In-situ construction of LiF/NaF-rich hybrid solid electrolyte interphase for dendrite-free and stable Li-Na alloy anodes

•Dendrite-free, low-expansion and long-cycle stable lithium metal anodes is realized.•LiF/NaF hybrid SEI film can alleviate the expansion and uniform Li+ deposition.•Na+ preferentially depositing on the surface of active Li crystals during cycling.•Lithium-sulfur batteries with high energy density and long stability is achieved. The uncontrolled growth of lithium dendrites and unstable solid-electrolyte interface (SEI) film limit the practical application of lithium metal anodes. In this study, we report a synergistic strategy utilizing a Li-Na alloy and fluoroethylene carbonate (FEC) for the in-situ construction of LiF/NaF-rich hybrid SEI film. This approach inhibits the growth of lithium dendrites and mitigates the volume expansion of lithium metal anodes during repeated plating/stripping. The results indicate that Na+ is precipitated from the Li-Na alloy anodes during cycling, with Na+ preferentially depositing on the surface of active Li crystals. Meanwhile, Li+ is forced to deposit in the inactive regions around Na, thereby inhibiting the growth of lithium dendrites. The LiF/NaF-rich hybrid SEI film, which process high ionic conductivity and mechanical strength, form on the surface of Li-Na alloy anodes, alleviating the volume expansion of lithium metal anodes. When the molar ratio of Li/Na is 16, the Li-Na alloy exhibits excellent cycling stability and low overpotential. The Li-Na alloy||Sulfurized polyacrylonitrile (SPAN) full cell demonstrates a high reversible capacity of 630 mAh g−1 at 1000 mA g−1, with 70 % capacity retention after 300 cycles, significantly higher than Li||SPAN (48 %). This strategy presents a potential solution for the practical application of long-life and high energy density lithium-sulfur batteries.