Aiming at improving the capacity retention abilities of LiMn2O4/graphite based Li-ion batteries (LIBs) at high temperatures, we report on 2-(triphenylphosphoranylidene) succinic anhydride (TPSA) as a new electrolyte additive. The unit cells with 0.1 wt.% of TPSA achieved a 43% capacity retention increase at high temperature operation (55 °C, 100 cycles, C/2 rate) compared to a control group without the additive. To understand the underlying principle on the enhanced capacity retention ability of the unit cells, the effect of TPSA was investigated by cyclic voltammetry, X-ray photoelectron spectroscopy, auger electron spectroscopy, and galvanostatic measurements. The results suggest that the electrochemical decomposition of TPSA takes place on both surfaces of the LiMn2O4 and graphite during precycling, thereby reducing the decomposition reaction of the electrolyte during subsequent cycles.