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

[0002] Conventionally, positive electrode active materials containing various additive elements have been used as positive electrode active materials for batteries having excellent resistance characteristics. In such positive electrode active materials for batteries, the particle shape of the positive electrode active material has been controlled.

[0003] For example, Patent Document 1 discloses a lithium manganese-based composite oxide having an average minor axis of 0.08 μm or more and 0.8 μm or less, an average major axis 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] JP 2014-139128 A Summary of the Invention [Problem to be solved by the invention]

[0005] When forming a positive electrode active material layer in a battery, the positive electrode active material has been conventionally pressed to fix it in place. However, when the positive electrode active material is pressed, a strong force is locally applied to the particles of the positive electrode active material, which may cause cracks in the particles, resulting in a decrease in the durability of the battery, and for example, a decrease in the battery capacity after repeated charge and discharge cycles. Therefore, there is a demand for batteries in which particle cracking in the positive electrode active material is suppressed, resulting in high durability.

[0006] The present disclosure has been made in consideration of the above circumstances, and has an object 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 problem]

[0007] Means for solving the above problems include the following aspects. <1> Li x Ni a Co b Mn c M d O y The positive electrode active material particles have a composition represented by The positive electrode active material has a sphericity standard deviation (Z) of 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, 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 sphericity standard deviation (Z) is 0.090≦Z≦0.800; <1> The positive electrode active material according to claim 1, <3> The present invention has a first active material particle having a coating material containing a compound of the element represented by M on its surface, and a second active material particle having no coating material on its surface. <1> or <2> The positive electrode active material according to claim 1, <4> <1> ~ <3> 13. A battery comprising the positive electrode active material according to claim 12. Effect of the Invention

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

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a positive electrode active material according to an embodiment of the present disclosure. [Diagram 2]FIG. 1 is a schematic cross-sectional view showing a conventional positive electrode active material layer. [Diagram 3] FIG. 1 is a schematic cross-sectional view showing a state in which a conventional positive electrode active material layer is pressed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] <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 M d O y The positive electrode active material particles have a composition represented by the following formula: and the standard deviation of sphericity (Z) 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, 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.)

[0011] According to the positive electrode active material according to the embodiment of the present disclosure, particle cracking 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 for use in a battery, the positive electrode active material has conventionally been pressed (applied pressure) to fix it in place. However, when the positive electrode active material is pressed, a strong force is applied locally to the particles of the positive electrode active material, which can cause cracks in the particles, resulting in a decrease in the durability of the battery (for example, a decrease in the battery capacity after repeated charge / discharge cycles). This is believed to be because the standard deviation of sphericity (Z) of particles in conventional positive electrode active materials is low (specifically, Z is less than 0.018). A low standard deviation of sphericity (Z) of particles in a positive electrode active material indicates that the particles of the positive electrode active material have uniform sphericity. If the particles of the positive electrode active material on the current collector have uniform sphericity, the gaps between the particles of the positive electrode active material become large, that is, the packing property becomes low. If the positive electrode active material is pressed in such a state where the gaps between the particles are large, a strong force is applied locally to the particles, and cracks occur in the positive electrode active material particles. It is believed that a battery having cracked positive electrode active material particles has poor durability.

[0013] In contrast, the positive electrode active material according to the embodiment of the present disclosure has a high standard deviation (Z) of sphericity of the positive electrode active material particles of 0.018 or more, which means that the sphericity is not uniform and positive electrode active material particles with different sphericity are included. The presence of positive electrode active material particles with different sphericity in this way fills the gaps between the particles of the positive electrode active material, making the gaps smaller, that is, increasing the packing property. Even if the positive electrode active material with high packing property is pressed, the application of strong force locally 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, the positive electrode active material according to the embodiment of the present disclosure will be described by giving a specific example and using the drawings.

[0015] First, examples of the conventional positive electrode active material having a low particle sphericity standard deviation (Z) (specifically, Z is less than 0.018) include positive electrode active materials having particles with 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 state in which the conventional positive electrode active material layer has been pressed. The positive electrode active material layer 20 shown in Fig. 2 is formed by stacking positive electrode active material particles 20A on a current collector 4. The positive electrode active material particles 20A are active material particles having a coating 220 containing a compound of the element represented by M (hereinafter also simply referred to as "M element") on the surface. All of the positive electrode active material particles 20A included in the positive electrode active material layer 20 are active material particles having a coating 220 containing a compound of the M element on the surface, and the standard deviation of sphericity (Z) of the positive electrode active material particles 20A is low and is less than 0.018. In a positive electrode active material layer 20 including only positive electrode active material particles 20A as an active material with a low standard deviation of sphericity (Z), the gaps between the positive electrode active material particles 20A become large as shown in Fig. 2. When this positive electrode active material layer 20 is pressed in the direction of arrow A as shown in Fig. 3, a strong force is locally applied to the positive electrode active material layer 20, which may cause 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 an embodiment of the present disclosure is shown in Fig. 1. Fig. 1 is a schematic cross-sectional view showing an example of a positive electrode active material according to an embodiment of the present disclosure. The positive electrode active material layer 2 shown in Fig. 1 is formed by stacking first active material particles 2A and second active material particles 2B on a current collector 4. The first active material particles 2A are active material particles having a coating 22 containing a compound of an element represented by M (M element) on their surfaces, and the second active material particles 2B are active material particles not having a coating 22 on their surfaces. 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 having a coating 22 on their surfaces and particles not having a coating 22 on their surfaces, and the sphericity standard deviation (Z) of the active material particles is high (specifically, 0.018 or more). In the positive electrode active material layer 2 containing active material particles with a high standard deviation of sphericity (Z), the gaps between the active material particles are small, as shown in Fig. 1. Therefore, even when the positive electrode active material layer 2 is pressed (pressurized), the application of a strong force locally to the active material particles is suppressed, and the occurrence of cracks in the active material (i.e., the first active material particles 20A and the second active material particles 20B) is suppressed.

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

[0018] (composition) The positive electrode active material according to the embodiment of the present disclosure is Li x Ni a Co b Mn c M d O y The positive electrode active material particles have 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, 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 Li ratio x is from 0.1 to 1.5, preferably from 0.3 to 1.4, and more preferably from 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 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 d of M is 0 or more and 0.1 or less, preferably 0.01 or more and 0.09 or less, and more preferably 0.03 or more and 0.07 or less. The total ratio 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)) In the positive electrode active material according to the embodiment of the present disclosure, the positive electrode active material particles have a sphericity standard deviation (Z) of Z ≧ 0.018. By having a sphericity standard deviation (Z) of Z ≧ 0.018, cracks occurring in the positive electrode active material particles are suppressed, and when the positive electrode active material is used in a battery, the durability of the battery can be improved. The standard deviation of sphericity (Z) of the positive electrode active material particles is preferably Z≧0.090, and more preferably Z≧0.100, from the viewpoint of suppressing cracking of the positive electrode active material particles. In addition, the upper limit of the sphericity standard deviation (Z) of the positive electrode active material particles is not particularly limited, but from the viewpoint of improving the function as a positive electrode, it is preferable that Z≦0.800, and more preferably Z≦0.650.

[0021] [Measurement of sphericity standard deviation (Z)] In the positive electrode active material, the standard deviation of sphericity (Z) of the positive electrode active material particles is measured by the following method. First, an image of the positive electrode active material is obtained by a scanning electron microscope (SEM, 5000 to 20000 magnifications), and the sphericity of the particles in the image is calculated using particle analysis software Expvrer4 (Kyokuto Boeki Co., Ltd.). This is repeated until sphericity data for 1000 particles is obtained. Next, the standard deviation is calculated from the sphericity calculated for the 1000 particles, and the sphericity standard deviation (Z) is obtained.

[0022] A positive electrode active material having particles with a standard deviation of sphericity (Z) of Z≧0.018 (that is, a positive electrode active material with a high standard deviation of sphericity (Z)) can be produced, for example, by the following method. (1) A method of obtaining a positive electrode active material by controlling the manufacturing method of active material particles to produce active material particles having a wide distribution of sphericity (i.e., having various degrees of sphericity). (2) A method in which two or more types of active material particles with different sphericity (average sphericity) are produced and then these two or more types of active material particles are mixed to obtain a positive electrode active material.

[0023] Here, an example will be described of obtaining a positive electrode active material according to an embodiment of the present disclosure by the above method (2) (a method of producing two or more types of active material particles having different sphericity (average sphericity) and then mixing these two or more types of active material particles to obtain a positive electrode active material).

[0024] (First active material particles and second active material particles) The positive electrode active material according to the embodiment of the present disclosure is preferably, for example, a positive electrode active material having first active material particles having a coating containing a compound of an element represented by M (M element) on the surface thereof, and second active material particles not having the coating on the surface thereof. A positive electrode active material having a first active material particle having a coating material containing a compound of M element on its surface and a second active material particle having no coating material on its surface has a configuration similar to that of the positive electrode active material contained in the positive electrode active material layer 2 shown in Fig. 1. As shown in Fig. 1, a first active material particle 2A having a coating material 22 containing a compound of M element on its surface has a distorted shape due to the presence of the coating material 22 on its surface, while a second active material particle 2B having no coating material 22 on its surface has a shape close to a sphere. Therefore, the standard deviation of sphericity (Z) of the positive electrode active material as a whole becomes large.

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

[0026] In addition, when the positive electrode active material has first active material particles having a coating on their surfaces and second active material particles having no coating on their surfaces, 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] In addition, in the case where the positive electrode active material has first active material particles having a coating on the surface thereof and second active material particles having no coating on the surface thereof, the composition of each particle will be described. The first active material particles having a coating on the surface thereof are x1 Ni a1 Co b1 Mn c1 M d1 O y1 It is preferable that the composition is represented by the following formula: (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, 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.)

[0028] The second active material particles having no coating on the surface are Li x2 Ni a2 Co b2 Mn c2 O y2 It is preferable that the composition is represented by the following formula: (In the composition of the second active material particles, 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] 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 the surface thereof, and second active material particles 2B having no coating 22 on the surface thereof, but the present disclosure is not limited to this embodiment. For example, in addition to the first active material particles and the second active material particles, the positive electrode active material may further include active material particles having a different sphericity (average sphericity) from the first active material particles and the second active material particles, that is, the positive electrode active material may include three or more types of active material particles having different sphericities (average sphericities).

[0030] (Method of manufacturing positive electrode active material) Here, an example of a method for producing a positive electrode active material according to an embodiment of the present disclosure will be described. In the following, as an example, a method for producing a positive electrode active material having a first active material particle having a coating material containing a compound of M element on its surface and a second active material particle having no coating material on its surface will be described.

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

[0032] Preparation of second active material particles without coating First, a method for preparing the second active material particles not having the coating on their surfaces will be described. The second active material particles can be prepared, for example, through 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 dissolution) (2) adding the solution to an alkaline solution to precipitate hydroxide (crystallization) (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 (Li raw material addition) (5) A firing step of firing the mixture (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 in which raw materials each containing Ni, Co, and Mn are dissolved. A solution is prepared in which a raw material containing Ni, a raw material containing Co, and a raw material containing Mn are dissolved. For example, a solution can be prepared by dissolving a raw material containing Ni, a raw material containing Co, and a raw material containing Mn in a solvent such as water. The concentration of the solution is preferably in the range of, for example, 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.

[0034] 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.

[0035] (2) A process of adding the solution to an alkaline solution to precipitate hydroxides Next, the solution is added to an alkaline solution to precipitate the hydroxide. As a result, particles containing hydroxides containing Ni, Co, and Mn are crystallized, and the particles are obtained as precipitates. 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 dropped, and the hydroxides of the transition metals are precipitated.

[0036] (3) A process for collecting the precipitate from the alkaline solution The precipitate is then collected from the alkaline solution. Examples of methods for collecting the precipitate particles include filtration and washing with water. First, the precipitate (particles) is taken out by filtration, washed with water, and the washed liquid is further filtered to take out the precipitate (particles). The precipitate (particles) after washing with water may be further dried.

[0037] (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. For example, the collected precipitate particles and the raw material containing Li can be mixed in a mortar. Examples of raw materials containing Li include Li2CO3 and LiOH.

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

[0039] In order to make the mixture have a predetermined particle size, the mixture after firing may be crushed, for example, by a crusher (such as a jet mill).

[0040] By going through these steps (1) to (5), it is possible to obtain second active material particles that have no coating on the surface.

[0041] Preparation of first active material particles having a coating In the above-mentioned "Preparation of second active material particles having no coating", by changing "(4) the step of mixing the precipitate with a raw material containing Li to obtain a mixture" to "(4') the step of mixing the precipitate with a raw material containing Li and a raw material containing M element to obtain a mixture" shown below, it is possible to prepare the first active material particles.

[0042] (4') A step of mixing the precipitate, a raw material containing Li, and a raw material containing M element to obtain a mixture. Next, a mixture is obtained by mixing the collected precipitate (particles), a raw material containing Li, and a raw material containing the element M. For example, the collected precipitate particles, the raw material containing Li, and the raw material containing the element M can be mixed in a mortar. Examples of raw materials containing Li 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) and hydroxides of each element (e.g., H3BO4).

[0043] Mixing the first active material particles with the second active material particles Next, the resulting first and second active material particles are mixed together to obtain the positive electrode active material according to an embodiment of the present disclosure.

[0044] In the above description of the manufacturing method, the positive electrode active material having two types of active material particles, the first active material particles having a coating containing a compound of M element on the surface and the second active material particles having no coating on the surface, is shown, but the present disclosure is not limited to this embodiment. For example, in addition to the first active material particles and the second active material particles, active material particles having a different sphericity (average sphericity) from the first active material particles and the 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 the present disclosure.

[0045] <Battery> A battery according to an embodiment of the present disclosure includes a positive electrode active material according to an embodiment of the present disclosure. 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, a liquid battery having a liquid electrolyte, or 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. The battery according to the embodiment of the present disclosure may be a liquid-based battery further comprising an electrolyte, with a non-aqueous electrolyte being particularly preferred.

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

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

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

[0049] (Synthesis of active material particles A) ·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%.

[0050] 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 controlling the pH inside the reaction vessel to a certain value (pH 10-12), and transition metal hydroxides were precipitated.

[0051] Washing, filtering, drying The precipitated transition metal hydroxide was removed by filtration, and ion-exchanged water was added thereto, and the mixture was stirred with a spoon to disperse the hydroxide, and then washed with water. The washed liquid 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.

[0052] Lithium raw material mixing The dried transition metal hydroxide was mixed with Li2CO3 and LiOH as Li raw materials in a mortar.

[0053] · Calcination and crushing The mixture of the transition metal hydroxide and the Li raw material was calcined in a calcination furnace (muffle furnace) at 800°C to 1100°C in an oxygen atmosphere for 10 hours. The calcined mixture was then pulverized in a pulverizer (jet mill) to a predetermined particle size. In this manner, active material particles A were obtained.

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

[0055] Mixture of Li and M raw materials The dried transition metal hydroxide, Li2CO3 and LiOH as Li sources, and H3BO4 as M source were mixed in a mortar.

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

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

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

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

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

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

[0062] In the present embodiment, the method of separately synthesizing active material particles synthesized with the addition of raw material M and active material particles synthesized without adding raw material M as in Examples 1 to 8, and then mixing both active material particles to obtain a positive electrode active material, is referred to as "synthesis method 1." In addition, in the present embodiment, a method in which only active material particles synthesized without adding the M raw material or only active material particles synthesized with the addition of the M raw material are synthesized and used as a positive electrode active material, as in Comparative Examples 1 to 3, is referred to as "synthesis method 2."

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

[0064] First, an image of the positive electrode active material was taken by SEM (magnification of 5000 to 20000), and the sphericity of the particles in the image was calculated using particle analysis software Expvrer4 (Kyokuto Boeki Co., Ltd.). This was repeated until sphericity data for 1000 particles was obtained. Next, the standard deviation was calculated from the sphericity calculated for the 1000 particles to obtain the sphericity standard deviation (Z).

[0065] [Cell Preparation] A cell was fabricated using the positive electrode active material 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%)

[0066] Preparation of electrodes A positive electrode and a negative electrode were applied onto 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.

[0067] [Measurement of resistance increase rate after cycling] The battery capacity of the cells obtained in each of the Examples and Comparative Examples was measured before and after cycling under the following test conditions. The results of the ratio of the battery capacity after cycling (capacity retention rate (%)) to the battery capacity before cycling is set as "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: Charge and discharge 300 cycles between SOC 0% and 100% at 60°C and 2C rate.

[0068] [Table 1]

[0069] As shown in Table 1, the positive electrode active materials of the examples having a high sphericity standard deviation (Z) of 0.018 or more maintained a high capacity retention rate after cycling compared to the positive electrode active materials of the comparative examples having 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 particle, 2B second active material particle, 20A positive electrode active material particle, 22, 220 coating, 4 current collector, 6 crack

Claims

1. Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ O ｙ The positive electrode active material particles have a composition represented by The positive electrode active material has a sphericity standard deviation (Z) of 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. 2. The positive electrode active material according to claim 1, wherein the sphericity standard deviation (Z) is 0.090≦Z≦0.

800.

3. 2. The positive electrode active material according to claim 1, comprising: first active material particles having a coating containing a compound of the element represented by M on their surfaces; and second active material particles not having the coating on their surfaces.

4. A battery comprising the positive electrode active material according to any one of claims 1 to 3.

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