High-Safety and High-Voltage Lithium Metal Batteries Enabled by a Nonflammable Ether-Based Electrolyte with Phosphazene as a Cosolvent

The high reactivity between lithium metal and traditional carbonate electrolytes is a great obstacle to realize the long-term cycling ability of lithium metal batteries. Ether-based electrolytes have good stability toward lithium metal anodes. However, the oxidation stability of ether-based electrolytes is generally lower than 4 V, which limits the application of high-voltage (>4 V) cathodes and restricts the energy density. The high flammability of ether is another key issue that hinders the commercialization of ether-based electrolytes. To address these issues, herein, we report a high-voltage, nonflammable ether-based electrolyte with F-, N-, and P-rich hexafluorocyclotriphosphazene (HFPN) as a cosolvent. HFPN can not only act as a highly efficient flame-retarding agent but also form a dense and homogeneous solid electrolyte interphase (SEI) layer rich in LiF and Li3N on the lithium metal anode, which stabilizes the lithium/electrolyte interface and inhibits the formation of lithium dendrites. Moreover, the HFPN-based electrolyte has a wider potential window than 4 V. As a result, with this electrolyte, high-voltage lithium metal batteries exhibit a capacity retention of ∼95% after 100 cycles. This study may provide a new pathway for developing safe, high-energy, and dendrite-free lithium metal batteries.