The performance of graphite/LiNi0.4Co0.2Mn0.4O2 pouch cells cycled to 4.35 V with 1.0 M LiPF6 in EC/DMC/EMC (1/1/1, v/v/v) solution with and without TMSP at room temperature has been investigated. Incorporation of 1% TMSP to the control electrolyte results in a significant improvement in cycling stability and rate performance of graphite/LiNi0.4Co0.2Mn0.4O2 cells. Electrochemical impedance spectra (EIS) results indicate that the use of TMSP can dramatically decrease the impedance of the graphite/LiNi0.4Co0.2Mn0.4O2 pouch cells. After 70 cycles, ex-situ analyses of the graphite/LiNi0.4Co0.2Mn0.4O2 cells were conducted via the combinations of XPS, SEM, TEM, and ICP-MS. The enhanced performance is ascribed to the modified cathode film with TMSP incorporated, which inhibits the continuous decomposition of the electrolyte on the cathode surface and provides the protection for the cathode bulk material upon cycling.