Toward a Stable Sodium Metal Anode in Carbonate Electrolyte: A Compact, Inorganic Alloy Interface

Development of the next-generation, high-energy-density, low-cost batteries will likely be fueled by sodium (Na) metal batteries because of their high capacity and the abundance of Na. However, their practical application is significantly plagued by the hyper-reactivity of Na metal, unstable solid electrolyte interphase (SEI), and dendritic Na growth, leading to continuous electrolyte decomposition, low Coulombic efficiency, large impedance, and safety concerns. Herein, we add a small amount of SnCl₂ additive in a common carbonate electrolyte so that the spontaneous reaction between SnCl₂ and Na metal enables in situ formation of a Na-Sn alloy layer and a compact NaCl-rich SEI. Benefitting from this design, rapid interfacial ion transfer is realized and direct exposure of Na metal to the electrolyte is prohibited, which jointly achieve a nondendritic deposition morphology and a markedly reduced voltage hysteresis in a Na/Na symmetric cell for over 500 h. The Na/SnCl₂-added electrolyte/Na₃V₂(PO₄)₃ full cell exhibits high capacity retention over cycling and excellent rate capability (101 mAh/g at 10 C). This work can provide an easily scalable and cost-effective approach for developing high-performance Na-metal batteries.