How to exert the high-voltage performance of LiNi0.5Mn1.5O4 and break through the bottleneck effect of corresponding electrolyte have become key points in advanced lithium-ion battery. Lithium bis(oxalato) borate (LiBOB) and sulfolane (SL) are chosen as additives to investigate their effects on the electrochemical performance of lithium-ion battery with LiNi0.5Mn1.5O4 cathode. The quantum chemistry calculation theory shows that oxidation potential of SL–BOB– is dramatically increased, consistent with the experimental result in CV measurement. Meanwhile, results of CV and charge–discharge cycling indicate that LiBOB and SL would be involved in the initial oxidation reaction to form an effective solid electrolyte interface film on surfaces of the cathode electrode thus enhance the cycling performance of LiNi0.5Mn1.5O4/Li cells. Electrochemical impedance spectroscopy data proves that SL is beneficial to resistance decrease. All these data will become important corroborations that the combined electrolyte LiBOB and SL have good oxidation resistances.