Lithium-ion secondary batteries

Abstract

A high-capacity positive electrode active material for lithium-ion secondary batteries with excellent charge-discharge cycle characteristics is provided. The positive electrode active material comprises lithium, cobalt, oxygen, and magnesium. The positive electrode active material includes a compound with a layered rock-salt type structure and a space group R-3m. Magnesium is substituted at the lithium and cobalt positions in a composite oxide containing lithium and cobalt. The compound is a particle, and the amount of substituted magnesium is greater in the region from the particle surface to 5 nm than in the region at a depth of more than 10 nm from the particle surface. Furthermore, more magnesium is substituted at the lithium positions than at the cobalt positions.