Designing electrolytes with high solubility of sulfides/disulfides for high-energy-density and low-cost K-Na/S batteries.

Alkaline metal sulfur (AMS) batteries offer a promising solution for grid-level energy storage due to their low cost and long cycle life. However, the formation of solid compounds such as MS and MS (M = Na, K) during cycling limits their performance. Here we unveil intermediate-temperature K-Na/S batteries utilizing advanced electrolytes that dissolve all polysulfides and sulfides (KS, x = 1-8), significantly enhancing reaction kinetics, specific capacity, and energy density. These batteries achieve near-theoretical capacity (1655 mAh g sulfur) at 75 °C with a 1 M sulfur concentration. At a 4 M sulfur concentration, they deliver 830 mAh g at 2 mA cm, retaining 71% capacity after 1000 cycles. This new K-Na/S battery with specific energy of 150-250 Wh kg only employs earth-abundant elements, making it attractive for long-duration energy storage.