Electrolyte Design for Intermediate-Temperature Na-S Batteries with High Energy Density

The conventional high-temperature sodium sulfur battery is an attractive technology for grid-level energy storage due to its low cost, high energy density and long cycle life. However, this system operates at 300-350 oC, which not only increases cost, but also accelerates degradation and causes potential safety issues. On the other side, Na-S batteries working at intermediate temperatures (e.g., 100-150 oC) are attractive, but the formation of solid Na2S2 and Na2S with low ionic diffusivity and electronic conductivity limits its specific capacity to ~500 mAh/g and thus energy density. In this study, we have developed a new electrolyte solvent that can dissolve all sulfides and polysulfides, which avoids the poor kinetics in Na2S2 and Na2S, and allows Na-S batteries to operate at ~75 ^oC. Based on the 1 mol L⁻¹ sulfur catholyte, a high specific capacity of 1640 mAhg⁻¹ is achieved at 0.13 mA cm⁻². When increased to the 4 mol L⁻¹, the capacity is achieved 830 mAhg⁻¹ and shows capacity retention of 70% over 1000 cycles. Such a system has potential applications for long-duration energy storage.