Positive electrode active material, and battery

Abstract

To provide a positive electrode active material in which cracking of positive electrode active material particles is suppressed.SOLUTION: A positive electrode active material includes positive electrode active material particles having a composition represented by LixNiaCobMncMdOy, the positive electrode active material particles having a standard deviation of sphericity (Z) of Z≥0.018. (In the composition, 0.1≤x≤1.5, 0.5≤a≤1.0, 0≤b≤0.3, 0≤c≤0.3, 0≤d≤0.1, a+b+c+d=1.0, 1.5≤y≤2.1, and M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material and a battery. [Background technology]

[0002] Conventionally, positive electrode active materials with various additive elements have been used as positive electrode active materials for batteries that possess excellent resistance characteristics. Furthermore, in these positive electrode active materials for batteries, the shape of the positive electrode active material particles and other characteristics are controlled.

[0003] For example, Patent Document 1 discloses a lithium manganese-based composite oxide having an average short diameter of 0.08 μm or more and 0.8 μm or less, an average long diameter of 2 μm or more and 10 μm or less, and an average aspect ratio of 8 or more and 80 or less. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2014-139128 [Overview of the project] [Problems that the invention aims to solve]

[0005] Conventionally, when forming the positive electrode active material layer in batteries, the positive electrode active material is fixed by pressing it. However, when the positive electrode active material is pressed, strong localized force is applied to the particles of the positive electrode active material, which can cause cracking of the particles. As a result, the durability of the battery may decrease, for example, the battery capacity may decrease after repeated charge-discharge cycles. Therefore, there is a demand for batteries that suppress particle breakage in the positive electrode active material, resulting in high durability.

[0006] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a positive electrode active material in which cracking of positive electrode active material particles is suppressed, and a battery having high durability. [Means for Solving the Problems]

[0007] The means for solving the above problems includes the following aspects. <1> Li x Ni a Co b Mn c M d O y The positive electrode active material includes positive electrode active material particles having a composition represented by, The positive electrode active material in which the standard deviation (Z) of the sphericity of the positive electrode active material particles is Z≧0.018. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, 0≦d≦0.1, a + b + c + d = 1.0, 1.5≦y≦2.1, and M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.) <2> The positive electrode active material according to <1>, in which the standard deviation (Z) of the sphericity is 0.090≦Z≦0.800. <3> The positive electrode active material according to <1> or <2>, comprising first active material particles having a coating containing a compound of the element represented by M on the surface, and second active material particles having no such coating on the surface. <4> A battery having the positive electrode active material according to any one of <1> to <3>. [Advantages of the Invention]

[0008] According to the present disclosure, a positive electrode active material in which cracking of positive electrode active material particles is suppressed, and a battery having high durability are provided. [Brief Description of the Drawings]

[0009] [Figure 1] It is a schematic cross-sectional view showing an example of a positive electrode active material according to an embodiment of the present disclosure. [Figure 2]It is a schematic cross-sectional view showing a conventional positive electrode active material layer. [Figure 3] It is a schematic cross-sectional view showing a state where pressing is performed on a conventional positive electrode active material layer.

Embodiments for Carrying Out the Invention

[0010] <Positive electrode active material> The positive electrode active material according to an embodiment of the present disclosure contains positive electrode active material particles having a composition represented by Li x Ni a Co b Mn c M d O y and the sphericity standard deviation (Z) of the positive electrode active material particles satisfies Z≧0.018. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, 0≦d≦0.1, a + b + c + d = 1.0, 1.5≦y≦2.1, and M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)

[0011] According to the positive electrode active material according to an embodiment of the present disclosure, cracking of particles is suppressed, and when the positive electrode active material is used in a battery, the durability of the battery can be improved.

[0012] When forming a positive electrode active material layer used in a battery, conventionally, pressing (applying pressure) is performed on a positive electrode active material to fix it. However, when pressing is performed on the positive electrode active material, a locally strong force is applied to the particles of the positive electrode active material, and cracks may occur in the particles, resulting in a decrease in the durability of the battery (for example, the battery capacity decreases after repeating charge and discharge cycles). This is thought to be because the standard deviation (Z) of particle sphericity in conventional positive electrode active materials is low (specifically, Z is less than 0.018). A low standard deviation (Z) of particle sphericity in positive electrode active materials indicates that the particles of the positive electrode active material are uniform in sphericity. When the particles of the positive electrode active material on the current collector are uniform in sphericity, the gaps between the particles of the positive electrode active material become larger, meaning that the packing efficiency decreases. When the positive electrode active material is pressed while the gaps between particles are large in this way, a strong localized force is applied to the particles, causing cracks in the positive electrode active material particles. It is thought that batteries with positive electrode active material particles that have cracked will have inferior battery durability.

[0013] In contrast, the positive electrode active material according to the embodiment of this disclosure has a high standard deviation (Z) of sphericity of positive electrode active material particles of 0.018 or higher, meaning that the sphericity is not uniform and that positive electrode active material particles with different degrees of sphericity are included. The presence of positive electrode active material particles with different degrees of sphericity fills the gaps between the particles of the positive electrode active material, reducing the size of the gaps and thus increasing the packing efficiency. Furthermore, even when pressing is performed on the positive electrode active material in a state of high packing efficiency, the application of strong localized forces to the particles is suppressed, and the occurrence of cracks in the positive electrode active material particles is suppressed. As a result, when this positive electrode active material is used in a battery, the durability of the battery can be improved (for example, the decrease in battery capacity after repeated charge-discharge cycles can be suppressed).

[0014] Here, a specific example of the positive electrode active material according to the embodiment of this disclosure will be described with reference to the drawings.

[0015] First, as a conventional positive electrode active material having a low particle sphericity standard deviation (Z) (specifically, Z being less than 0.018), examples include positive electrode active materials having particles with the shapes shown in Figures 2 and 3. Figure 2 is a schematic cross-sectional view showing a conventional positive electrode active material layer, and Figure 3 is a schematic cross-sectional view showing a conventional positive electrode active material layer after pressing. The positive electrode active material layer 20 shown in Figure 2 is formed by stacking positive electrode active material particles 20A on the current collector 4. The positive electrode active material particles 20A are active material particles having a coating 220 on their surface that contains a compound of the element represented by M (hereinafter also simply referred to as "element M"). All positive electrode active material particles 20A contained in the positive electrode active material layer 20 are active material particles having a coating 220 on their surface that contains a compound of element M, and the sphericity standard deviation (Z) of the positive electrode active material particles 20A is low, less than 0.018. In a positive electrode active material layer 20 containing only positive electrode active material particles 20A with a low sphericity standard deviation (Z), the voids between the positive electrode active material particles 20A become larger, as shown in Figure 2. Furthermore, when this positive electrode active material layer 20 is pressed in the direction of arrow A, as shown in Figure 3, a strong localized force is applied to the positive electrode active material layer 20, sometimes causing cracks 6 to occur in the positive electrode active material particles 20A.

[0016] In contrast, an example of a positive electrode active material according to the embodiment of this disclosure is shown in Figure 1. Figure 1 is a schematic cross-sectional view showing an example of a positive electrode active material according to the embodiment of this disclosure. The positive electrode active material layer 2 shown in Figure 1 is formed by stacking first active material particles 2A and second active material particles 2B on the current collector 4. The first active material particles 2A are active material particles having a coating 22 on their surface containing a compound of the element represented by M (element M) as described above, while the second active material particles 2B are active material particles without a coating 22 on their surface. The active material contained in the positive electrode active material layer 2 (i.e., the first active material particles 20A and the second active material particles 20B) is a mixture of particles with a coating 22 on their surface and particles without a coating 22 on their surface, and the sphericity standard deviation (Z) of the active material particles is high (specifically, 0.018 or higher). In the positive electrode active material layer 2, which contains active material particles with a high sphericity standard deviation (Z), the voids between the active material particles become smaller, as shown in Figure 1. Therefore, even when pressing (pressure) is applied to this positive electrode active material layer 2, the application of strong local forces to the active material particles is suppressed, and the occurrence of cracks in the active material (i.e., the first active material particle 20A and the second active material particle 20B) is suppressed.

[0017] Next, the positive electrode active material according to the embodiment of this disclosure will be described in detail.

[0018] (composition) The cathode active material according to the embodiments of this disclosure is Li x Ni a Co b Mn c M d O y It contains positive electrode active material particles having the composition represented by [formula]. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, 0≦d≦0.1, a+b+c+d=1.0, and 1.5≦y≦2.1, and M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)

[0019] In the above composition, the ratio x of Li is preferably 0.1 to 1.5, more preferably 0.3 to 1.4, and more preferably 0.5 to 1.2. 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. 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. The ratio c of Mn is between 0 and 0.3, preferably between 0 and 0.2, and more preferably between 0.1 and 0.2. The ratio d of M is 0 or greater and 0.1 or less, preferably 0.01 or greater and 0.09 or less, and more preferably 0.03 or greater and 0.07 or less. The sum of the ratios of Ni, Co, Mn, and M (a+b+c+d) is 1.0. The ratio y of O is 1.5 or more and 2.1 or less, preferably 1.7 or more and 2.1 or less, and more preferably 1.9 or more and 2.0 or less.

[0020] (Sphericity standard deviation (Z)) The positive electrode active material according to the embodiments of this disclosure has a sphericity standard deviation (Z) of positive electrode active material particles of Z ≥ 0.018. A sphericity standard deviation (Z) of Z ≥ 0.018 suppresses cracking of the positive electrode active material particles, thereby improving the durability of the battery when the positive electrode active material is used in a battery. From the viewpoint of suppressing cracking of the positive electrode active material particles, the standard deviation of sphericity (Z) is preferably Z ≥ 0.090, and more preferably Z ≥ 0.100. Furthermore, while there are no particular limitations on the upper limit of the standard deviation (Z) of the sphericity of the positive electrode active material particles, it is preferable that Z ≤ 0.800, and more preferably Z ≤ 0.650, from the viewpoint of improving the function as a positive electrode.

[0021] [Measurement of Sphericity Standard Deviation (Z)] The standard deviation (Z) of the sphericity of the positive electrode active material particles is measured by the following method. First, images of the positive electrode active material are acquired using a scanning electron microscope (SEM, 5000-20000x magnification), and the sphericity of the particles in the images is calculated using particle analysis software from Exvrer4 (Kyokuto Trading Co., Ltd.). This process is repeated until data for the sphericity of 1000 particles is obtained. Next, the standard deviation is calculated from the sphericity of the 1000 particles to obtain the sphericity standard deviation (Z).

[0022] A positive electrode active material having a sphericity standard deviation (Z) of Z ≥ 0.018 (i.e., a positive electrode active material with a high sphericity standard deviation (Z)) can be created, for example, by the following method. (1) A method for obtaining a positive electrode active material by controlling the manufacturing method of the active material particles to produce active material particles with a wide distribution of sphericity (i.e., having various degrees of sphericity). (2) A method for obtaining a positive electrode active material by first preparing two or more active material particles with different degrees of sphericity (average sphericity), and then mixing these two or more active material particles.

[0023] Here, an example will be given of how to obtain the positive electrode active material according to the embodiment of this disclosure by the method described in (2) above (a method of obtaining a positive electrode active material by mixing two or more active material particles with different sphericities (average sphericities) after producing two or more types of active material particles).

[0024] (First active material particle and second active material particle) The positive electrode active material according to the embodiments of this disclosure is preferably a positive electrode active material comprising, for example, first active material particles having a coating on its surface containing a compound of an element represented by M (element M), and second active material particles not having the coating on their surface. A positive electrode active material having first active material particles with a coating containing a compound of element M on its surface, and second active material particles without the coating on its surface, has a configuration similar to the positive electrode active material contained in the positive electrode active material layer 2 shown in Figure 1. As shown in Figure 1, the first active material particles 2A, which have a coating 22 containing a compound of element M on their surface, have an irregular shape due to the presence of the coating 22 on their surface, while the second active material particles 2B, which do not have a coating 22 on their surface, have a shape that is close to spherical. Therefore, the sphericity standard deviation (Z) of the positive electrode active material as a whole becomes large.

[0025] Therefore, the positive electrode active material according to the embodiment of this disclosure is preferably a positive electrode active material having first active material particles having a coating on its surface containing a compound of an element represented by M (element M), and second active material particles not having the coating on its surface, from the viewpoint that the sphericity standard deviation (Z) can be easily controlled to the aforementioned range, and as a result cracking of the positive electrode active material particles is suppressed.

[0026] Furthermore, if the positive electrode active material comprises first active material particles having a coating on its surface and second active material particles not having a coating on its surface, the mass ratio of the first active material particles to the second active material particles (first active material particles / second active material particles (mass%)) is preferably 10 to 90 mass%, more preferably 20 to 80 mass%, and even more preferably 40 to 60 mass%.

[0027] Furthermore, the composition of each particle in the case where the positive electrode active material comprises first active material particles having a coating on its surface and second active material particles not having a coating on its surface will be described. The first active material particle having a coating on its surface is Li x1 Ni a1 Co b1 Mn c1 M d1 O y1 It is preferable to have a composition represented by . (In the composition of the first active material particles, 0.1≦x1≦1.5, 0.5≦a1≦1.0, 0≦b1≦0.3, 0≦c1≦0.3, 0≦d1≦0.1, a1+b1+c1+d1=1.0, and 1.5≦y1≦2.1, where M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)

[0028] The second active material particles, which do not have a coating on their surface, are Li x2 Ni a2 Co b2 Mn c2 O y2 It is preferable to have a composition represented by . (In the composition of the second active material particle, 0.1 ≤ x² ≤ 1.5, 0.5 ≤ a² ≤ 1.0, 0 ≤ b² ≤ 0.3, 0 ≤ c² ≤ 0.3, a² + b² + c² = 1.0, and 1.5 ≤ y² ≤ 2.1.)

[0029] Furthermore, Figure 1 shows a positive electrode active material having two types of active material particles: first active material particles 2A having a coating 22 containing a compound of element M on its surface, and second active material particles 2B not having a coating 22 on its surface. However, the disclosure is not limited to this embodiment. For example, in addition to the first and second active material particles, the material may also contain active material particles with different degrees of sphericity (average sphericity) from the first and second active material particles, that is, it may contain three or more types of active material particles with different degrees of sphericity (average sphericity).

[0030] (Method for manufacturing positive electrode active material) Here, an example of a method for producing a positive electrode active material according to the embodiments of this disclosure will be described. In the following, as an example, a method for producing a positive electrode active material having first active material particles having a coating containing a compound of element M on its surface, and second active material particles not having the coating on its surface will be described.

[0031] The positive electrode active material having the first active material particles and the second active material particles can be manufactured, for example, by separately preparing the first active material particles and the second active material particles, and then mixing the two particles.

[0032] • Preparation of second active material particles without a coating. First, a method for preparing second active material particles that do not have the aforementioned coating on their surface will be described. The second active material particles can be prepared, for example, by following the steps (1) to (5) below. (1) Step of preparing a solution by dissolving raw materials containing Ni, Co, and Mn respectively (raw material dissolution) (2) Adding the aforementioned solution to an alkaline solution and precipitating the hydroxide (crystallization) (3) Step of collecting precipitate from the alkaline solution (4) A step of mixing the precipitate with a raw material containing Li to obtain a mixture (addition of Li raw material) (5) A firing process for firing the polymer (firing) (Note that M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)

[0033] (1) A step of preparing a solution by dissolving raw materials containing Ni, Co, and Mn, respectively. Prepare solutions by dissolving raw materials containing Ni, Co, and Mn. For example, a solution can be prepared by dissolving raw materials containing Ni, Co, and Mn in a solvent such as water. The concentration of the solution is preferably in the range of 10 to 40% by mass. The ratio of Ni / Co / Mn is preferably 1.0 / 0.8~1.2 / 0.8~1.2 (atm%) for Ni:1.0.

[0034] 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.

[0035] (2) Adding the solution to the alkaline solution and allowing the hydroxide to precipitate. Next, the solution is added to an alkaline solution to precipitate the hydroxide. This causes the hydroxide particles containing Ni, Co, and Mn to crystallize, and these particles are obtained as a precipitate. In this step, for example, the hydroxide of the transition metal is precipitated by adding the solution and NH3 dropwise to the alkaline solution in which the hydroxide has precipitated while controlling the pH to a constant level (e.g., pH 10-12).

[0036] (3) Steps to collect precipitate from alkaline solution Next, the precipitate is collected from the alkaline solution. Methods for collecting precipitate particles include, for example, filtration and washing. One method involves first removing the precipitate (particles) by filtration and washing with water, and then filtering the washed liquid to remove the precipitate (particles). The precipitate (particles) after washing may be further dried.

[0037] (4) A step of mixing the precipitate with a Li-containing raw material to obtain a mixture. Next, the collected precipitate (particles) and the Li-containing raw material are mixed to obtain a mixture. For example, the collected precipitate particles and the Li-containing raw material can be mixed in a mortar. Examples of raw materials containing lithium include Li2CO3 and LiOH.

[0038] (5) Firing process in which the mixture is fired. Next, the mixture of the collected precipitate (particles) and the Li-containing raw material is calcined. For example, the mixture can be calcined in a calcination furnace (such as a muffle furnace). The calcination conditions can be, for example, a temperature of 800°C to 1100°C, under an oxygen atmosphere, and for a time of 5 to 20 hours.

[0039] Furthermore, in order to obtain a predetermined particle size from the mixture, the mixture may be crushed after calcination. Methods of crushing include, for example, using a pulverizer (e.g., a jet mill).

[0040] By going through these steps (1) to (5), second active material particles without a surface coating can be obtained.

[0041] Preparation of first active material particles having a coating. In the aforementioned "Preparation of second active material particles without a coating," the first active material particles can be prepared by changing "(4) the step of mixing the precipitate with a Li-containing raw material to obtain a mixture" to "(4') the step of mixing the precipitate with a Li-containing raw material and a M-containing raw material to obtain a mixture," as shown below.

[0042] (4') A step of mixing the precipitate, a Li-containing raw material, and a M-containing raw material to obtain a mixture. Next, the collected precipitate (particles), the Li-containing raw material, and the M-containing raw material are mixed to obtain a mixture. For example, the collected precipitate particles, the Li-containing raw material, and the M-containing raw material can be mixed in a mortar. Examples of raw materials containing lithium include Li2CO3 and LiOH. Examples of raw materials containing M (i.e., at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn) include oxides of each element (e.g., Nb2O5, W2O3, SrO, Pr2O3, and La2O3), as well as hydroxides of each element (e.g., H3BO4).

[0043] • Mixing of first active material particles and second active material particles Next, by mixing the obtained first active material particles and second active material particles, a positive electrode active material according to the embodiment of this disclosure can be obtained.

[0044] In the above description of the manufacturing method, a positive electrode active material having two types of active material particles was shown: first active material particles having a coating containing a compound of element M on their surface, and second active material particles not having a coating on their surface. However, this disclosure is not limited to this embodiment. For example, in addition to the first and second active material particles, active material particles with a different degree of sphericity (average sphericity) from the first and second active material particles may be prepared, and these three types of active material particles may be mixed to obtain the positive electrode active material according to the embodiment of this disclosure.

[0045] <Battery> A battery according to an embodiment of this disclosure has a positive electrode active material according to an embodiment of this disclosure. The battery has, for example, a negative electrode, a positive electrode, a separator, and an electrolyte. The battery according to the embodiment of this disclosure may be a solid-state battery having a solid electrolyte, or a liquid-state battery having a liquid electrolyte. It may also 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 both a positive electrode current collector and a negative electrode current collector. The battery according to the embodiment of this disclosure may further be a liquid-type battery having an electrolyte. A non-aqueous electrolyte is particularly preferred.

[0046] (Application) Applications of batteries include, for example, power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). [Examples]

[0047] The present disclosure will be described below based on examples, but the present disclosure is not limited in any way to these examples.

[0048] <Example 1> (Synthesis of positive electrode active material) Li x Ni a Co b Mn c O y Active material particle A having a composition represented by and where x, a, b, c, and y are in the ratios shown in Table 1, and Li x Ni a Co b Mn c M d O y A positive electrode active material having active material particles B having the composition represented by , where x, a, b, c, d, and y are in the ratios shown in Table 1, and the element represented by M (element M) is one of the elements listed in Table 1, was synthesized by the method shown below.

[0049] (Synthesis of active material particle A) ·Raw material solution NiSO4, CoSO4, and MnSO4 were dissolved in deionized 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.

[0050] • Crystallization A fixed amount of NH3 aqueous solution was added to the reaction vessel, and the mixture was purged with nitrogen while stirring with a stirrer. NaOH was added to the reaction vessel to make the pH alkaline. Then, while controlling the pH of the reaction vessel to a constant level (pH 10-12), the raw material solution and NH3 were added dropwise to precipitate the transition metal hydroxide.

[0051] • Wash, filter, dry The precipitated transition metal hydroxide was removed by filtration, deionized water was added, and the mixture was dispersed by stirring with a spoon, then washed with water. Next, the rinsed solution was filtered to extract the transition metal hydroxide. Next, the filtered transition metal hydroxide was dried at 120°C for 16 hours to evaporate the water.

[0052] • Mixing of Li raw materials Dried transition metal hydroxides were mixed with Li2CO3 and LiOH as Li raw materials in a mortar.

[0053] • Firing and crushing A mixture of transition metal hydroxide and Li raw material was calcined in a muffle furnace at 800°C to 1100°C in an oxygen atmosphere for 10 hours. Subsequently, the calcined mixture was crushed to a predetermined particle size using a jet mill. In this way, we obtained the active material particle A.

[0054] (Synthesis of active material particle B) Active material particle B was obtained in the same manner as in (synthesis of active material particle A), except that the step of "mixing Li raw material" was changed to "mixing Li raw material and M raw material" as described below.

[0055] • Mixing of Li raw material and M raw material Dried transition metal hydroxides, Li2CO3 and LiOH as Li raw materials, and H3BO4 as M raw material were mixed in a mortar.

[0056] (Mixing of active material particles A and active material particles B) The obtained active material particles A and active material particles B were mixed at the particle ratio A / B (mass ratio of active material particles A to active material particles B (mass%)) shown in Table 1 to obtain the positive electrode active material of Example 1.

[0057] <Examples 2-5 and Example 7> In Example 1, the cathode active materials for each example were obtained in the same manner as in Example 1, except that the raw material M used in the synthesis of active material particle B was changed from H3BO4 to Nb2O5 (Example 2), W2O3 (Example 3), SrO (Example 4), Pr2O3 (Example 5), and La2O3 (Example 7).

[0058] <Examples 6 and 8> In Example 7, the positive electrode active material for each example was obtained in the same manner as in Example 7, except that the particle ratio A / B (mass ratio of active material particle A to active material particle B (mass%)) of active material particle A to active material particle B was changed to the particle ratio shown in Table 1.

[0059] <Comparative Example 1> The "active material particle A" synthesized in Example 1 was used as the positive electrode active material in Comparative Example 1.

[0060] <Comparative Example 2> The "active material particle B" synthesized in Example 2 was used as the positive electrode active material in Comparative Example 2.

[0061] <Comparative Example 3> The "active material particle B" synthesized in Example 3 was used as the positive electrode active material in Comparative Example 3.

[0062] In this embodiment, the method of separately synthesizing active material particles with and without the addition of the M raw material, as in Examples 1 to 8, and then mixing these two types of active material particles to obtain a positive electrode active material, is referred to as "Synthesis Method 1". Furthermore, as shown in Comparative Examples 1 to 3, a method of synthesizing only active material particles synthesized without the addition of M raw material, or only active material particles synthesized with the addition of M raw material, and using them as positive electrode active material, is referred to in this embodiment as "Synthesis Method 2".

[0063] [Measurement of Sphericity Standard Deviation (Z)] For the positive electrode active materials obtained in Examples 1-8 and Comparative Examples 1-3, the standard deviation (Z) of the sphericity of the positive electrode active material particles was measured by the following method.

[0064] First, images of the positive electrode active material were acquired using SEM (5000-20000x magnification), and the sphericity of the particles in the images was calculated using particle analysis software from Exvrer4 (Kyokuto Trading Co., Ltd.). This process was repeated until sphericity data for 1000 particles was obtained. Next, the standard deviation was calculated from the sphericity of the 1000 particles to obtain the sphericity standard deviation (Z).

[0065] [Cell creation] Cells were fabricated using the positive electrode active materials obtained in each example and comparative example. • Cell configuration Winding 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) / Carboxymethylcellulose (CMC) Electrolyte composition: Electrolyte = LiPF6 (1M), Solvent = Ethylene carbonate (EC) / Dimethyl carbonate (DMC) / Ethyl methyl carbonate (EMC) = 3 / 4 / 3 (volume %)

[0066] • Electrode fabrication A cell was fabricated by coating the positive and negative electrodes onto the current collector using a film applicator with film thickness adjustment function (Allgood Co., Ltd.), and then drying them in a dryer at 80°C for 5 minutes.

[0067] [Measurement of resistance increase rate after cycle] For 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 percentage of the battery capacity after cycling (capacity retention rate (%)) relative to the battery capacity before cycling, which is set to "100%". A capacity retention rate closer to 100% indicates better battery characteristics. Test conditions: Charge and discharge were performed 300 times between 0% and 100% SOC at 60°C and a 2C rate.

[0068] [Table 1]

[0069] As shown in Table 1, the positive electrode active materials of each example with a high sphericity standard deviation (Z) of 0.018 or higher maintained a higher capacity retention rate after cycling compared to the positive electrode active materials of each comparative example with a low sphericity standard deviation (Z) of less than 0.018. [Explanation of symbols]

[0070] 2, 20 Positive electrode active material layer, 2A First active material particles, 2B Second active material particles, 20A Positive electrode active material particles, 22, 220 Coating, 4 Current collector, 6 Crack

Claims

1. Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ O ｙ It contains positive electrode active material particles having a composition represented by, The sphericity standard deviation (Z) calculated from the sphericity of the positive electrode active material particles is Z ≥ 0.

018. A positive electrode active material comprising first active material particles having a coating on its surface containing a compound of the element represented by M, and second active material particles not having the coating on their surface. (In the above composition, 0.1 ≤ x ≤ 1.5, 0.5 ≤ a ≤ 1.0, 0 ≤ b ≤ 0.3, 0 ≤ c ≤ 0.3, 0 ≤ d ≤ 0.1, a + b + c + d = 1.0, and 1.5 ≤ y ≤ 2.1, and M represents at least one element selected from the group consisting of B, Nb, W, Sr, Pr, La, Ba, Mg, Al, Zr, Sc, Ti, Y, Hf, and Sn.)

2. The positive electrode active material according to claim 1, wherein the sphericity standard deviation (Z) is 0.090 ≤ Z ≤ 0.

800.

3. A battery having the positive electrode active material according to claim 1 or claim 2.

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