Insight into the probability of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as the functional electrolyte additive in lithium–sulfur batteries

Enhancing the flame retardancy of electrolytes and the stability of lithium anodes is of great significance to improve the safety performance of lithium-sulfur (Li-S) batteries. It is well known that the most commonly used ether based electrolyte solvents in Li-S batteries have a lower flash point and higher volatility than the ester electrolyte solvents in Li-ion batteries. Hence, lithium-sulfur batteries have greater safety risks than lithium-ion batteries. Herein, ethoxy(pentafluoro)cyclotriphosphazene (PFPN), which is commonly used as a flame retardant for ester electrolytes in lithium-ion batteries, is utilized as both a film-forming electrolyte additive and a flame retardant additive for the ether electrolyte to investigated its applicability in Li-S batteries. It is found that the ether electrolyte containing PFPN not only has good flame retardant properties and a wide potential window of about 5 V, but also can form a stable SEI film on the surface of a lithium anode. As a result, with the ether-based electrolyte containing 10 wt% PFPN, Li-Cu and Li-S batteries all delivered a stable cycling performance with a high coulombic Efficiency and a long-lifespan performance, which were all superior to the batteries using the ether-based electrolyte without PFPN. This study demonstrates an effective solution to solve the problems of flammable ether-based electrolytes and reactive lithium anodes, and it may contribute to the development of safe Li-S batteries.