POSITIVE ELECTRODE ACTIVE MATERIAL FOR ALL-SOLID-STATE LITHIUM ION BATTERIES, POSITIVE ELECTRODE FOR ALL-SOLID-STATE LITHIUM ION BATTERIES, ALL-SOLID-STATE LITHIUM ION BATTERY, AND METHOD FOR PRODUCING POSITIVE ELECTRODE ACTIVE MATERIAL FOR ALL-SOLID-STATE LITHIUM ION BATTERIES

Provided are a positive electrode active material for all-solid lithium ion batteries, which suppresses generation of ahigh-resistivity layer at an interface with a sulfide-based solid electrolyte, has improved electron conductivity, and exhibits good output characteristics when applied to all-solid lithium ion batteries; a positive electrode for all-solid lithium ion batteries using the same; an all-solid lithium ion battery; and a method for producing a positive electrode active material for all-solid lithium ion batteries. A positive electrode active material for all-solid lithium ion batteries includes: a core positive electrode active material having a composition represented by the following formula: LiaNibCocMdO2 in which M is at least one element selected from Mn, V, Mg, Ti and Al, 1.00 ≤ a ≤ 1.02, 0.8 ≤ b ≤ 0.9, and b + c + d = 1; and a coated portion formed on a surface of the core positive electrode active material, wherein the coated portion comprises a layer of lithium niobate and a carbon layer in this order, from the surface of the core positive electrode active material.