Development of Stable All-Solid-State Sodium-Sulfur Batteries

Sodium-sulfur (Na-S) batteries have been commercialized for stationary energy storage systems because of their low-cost fabrication and long-cycling stability. However, the Na-S batteries need to operate at high temperatures (> 300 o C) to maintain the molten state of sodium anode and sulfur cathode, which requires extra energy and raises safety issues when the solid electrolyte membrane fails. Therefore, low-temperature all-solid-state Na-S batteries (such as 60 o C) are highly desirable because of the improved safety and a lower operating temperature. To achieve high sulfur-specific capacity and long-cycling stability, the stable interfaces between electrodes and solid-state electrolyte are important for all-solid-state Na-S batteries. Herein, we demonstrate a strategy using Na alloy anodes to replace Na metal anode to solve the issues of unstable interface between Na metal and solid-state electrolytes. Na-Sn and Na-Sb alloys are first compared in terms of their alloying/dealloying process to identify the advantage of Na-Sb alloy as the anode. The Na-Sb alloy anode shows its superior stability against the solid electrolyte and undergoes a stable Na alloying/dealloying process at 0.04 mA cm -2 for over 500 hours. Sulfur-carbon composites with confined sulfur nanostructure were synthesized via the sulfur vapor deposition method and further utilized in full cells. Combining the optimized Na-Sb alloy anode and sulfur composite cathodes, the all-solid-state Na alloy-S battery shows improved rate performance and a high sulfur-specific capacity of 1377 mAh g -1 with good capacity retention of 70 % after 180 cycles at 60 o C (Figure 1a and b). Postmortem analysis of both the anode and cathode shows a reversible discharge/charge process after the first cycle and undergoes significantly rearranged distributions of carbon and solid electrolytes after 180 cycles due to severe volume change induced by repeated sodiation/desodiation process. These research results show a high sulfur specific capacity with long-cycling stability and shed light on further improving next-generation Na-S energy storage systems. Figure 1