The effects of methylene methanedisulfonate (MMDS) on the high-temperature (~50 °C) cycle performance of LiMn2O4/graphite cells are investigated. By addition of 2 wt% MMDS into a routine electrolyte, the high-temperature cycling performance of LiMn2O4/graphite cells can be significantly improved. The analysis of differential capacity curves and energy-dispersive X-ray spectrometry (EDX) indicates that MMDS decomposed on both cathode and anode. The three-electrode system of pouch cell is used to reveal the capacity loss mechanism in the cells. It is shown that the capacity fading of cells without MMDS in the electrolytes is due to irreversible lithium consumption during cycling and irreversible damage of LiMn2O4 material, while the capacity fading of cell with 2 wt% MMDS in electrolytes mainly originated from irreversible lithium consumption during cycling.