Positive electrode active material powder and lithium secondary battery

Abstract

To provide positive electrode active material powder capable of reducing the rate of increase in resistance of a lithium secondary battery.SOLUTION: Positive electrode active material powder of the present disclosure includes a plurality of particles A composed of a positive electrode active material. The particles A include a plurality of primary particles A1 and a plurality of secondary particles A2 formed by sintering together the primary particles A1. In a scanning micrograph of a field of view in which 50 or more of the particles A are contained, the ratio of coarse secondary particles is 45 number % or less with respect to the total number of the particles A in the field of view. The coarse secondary particles are, among the secondary particles A2, secondary particles A2 formed by sintering together five or more of the primary particles A1. The ratio of a plurality of microparticles is 1.5 number % or less with respect to the total number of the particles A in the field of view. The microparticles are, among the particles A, particles A having a circle-equivalent diameter of 0.8 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material powder and a lithium secondary battery. [Background technology]

[0002] Conventionally, from the viewpoint of improving battery performance, the particle size and crystallinity of positive electrode active material powder have been adjusted.

[0003] Patent Document 1 discloses a positive electrode active material. The positive electrode active material is a lithium transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn). The nickel (Ni) content is within a specific range. The lithium transition metal oxide is doped with a doping element. The doping element is at least one selected from the group consisting of B, Zr, Mg, Ti, Sr, W, and Al. The average particle size (D50) after rolling at a specific rolling density is 4 μm to 10 μm. The positive electrode active material has a single particle form (i.e., primary particles). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-001232 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, single-crystal positive electrode active material powders contain secondary particles in addition to primary particles. The secondary particles are formed by sintering multiple primary particles. When a lithium secondary battery equipped with a positive electrode containing the positive electrode active material powder is charged or discharged (hereinafter also referred to as "during durability"), the interfaces of the secondary particles tend to crack, and new surfaces tend to form. This accelerates the deterioration of the positive electrode. In addition, the positive electrode active material powders contain multiple fine particles generated during their production. The fine particles, whether primary or secondary particles, have a circle-equivalent diameter of 0.8 μm or less. The fine particles tend to cause interruption of the conductive path in the positive electrode during durability. Furthermore, the fine particles lack excellent structural stability due to insufficient crystal growth. As a result, conventional positive electrode active material powders may not be able to reduce the resistance increase rate of lithium secondary batteries.

[0006] Patent Document 1 does not disclose the proportion of secondary particles and the proportion of fine particles. Therefore, when a lithium secondary battery equipped with a positive electrode containing the positive electrode active material disclosed in Patent Document 1 is repeatedly charged and discharged, the resistance of the lithium secondary battery may easily increase. Therefore, there is a need for a positive electrode active material powder that can suppress the increase in resistance of a lithium secondary battery even when the lithium secondary battery is repeatedly charged and discharged (i.e., a positive electrode active material powder that can reduce the rate of increase in resistance of a lithium secondary battery).

[0007] The present disclosure has been made in consideration of the above circumstances. An object of one embodiment of the present disclosure is to provide a positive electrode active material powder that can reduce the rate of increase in resistance of a lithium secondary battery. Another problem to be solved by another embodiment of the present disclosure is to provide a lithium secondary battery with a reduced rate of increase in resistance. [Means for solving the problem]

[0008] The means for solving the above problems include the following embodiments. <1> A positive electrode active material powder for a lithium secondary battery, comprising a plurality of particles (A) made of a positive electrode active material, the plurality of particles (A) are composed of a plurality of primary particles (A1) and a plurality of secondary particles (A2) formed by sintering the primary particles (A1), In a scanning microscope photograph of the positive electrode active material powder in a field of view that includes 50 or more of the particles (A), the proportion of a plurality of coarse secondary particles is 45% by number or less with respect to the total number of the plurality of particles (A) in the field of view, the plurality of coarse secondary particles represent a plurality of secondary particles (A2) formed by sintering five or more of the primary particles (A1), among the plurality of secondary particles (A2), the ratio of the plurality of fine particles to the total number of the plurality of particles (A) within the field of view is 1.5% by number or less, The positive electrode active material powder, wherein the plurality of fine particles are a plurality of particles (A) among the plurality of particles (A) having a circle-equivalent diameter of 0.8 μm or less. <2> the proportion of the coarse secondary particles is 28% by number or less relative to the total number of the plurality of particles (A) within the field of view, the proportion of the fine particles is 0.8% by number or less with respect to the total number of the plurality of particles (A) within the field of view; <1> The positive electrode active material powder according to claim 1. <3> the positive electrode active material is a lithium transition metal oxide having a layered crystal structure; <1> or <2> The positive electrode active material powder according to claim 1. <4> The lithium transition metal oxide contains at least one of Ni, Co, and Mn. <3> The positive electrode active material powder according to claim 1. <5> The aforementioned <1> ~ <4> 10. A lithium secondary battery comprising a positive electrode containing the positive electrode active material powder according to any one of claims 1 to 9. [Effects of the Invention]

[0009] According to one embodiment of the present disclosure, there is provided a positive electrode active material powder that can reduce the resistance increase rate of a lithium secondary battery. According to another embodiment of the present disclosure, there is provided a lithium secondary battery with a reduced rate of increase in resistance. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 1 is a schematic diagram of a primary particle. [Figure 2] FIG. 2 is a schematic diagram of a secondary particle. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the Examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, when multiple substances corresponding to each component are present, the amount of each component means the total amount of multiple substances unless otherwise specified. In the present disclosure, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0012] (1) Positive electrode active material powder The cathode active material powder of the present disclosure is a cathode active material powder for a lithium secondary battery. The cathode active material powder comprises a plurality of particles (A) composed of a cathode active material. The plurality of particles (A) comprises a plurality of primary particles (A1) and a plurality of secondary particles (A2). The secondary particles (A2) are formed by sintering the primary particles (A1). In a scanning electron microscope photograph of the cathode active material powder in a field of view (hereinafter also referred to as a "specific field of view") that contains 50 or more of the particles (A), the proportion of multiple coarse secondary particles (hereinafter also referred to as the "coarse secondary particle proportion") is 45% or less by number relative to the total number of the multiple particles (A) in the field of view. The multiple coarse secondary particles refer to multiple secondary particles (A2) that are formed by sintering five or more of the primary particles (A1) among the multiple secondary particles (A2). The proportion of multiple fine particles (hereinafter also referred to as the "fine particle proportion") is 1.5% or less by number relative to the total number of the multiple particles (A) in the field of view. The plurality of fine particles refers to the plurality of particles (A) among the plurality of particles (A) having a circle-equivalent diameter of 0.8 μm or less.

[0013] In this disclosure, "primary particles" refer to single-crystal particles, which are the smallest particle unit and exist independently (see Figure 1). In Figure 1, the symbol "1" indicates a primary particle. "Single crystal" refers to a crystal whose orientation does not change. "Secondary particles" refer to particles formed by primary particles sintering together to form a single particle (see Figure 2). In Figure 2, the symbol "1" indicates a primary particle, and the symbol "2" indicates a secondary particle. Primary particles and secondary particles can be distinguished using SEM images. "Fine particles" refer to particles with a circle-equivalent diameter of 0.8 μm or less, regardless of whether they are primary particles or secondary particles. "Circle-equivalent diameter" refers to the diameter of a circle having an area equal to the area of a particle. The circle-equivalent diameter is measured in the same manner as in the Examples.

[0014] The positive electrode active material powder of the present disclosure has the above-described configuration, and therefore can reduce the rate of increase in resistance of a lithium secondary battery. This effect is presumably due to, but not limited to, the following reasons. In general, when coarse secondary particles in a positive electrode composite layer are repeatedly charged and discharged, the interfaces of the coarse secondary particles may crack, which may increase the resistance of the lithium secondary battery (hereinafter also referred to as "battery resistance"). The increase in battery resistance is more pronounced when the proportion of coarse secondary particles is high. In addition, when repeated charging and discharging are performed, the fine particles in the positive electrode active material tend to become isolated without contact with their surroundings, which may increase the battery resistance. The increase in battery resistance is more pronounced when the proportion of fine particles is high. On the other hand, in the present disclosure, the proportion of coarse secondary particles is 45% by number or less. In other words, the proportion of coarse secondary particles is low. Furthermore, in the present disclosure, the proportion of fine particles is 1.5% by number or less. In other words, the proportion of fine particles is low. As a result, it is presumed that the positive electrode active material powder of the present disclosure can reduce the resistance increase rate of a lithium secondary battery.

[0015] The average particle diameter (D50) of the positive electrode active material powder is not particularly limited and may be 0.05 μm to 20 μm or less. The average particle diameter (D50) of the positive electrode active material powder indicates the particle diameter (particle size distribution D50, median diameter) corresponding to a cumulative 50% by volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution measuring device based on a laser light diffraction scattering method.

[0016] (1.1) Particle (A) The particles (A) are composed of a positive electrode active material.

[0017] The positive electrode active material is preferably a lithium transition metal oxide having a layered crystal structure, whereby the positive electrode active material powder of the present disclosure can further reduce the rate of increase in resistance of a lithium secondary battery.

[0018] "Lithium transition metal oxide" refers to a compound in which lithium and transition metals are cations and oxide ions are anions. "Transition metal" refers to elements in groups 3A to 7A, 8, and 1B of the periodic table. "Layered crystal structure" refers to a crystal structure in which transition metal layers containing lithium and layers of lithium alone are alternately stacked, with oxide ions interposed between them.

[0019] The lithium transition metal oxide preferably contains at least one of nickel (Ni), cobalt (Co), and manganese (Mn). Thereby, the positive electrode active material powder of the present disclosure can further reduce the resistance increase rate of the lithium secondary battery.

[0020] Examples of the lithium transition metal oxide containing at least one of Ni, Co, and Mn include compounds represented by the following (1), LiNiCoMnO2 (lithium nickel cobalt manganese composite oxide), LiNiO2 (lithium nickel oxide), LiCoO2 (lithium cobalt oxide), or LiMn2O4 (lithium manganate), etc. Formula (1): Li 1+x Ni y Co z Mn (1-y-z) M γ O2 In formula (1), the relationship of 0≦x≦0.2, 0.1<y<0.9, 0.1<z<0.4, 0≦γ≦0.01 is satisfied. M is at least one additive element selected from the group consisting of Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F.

[0021] The plurality of particles (A) contained in the positive electrode active material powder may be of one type or at least two types.

[0022] The shape of the particle (A) is not particularly limited, and may be, for example, spherical, columnar, or massive.

[0023] (1.2) Primary particles (A1) and secondary particles (A2) The plurality of particles (A) consists of a plurality of primary particles (A1) and a plurality of secondary particles (A2).

[0024] The size of each of the primary particles (A1) and the secondary particles (A2) is not particularly limited. The average particle diameter (D50) of the primary particles (A1) may be larger than the average particle diameter (D50) of the secondary particles (A2).

[0025] The plurality of secondary particles (A2) includes a plurality of coarse secondary particles.

[0026] The proportion of coarse secondary particles is 45% by number or less relative to the total number of the plurality of particles (A) within a specific field of view. The proportion of coarse secondary particles may be 27% by number or less, 20% by number or less, 15% by number or more, or 15% to 45% by number.

[0027] The proportion (by number %) of the multiple primary particles (A1) in the positive electrode active material powder and the proportion (by number %) of the multiple secondary particles (A2) in the positive electrode active material powder are not particularly limited, as long as the proportion of coarse secondary particles is 45% by number or less.

[0028] (1.3) Fine particles and coarse particles The plurality of particles (A) consists of a plurality of fine particles and a plurality of coarse particles. The term "coarse particles" refers to a plurality of particles (A) having an equivalent circle diameter of more than 0.8 μm.

[0029] The proportion of fine particles is 1.5% by number or less relative to the total number of the plurality of particles (A) within the specific field of view. The proportion of fine particles may be 1.0% by number or less, 0.7% by number or more, or 0.7% to 1.5% by number.

[0030] (1.3) Preferred Embodiments It is preferable that the proportion of the coarse secondary particles is 28% by number or less of the total number of the plurality of particles (A) within the field of view, and the proportion of the fine particles is 0.8% by number or less of the total number of the plurality of particles (A) within the field of view, thereby enabling the positive electrode active material powder of the present disclosure to further reduce the rate of increase in resistance of a lithium secondary battery.

[0031] (2) Lithium secondary battery The lithium secondary battery of the present disclosure includes a positive electrode containing the positive electrode active material powder of the present disclosure. Because the lithium secondary battery of the present disclosure has the above-described configuration, the rate of increase in resistance is reduced.

[0032] The lithium secondary battery of the present disclosure typically further comprises, in addition to a positive electrode, a negative electrode and an ionically conductive medium. The ionically conductive medium is interposed between the positive electrode and the negative electrode and conducts carrier ions. Examples of the ionically conductive medium include a non-aqueous electrolyte solution, a non-aqueous gel electrolyte solution, a solid ionically conductive polymer, and an inorganic solid electrolyte.

[0033] A lithium secondary battery using a non-aqueous electrolyte solution (hereinafter also referred to as a "nonaqueous battery") will be described below.

[0034] (2.1) Non-aqueous batteries The non-aqueous battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0035] (2.1.1) Positive electrode The positive electrode has a positive electrode mixture layer and may further have a positive electrode current collector (e.g., aluminum foil, etc.). The positive electrode mixture layer is laminated on at least one main surface of the positive electrode current collector. The positive electrode mixture layer contains the positive electrode active material powder of the present disclosure.

[0036] The positive electrode mixture layer contains the positive electrode active material powder of the present disclosure. The positive electrode mixture layer may further contain a known conductive material (e.g., acetylene black, etc.), trilithium phosphate, or a binder (e.g., polyvinylidene fluoride (PVDF), etc.).

[0037] (2.1.2) Negative electrode The negative electrode has a negative electrode current collector, and may or may not further have a negative electrode mixture layer.

[0038] When the negative electrode does not have a negative electrode composite layer, the negative electrode current collector includes a main surface on which lithium metal is deposited during charging. Specifically, lithium ions contained in the nonaqueous electrolyte solution receive electrons on the negative electrode current collector during charging, causing lithium metal to deposit. The deposited lithium metal dissolves as lithium ions in the nonaqueous electrolyte solution upon discharge. The lithium ions contained in the nonaqueous electrolyte solution may be at least one of ions derived from a lithium salt described below and ions supplied from the positive electrode active material during charging.

[0039] When the negative electrode has a negative electrode mixture layer, the negative electrode mixture layer is laminated on at least one main surface of a negative electrode current collector (e.g., copper foil, etc.). The negative electrode mixture layer contains a negative electrode layer active material capable of absorbing and releasing charge carriers (e.g., carbon (e.g., natural graphite, etc.) or a compound capable of alloying with lithium (e.g., silicon, tin, etc.)). The negative electrode mixture layer may further contain, as necessary, a conductive material (e.g., acetylene black, etc.) for increasing electronic conductivity, a binder (e.g., carboxymethyl cellulose (CMC) or styrene butadiene rubber (SBR)), an electrolyte supporting salt (lithium salt) for increasing ionic conductivity, a polymer electrolyte, or an additive (e.g., trifluoropropylene carbonate, etc.). The negative electrode may have a known configuration.

[0040] (2.1.3) Separator The separator maintains the distance between the positive electrode and the negative electrode to prevent short-circuiting and allows lithium ions to pass through. Examples of the separator include a porous resin sheet or a nonwoven fabric. Examples of materials for the porous resin sheet include polyolefins (e.g., polypropylene (PP), polyethylene (PE), etc.). Examples of materials for the nonwoven fabric include polypropylene, polyethylene terephthalate, and methyl cellulose. The separator may have a known configuration.

[0041] (2.1.4) Non-aqueous electrolyte The non-aqueous electrolyte may include a non-aqueous solvent and a lithium salt. Examples of lithium salts include LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2. Examples of non-aqueous solvents include cyclic carbonates (e.g., ethylene carbonate (EC)), chain carbonates (e.g., dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC)), cyclic esters (e.g., γ-butyrolactone and γ-valerolactone), chain esters (e.g., methyl formate and methyl acetate), and ethers (e.g., dimethoxyethane and ethoxymethoxyethane). The non-aqueous electrolyte may include additives (e.g., vinylene carbonate and lithium bis(oxalato)borate).

[0042] (2.1.5) Case A non-aqueous battery usually has a case that houses a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The case is not particularly limited, and examples thereof include a laminate film (e.g., an aluminum sheet) and a battery can (e.g., a cylindrical, rectangular, or coin-shaped can). [Example]

[0043] The present disclosure will be described in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0044] [1] Examples 1 to 4 and Comparative Examples 1 and 2 [1.1] Crystallization Nickel sulfate (NiSO4), cobalt sulfate (CoSO4), and manganese sulfate (MnSO4) were dissolved in ion-exchanged water to obtain an NCM aqueous solution. The molar ratio of Ni to Co to Mn (Ni:Co:Mn) was 8:1:1. The total content of nickel sulfate, cobalt sulfate, and manganese sulfate in the NCM aqueous solution was 30 mass% of the total amount of the NCM aqueous solution.

[0045] The gas in the reaction vessel was replaced with nitrogen (N2). A certain amount of aqueous ammonia (NH3) solution was placed in the reaction vessel, and the solution was stirred with a stirrer at the stirring speed shown in Table 1. Sodium hydroxide (NaOH) was added to the reaction vessel to make the solution alkaline. While controlling the pH of the solution to the pH shown in Table 1, the NCM aqueous solution and NH3 were added dropwise to the reaction vessel. The molar ratio of NH3 to the transition metals (i.e., Ni, Co, and Mn) in the NCM aqueous solution was the ratio (NH3 / TM) shown in Table 1. This resulted in the precipitation of a crystallized product. The crystallized product was removed from the solution by filtration, dispersed in ion-exchanged water with a spoon, and washed with water. The washed product was then filtered. The obtained transition metal hydroxide was dried at 120°C for 16 hours to evaporate the water. This yielded the transition metal hydroxide.

[0046] [1.2] Firing A transition metal hydroxide and a Li raw material (Li2CO3) were mixed in a mortar. The molar ratio of Li in the Li raw material to the transition metal in the transition metal hydroxide was the ratio (Li / TM) shown in Table 1. The resulting mixture was fired in an oxygen atmosphere in a firing furnace (muffle furnace). The mixture was fired at the temperatures and times shown in Table 1, with first, second, and third firings being carried out in this order. The resulting fired product was pulverized in a pulverizer (jet mill) and crushed to a predetermined particle size. This yielded a positive electrode active material powder precursor.

[0047] [1.3] Post-processing The positive electrode active material powder precursor was crushed using a mortar and passed through a sieve with 32 μm openings. The powder that passed through the sieve was then subjected to the post-treatment shown in Table 1. The post-treatment refers to washing, firing, and sieving, performed in this order. "Washing" refers to washing with water. "Firing" refers to firing in an oxygen atmosphere at the temperature and for the time shown in Table 1. "Sieving" refers to a process for removing particles that do not pass through the sieve with the openings shown in Table 1. This resulted in the production of a positive electrode active material powder. The positive electrode active material powder was an aggregate of lithium transition metal oxide particles having a layered crystal structure. The lithium transition metal oxide contained Ni, Co, and Mn.

[0048] [Table 1]

[0049] In Table 1, "-" for the second and third firings indicates that the second and third firings were not performed. "-" for the firing treatment indicates that the firing treatment was not performed. "-" for the sieving treatment indicates that the sieving treatment was not performed.

[0050] [1.4] Measurement of coarse secondary particle ratio and fine particle ratio The obtained positive electrode active material powder was subjected to a scanning microscope (manufactured by Thermo Fisher Scientific) to take a scanning microscope photograph of the positive electrode active material powder in a field of view that could accommodate 50 or more particles (A). The number of particles (A) in the scanning microscope photograph and the number of coarse secondary particles were counted. Using particle analysis software (manufactured by Thermo Fisher Scientific), the circle-equivalent diameter of the particles in the scanning microscope photograph was calculated, and the number of particles (A) having a circle-equivalent diameter of 0.8 μm or less was counted. The proportion of coarse secondary particles was calculated from the following formula (i): The proportion of fine particles was calculated from the following formula (ii): The results are shown in Table 2. Formula (i): Coarse secondary particle ratio (number%) = [number of coarse secondary particles ÷ number of particles (A)] × 100 Formula (ii): Fine particle ratio (number %) = [number of particles having a circle equivalent diameter of 0.8 μm or less ÷ number of particles (A)] × 100

[0051] [2] Evaluation of the rate of increase in resistance A lithium secondary battery (hereinafter also referred to as a "battery for evaluation") was fabricated using the positive electrode active material powder as follows, and the resistance increase rate was measured.

[0052] [2.1] Manufacturing of evaluation batteries The positive electrode was fabricated using the obtained positive electrode active material powder, acetylene black (Denka Co., Ltd.) as a conductive material, PVDF (Kureha Co., Ltd.) as a binder, and aluminum foil (thickness: 15 μm) as a positive electrode current collector.

[0053] The negative electrode was fabricated using natural graphite (Hitachi Chemical Co., Ltd.) as the active material, CMC (Nippon Paper Industries Co., Ltd.) and SBR (JSR Corporation) as binders, and copper foil (thickness: 10 μm) as the negative electrode current collector.

[0054] A porous resin (PP / PE / PP) (thickness: 24 μm) was prepared as a separator, with PP layers laminated on both sides of a PE layer. The positive electrode, separator, and negative electrode were laminated so that the separator separated the positive electrode and negative electrode. This formed an electrode assembly.

[0055] A pouch made of laminated film was prepared as the exterior body. The electrode body was housed in the exterior body. A non-aqueous electrolyte solution was used in which a supporting salt (LiPF6) was dissolved at a concentration of 1 mol / L in a mixed solvent containing EC, DMC, and EMC. The electrolyte solution was injected into the exterior body. After the electrolyte solution was injected, the exterior body was sealed. In this way, a battery for evaluation was produced.

[0056] [2.2] Measurement of the rate of increase in resistance [2.2.1] Initial resistance The SOC (State of Charge) of the evaluation battery was adjusted to 50% by constant current-constant voltage (CC-CV) charging in a temperature environment of 25°C. The current during constant current (CC) charging was 1 It. "1 It" is defined as the current that drains the rated capacity of the battery in 1 hour. At 50% SOC, the battery voltage was 3.7 V. After adjusting the SOC, the battery was discharged for 10 seconds at a current of 10 It, followed by a 30-minute rest. The initial discharge resistance (initial resistance) was calculated using the following formula (iii):

[0057] Formula (iii):r=(V0-V 10 ) / current In formula (iii), "r" represents the discharge resistance. "V0" represents the voltage at the start of discharge. 10 " indicates the voltage 10 seconds after the start of discharge.

[0058] [2.2.2] Resistance Increase Rate After measuring the initial resistance, the battery's SOC was adjusted to 80% by CC-CV charging at 25°C. The current during CC charging was 1 It. After adjusting the SOC, a cycle test was conducted at 25°C. Specifically, the following discharge and charge cycles were repeated alternately for 15 days. Discharge: Current = 1 It, Discharge capacity = Capacity equivalent to 20% SOC Charging: Current = 1 It, charging capacity = capacity equivalent to 20% SOC

[0059] After the cycle test, the discharge resistance after the cycle (post-cycle resistance) was measured in the same manner as the initial resistance. The resistance increase rate (percentage) was calculated by dividing the post-cycle resistance by the initial resistance. The results are shown in Table 2. The resistance increase rates in Table 2 are relative values. The resistance increase rate of Comparative Example 1 is defined as 1. The acceptable range of the resistance increase rate is less than 1.

[0060] [Table 2]

[0061] [3] Results In Comparative Example 1, the proportion of coarse secondary particles was not 45% by number or less. In Comparative Example 2, the proportion of fine particles was measured to be 1.5% by number or less. Therefore, in Comparative Examples 1 and 2, the resistance increase rate was not less than 1. From these results, it was found that the positive electrode active material powders of Comparative Examples 1 and 2 are not "positive electrode active material powders capable of reducing the resistance increase rate of lithium secondary batteries."

[0062] In Examples 1 to 4, the proportion of coarse secondary particles was 45% by number or less, and the proportion of fine particles was 1.5% by number or less. Therefore, in Examples 1 to 4, the resistance increase rate was less than 1. From these results, it was found that the positive electrode active material powders of Examples 1 to 4 were "positive electrode active material powders capable of reducing the resistance increase rate of lithium secondary batteries."

Claims

1. A positive electrode active material powder for a lithium secondary battery, comprising a plurality of particles (A) made of a positive electrode active material, the plurality of particles (A) comprises a plurality of primary particles (A1) and a plurality of secondary particles (A2) formed by sintering the primary particles (A1), In a scanning microscope photograph of the positive electrode active material powder in a field of view that includes 50 or more of the particles (A), a ratio of a plurality of coarse secondary particles to the total number of the plurality of particles (A) in the field of view is 45% by number or less, the plurality of coarse secondary particles represent a plurality of secondary particles (A2) formed by sintering five or more of the primary particles (A1), among the plurality of secondary particles (A2), the ratio of the plurality of fine particles to the total number of the plurality of particles (A) within the field of view is 1.5% by number or less, The positive electrode active material powder, wherein the plurality of fine particles are, among the plurality of particles (A), a plurality of particles (A) having an equivalent circle diameter of 0.8 μm or less.

2. the proportion of the coarse secondary particles is 28% by number or less relative to the total number of the plurality of particles (A) within the field of view, 2. The positive electrode active material powder according to claim 1, wherein the proportion of the fine particles is 0.8% by number or less with respect to the total number of the plurality of particles (A) within the field of view.

3. 2. The positive electrode active material powder according to claim 1, wherein the positive electrode active material is a lithium transition metal oxide having a layered crystal structure.

4. The positive electrode active material powder according to claim 3 , wherein the lithium transition metal oxide contains at least one of Ni, Co, and Mn.

5. A lithium secondary battery comprising a positive electrode containing the positive electrode active material powder according to any one of claims 1 to 4.

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