The positive electrode and the negative electrode were stacked via a polyolefin separator to fabricate a laminated electrode body. The electrode body was housed in an outer packaging body composed of an aluminum laminate sheet, and after injecting the above non-aqueous electrolyte, the opening of the outer packaging body was sealed to obtain a test battery cell. For the above test battery cell, at a temperature of 25°C, a constant current charge was carried out at a current density of 0.2 mA / cm 2 until it reached 4.25 V, and then a constant voltage charge was carried out at 4.25 V until the current density became 0.04 mA / cm 2 , and the charge capacity was determined. After a rest of 10 minutes, a constant current discharge was carried out at a current density of 0.2 mA / cm 2 until it reached 3.0 V, and the discharge capacity was determined. For the test battery cell, the output resistance and the amount of gas generated during storage were evaluated. The results are shown in Table 1. <Examples 2 to 4, Comparative Examples 1 to 3> A test battery cell was fabricated in the same manner as in Example 1, except that small particles and large particles shown in Table 1 were used. D1(90), D1(50), D1(10) of the small particle group, D2(50) of the large particle group, and the particle size distribution width [[D1(90) - D1(10)] / D1(50)] were controlled by adjusting the particle size (D50) and the particle size distribution width of the transition metal compound. The c / a ratio of the lattice of the large particles was controlled by adjusting the firing parameters of the firing process.

Table 1

Claims

1. The positive electrode active material is a positive electrode active material including a small particle group and a large particle group. The small particle group is composed of a plurality of small particles, and the large particle group is composed of a plurality of large particles. When the particle diameters at which the cumulative particle volume from the small particle diameter side in the volume-based particle size distribution of the small particle group becomes 90%, 50%, and 10% of the total particle volume are respectively set as D1(90), D1(50), and D1(10), the following formula (1) is satisfied: (1) 1.0 ≤ [D1(90) - D1(10)] / D1(50), The small particles include single particles, and the BET specific surface area of the small particles is 0.6 or more and 0.85 or less. The large particles include aggregated particles, and the aggregated particles are formed by aggregation of a plurality of primary particles. The ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles is 4.9620 or more.

2. The positive electrode active material according to claim 1, wherein, The following formula (2) is satisfied: (2) [D1(90) - D1(10)] / D1(50) ≤ 1.

4.

3. The positive electrode active material according to claim 1, wherein, The D1(50) is 3 μm or more and 6 μm or less.

4. The positive electrode active material according to claim 1, wherein When the particle diameter at which the cumulative particle volume from the small particle diameter side in the volume-based particle size distribution of the large particle group becomes 50% of the total particle volume is set as D2(50), D2(50) is 14 μm or more.

5. A non-aqueous electrolyte secondary battery comprising the positive electrode active material according to claim 1.

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