Dimethylacetamide (DMAc) and triethyl(2-methoxyethyl)phosphonium bis(trifluoromethylsulfonyl)imide (TEMEP-TFSI) are investigated as protective additives for high voltage LiNi0.5Mn1.5O4 cathode materials. The capacity retention of Li/LiNi0.5Mn1.5O4 cells is improved by adding these additives to the base liquid electrolyte. The self-extinguishing time and DSC results demonstrate that the addition of TEMEP-TFSI to the base electrolyte is effective in reducing the flammability of the electrolyte solution and improving the thermal stability of the LiNi0.5Mn1.5O4 electrode. TEM and XPS analyses of the LiNi0.5Mn1.5O4 electrode reveal that DMAc and TEMEP-TFSI participate in the formation of a stable solid electrolyte interphase (SEI) on the LiNi0.5Mn1.5O4 electrode, and the resulting SEI layer effectively suppresses electrolyte decomposition.