A lithium ion secondary battery comprising: a positive electrode; a negative electrode; and a non-aqueous electrolyte, the positive electrode comprising a positive electrode active material and a binder, the positive electrode active material comprising a lithium-containing composite oxide represented by the chemical formula: Li.sub.a(Co.sub.1-x-yMg.sub.xM.sub.y).sub.bO.sub.c where M is at least one selected from Ni, Mn and Al, 0.ltoreq.a.ltoreq.1.05, 0.005.ltoreq.x.ltoreq.0.025, 0.ltoreq.y.ltoreq.0.25, 0.85.ltoreq.b.ltoreq.1.1, and 1.8.ltoreq.c.ltoreq.2.1.