An artificial metal-alloy interphase for high-rate and long-life sodium-sulfur batteries

Room-temperature sodium-sulfur battery is considered to be a promising candidate for next-generation batteries due to its high theoretical energy density (similar to 1274 Wh kg(-1)) and natural abundance of elements. There are however, a number of concomitant challenges, including large volume change, low ionic conductivity, rapid dendrite growth, and high chemical reactivity, which limit the viability of sodium anodes. Solid electrolyte interphases that address all 4 challenges simultaneously to enable high-rate cycling of sodium anodes remain scarce in the literature. Here we report an artificial metal-alloy interphase (MAI) comprising sodium-tin alloy, which was synthesized using a facile solid-vapor reaction of metallic sodium with tin tetrachloride vapors, instead of using typical liquid electrolytes with tin-based additives (solid-liquid reaction). The MAI was found to facilitate reversible deposition of sodium at relatively high current densities (2-7 mA cm(-2)), and allows sodium electrodes to cycle stably for over 650 cycles at 2 mA cm(-2) in sodium symmetric cells. Owing to the unique properties of MAI, such as strong electrode adhesion (to accommodate volume change), high ionic conductivity (to minimize overpotential), high Young's modulus (to suppress dendrite growth), and low electrolyte permeability (to minimize electrolyte reduction), the sodium anode with MAI can endure extended cycling test in sodium-sulfur batteries for over 500 cycles with a high Coulombic efficiency of 99.7%. This solid-vapor chemistry concept to synthesize MAIs can also be generalized to other material systems such as sodium-silicon and sodium-titanium alloys.