Fluorinated phosphazene derivative – A promising electrolyte additive for high voltage lithium ion batteries: From electrochemical performance to corrosion mechanism

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 (LiNi_0.5Mn_1.5O₄) 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 LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄. 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. © 2018