A spinel-type of lithium-rich lithium manganese oxide, Li1.1Mn1.9O4, was prepared and studied electrochemically and structurally as a positive electrode of a Li-ion battery. In cycle performance tests, capacity losses were 3 and 11 mAh/g after 75 cycles at 25 and 55degreesC, respectively, corresponding to 15 and 50 mAh/g for a normal lithium manganese oxide, LiMn2O4. Their losses were divided into two parts; one resulting from the deterioration of the active material itself and the other due to the poor conduction of the active material to the collector. The former and latter contributions to the losses at 55degreesC were 2 and 9 mAh/g, respectively for the Li-rich spinel, and 18 and 32 mAh/g, respectively for the normal spinel. The capacity loss by the poor conduction was apparently much larger for both spinels of positive electrodes. Cycle performance tests in each region of the charge states divided into five regions show that relatively larger capacity losses were observed in lower potential states. However, in every region, a capacity loss for the Li-rich spinel was much smaller than that of the normal spinel. There was a close relation found between capacity losses in the storage and those in the cycle. Electron densities spreading between manganese and oxygen atoms were larger in the Li-rich than in the normal spinels. Thus, it turned out that the structure of the Li-rich spinel was made more stable because of strong Mn-O bonds. (C) 2003 The Electrochemical Society.