The development of novel electrolytes for next-generation high voltage lithium ion battery is of primary importance. In this work, a fluorinated phosphazene derivative, ethoxy-(pentafluoro)-cyclotriphosphazene (PFN), is proposed as a novel electrolyte additive for improving the electrochemical performance and safety of lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode. With the addition of PFN, the electrolyte can be preferentially oxidized and decomposed, thus producing some linear polymers, multi-ring polymers, LiNO3, RONO2Li (RONO2: nitrate ester functional group, with R standing for any organic residue), Li3PO4, and ROPO3Li (ROPO3: monoester phosphate) simultaneously. These as-generated materials form a dense, uniform, and thin protective layer on the surface of the cathode material, which suppresses the decomposition of electrolyte and electrode corrosion, correspondingly protecting the LiNi0.5Mn1.5O4 from structural destruction. Due to the coverage by the protective film and corrosion suppression, charge and discharge tests demonstrate that PFN is effective for improving the cycling stability of LiNi0.5Mn1.5O4. The discharge capacity of a battery with 5 wt% PFN is 124.4 mAh g−1 and 99.8 mAh g−1 after 100 cycles at the current rates of 0.2 C and 1 C, respectively, which is much better than the performance without PFN. Meanwhile, because of the combined structure of the nonflammable cyclophosphazene and fluorine, the PFN creates a highly synergistic flame retardant effect, and a low content of PFN can almost completely extinguish burning electrolyte, leading to excellent safety performance for the lithium ion battery.