Positive electrode active material and method for producing positive electrode active material

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 same.

Background Art

[0002] Conventionally, in positive electrode active materials used in batteries, methods for controlling crystal grains of particles have been attempted. For example, in Patent Document 1, steps of preparing a mixed solution of a nickel salt, a cobalt salt, and a manganese salt, adding a precipitating agent and a complexing agent to the mixed solution, adjusting the pH of the mixed solution to 10.5 to 12, and precipitating to obtain a precursor A, steps of mixing the washed precursor A and a lithium salt by a ball mill to obtain a precursor B, and sintering the precursor B in an atmosphere of air or oxygen, where the sintering involves heating from 400 to 800 °C at a rate of 5 to 15 °C / min, performing isothermal sintering for 1 to 6 h, then further heating from 900 to 980 °C at a rate of 1 to 10 °C / min, and performing isothermal sintering for 8 to 10 h, steps of cooling to obtain a large crystal grain aggregate ternary positive electrode material, are disclosed for a method for manufacturing a large crystal grain aggregate ternary positive electrode material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Batteries are required to have a performance in which the battery capacity does not decrease even after repeated charge and discharge (i.e., the maintainability of the battery capacity). However, in a battery including a positive electrode containing a positive electrode active material, the battery capacity may decrease after repeated charge and discharge. Therefore, there is a need for a positive electrode active material that can suppress a decrease in battery capacity when used in a battery.

[0005] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a positive electrode active material capable of suppressing a decrease in battery capacity when used in a battery, and a method for producing the positive electrode active material.

Means for Solving the Problems

[0006] Means for solving the above problems include the following aspects. <1>Li x Ni a Co b Mn c O y A positive electrode active material having a composition represented by the formula, wherein the crystallite size in 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, 1.5 ≦ y ≦ 2.1.) <2>The positive electrode active material according to <1>, wherein 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. <3>The positive electrode active material according to <1> or <2>, further comprising at least one element selected from the following group X and the following group Y in the above composition. 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>The positive electrode active material according to <3>, comprising at least one element selected from the group X and at least one element selected from the group Y (provided that the element selected from the group X is not only Zr and the element selected from the group Y is not only Zr, and the element selected from the group X is not only Sn and the element selected from the group Y is not only Sn). <5>A step of mixing a raw material containing Ni, Co, and Mn respectively, and a raw material containing Li to obtain a mixture. The mixture is subjected to a step firing process in which a low-temperature firing treatment is performed at a temperature of 400°C or higher and 600°C or lower, a medium-temperature firing treatment is performed at a temperature of 500°C or higher and 800°C or lower and higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment is performed at a temperature of 600°C or higher and 1000°C or lower and 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, which produces a positive electrode active material having a composition represented by (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, 1.5 ≦ y ≦ 2.1.)

Advantages of the Invention

[0007] According to the present disclosure, there are provided a positive electrode active material capable of suppressing a decrease in battery capacity when used in a battery, and a method for producing the positive electrode active material.

Embodiments for Carrying Out the Invention

[0008] <Positive Electrode Active Material> The positive electrode active material according to an embodiment of the present disclosure has a composition represented by Li x Ni a Co b Mn c O y And 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, 1.5 ≦ y ≦ 2.1.)

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

[0010] One of the required performances of a battery is that the decrease in battery capacity is suppressed even after repeated charge and discharge (that is, the maintainability of battery capacity). However, in a battery equipped with a positive electrode containing a positive electrode active material, the capacity of the battery may decrease after repeated charge and discharge. One of the factors is that a reaction occurs between the positive electrode active material and the electrolyte solution, and Li is consumed by the film, which may cause a decrease in battery capacity. Therefore, it is required to suppress the decrease in battery capacity due to the reaction between the positive electrode active material and the electrolyte solution.

[0011] On the other hand, for the positive electrode active material according to the embodiment of the present disclosure, the crystallite size in the crystal within the primary particle is within the above range. When the crystallite size in the particles of the positive electrode active material is small (that is, when the crystallite size is less than 300 nm), it means that crystal growth in the positive electrode active material particles has not progressed. And in a positive electrode active material with a small crystallite size, the reaction area becomes large, and a reaction with the electrolyte solution occurs in the battery. On the other hand, the positive electrode active material according to the embodiment of the present disclosure has a crystallite size of 300 nm or more, and crystal growth has sufficiently progressed. Therefore, the reaction area of the positive electrode active material is small, and the reaction with the electrolyte solution in the battery is suppressed. As a result, the consumption of Li due to the formation of the film is suppressed, and the decrease in capacity in the battery is suppressed.

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

[0013] (Crystallite size) The crystallite size of the crystal in the particles (primary particles) of the positive electrode active material is 300 nm or more and 1700 nm or less. When the crystallite size is 300 nm or more, the reaction area of the positive electrode active material is small, the reaction with the electrolyte solution in the battery is suppressed, the consumption of Li due to the formation of the film is suppressed, and the decrease in capacity in the battery is suppressed. On the other hand, when the crystallite size is 1700 nm or less, the process for increasing the crystallite size, specifically, the firing process can be simplified, and the complication of manufacturing can be suppressed.

[0014] The lower limit of the crystallite size in the particles (primary particles) of the positive electrode active material is preferably 500 nm or more, more preferably 800 nm or more, from the viewpoint of suppressing capacity degradation in the battery. On the other hand, the upper limit of the crystallite size is preferably 1500 nm or less, 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 performing firing while increasing the temperature stepwise from low temperature to high temperature in the firing step when manufacturing the positive electrode active material. More preferably, a low-temperature firing treatment performed at a temperature of 400 °C or more and 600 °C or less, a medium-temperature firing treatment performed at a temperature of 500 °C or more and 800 °C or less and higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment performed at a temperature of 600 °C or more and 1000 °C or less and higher than the temperature in the medium-temperature firing treatment are sequentially performed through a stepwise firing process.

[0016] [Crystallite Size Calculation Method] Here, the method for calculating the crystallite size in the particles (primary particles) of the positive electrode active material will be described. For the positive electrode active material, measurement of the crystallite size is performed using an XRD (X-ray diffraction) measurement device (manufactured by Rigaku Corporation, SmartLab (registered trademark)). The crystallite size is calculated from the angle (θ) and the half-width (β) of the peak existing between 17° and 19° by the following formula. Formula: L = 0.9 × λ / (β cos θ) (In the formula, λ represents the wavelength (Å) of X-rays.) The measurement conditions are as follows. Angle: 10° - 120° Interval: 0.02° / step Speed: 10° / min

[0017] (Composition) The cathode active material according to an embodiment of the present disclosure contains at least Li, Ni, and O, and may contain Co and Mn. The ratios of these components are Li x Ni a Co b Mn c and has a composition represented by O2. Further, the cathode 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, 1.5 ≦ y ≦ 2.1.)

[0018] In the composition of the cathode active material, from the viewpoint of suppressing capacity degradation in the battery, etc., the ratio x of Li is 0.1 or more and 1.5 or less, preferably 0.3 or more and 1.4 or less, and more preferably 0.5 or more and 1.2 or less. The ratio a of Ni is 0.5 or more and 1.0 or less, preferably 0.6 or more and 0.9 or less, and more preferably 0.7 or more and 0.8 or less, from the viewpoint of suppressing capacity degradation in the battery, etc. The ratio b of Co is 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, and more preferably 0.1 or more and 0.2 or less, from the viewpoint of suppressing capacity degradation in the battery, etc. The ratio c of Mn is 0 or more and 0.3 or less, preferably 0 or more and 0.2 or less, and more preferably 0.1 or more and 0.2 or less, from the viewpoint of suppressing capacity degradation in the battery, etc. Note that the total of the ratios of Ni, Co, and Mn (a + b + c) is 1.0.

[0019] The cathode active material may further contain other additive elements. In particular, from the viewpoint of suppressing capacity degradation in the battery, etc., it is preferable that the cathode active material further contains 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

[0020] Further, from the viewpoint of suppressing capacity degradation in the battery, etc., the positive electrode active material preferably contains at least one element selected from the group X and at least one element selected from the group Y. However, it is not the case that the element selected from the group X is only Zr and the element selected from the group Y is only Zr, and it is not the case that the element selected from the group X is only Sn and the element selected from the group Y is only Sn.

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

[0022] · Preferred combinations of the element of group X and the element of group Y Ba-W, Pr-W, La-W, Y-W, 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 ratio (mass %) of the element selected from the group X in the positive electrode active material is 0.0005 or more and 0.05 or less, preferably 0.001 or more and 0.040 or less, and more preferably 0.003 or more and 0.030 or less, from the viewpoint of suppressing capacity degradation in the battery, etc. The content ratio (mass %) of the element selected from the group Y in the positive electrode active material is 0.0005 or more and 0.05 or less, preferably 0.001 or more and 0.040 or less, and more preferably 0.003 or more and 0.030 or less, from the viewpoint of suppressing capacity degradation in the battery, etc.

[0024] (Secondary particles) The positive electrode active material according to the embodiment of the present disclosure preferably has a plurality of primary particles aggregated to form secondary particles. And from the viewpoint of suppressing capacity degradation in the battery, etc., the average number of primary particles constituting one secondary particle is preferably 5 or less. Note that whether the positive electrode active material forms secondary particles can be confirmed by observing a cross-section of the positive electrode active material layer with a scanning electron microscope (SEM). Further, the average number of primary particles constituting the secondary particles in the positive electrode active material is measured for any 50 secondary particles in the observation with the above microscope, the number of primary particles constituting each secondary particle is measured, and the arithmetic mean value is calculated.

[0025] <Manufacturing method of positive electrode active material> Next, the manufacturing method of the positive electrode active material according to the embodiment of the present disclosure will be described. Note that the positive electrode active material according to the embodiment of the present disclosure described above can be manufactured by the manufacturing method of the positive electrode active material according to the embodiment of the present disclosure shown below.

[0026] The manufacturing method of the positive electrode active material according to the embodiment of the present disclosure includes a step of mixing raw materials containing Ni, Co, and Mn respectively, and a raw material containing Li to obtain a mixture, and performing a low-temperature firing treatment on the mixture at a temperature of 400°C or higher and 600°C or lower, a medium-temperature firing treatment at a temperature of 500°C or higher and 800°C or lower and higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment at a temperature of 600°C or higher and 1000°C or lower and higher than the temperature in the medium-temperature firing treatment, in this order. And a positive electrode active material having a composition represented by x Ni a Co b Mn c O y is manufactured. (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, 1.5 ≦ y ≦ 2.1.)

[0027] In the method for manufacturing a positive electrode active material according to an embodiment of the present disclosure, as described above, there is a step firing process in which firing is performed while gradually increasing the temperature from a low temperature to a high temperature. Therefore, crystal growth in the particles of the positive electrode active material can be promoted, and the crystallite size can be increased to 300 nm or more. As a result, a positive electrode active material with a small reaction area in the positive electrode active material and capable of suppressing the reaction with the electrolyte in the battery can be obtained. And by using this positive electrode active material in a battery, the consumption of Li due to the formation of a film is suppressed, and the decrease in the capacity of the battery is suppressed.

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

[0029] Note that when adding an additive element to the positive electrode active material, it is preferable to further add a raw material containing the additive element in the (4) mixing step. Examples of the additive element include elements selected from the aforementioned group X and elements selected from group Y. Hereinafter, each step will be described in detail.

[0030] (1) Step of preparing a solution in which raw materials containing Ni, Co, and Mn are dissolved Prepare a solution by dissolving raw materials containing Ni, raw materials containing Co, and raw materials containing Mn. For example, a solution can be prepared by dissolving raw materials containing Ni, raw materials containing Co, and raw materials containing Mn in a solvent such as water. The concentration of the solution is preferably in the range of, for example, 10 to 40% by mass. As for the ratio of Ni / Co / Mn, it is preferably in the ratio of 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, examples of raw materials containing Co include sulfates such as CoSO4, and examples of raw materials containing Mn include sulfates such as MnSO4.

[0032] (2) Step of adding the solution to an alkaline solution to precipitate hydroxides Next, add the solution to the alkaline solution to precipitate hydroxides. As a result, particles of hydroxides containing Ni, Co, and Mn are crystallized, and these particles are obtained as a precipitate. In this step, for example, while controlling the alkaline solution in which the hydroxides have precipitated to a certain pH (for example, pH 10 - 12), the solution and NH3 are added dropwise so that the hydroxides of transition metals precipitate.

[0033] (3) Step of collecting the precipitate from the alkaline solution Next, collect the precipitate from the alkaline solution. Examples of the method for collecting the precipitate particles include, for example, filtration and washing with water. First, a method of taking out the precipitate (particles) by filtration, washing with water, and then filtering the washed liquid to take out the precipitate (particles) can be mentioned. Note that the precipitate (particles) after washing with water may be further dried.

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

[0035] Examples of the raw material containing Li include Li2CO3 and LiOH. Examples of the raw material containing at least one element selected from the aforementioned group X (that is, at least one element selected from the group consisting of Ba, Pr, La, Y, Sr, Ce, Se, Hf, Rh, Zr, Sn, and Mg) and at least one element selected from the aforementioned group Y (that is, 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 (for example, BaO, Pr2O3, La2O3, SrO, W2O3, MoO3, and NbO).

[0036] (5) 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 step firing process is performed in which the following firing treatments (a) to (c) are sequentially performed. (a) Low-temperature firing treatment in which firing is performed at a temperature of 400°C or higher and 600°C or lower (b) Medium-temperature firing treatment in which firing is performed at a temperature of 500°C or higher and 800°C or lower and at a temperature higher than the temperature in the low-temperature firing treatment (c) High-temperature firing treatment in which firing is performed at a temperature of 600°C or higher and 1000°C or lower and at a temperature higher than the temperature in the medium-temperature firing treatment

[0038] By undergoing the above-described stepwise firing process, crystal growth within the particles of the positive electrode active material can be promoted, and the crystallite size can be increased to 300 nm or more.

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

[0040] The firing is preferably performed in an oxygen atmosphere. In order to obtain a positive electrode active material having a predetermined particle size, the fired mixture may be pulverized. Examples of the pulverization method include pulverization using a pulverizer (e.g., jet mill).

[0041] By passing 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 preferably used in a lithium ion battery. The battery has, 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 battery having a solid electrolyte or a liquid battery having a liquid electrolyte, but a liquid battery is preferred. Further, it may be a bipolar type battery having a positive electrode active material layer and a negative electrode active material layer on both surfaces of a current collector having the functions of 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 membrane. The separator electrically isolates the positive electrode and the negative electrode. The battery according to the embodiment of the present disclosure may further be a liquid-based battery having an electrolyte. Particularly, a non-aqueous electrolyte is preferred. Examples of the uses of the battery include power sources for hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), battery electric vehicles (BEV), and the like.

Examples

[0044] Hereinafter, the present disclosure will be described based on examples, but the present disclosure is not limited to these examples at all.

[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% by mass.

[0046] ·Crystallization A certain amount of aqueous ammonia solution was placed in a reaction vessel and purged with nitrogen while stirring with a stirrer. NaOH was added to the reaction vessel to make the pH alkaline. Subsequently, while controlling the inside of the reaction vessel to a certain pH (pH 10 - 12), a raw material solution and ammonia were dropped to precipitate a transition metal hydroxide.

[0047] ·Washing with water, filtration, drying The precipitated transition metal hydroxide was taken out by filtration, ion-exchanged water was added, and it was dispersed by stirring with a spoon and washed with water. Subsequently, the washed liquid was filtered to take out the transition metal hydroxide. Then, the filtered transition metal hydroxide was dried at 120 °C for 16 hours to evaporate the moisture.

[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 the raw material of additive element 1, and Al₂O₃ as the raw material of additive element 2 were mixed in a mortar.

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

[0050] Subsequently, the fired mixture was pulverized with a pulverizer (jet mill) to be crushed to a predetermined particle size. Thus, the positive electrode active material of Example 1 was obtained.

[0051] The positive electrode active material of Example 1 contained Li, Ni, Co, Mn, O, Mg, and Al, and its ratio (mass ratio) was the ratio shown in Table 1. Also, the obtained positive electrode active material had a plurality of primary particles aggregated to form secondary particles, and the average number of primary particles constituting one secondary particle was 5 or less.

[0052] <Examples 2 - 6> The raw materials of additive element 1 in Example 1 were changed from MgO to La2O3 (Example 2), SrO (Example 3), and Pr2O3 (Example 4), and the raw materials of additive element 2 were changed from Al2O3 to W2O3 (Examples 2 and 4), and NbO (Example 3). The positive electrode active materials of each example were obtained in the same manner as in Example 1, except for the above changes. The elements contained in the positive electrode active material of each example and their ratios (mass ratios) are shown in Table 1. Further, the obtained positive electrode active material had a plurality of primary particles aggregated to form secondary 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 additive element 1 and additive element 2 in Example 1 were not added, and the firing conditions were changed to firing for 10 hours in an oxygen atmosphere at a temperature of 900°C. The elements contained in the positive electrode active material of Comparative Example 1 and their ratios (mass ratios) are shown in Table 1. Further, the obtained positive electrode active material had a plurality of primary particles aggregated to form secondary particles, and the average number of primary particles constituting one secondary particle was 5 or less.

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

[0055] [Fabrication of Cells] Cells were fabricated using the positive electrode active materials obtained in each example and each comparative example. ·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] · Fabrication of electrodes Using a film applicator with a film thickness adjustment function (All Good Co., Ltd.), the positive and negative electrodes were coated on the current collector and dried at 80 °C for 5 minutes in a dryer to fabricate the cell.

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

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

[0059] Note that the "Synthesis Method 2" shown in Table 1 means a synthesis method having a step firing process in which a low-temperature firing process of firing at a temperature of 400°C or higher and 600°C or lower, a medium-temperature firing process of firing at a temperature of 500°C or higher and 800°C or lower and higher than the temperature in the low-temperature firing process, and a high-temperature firing process of firing at a temperature of 600°C or higher and 1000°C or lower and higher than the temperature in the medium-temperature firing process are performed in this order. On the other hand, the "Synthesis Method 1" means a synthesis method not having the above step firing process.

[0060]

Table 1

[0061] As shown in Table 1, it can be seen that the positive electrode active materials of the examples in which the crystallite size falls within a specific range are superior in battery capacity retention compared to the positive electrode active materials of the comparative examples in which the crystallite size is below the specific range.

Claims

1. Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ A positive electrode active material having a composition represented by and having a crystallite size in the primary particles of 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, 1.5 ≦ y ≦ 2.1.)

2. The 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 of the secondary particles is 5 or less, according to Claim 1.

3. 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 in the above composition. 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. The positive electrode active material according to Claim 3, comprising at least one element selected from the group X and at least one element selected from the group Y (provided that the element selected from the group X is not only Zr and the element selected from the group Y is not only Zr, and the element selected from the group X is not only Sn and the element selected from the group Y is not only Sn).

5. A step of mixing a raw material containing Ni, Co, and Mn respectively, and a raw material containing Li to obtain a mixture; A step firing process in which the mixture is subjected to a low-temperature firing treatment at a temperature of 400°C or higher and 600°C or lower, a medium-temperature firing treatment at a temperature of 500°C or higher and 800°C or lower and higher than the temperature in the low-temperature firing treatment, and a high-temperature firing treatment at a temperature of 600°C or higher and 1000°C or lower and higher than the temperature in the medium-temperature firing treatment, in this order; (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, 1.5 ≦ y ≦ 2.1.) Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ A method for producing a positive electrode active material, which produces a positive electrode active material having a composition represented by (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, 1.5 ≦ y ≦ 2.1.)

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