Unlocking Solid-State Sodium–Metal Batteries at −15 °C by Electrolyte Optimization and Interface Regulation

Beta-Al2O3-based solid-state sodium metal batteries are some of the best options for large-scale energy storage systems because of their high energy density, high-level safety, and low cost. Nevertheless, their room-/low-temperature operation remains challenging due to low ionic conductivity of Beta-Al2O3 electrolyte and weak solid–solid contact of the Na/Beta-Al2O3 interface. Herein, an integrated strategy was developed via electrolyte optimization and interface regulation, in which Cu2+ as a stabilizing agent was incorporated into Beta-Al2O3 to improve density and ionic conductivity and the In2S3 interface layer was introduced between the Na anode and solid electrolyte to induce the in situ formation of a mixed conductive layer (Na–In alloy and Na2S). The integrated strategy bolstered the interfacial electrochemical stability and promoted fluent Na+ transport, allowing the symmetric battery to cycle steadily for more than 2670 h at room temperature with a current density of 0.2 mA cm–2. Impressively, it demonstrated remarkable endurance, cycling at 0.025 mA cm–2 for more than 3315 h at −15 °C. The Na3V2(PO4)3|Beta-Al2O3-0.5 wt.% Cu2+@In2S3|Na full battery demonstrated outstanding cyclic stability and rate performance at −15 °C and room temperature, underscoring its potential for low-temperature solid-state sodium–metal batteries.