Fire-Retardant\nPhosphate-Based Electrolytes for High-Performance\nLithium Metal Batteries

Abstract

Rechargeable lithium metal batteries\n(LMBs) are considered as promising\ncandidates for high-energy storage systems, but their practical applications\nare plagued by the severe safety concerns and poor cyclability. Here\nwe report a fire-retardant electrolyte consisting of 2.8 M lithium\nbis­(trifluoro­methane­sulfonyl)­imide in triethyl phosphate\n(TEP) with 10 vol % fluoroethylene carbonate (FEC) for high-performance\nLMBs. This concentrated system with almost no free solvent molecules\nand anions can alleviate the electrolyte reactivity toward Li anode\nand improve the anodic stability up to 5.0 V. FEC cosolvents as functional\nadditives can enhance the ionic conductivity and reduce the viscosity\nof the concentrated electrolyte. Moreover, the presence of FEC in\nthe TEP-based electrolyte favors to form a stable LiF-rich solid electrolyte\ninterphase, suppressing the parasitic reactions between Li and TEP\nsolvent and enabling a dendrite-free cycling of Li anode. A Li||LiFePO₄ battery using this electrolyte shows a high cyclic stability\nwith 90% capacity retention over 2000 cycles. In addition, significantly\nimproved cycling performances of LMBs employing 4 V class cathodes\n(e.g., LiNi_0.8Co_0.1Mn_0.1O₂, LiNi_0.6Co_0.2Mn_0.2O₂, and LiCoO₂) have also been realized in the tailored\nelectrolyte. The combination of high safety, high reduction/oxidation\nstability, and excellent electrochemical performance indicates this\nnonflammable electrolyte system is promising for LMBs.