A lithium-containing complex oxide represented by General Formula: Li.sub.1+x+.alpha.Ni.sub.(1-x-y+.delta.)/2Mn.sub.(1-x-y-.delta.)/2M.sub.y- O.sub.2 (where 0.ltoreq.x.ltoreq.0.15, -0.05.ltoreq.x+.alpha..ltoreq.0.2, 0.ltoreq.y.ltoreq.0.4; -0.1.ltoreq..delta..ltoreq.0.1 (when 0.ltoreq.y.ltoreq.0.2) or -0.24.ltoreq..delta..ltoreq.0.24 (when 0.2