To seek a promising candidate for 5V electrolytes, fluorine-free lithium bis(oxalato) borat (LiBOB) is chosen as the lithium salt, and lithium difluoro(sulfato) borate (LiBSO4F2) is investigated as an additive for the stabilization of a high-voltage LiNi0.5Mn1.5O4 cathode. Cyclic voltammetry test, AC impedance measurement and scanning electron microscopy (SEM) analysis are used to examine the electrochemical stability and the compatibility between electrolytes and LiNi0.5Mn1.5O4 cathode. It is found that the addition of 0.1M LiBF2SO4 to 0.7M LiBOB-based electrolyte could significantly decrease the interfacial impedance of LiNi0.5Mn1.5O4/Li cells, due to the fact that LiBSO4F2 is involved in the formation of protective film on cathode surface, as well as the prior oxidation reactions of LiBOB. Moreover, LiBSO4F2 additive could obviously improve both of the capacity retention and the rate performance of lithium-ion cells. These results demonstrate that using blend salts can combine the advantages of LiBOB and LiBSO4F2 to maximize the electrochemical performances of lithium-ion cells.