Positive electrode active material and method for producing the same

Abstract

To provide a positive electrode active material which can suppress a decrease in battery capacity when used in a battery.SOLUTION: The positive electrode active material has a composition represented by LixNiaCobMncOy and has a crystallite size in primary particles of 300 nm or more and 1,700 nm or less. In the composition, 0.1≤x≤1.5, 0.5≤a≤1.0, 0≤b≤0.3, 0≤c≤0.3, a+b+c=1.0 and 1.5≤y≤2.1 are satisfied.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material and a method for manufacturing the positive electrode active material. [Background technology]

[0002] Conventionally, methods for controlling the crystal grain size of particles in positive electrode active materials used in batteries have been attempted. For example, Patent Document 1 discloses a method for producing a large crystal grain agglomerate ternary positive electrode material, which includes the steps of preparing a mixed solution of a nickel salt, a cobalt salt, and a manganese salt, adding a precipitant and a coordinating agent to the mixed solution, adjusting the pH of the mixed solution to 10.5 to 12, and precipitating the mixture to obtain precursor A, mixing the washed precursor A with a lithium salt in a ball mill to obtain precursor B, sintering precursor B in an air or oxygen atmosphere by heating the precursor B to 400 to 800°C at a rate of 5 to 15°C / min, isothermally sintering for 1 to 6 hours, and then further heating the precursor B to 900 to 980°C at a rate of 1 to 10°C / min, isothermally sintering for 8 to 10 hours, and finally cooling the resulting mixture to obtain a large crystal grain agglomerate ternary positive electrode material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-36570 Summary of the Invention [Problem to be solved by the invention]

[0004] Batteries are required to have a capacity that does not decrease even after repeated charge and discharge (i.e., the ability to maintain battery capacity). However, in batteries equipped with a positive electrode containing a positive electrode active material, the capacity of the battery sometimes decreases after repeated charge and discharge. Therefore, there is a demand for a positive electrode active material that can suppress the decrease in battery capacity when used in a battery.

[0005] The present disclosure has been made in view of the above circumstances, and aims to provide a positive electrode active material that can suppress a decrease in battery capacity when used in a battery, and a method for producing the positive electrode active material. [Means for solving the problem]

[0006] Means for solving the above problems include the following aspects. <1> Li x Ni a Co b Mn c O y and the crystallite size within the primary particles is 300 nm or more and 1700 nm or less. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.) <2> A positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles, and the average number of the primary particles constituting one secondary particle is 5 or less. <1> The positive electrode active material according to claim 1. <3> The composition further contains at least one element selected from the group consisting of the following group X and the following group Y: <1> or <2> The positive electrode active material according to claim 1. X:Ba, Pr, La, Y, Sr, Ce, Se, Hf, Rh, Zr, Sn, Mg Y:W, Re, Sb, Sn, Ta, Os, Ir, Mo, Nb, Tc, Ru, Ga, Ag, Pd, Ge, As, Zr, In, Pt, Al, Ti <4> at least one element selected from group X and at least one element selected from group Y (however, the element selected from group X is not Zr alone and the element selected from group Y is not Zr alone, and the element selected from group X is not Sn alone and the element selected from group Y is not Sn alone); <3> The positive electrode active material according to claim 1. <5> a step of mixing raw materials containing Ni, Co, and Mn, and a raw material containing Li, to obtain a mixture; a stepwise firing process in which the mixture is subjected to a low-temperature firing treatment at a temperature of 400°C or more and 600°C or less, a medium-temperature firing treatment at a temperature of 500°C or more and 800°C or less, which is higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment at a temperature of 600°C or more and 1000°C or less, which is higher than the temperature in the medium-temperature firing treatment, in this order; Li x Ni a Co b Mn c O y A method for producing a positive electrode active material having a composition represented by the formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.) [Effects of the Invention]

[0007] According to the present disclosure, there are provided a positive electrode active material that can suppress a decrease in battery capacity when used in a battery, and a method for producing the positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0008] <Cathode active material> The positive electrode active material according to the embodiment of the present disclosure is Li x Ni a Co b Mn c O y The positive electrode active material has a composition represented by the following formula: The crystallite size in the primary particles of the positive electrode active material is 300 nm or more and 1700 nm or less. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0009] The positive electrode active material according to the embodiment of the present disclosure suppresses the decrease in capacity of the battery. The reason for this effect is presumed to be as follows.

[0010] One of the performance requirements for batteries is that the decrease in battery capacity be suppressed even after repeated charge / discharge (i.e., the ability to maintain battery capacity). However, in batteries equipped with a positive electrode containing a positive electrode active material, the battery capacity has sometimes decreased after repeated charge / discharge. One of the reasons for this is thought to be that the positive electrode active material reacts with the electrolyte, resulting in the consumption of Li in the coating, resulting in a decrease in battery capacity. Therefore, it is necessary to suppress the decrease in battery capacity that occurs due to the reaction of the positive electrode active material with the electrolyte.

[0011] In contrast, the positive electrode active material according to an embodiment of the present disclosure has a crystallite size within the crystals in the primary particles that falls within the above range. When the crystallite size of the positive electrode active material particles is small (i.e., when the crystallite size is less than 300 nm), this means that crystal growth in the positive electrode active material particles is not progressing. A positive electrode active material with a small crystallite size has a large reaction area, which causes a reaction with the electrolyte in the battery. On the other hand, the positive electrode active material according to an embodiment of the present disclosure has a crystallite size of 300 nm or more, and crystal growth is sufficiently advanced. Therefore, the reaction area in the positive electrode active material is small, and the reaction with the electrolyte in the battery is suppressed. This suppresses Li consumption due to the formation of a coating, thereby suppressing a decrease in the battery capacity.

[0012] Next, the positive electrode active material according to an embodiment of the present disclosure will be described in detail.

[0013] (crystallite size) The crystallite size of the crystals in the particles (primary particles) of the positive electrode active material is 300 nm or more and 1700 nm or less. A crystallite size of 300 nm or more reduces the reaction area of ​​the positive electrode active material, suppressing reaction with the electrolyte in the battery and reducing Li consumption due to the formation of a coating, thereby suppressing a decrease in battery capacity. On the other hand, a crystallite size of 1700 nm or less simplifies the process for increasing the crystallite size, specifically the firing process, and prevents the manufacturing process from becoming complicated.

[0014] The lower limit of the crystallite size of the particles (primary particles) of the positive electrode active material is preferably 500 nm or more, and more preferably 800 nm or more, from the viewpoint of suppressing capacity reduction in the battery, while the upper limit of the crystallite size is preferably 1500 nm or less, and more preferably 1000 nm or less, from the viewpoint of simplifying the process for increasing the crystallite size.

[0015] The method for controlling the crystallite size of the crystals in the particles (primary particles) of the positive electrode active material is not particularly limited. For example, in order to increase the crystallite size to 300 nm or more, it is preferable to promote crystal growth by gradually increasing the temperature from low to high in the firing process when producing the positive electrode active material. More preferably, it is preferable to undergo a stepwise firing process in which a low-temperature firing process is performed at a temperature of 400 ° C. to 600 ° C., a medium-temperature firing process is performed at a temperature of 500 ° C. to 800 ° C. and higher than the temperature in the low-temperature firing process, and a high-temperature firing process is performed at a temperature of 600 ° C. to 1000 ° C. and higher than the temperature in the medium-temperature firing process in this order.

[0016] [Crystallite size calculation method] Here, we will explain how to calculate the crystallite size of particles (primary particles) of the positive electrode active material. The crystallite size of the positive electrode active material is measured using an XRD (X-ray diffraction) measurement device (Rigaku Corporation, SmartLab (registered trademark)). The crystallite size is calculated using the following formula from the angle (θ) of the peak between 17° and 19° and the half-width (β). Formula: L = 0.9 × λ / (β cos θ) (In the formula, λ represents the wavelength of X-rays (Å).) The measurement conditions are as follows: Angle: 10°~120° Interval: 0.02° / step Speed: 10° / min

[0017] (composition) The positive electrode active material according to the embodiment of the present disclosure contains at least Li, Ni, and O, and may contain Co and Mn, and the ratio of these components is Li x Ni a Co b Mn c The positive electrode active material has a composition represented by O2. The positive electrode active material may further contain other additive elements. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0018] In the composition of the positive electrode active material, from the viewpoint of suppressing capacity reduction in the battery, the Li ratio x is 0.1 to 1.5, preferably 0.3 to 1.4, and more preferably 0.5 to 1.2. From the viewpoint of suppressing capacity reduction in the battery, the Ni ratio a is 0.5 to 1.0, preferably 0.6 to 0.9, and more preferably 0.7 to 0.8. From the viewpoint of suppressing capacity reduction in the battery, the Co ratio b is 0 to 0.3, preferably 0 to 0.2, and more preferably 0.1 to 0.2. From the viewpoint of suppressing capacity reduction in the battery, the Mn ratio c is 0 to 0.3, preferably 0 to 0.2, and more preferably 0.1 to 0.2. The sum of the ratios of Ni, Co, and Mn (a+b+c) is 1.0.

[0019] The positive electrode active material may further contain other additive elements. In particular, from the viewpoint of suppressing capacity reduction in the battery, it is preferable that the positive electrode active material further contains at least one element selected from the group consisting of Group X and Group Y below. X:Ba, Pr, La, Y, Sr, Ce, Se, Hf, Rh, Zr, Sn, Mg Y:W, Re, Sb, Sn, Ta, Os, Ir, Mo, Nb, Tc, Ru, Ga, Ag, Pd, Ge, As, Zr, In, Pt, Al, Ti

[0020] Furthermore, from the viewpoint of suppressing capacity reduction in a battery, the positive electrode active material preferably contains at least one element selected from Group X and at least one element selected from Group Y. However, the element selected from Group X is not Zr alone and the element selected from Group Y is not Zr alone, and the element selected from Group X is not Sn alone and the element selected from Group Y is not Sn alone.

[0021] Combinations of elements selected from Group X and elements selected from Group Y that are preferably contained in the positive electrode active material are shown below. From the viewpoint of suppressing capacity reduction in the battery, the positive electrode active material preferably contains one or more of the combinations of elements shown below. In the combinations shown below, the element written before "-" represents an element selected from Group X, and the element written after "-" represents an element selected from Group Y.

[0022] -Preferable combinations of elements in group X and elements in group Y Ba-W, Pr-W, La-W, YW, Sr-W, Ce-W, Pr-Re, Ba-Re, Sr-Sb, Se-W, Y-Re, Hf-W, Sr-Re, Rh-W, Zr-W, Sr-Sn, Y -Ta, Pr-Ta, Y-Sb, Sr-Os, Sr-Ta, Ce-Re, La-Re, Ba-Ta, Sr-Ir, Sn-W, Sr-Mo, Sr-Nb, Ba-Ti, Ba-Zr, Ba-Al

[0023] The content (% by mass) of the element selected from group X in the positive electrode active material is from 0.0005 to 0.05, preferably from 0.001 to 0.040, and more preferably from 0.003 to 0.030, from the viewpoint of suppressing capacity reduction in the battery. The content (% by mass) of the element selected from group Y in the positive electrode active material is from 0.0005 to 0.05, preferably from 0.001 to 0.040, and more preferably from 0.003 to 0.030, from the viewpoint of suppressing capacity reduction in the battery.

[0024] (Secondary particles) In the positive electrode active material according to the embodiment of the present disclosure, a plurality of primary particles are preferably aggregated to form secondary particles, and from the viewpoint of suppressing capacity reduction in a battery, the average number of primary particles constituting one secondary particle is preferably 5 or less. The formation of secondary particles in the positive electrode active material can be confirmed by observing a cross section of the positive electrode active material layer with a scanning electron microscope (SEM). The average number of primary particles constituting the secondary particles in the positive electrode active material is calculated by measuring 50 randomly selected secondary particles in the microscope observation, counting the number of primary particles constituting each secondary particle, and determining the arithmetic mean value.

[0025] <Method of manufacturing positive electrode active material> Next, a method for producing a positive electrode active material according to an embodiment of the present disclosure will be described. The positive electrode active material according to the embodiment of the present disclosure described above can be produced by the method for producing a positive electrode active material according to an embodiment of the present disclosure shown below.

[0026] A method for producing a positive electrode active material according to an embodiment of the present disclosure includes a step of mixing raw materials containing Ni, Co, and Mn, and a raw material containing Li, to obtain a mixture, and a stepwise firing step of subjecting the mixture to a low-temperature firing treatment at a temperature of 400°C to 600°C, a medium-temperature firing treatment at a temperature of 500°C to 800°C, which is higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment at a temperature of 600°C to 1000°C, which is higher than the temperature in the medium-temperature firing treatment, in this order. x Ni a Co b Mn c O y A positive electrode active material having a composition represented by the formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0027] As described above, the manufacturing method of the cathode active material according to the embodiment of the present disclosure includes a stepwise calcination process in which the calcination temperature is gradually increased from low to high. This promotes crystal growth within the particles of the cathode active material, enabling the crystallite size to be increased to 300 nm or greater. This allows for a cathode active material with a small reaction area and capable of suppressing reaction with the electrolyte in the battery. Furthermore, by using this cathode active material in a battery, Li consumption due to the formation of a coating is suppressed, thereby suppressing a decrease in the battery capacity.

[0028] The method for producing a positive electrode active material according to an embodiment of the present disclosure preferably includes the following steps (1) to (5). (1) A step of preparing a solution in which raw materials containing Ni, Co, and Mn are dissolved (raw material dissolving step) (2) A step of adding the solution to an alkaline solution to precipitate hydroxide (crystallization step) (3) collecting the precipitate from the alkaline solution (4) A step of mixing the precipitate with a raw material containing Li to obtain a mixture (mixing step). (5) A step of firing the mixture (firing step)

[0029] When an additive element is contained in the positive electrode active material, it is preferable to further add a raw material containing the additive element in the mixing step (4). Examples of the additive element include elements selected from the above-mentioned group X and group Y. Each step will be described in detail below.

[0030] (1) A step of preparing a solution in which raw materials containing Ni, Co, and Mn are dissolved. A solution is prepared by dissolving a raw material containing Ni, a raw material containing Co, and a raw material containing Mn. For example, the solution can be prepared by dissolving the raw material containing Ni, the raw material containing Co, and the raw material containing Mn in a solvent such as water. The concentration of the solution is preferably in the range of 10 to 40 mass %. The ratio of Ni / Co / Mn is preferably 1.0 / 0.8-1.2 / 0.8-1.2 (atm %) with respect to Ni:1.0.

[0031] Examples of raw materials containing Ni include sulfates such as NiSO4, raw materials containing Co include sulfates such as CoSO4, and raw materials containing Mn include sulfates such as MnSO4.

[0032] (2) Adding the solution to an alkaline solution to precipitate hydroxide Next, the solution is added to an alkaline solution to precipitate the hydroxides. This causes the generated particles of hydroxides containing Ni, Co, and Mn to crystallize, and these particles are obtained as a precipitate. In this process, for example, the alkaline solution in which the hydroxides have precipitated is controlled to a constant pH (e.g., pH 10 to 12) while the solution and NH3 are added dropwise, thereby precipitating the transition metal hydroxides.

[0033] (3) A step of collecting the precipitate from the alkaline solution Next, the precipitate is collected from the alkaline solution. Examples of methods for collecting the precipitate particles include filtration and washing with water. First, the precipitate (particles) are collected by filtration and washed with water, and the washed solution is further filtered to collect the precipitate (particles). The precipitate (particles) after washing with water may be further dried.

[0034] (4) A step of mixing the precipitate with a raw material containing Li to obtain a mixture. Next, the collected precipitate (particles) and a raw material containing Li are mixed to obtain a mixture. When an additive element is to be contained in the positive electrode active material, it is preferable to further add a raw material containing the additive element. Examples of the additive element include an element selected from the aforementioned group X and an element selected from group Y. Examples of mixing methods include mixing the collected precipitate particles, the raw material containing Li, and a raw material containing the additive element (for example, a raw material containing an element selected from the aforementioned group X and an element selected from group Y) in a mortar.

[0035] Examples of raw materials containing Li include Li2CO3 and LiOH, etc. Examples of raw materials containing an element selected from the aforementioned group X (i.e., at least one element selected from the group consisting of Ba, Pr, La, Y, Sr, Ce, Se, Hf, Rh, Zr, Sn, and Mg) and an element selected from the aforementioned group Y (i.e., at least one element selected from the group consisting of W, Re, Sb, Sn, Ta, Os, Ir, Mo, Nb, Tc, Ru, Ga, Ag, Pd, Ge, As, Zr, In, Pt, Al, and Ti) include oxides of each element (e.g., BaO, Pr2O3, La2O3, SrO, W2O3, MoO3, and NbO).

[0036] (5) A step of firing the mixture Next, the mixture of the collected precipitate (particles) and the raw material containing Li is fired. For example, the mixture can be fired in a firing furnace (such as a muffle furnace).

[0037] In the method for producing a positive electrode active material according to an embodiment of the present disclosure, a stepwise firing process is performed in which the following firing treatments (a) to (c) are performed in this order. (a) Low-temperature firing at temperatures between 400°C and 600°C (b) Medium-temperature firing at a temperature of 500°C to 800°C, which is higher than the temperature in the low-temperature firing. (c) High-temperature firing at a temperature between 600°C and 1000°C, which is higher than the temperature in the medium-temperature firing.

[0038] By undergoing the above-mentioned stepwise firing process, it is possible to promote the growth of crystals in the particles of the positive electrode active material, and to increase the crystallite size to 300 nm or more.

[0039] The temperature in the (a) low-temperature firing treatment is 400° C. or higher and 600° C. or lower, and from the viewpoint of suppressing capacity reduction in the battery, it is preferably 420° C. or higher and 580° C. or lower, and more preferably 450° C. or higher and 550° C. or lower. The heating time at the above temperature in the (a) low-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of suppressing capacity reduction in the battery. The temperature in the (b) medium-temperature firing treatment is 500° C. or higher and 800° C. or lower, and from the viewpoint of suppressing capacity reduction in the battery, it is preferably 550° C. or higher and 750° C. or lower, and more preferably 600° C. or higher and 700° C. The heating time at the above temperature in the (b) medium-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of suppressing capacity reduction in the battery. The temperature in the (c) high-temperature firing treatment is 600° C. or higher and 1000° C. or lower, and from the viewpoint of suppressing capacity reduction in the battery, it is preferably 550° C. or higher and 750° C. or lower, and more preferably 600° C. or higher and 700° C. The heating time at the above temperature in the (c) high-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of suppressing capacity reduction in the battery.

[0040] The calcination is preferably carried out in an oxygen atmosphere. In order to obtain a predetermined particle size of the positive electrode active material, the calcined mixture may be crushed. Examples of the crushing method include crushing using a crusher (e.g., a jet mill).

[0041] By going through these steps, the positive electrode active material according to the embodiment of the present disclosure can be obtained.

[0042] <Battery> The positive electrode active material according to the embodiment of the present disclosure can be used in a battery, and is particularly suitable for use in a lithium-ion battery. The battery includes, for example, a negative electrode, a positive electrode, a separator, and an electrolyte. The battery according to the embodiment of the present disclosure may be a solid-state battery having a solid electrolyte or a liquid battery having a liquid electrolyte, but is preferably a liquid battery. Alternatively, the battery may be a bipolar battery having a positive electrode active material layer and a negative electrode active material layer on both sides of a current collector that functions as a positive electrode current collector and a negative electrode current collector.

[0043] The positive electrode includes, for example, a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. The separator is an electrically insulating porous film. The separator electrically isolates the positive electrode and the negative electrode. The battery according to the embodiment of the present disclosure may be a liquid-based battery further including an electrolyte solution. A non-aqueous electrolyte solution is particularly preferred. Examples of applications of batteries include power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). [Example]

[0044] The present disclosure will be described below based on examples, but the present disclosure is not limited to these examples in any way.

[0045] Example 1 (Synthesis of positive electrode active material) ·Raw material solution NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to obtain a raw material solution. The ratio of Ni / Co / Mn was 1 / 1 / 1 (atm %), and the concentration of the aqueous solution was 30 mass %.

[0046] Crystallization A certain amount of NH3 aqueous solution was placed in a reaction vessel, and the atmosphere was replaced with nitrogen while stirring with a stirrer. NaOH was added to the reaction vessel to make the pH alkaline. Next, the raw material solution and NH3 were added dropwise while maintaining a constant pH (pH 10-12) in the reaction vessel, causing the transition metal hydroxide to precipitate.

[0047] Washing, filtering, drying The precipitated transition metal hydroxide was filtered out, and ion-exchanged water was added and stirred with a spoon to disperse the hydroxide, followed by washing with water. The washed solution was then filtered to remove the transition metal hydroxide. The filtered transition metal hydroxide was then dried at 120°C for 16 hours to evaporate the water.

[0048] Mixing of Li raw material and raw materials of additive elements The dried transition metal hydroxide, Li2CO3 and LiOH as Li raw materials, MgO as a raw material for additional element 1, and Al2O3 as a raw material for additional element 2 were mixed in a mortar.

[0049] · Calcination and crushing The mixture of the transition metal hydroxide, the Li raw material, and the raw material of the additive element was fired in a firing furnace (muffle furnace) in a stepwise firing process in which a low-temperature firing treatment at 500°C, a medium-temperature firing treatment at 700°C, and a high-temperature firing treatment at 900°C were carried out in this order in an oxygen atmosphere for 3 hours each.

[0050] The fired mixture was then crushed in a crusher (jet mill) to a predetermined particle size, thereby obtaining the positive electrode active material of Example 1.

[0051] The positive electrode active material of Example 1 contained Li, Ni, Co, Mn, O, Mg, and Al in the ratios (mass ratios) shown in Table 1. Furthermore, the obtained positive electrode active material had secondary particles formed by aggregation of a plurality of primary particles, and the average number of primary particles constituting one secondary particle was five or less.

[0052] <Examples 2 to 6> The positive electrode active materials of each example were obtained in the same manner as in Example 1, except that the raw material of the additional element 1 in Example 1 was changed from MgO to La2O3 (Example 2), SrO (Example 3), and Pr2O3 (Example 4), and the raw material of the additional element 2 was changed from Al2O3 to W2O3 (Examples 2 and 4), and NbO (Example 3). The elements contained in the positive electrode active material of each example and their ratios (mass ratios) are shown in Table 1. Furthermore, the obtained positive electrode active material had secondary particles formed by aggregation of multiple primary particles, and the average number of primary particles constituting one secondary particle was 5 or less.

[0053] <Comparative Example 1> The positive electrode active material of Comparative Example 1 was obtained in the same manner as in Example 1, except that the raw materials of the additional element 1 and the raw materials of the additional element 2 in Example 1 were not added and the firing conditions were changed to firing at a temperature of 900°C in an oxygen atmosphere for 10 hours. The elements contained in the positive electrode active material of Comparative Example 1 and their ratios (mass ratios) are shown in Table 1. Furthermore, in the obtained positive electrode active material, a plurality of primary particles were aggregated to form secondary particles, and the average number of primary particles constituting one secondary particle was 5 or less.

[0054] <Comparative Example 2> A positive electrode active material of Comparative Example 2 was obtained in the same manner as in Example 1, except that the firing conditions in Example 1 were changed to firing at a temperature of 900° C. in an oxygen atmosphere for 10 hours. The elements contained in the positive electrode active material of Comparative Example 2 and their ratios (mass ratios) are shown in Table 1. Furthermore, in the obtained positive electrode active material, a plurality of primary particles were aggregated to form secondary particles, and the average number of primary particles constituting one secondary particle was 5 or less.

[0055] [Cell preparation] Cells were fabricated using the positive electrode active materials obtained in each of the examples and comparative examples. Cell configuration Wound cylinder Positive electrode composition: Positive electrode active material / acetylene black (conductive material) / polyvinylidene fluoride = 88 / 10 / 2 (mass%) Negative electrode composition: natural graphite / styrene butadiene rubber (SBR) / carboxymethyl cellulose (CMC) Electrolyte composition: electrolyte = LiPF6 (1M), solvent = ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3 / 4 / 3 (volume%)

[0056] Electrode preparation A positive electrode and a negative electrode were applied to a current collector using a film applicator with a film thickness adjustment function (All Good Co., Ltd.), and the applied film was dried in a dryer at 80°C for 5 minutes to prepare a cell.

[0057] [Crystallite size calculation method] The crystallite size of the positive electrode active materials obtained in each example and comparative example was measured using an XRD (X-ray diffraction) measurement device (Rigaku Corporation, SmartLab (registered trademark)). The crystallite size was calculated from the angle (θ) of the peak between 17° and 19° and the half-width (β) using the following formula. Formula: L = 0.9 × λ / (β cos θ) (In the formula, λ represents the wavelength of X-rays (Å).) The measurement conditions were as follows: Angle: 10°~120° Interval: 0.02° / step Speed: 10° / min

[0058] [Measurement of capacity retention rate after cycling] The battery capacity of the cells obtained in each example and comparative example was measured before and after cycling under the following test conditions. The results of the percentage of battery capacity after cycling (capacity retention rate (%)) when the battery capacity before cycling is set to "100%" are shown in Table 1. It can be said that the closer the capacity retention rate is to 100%, the better the battery characteristics are. Test conditions: 300 cycles of charge and discharge between SOC 0% and 100% at 60°C and 2C rate.

[0059] Note that "Synthesis method 2" in Table 1 refers to a synthesis method having a stepwise firing process in which a low-temperature firing process is performed at a temperature of 400°C to 600°C, a medium-temperature firing process is performed at a temperature of 500°C to 800°C but higher than the temperature in the low-temperature firing process, and a high-temperature firing process is performed at a temperature of 600°C to 1000°C but higher than the temperature in the medium-temperature firing process, in this order. On the other hand, "Synthesis method 1" refers to a synthesis method that does not have the above stepwise firing process.

[0060] [Table 1]

[0061] As shown in Table 1, the positive electrode active materials of each example, whose crystallite size falls within a specific range, have superior battery capacity retention properties compared to the positive electrode active materials of each comparative example, whose crystallite size falls below the specific range.

Claims

1. A positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles, Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ The composition is represented by the formula: wherein the crystallite size within the primary particles is 300 nm or more and 1700 nm or less; a positive electrode active material in which the average number of the primary particles constituting one secondary particle is 5 or less; (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

2. The positive electrode active material according to claim 1 , further comprising at least one element selected from the group consisting of the following group X and the following group Y: X: Ba, Pr, La, Y, Sr, Ce, Se, Hf, Rh, Zr, Sn, Mg Y: W, Re, Sb, Sn, Ta, Os, Ir, Mo, Nb, Tc, Ru, Ga, Ag, Pd, Ge, As, Zr, In, Pt, Al, Ti

3. 3. The positive electrode active material according to claim 2, comprising at least one element selected from Group X and at least one element selected from Group Y (provided that the element selected from Group X is not Zr alone and the element selected from Group Y is not Zr alone, and the element selected from Group X is not Sn alone and the element selected from Group Y is not Sn alone).

4. A method for producing a positive electrode active material in which a plurality of primary particles are aggregated to form secondary particles, comprising: A step of mixing raw materials containing Ni, Co, and Mn, and a raw material containing Li to obtain a mixture; a stepwise firing process in which the mixture is subjected to a low-temperature firing treatment at a temperature of 400° C. to 600° C. for 1 hour to 5 hours, a medium-temperature firing treatment at a temperature of 500° C. to 800° C., which is higher than the temperature in the low-temperature firing treatment, for 1 hour to 5 hours, and a high-temperature firing treatment at a temperature of 600° C. to 1000° C., which is higher than the temperature in the medium-temperature firing treatment, for 1 hour to 5 hours, in this order; The positive electrode active material is Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ It has a composition represented by The crystallite size in the primary particles is 300 nm or more and 1700 nm or less, a method for producing a positive electrode active material, wherein an average number of the primary particles constituting one secondary particle is 5 or less. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

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