Molten Salt Electrolyte for Enhanced Low‐Temperature Sodium Liquid Metal Batteries

Liquid metal batteries (LMBs) are attractive for grid energy storage by virtue of their long lifespan and low cost. Nevertheless, the battery based on traditional inorganic molten salt design operates at a high temperature (typically >300 °C), which poses a challenge to its long‐term stable cycling. Herein, a design strategy for a low‐melting‐point, high‐conductivity, and solvent‐free molten salt electrolyte based on large‐sized anions is presented. Combined with molecular dynamics (MD) and density functional theory (DFT) simulations under the constraints of thermodynamics and Na + transport, a promising Na/KFSI (1:1 mol%) electrolyte was creatively designed and applied for the first time in a sodium‐based liquid metal battery. This electrolyte exhibits a low melting point of 69 °C and a high ionic conductivity of 42.07 mS cm −1 , coupled with non‐volatility and non‐flammability. Remarkably, coupled with a Bi‐Pb‐In cathode, the Na/KFSI electrolyte facilitates the formation of a uniform NaF‐rich cathode electrolyte interphase (CEI) layer on the cathode. Benefiting from these advantages, Na||Bi 5 Pb 3 In 2 exhibit stable cycling at 140 °C, achieving a coulombic efficiency of 99.76% and retaining the high capacity of 9.95 mAh cm −2 (at 1.57 mA cm −2 ) after 70 cycles. These results demonstrate that sodium liquid metal batteries (Na‐LMBs) can operate stably at 140 °C with the NaFSI–KFSI molten salt electrolyte. This design surpasses conventional electrolytes and offers a viable new approach for achieving low‐temperature operation in liquid metal batteries.