(Pentafluorophenyl)diphenylphosphine (PFPDPP) is studied as an effective dual-functional additive for high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes. The additive can stabilize the LiPF6 salt and form a good cathode–electrolyte interphase (CEI). Hydrolysis of LiPF6 in the electrolyte is investigated by 19F and 31P nuclear magnetic resonance spectroscopy, which reveal that PFPDPP suppresses the decomposition of LiPF6 into LiF, PO3F2−, and PO2F2 −. Cyclic voltammetry results reveal that PFPDPP is anodically decomposed at 3.5 V vs. Li/Li+ prior to the oxidation of carbonate electrolytes to form a CEI film. The results of scanning electron microscopy, transmission electron microscopy, and AC impedance analyses show that PFPDPP forms a thin CEI film with low impedance. X-ray photoelectron spectroscopy results also confirm that the addition of PFPDPP suppresses the formation of LiF in the CEI film. Furthermore, the Li|1.0 M LiPF6-EC/DEC (1:1, v/v)|LNMO cell with 0.2 wt% PFPDPP shows a retention capacity of 71% after 300 cycles at a charge–discharge rate of 2C between 3.0 and 5.0 V, whereas the cell without the additive shows a retention capacity of only 53.4% after 300 cycles. Furthermore, self-discharge of the battery is suppressed upon the addition of PFPDPP.