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

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.