We investigate irreversible capacity losses in lithium ion manganese spinel cells during 60 degreesC storage in the charged state and propose a mechanism based on acid generation to explain the origin of the irreversible loss. Various approaches such as surface, structural, electrode formulation, and electrolyte modification were applied to address the instability issue. Based on understanding of the mechanism, we developed a method of coating the spinel surface with a carbonate layer to minimize the detrimental effect of acid generation in cells. As a result, irreversible losses were reduced from >70% after I weeks of storage at 60 degreesC to about 20%.