Positive electrode active material and method for producing the same

Abstract

To provide a positive electrode active material which can reduce resistance of a battery when used in the battery.SOLUTION: The positive electrode active material has a composition represented by LixNiaCobMncOy, has a compound A containing La and Ni and a compound B containing Li and W on the surface of primary particles, and contains La and W inside the primary particles. In the composition, 0.1≤x≤1.5, 0.5≤a≤1.0, 0≤b≤0.3, 0≤c≤0.3, a+b+c=1.0 and 1.5≤y≤2.1 are satisfied.SELECTED DRAWING: None

Description

[Technical Field]

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

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

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

[0004] Batteries are required to have low resistance, but in batteries equipped with a positive electrode containing a positive electrode active material, the resistance can sometimes be high due to the positive electrode active material.

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

[0006] Means for solving the above problems include the following aspects. <1> Li x Ni a Co b Mn c O y The positive electrode active material has a composition represented by the following formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.) <2> The compound A is a granular compound, and the compound B is a layered compound. <1> The positive electrode active material according to claim 1. <3> The compound A includes at least one of La4LiNiO8 and LaNiO3, and the compound B includes Li6WO6. <1> or <2> The positive electrode active material according to claim 1. <4> a step of mixing raw materials each containing Ni, Co, and Mn, a raw material containing Li, a raw material containing La, and a raw material containing W to obtain a mixture; a stepwise firing process in which the mixture is subjected to a high-temperature firing process at a temperature of 600°C or more and 1000°C or less, a low-temperature firing process at a temperature of 400°C or more and 600°C or less, and a medium-temperature firing process at a temperature of 500°C or more and 800°C or less, which is lower than the temperature in the high-temperature firing process but higher than the temperature in the low-temperature firing process, in this order; Li x Ni a Co b Mn c O y and a compound A containing La and Ni and a compound B containing Li and W on the surface of primary particles, and a method for producing a positive electrode active material having a composition represented by the following formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.) [Effects of the Invention]

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

[0008] <Cathode active material> The positive electrode active material according to the embodiment of the present disclosure is Li x Ni a Co b Mn c O y The primary particles have a composition represented by the formula: Compound A containing La and Ni and Compound B containing Li and W on the surface thereof, and La and W are contained inside the primary particles. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

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

[0010] One of the performance requirements for batteries is low resistance. However, in batteries equipped with a positive electrode containing a positive electrode active material, the positive electrode active material can cause high resistance. Therefore, there is a need to reduce the resistance of the positive electrode active material, thereby reducing the resistance of the battery.

[0011] A positive electrode active material according to an embodiment of the present disclosure has a compound A containing La and Ni and a compound B containing Li and W on the surface of primary particles, and also contains La and W inside the primary particles. That is, the particle surface has a compound A with high electron conductivity and a compound B with high Li conductivity, and the particle interior further contains La and W, thereby reducing the surface and internal resistance of the particles. By reducing the surface and internal resistance of the positive electrode active material particles in this way, using the positive electrode active material in a battery can reduce the resistance in the battery.

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

[0013] The positive electrode active material according to the embodiment of the present disclosure contains at least Li, Ni, and O, and may contain Co and Mn, and the ratio of these components is Li x Ni a Co b Mn c The positive electrode active material has a composition represented by the formula: O2. The positive electrode active material also contains La and W as additive elements. The positive electrode active material has, on the surface of the particles (primary particles), a compound A containing La and Ni, and a compound B containing Li and W. The particles also contain La and W inside. The positive electrode active material may further contain other additive elements.

[0014] (Compound A) The positive electrode active material has a compound A containing La and Ni on the surface of each particle (primary particle). The compound A containing La and Ni includes an oxide containing La and Ni, and preferably contains at least one of La4LiNiO8 and LaNiO3, for example. Compound A is preferably a granular compound, that is, the positive electrode active material preferably has granular compound A attached to the surface of particles.

[0015] (Compound B) The positive electrode active material has on the surface of the particles (primary particles) a compound B containing Li and W. The compound B containing Li and W includes an oxide containing Li and W, and preferably contains, for example, Li6WO6. Compound B is preferably a layered compound, that is, at least a part of the surface of the particles of the positive electrode active material is preferably covered with compound B in a layered form.

[0016] (inside the particle) The positive electrode active material contains La and W inside the particles (primary particles).

[0017] Thus, the positive electrode active material has compound A containing La and Ni and compound B containing Li and W on the surface of the particles (primary particles), and also contains La and W inside the particles. That is, La and Ni, which have high electron conductivity, and Li and W, which have high Li conductivity, are arranged on the surface of the particles, and La and W are also contained inside the particles, reducing the resistance on the surface and inside the particles.

[0018] Here, a method for confirming each compound and each element on the particle surface and inside the particle will be described. Regarding the method for confirming compound A and compound B on the particle surface, the presence of compound A containing La and Ni and the presence of compound B containing Li and W can be confirmed by observing the cross section of the positive electrode active material layer with a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA). Furthermore, in the present disclosure, the interior of a particle (primary particle) refers to the area within 50% of the inside of the particle, determined by defining the outline of the primary particle in an SEM image of the cross section of the positive electrode active material layer. Regarding the method for confirming La and W inside the particle, the presence of La and W can be confirmed by observing the cross section of the positive electrode active material layer with a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA) for the inside of the particle (i.e., the area within 50% of the inside of the primary particle from the outline of the primary particle).

[0019] The method for controlling the placement of compound A containing La and Ni and compound B containing Li and W on the surface of particles (primary particles) of a positive electrode active material and the incorporation of La and W into the particles is not particularly limited, but can be controlled, for example, by the following method. In the calcination process for producing a positive electrode active material, calcination at high temperature is performed, followed by stepwise calcination at low and medium temperatures. Specifically, a stepwise calcination process is preferably performed in this order: a high-temperature calcination treatment at a temperature of 600°C to 1000°C, a low-temperature calcination treatment at a temperature of 400°C to 600°C, and a medium-temperature calcination treatment at a temperature of 500°C to 800°C, which is lower than the temperature in the high-temperature calcination treatment but higher than the temperature in the low-temperature calcination treatment. The initial high-temperature calcination treatment generates compound A and compound B on the surface of the positive electrode active material particles, and then the low-temperature and medium-temperature calcination treatments allow the elements La and W to penetrate into the interior of the positive electrode active material particles. This makes it possible to obtain the positive electrode active material according to the embodiment of the present disclosure having the above-described configuration.

[0020] (composition) The positive electrode active material according to the embodiment of the present disclosure contains at least Li, Ni, and O, and may contain Co and Mn, and the ratio of these components is Li x Ni a Co b Mn c O2. The positive electrode active material also contains additive elements La and W. The positive electrode active material may further contain other additive elements. (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0021] In the composition of the positive electrode active material, from the viewpoint of reducing the resistance of the battery, the Li ratio x is 0.1 to 1.5, preferably 0.3 to 1.4, and more preferably 0.5 to 1.2. From the viewpoint of reducing the resistance of the battery, the Ni ratio a is 0.5 to 1.0, preferably 0.6 to 0.9, and more preferably 0.7 to 0.8. From the viewpoint of reducing the resistance of the battery, the Co ratio b is 0 to 0.3, preferably 0 to 0.2, and more preferably 0.1 to 0.2. From the viewpoint of reducing the resistance of the battery, the Mn ratio c is 0 to 0.3, preferably 0 to 0.2, and more preferably 0.1 to 0.2. The sum of the ratios of Ni, Co, and Mn (a+b+c) is 1.0.

[0022] In the positive electrode active material, the content (mass%) of La contained as an additive element is 0.0005 to 0.05, preferably 0.001 to 0.040, and more preferably 0.003 to 0.030, from the viewpoint of reducing the resistance of the battery. In the positive electrode active material, the content (mass%) of W contained as an additive element is 0.0005 to 0.05, preferably 0.001 to 0.040, and more preferably 0.003 to 0.030, from the viewpoint of reducing the resistance of the battery. Note that La and W refer to the amounts contained as compound A and compound B on the surface of the positive electrode active material particles, and the total amounts of both contained in the particles.

[0023] <Method of manufacturing positive electrode active material> Next, a method for producing a positive electrode active material according to an embodiment of the present disclosure will be described. The positive electrode active material according to the embodiment of the present disclosure described above can be produced by the method for producing a positive electrode active material according to an embodiment of the present disclosure shown below.

[0024] A method for producing a positive electrode active material according to an embodiment of the present disclosure includes a step of mixing raw materials containing Ni, Co, and Mn, a raw material containing Li, a raw material containing La, and a raw material containing W to obtain a mixture, and a stepwise firing step of subjecting the mixture to a high-temperature firing treatment at a temperature of 600°C to 1000°C, a low-temperature firing treatment at a temperature of 400°C to 600°C, and a medium-temperature firing treatment at a temperature of 500°C to 800°C, which is lower than the temperature in the high-temperature firing treatment but higher than the temperature in the low-temperature firing treatment, in this order. x Ni a Co b Mn c O y The positive electrode active material has a composition represented by the following formula: (In the above composition, 0.1≦x≦1.5, 0.5≦a≦1.0, 0≦b≦0.3, 0≦c≦0.3, a+b+c=1.0, and 1.5≦y≦2.1.)

[0025] The first high-temperature firing produces Compound A and Compound B on the surface of the positive electrode active material particles, and the subsequent low-temperature and medium-temperature firings allow the elements La and W to penetrate into the positive electrode active material particles, thereby obtaining the positive electrode active material according to the embodiment of the present disclosure having the above-described configuration.

[0026] The method for producing a positive electrode active material according to an embodiment of the present disclosure preferably includes the following steps (1) to (5). (1) A step of preparing a solution in which raw materials containing Ni, Co, and Mn are dissolved (raw material dissolving step) (2) A step of adding the solution to an alkaline solution to precipitate hydroxide (crystallization step) (3) collecting the precipitate from the alkaline solution (4) A step of mixing the precipitate with a raw material containing Li, a raw material containing La, and a raw material containing W to obtain a mixture (mixing step). (5) A step of firing the mixture (firing step) Each step will be described in detail below.

[0027] (1) A step of preparing a solution in which raw materials 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 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.

[0028] 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.

[0029] (2) Adding the solution to an alkaline solution to precipitate hydroxide Next, the solution is added to an alkaline solution to precipitate hydroxides. This causes the generated particles of hydroxides containing Ni, Co, and Mn to crystallize, and these particles are obtained as a precipitate. In this process, for example, the alkaline solution in which the hydroxides have precipitated is controlled to a constant pH (e.g., pH 10 to 12) while the solution and NH3 are added dropwise, thereby precipitating the transition metal hydroxides.

[0030] (3) A step of collecting the precipitate from the alkaline solution Next, the precipitate is collected from the alkaline solution. Examples of methods for collecting the precipitate particles include filtration and washing with water. First, the precipitate (particles) are collected by filtration and washed with water, and the washed solution is further filtered to collect the precipitate (particles). The precipitate (particles) after washing with water may be further dried.

[0031] (4) A step of mixing the precipitate with a raw material containing Li, a raw material containing La, and a raw material containing W to obtain a mixture. Next, a mixture is obtained by mixing the collected precipitate (particles) with a raw material containing Li, a raw material containing La, and a raw material containing W. When other additive elements are further contained in the positive electrode active material, raw materials containing the additive elements may be added. As a method of mixing, for example, a method of mixing collected precipitate particles with a raw material containing Li, a raw material containing La, and a raw material containing W in a mortar can be mentioned.

[0032] Examples of raw materials containing Li include Li2CO3 and LiOH. Examples of raw materials containing La include La2O3. Examples of raw materials containing W include W2O3.

[0033] (5) A step of firing the mixture Next, the mixture of the collected precipitate (particles) and the raw material containing Li, the raw material containing La, and the raw material containing W is fired. For example, the mixture can be fired in a firing furnace (such as a muffle furnace).

[0034] In the method for producing a positive electrode active material according to an embodiment of the present disclosure, a stepwise firing process is performed in which the following firing treatments (a) to (c) are performed in this order. (a) High-temperature firing at temperatures between 600°C and 1000°C (b) Low-temperature firing at temperatures between 400°C and 600°C (c) Medium-temperature firing, which is performed at a temperature between 500°C and 800°C, lower than the temperature in the high-temperature firing and higher than the temperature in the low-temperature firing.

[0035] The temperature in the (a) high-temperature firing treatment is 600° C. or higher and 1000° C. or lower, and from the viewpoint of reducing the resistance in the battery, it is preferably 550° C. or higher and 750° C. or lower, and more preferably 600° C. or higher and 700° C. The heating time at the above temperature in the (a) high-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of reducing the resistance in the battery, etc. The temperature in the (b) low-temperature firing treatment is 400° C. or higher and 600° C. or lower, and from the viewpoint of reducing the resistance in the battery, it is preferably 420° C. or higher and 580° C. or lower, and more preferably 450° C. or higher and 550° C. or lower. The heating time at the above temperature in the (b) low-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of reducing the resistance in the battery, etc. The temperature in the (c) medium-temperature firing treatment is 500° C. or higher and 800° C. or lower, and from the viewpoint of reducing the resistance in the battery, it is preferably 550° C. or higher and 750° C. or lower, and more preferably 600° C. or higher and 700° C. The heating time at the above temperature in the (c) medium-temperature firing treatment is preferably 1 hour or higher and 5 hours or lower, and more preferably 2 hours or higher and 4 hours or lower, from the viewpoint of reducing the resistance in the battery, etc.

[0036] The calcination is preferably carried out in an oxygen atmosphere. In order to obtain a predetermined particle size of the positive electrode active material, the calcined mixture may be crushed. Examples of the crushing method include crushing using a crusher (e.g., a jet mill).

[0037] By going through these steps, the positive electrode active material according to the embodiment of the present disclosure can be obtained.

[0038] <Battery> The positive electrode active material according to the embodiment of the present disclosure can be used in a battery, and is particularly suitable for use in a lithium-ion battery. The battery includes, for example, a negative electrode, a positive electrode, a separator, and an electrolyte. The battery according to the embodiment of the present disclosure may be a solid-state battery having a solid electrolyte or a liquid battery having a liquid electrolyte solution, but a liquid battery is preferred. The battery 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 functioning as a positive electrode current collector and a negative electrode current collector. The positive electrode includes, for example, a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. The separator is an electrically insulating porous film. The separator electrically isolates the positive electrode and the negative electrode. The battery according to the embodiment of the present disclosure may also be a liquid-based battery further having an electrolyte solution. A nonaqueous electrolyte solution is particularly preferred. Examples of applications of batteries include power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). [Example]

[0039] The present disclosure will be described below based on examples, but the present disclosure is not limited to these examples in any way.

[0040] Example 1 (Synthesis of positive electrode active material) ·Raw material solution NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to obtain a raw material solution. The ratio of Ni / Co / Mn was 1 / 1 / 1 (atm %), and the concentration of the aqueous solution was 30 mass %.

[0041] Crystallization A certain amount of NH3 aqueous solution was placed in a reaction vessel, and the atmosphere was replaced with nitrogen while stirring with a stirrer. NaOH was added to the reaction vessel to make the pH alkaline. Next, the raw material solution and NH3 were added dropwise while maintaining a constant pH (pH 10-12) in the reaction vessel, causing the transition metal hydroxide to precipitate.

[0042] Washing, filtering, drying The precipitated transition metal hydroxide was filtered out, and ion-exchanged water was added and stirred with a spoon to disperse the hydroxide, followed by washing with water. The washed solution was then filtered to remove the transition metal hydroxide. The filtered transition metal hydroxide was then dried at 120°C for 16 hours to evaporate the water.

[0043] Mixing of Li raw material and raw materials of additive elements The dried transition metal hydroxide, Li2CO3 and LiOH as Li raw materials, and La2O3 and W2O3 as additive element raw materials were mixed in a mortar. The amount of La added by La2O3 was adjusted so that La4LiNiO8 was formed on the particle surface of the positive electrode active material.

[0044] · Calcination and crushing The mixture of the transition metal hydroxide, the Li raw material, and the raw material of the additive element was fired in a firing furnace (muffle furnace). This firing was a step-by-step firing process in which a high-temperature firing treatment at 900°C, a low-temperature firing treatment at 500°C, and a medium-temperature firing treatment at 700°C were carried out in this order in an oxygen atmosphere for 3 hours each.

[0045] The fired mixture was then crushed in a crusher (jet mill) to a predetermined particle size, thereby obtaining the positive electrode active material of Example 1.

[0046] The positive electrode active material of Example 1 contained Li, Ni, Co, Mn, O, La, and W, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had, on the surfaces of the primary particles, granular compound A having the composition shown in Table 1 and layered compound B having the composition shown in Table 1. It was also confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had La and W inside the primary particles.

[0047] <Example 2> The positive electrode active materials of each example were obtained in the same manner as in Example 1, except that the amount of La2O3 added as a raw material for the additive element was adjusted to a ratio at which LaNiO3 was formed on the particle surfaces of the positive electrode active material.

[0048] The positive electrode active material of Example 2 contained Li, Ni, Co, Mn, O, La, and W, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had, on the surfaces of the primary particles, granular compound A having the composition shown in Table 1 and layered compound B having the composition shown in Table 1. It was also confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had La and W inside the primary particles.

[0049] <Comparative Example 1> The positive electrode active material of Comparative Example 1 was obtained in the same manner as in Example 1, except that La2O3 and W2O3 were not added as raw materials for the added elements, and the firing conditions were changed to firing at a temperature of 900°C in an oxygen atmosphere for 10 hours.

[0050] The positive electrode active material of Comparative Example 1 contained Li, Ni, Co, Mn, and O, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. In the obtained positive electrode active material, Compound A and Compound B were not present on the surfaces of the primary particles, and La and W were not present inside the primary particles.

[0051] <Comparative Example 2> The positive electrode active material of Comparative Example 2 was obtained in the same manner as in Example 1, except that only La2O3 was added as a raw material for the additive element, and the firing conditions were changed to firing at a temperature of 900°C in an oxygen atmosphere for 10 hours.

[0052] The positive electrode active material of Comparative Example 2 contained Li, Ni, Co, Mn, O, and La, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had granular compound A having the composition shown in Table 1 on the surfaces of the primary particles. Furthermore, La and W were not present inside the primary particles.

[0053] <Comparative Example 3> The positive electrode active material of Comparative Example 3 was obtained in the same manner as in Example 1, except that in Example 1, only W2O3 was added as a raw material for the additive element, and the firing conditions were changed to firing at a temperature of 900°C in an oxygen atmosphere for 10 hours.

[0054] The positive electrode active material of Comparative Example 3 contained Li, Ni, Co, Mn, O, and W, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had layered compound B having the composition shown in Table 1 on the surfaces of the primary particles. Furthermore, La and W were not present inside the primary particles.

[0055] <Comparative Example 4> A positive electrode active material of Comparative Example 4 was obtained in the same manner as in Example 1, except that the firing conditions were changed to firing at a temperature of 900° C. in an oxygen atmosphere for 10 hours.

[0056] The positive electrode active material of Comparative Example 4 contained Li, Ni, Co, Mn, O, La, and W, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had granular compound A having the composition shown in Table 1 and layered compound B having the composition shown in Table 1 on the surfaces of the primary particles. Furthermore, La and W were not present inside the primary particles.

[0057] <Comparative Example 5> A positive electrode active material of Comparative Example 5 was obtained in the same manner as in Example 1, except that only La2O3 was added as a raw material for the additive element.

[0058] The positive electrode active material of Comparative Example 5 contained Li, Ni, Co, Mn, O, and La, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had granular compound A having the composition shown in Table 1 on the surfaces of its primary particles. It was also confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had La inside its primary particles.

[0059] <Comparative Example 6> A positive electrode active material of Comparative Example 6 was obtained in the same manner as in Example 2, except that only La2O3 was added as a raw material for the additive element.

[0060] The positive electrode active material of Comparative Example 6 contained Li, Ni, Co, Mn, O, and La, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had granular compound A having the composition shown in Table 1 on the surfaces of its primary particles. It was also confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had La inside its primary particles.

[0061] <Comparative Example 7> A positive electrode active material of Comparative Example 7 was obtained in the same manner as in Example 1, except that only W2O3 was added as a raw material for the additive element.

[0062] The positive electrode active material of Comparative Example 7 contained Li, Ni, Co, Mn, O, and W, and the ratios (mass ratios) of Li, Ni, Co, Mn, and O were as shown in Table 1. It was confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had layered compound B having the composition shown in Table 1 on the surfaces of the primary particles. It was also confirmed by the above-mentioned confirmation method that the obtained positive electrode active material had W inside the primary particles.

[0063] [Cell preparation] Cells were fabricated using the positive electrode active materials obtained in each of the examples and comparative examples. Cell configuration Wound cylinder Positive electrode composition: Positive electrode active material / acetylene black (conductive material) / polyvinylidene fluoride = 88 / 10 / 2 (mass%) Negative electrode composition: natural graphite / styrene butadiene rubber (SBR) / carboxymethyl cellulose (CMC) Electrolyte composition: electrolyte = LiPF6 (1M), solvent = ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3 / 4 / 3 (volume%)

[0064] Electrode preparation A positive electrode and a negative electrode were applied to a current collector using a film applicator with a film thickness adjustment function (All Good Co., Ltd.), and the applied film was dried in a dryer at 80°C for 5 minutes to prepare a cell.

[0065] [Initial resistance] The initial battery resistance was measured for the cells obtained in each example and comparative example. The percentages (%) of the battery resistance for each example and comparative example, with the battery resistance for Comparative Example 1 being set as "100%", are shown in Table 1.

[0066] The "ratio" of compound A shown in Table 1 means the ratio of the total amount of La contained in the positive electrode active material, and the "ratio" of compound B means the ratio of the total amount of W contained in the positive electrode active material.

[0067] Note that "Synthesis method 2" in Table 1 refers to a synthesis method having a stepwise firing process in which a high-temperature firing process is performed at a temperature of 600°C to 1000°C, which is higher than the temperature in the medium-temperature firing process, a low-temperature firing process is performed at a temperature of 400°C to 600°C, and a medium-temperature firing process is performed at a temperature of 500°C to 800°C, which is higher than the temperature in the low-temperature firing process, in this order. On the other hand, "Synthesis method 1" refers to a synthesis method that does not have the above stepwise firing process.

[0068] [Table 1]

[0069] As shown in Table 1, the positive electrode active materials of the examples, which have compound A containing La and Ni and compound B containing Li and W on the surface of the primary particles and contain La and W inside the primary particles, have reduced initial resistance of the battery compared to the positive electrode active materials of the comparative examples, which do not satisfy these requirements.

Claims

1. Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ and a compound A containing La and Ni and a compound B containing Li and W on the surface of primary particles, and the primary particles contain La and W inside. (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 claim 1 , wherein the compound A is a granular compound and the compound B is a layered compound.

3. The compound A is La ４ LiNiO ８ and LaNiO ３ and the compound B contains Li ６ WO ６ The positive electrode active material of claim 1 , comprising:

4. a step of mixing raw materials each containing Ni, Co, and Mn, a raw material containing Li, a raw material containing La, and a raw material containing W to obtain a mixture; a stepwise firing process in which the mixture is subjected to a high-temperature firing process at a temperature of 600°C or more and 1000°C or less, a low-temperature firing process at a temperature of 400°C or more and 600°C or less, and a medium-temperature firing process at a temperature of 500°C or more and 800°C or less, which is lower than the temperature in the high-temperature firing process but higher than the temperature in the low-temperature firing process, in this order; Li ｘ Ni ａ Co ｂ Mn ｃ O ｙ and a compound A containing La and Ni and a compound B containing Li and W on the surfaces of primary particles, and a method for producing a positive electrode active material containing La and W inside the primary particles. (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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