Lithium magnesium cobalt oxide (LMCO) was synthesized with heating a reaction mixture of dehydrated magnesium cobalt hydroxide and Li2CO3 to stabilized crystal structure of LiCoO2. The sample was characterized by XRD. All diffraction lines could be indexed by assuming a hexagonal lattice. Rietvelt analysis indicated that magnesium were located at cobalt site in the layered structure. The voltage plateau around 4.1 V (versus Li) for LiCoO2, was not observed for LMCO. The prismatic lithium-ion batteries of lithium magnesium cobalt oxide and graphite with gamma-butyrolactone (GBL) were fabricated and examined in terms of rate capability, low temperature discharge characteristics, high-temperature storage, and safety inspection by so-called hot pot test of the positive electrolyte. Interesting phenomenon on thermal stability of the lithium magnesium cobalt oxide was reported. (c) 2005 Elsevier B.V. All rights reserved.