Active material, and positive electrode mixture and solid-state battery that use said active material

An active material for use in a solid-state battery, exhibiting at least one peak in a range of from 0.145 nm to 0.185 nm and at least one peak in a range of from 0.28 nm to 0.31 nm in a radial distribution function obtained through measurement of an X-ray absorption fine structure of the active material,

• wherein the active material comprises a core particle and a coating layer located on a surface of the core particle,
• the core particle comprises a lithium-metal complex oxide,
• the lithium-metal complex oxide comprises a layered rock salt-type compound or a spinel-type compound,
• the coating layer comprises an oxide containing Li and Nb,
• the active material has a BET specific surface area A of 0.5 m2/g or more and 5.0 m2/g or less,
• a ratio B/A is from 308 to 778 when the lithium-metal complex oxide is a layered rock salt-type compound, and from 223 to 1257 when the lithium-metal complex oxide is a spinel-type compound, wherein A represents the BET specific surface area and B represents a moisture content (mass ppm) as measured up to 110° C. by the Karl-Fischer method,
• a proportion of the oxide contained in the active material, in terms of a ratio of a mass of niobium to a mass of the active material is at least 0.01% by mass and at most 3% by mass, and
• an amount of carbonic acid ions on a surface of the active material is less than 2.0% by mass, based on the active material.