The electrolyte additive, 3,9-divinyl-2,4,8,10-tetraoxaspiro[5,5] undecane (TOS), was investigated as a means to improve the life of mesocarbon microbead (MCMB)/Li1.1[Ni1/3Co1/3Mn1/3]0.9O2 (NCM) cells for high-power applications. With the addition of an appropriate amount of TOS (no more than 1wt%) to MCMB/NCM cells, the capacity retention was significantly improved at 55°C compared with cells containing pristine electrolyte. Aging tests at 55°C indicated that the capacity retention of the negative electrode had benefited as a result of the formation of a stable passivation film at the surface of the carbon electrode due to TOS reduction. Electrochemical impedance spectroscopy showed that a TOS addition of more than 0.5wt% increased the cell interfacial impedance. Differential scanning calorimetry showed that the thermal stability of lithiated MCMB was also improved with the TOS addition.