To satisfy the need of 5V high-voltage cathode materials, lithium difluoro(oxalato)borate (LiDFOB) with excellent film-forming property has been chosen as lithium salt, and adiponitrile (ADN) with stable electrochemical performance has been evaluated as functional additive. An electrochemical investigation of this novel electrolyte solution shows a wide electrochemical oxidation window of nearly 6.0V vs Li+/Li. Due to the positive effect of ADN on the cathode/electrolyte interface, LiNi0.5Mn1.5O4/Li half-cell with ADN-adding electrolyte exhibits prominent rate capability and cyclability. It is believed that the improvement of electrochemical performances has benefitted greatly from interfacial impedance reducing. Two modeling hypotheses have been used to explain interfacial interaction between ADN and LiNi0.5Mn1.5O4, by the means of analysis of X-ray photoelectron spectroscopy and theoretical calculation. Results show that ADN can enrich the surface of cathode material to hinder the further decomposition of the electrolyte, and contribute to the formation of a cathode-electrolyte interface passivation film with protective and conductive properties.