Positive electrode active material for all-solid-state lithium ion secondary battery and method for manufacturing the same

1. A positive electrode active material for an all-solid-state lithium ion secondary battery, comprising a lithium-nickel composite oxide particle and a coating layer coating a surface of the particle, wherein

the lithium-nickel composite oxide particle

has a crystal structure belonging to a space group R-3m,

contains at least Li, Ni, an element M, and Nb,

a molar ratio among the elements being represented by Li:Ni:M:Nb=a:(1-x-y):x:y

(0.98≤ a≤1.15,

0<x≤0.5,

0<y≤0.03,

0<x+y≤0.5, and

the element M is at least one selected from the group consisting of Co, Al, Mn, Zr, Si, Zn, and Ti),

has a crystallite diameter of 140 nm or less as calculated by a Scherrer method from a diffraction peak attributed to a (003) plane measured by XRD, and

has an eluted lithium ion amount of 0.30% by mass or more and 1.00% by mass or less with respect to a total amount of the lithium-nickel composite oxide particles as determined by neutralization titration, and

the coating layer is a composite oxide containing Li and at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W.

2. The positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 1 , wherein

the lithium-nickel composite oxide particles

include a secondary particle formed by aggregation of a plurality of primary particles,

have a porous structure having a plurality of voids in which no primary particle is present in the secondary particle, and

have a specific surface area of 0.3 m 2 /g or more and 2.0 m 2 /g or less as measured by a nitrogen adsorption BET method.

3. The positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 2 , wherein at least a part of niobium contained in the lithium-nickel composite oxide particles is segregated at an interface between the primary particles.

4. The positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 1 , wherein

the lithium-nickel composite oxide particles have a particle size (D50) corresponding to an integrated volume ratio of 50% in an integrated volume distribution curve of particle size distribution of 7 μm or less.

5. The positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 1 , wherein

the coating layer has an average thickness of 2 nm or more and 15 nm or less.

6. A method for manufacturing the positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 1 , the method comprising:

mixing a nickel composite compound, a niobium compound, and a lithium compound to obtain a mixture;

firing the mixture to obtain the lithium-nickel composite oxide particles; and

forming the coating layer by attaching a coating liquid containing at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W to surfaces of the lithium-nickel composite oxide particles.

7. The method for manufacturing a positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 6 , wherein

the nickel composite compound contains a nickel composite oxide, and

the method further includes oxidation-roasting a nickel composite hydroxide adjusted by a crystallization reaction to obtain the nickel composite oxide.

8. The method for manufacturing a positive electrode active material for an all-solid-state lithium ion secondary battery according to claim 6 , the method further comprising heat-treating the lithium-nickel composite oxide particles having the coating layer formed on surfaces of the particles at 300° C. or higher after the coating.