Solid state secondary battery and positive electrode active material

Abstract

The initial capacity and cycle durability are improved in a secondary battery which uses a sulfide solid electrolyte containing sulfur and phosphorus and high nickel-type positive electrode active material. In the secondary battery having a power generating element containing, laminated in this order, a positive electrode with a positive electrode active material layer containing a positive electrode active material composed of a so-called high nickel-type lithium-containing composite oxide, an electrolyte layer which contains a sulfide solid electrolyte containing sulfur and phosphorus, and a negative electrode with a negative electrode active material layer containing a negative electrode active material, one or more added element selected from the group consisting of B, P, S and Si is made to be present in a molar concentration larger than that of Ni, into a surface layer region having a depth within 100 nm from the surface of a particle of the lithium-containing composite oxide.