Fire-Retardant Phosphate-Based Electrolytes for High-Performance Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are considered as promising candidates for high-energy storage systems, but their practical applications are plagued by the severe safety concerns and poor cyclability. Here we report a fire-retardant electrolyte consisting of 2.8 M lithium bis(trifluoromethanesulfonyl)imide in triethyl phosphate (TEP) with 10 vol % fluoroethylene carbonate (FEC) for high-performance LMBs. This concentrated system with almost no free solvent molecules and anions can alleviate the electrolyte reactivity toward Li anode and improve the anodic stability up to 5.0 V. FEC cosolvents as functional additives can enhance the ionic conductivity and reduce the viscosity of the concentrated electrolyte. Moreover, the presence of FEC in the TEP -based electrolyte favors to form a stable LiF-rich solid electrolyte interphase, suppressing the parasitic reactions between Li and TEP solvent and enabling a dendrite-free cycling of Li anode. A LiIILiFePO4 battery using this electrolyte shows a high cyclic stability with 90% capacity retention over 2000 cycles. In addition, significantly improved cycling performances of LMBs employing 4 V class cathodes (e.g., LiNi0.8Co0.1Mn0.1O2, LiNi0.6Co0.2Mn0.2O2, and LiCoO2) have also been realized in the tailored electrolyte. The combination of high safety, high reduction/oxidation stability, and excellent electrochemical performance indicates this nonflammable electrolyte system is promising for LMBs.