Magnesium trifluoromethanesulfonate (MFS) was used as a bifunctional electrolyte additive to improve the cycle performance of Li/LiNi0.5Mn1.5O4(Li/LNMO) cells. The electrochemical performance was measured by linear sweep voltammetry (LSV), cyclic voltammetry (CV), charge-discharge, and electrochemical impedance spectroscopy (EIS). The electrode surface morphology and composition of before and after cycle Li/LNMO cells with different electrolytes were characterized by SEM, XPS, and FTIR. This indicates that MFS can be oxidized in preference to solvents during the charge-discharge process. MFS-containing electrolyte interface films can form simultaneously on both electrodes, suppressing the decomposition of electrolytes and provide protection for the electrodes. The discharge-specific capacity of Li/LNMO cells with the electrolytes containing a mass fraction of 0.3% MFS decreased from 135.12 mA·h/g to 123.86 mA·h/g at 1 C rate after 300 cycles, and the capacity retention rate was as high as 91.67%. Compared with cells without MFS in the electrolyte, the increase of charge transfer resistance is restrained, and the electrochemical properties are improved significantly.