High‐Energy and Long‐Lifespan Potassium–Sulfur Batteries Enabled by Concentrated Electrolyte

Abstract Potassium–sulfur (K–S) batteries are emerging as low‐cost and high‐capacity energy‐storage technology. However, conventional K–S batteries suffer from two critical issues that have not yet been successfully resolved: the dissolution of potassium polysulfides (KPS) into the liquid electrolyte and the formation of K dendrites on the K metal anode, which lead to inadequate cycling efficiencies with a low reversible capacity. Herein, a high‐capacity and long cycle‐life K–S battery consisting of a highly concentrated electrolyte (HCE) (4.34 mol kg −1 potassium bis(fluorosulfonyl)imide in a 1,2‐Dimethoxyethane) and a sulfurized polyacrylonitrile (SPAN) cathode is presented The application of a HCE efficiently suppresses the dendritic growth of K, as evidenced by operando optical imaging and phase field modeling, owing to the reduced K‐ion depletion on the electrode surface and a uniform Faradaic current density over the K metal anode surface. Additionally, because S is covalently bonded to the C backbone of PAN in the SPAN structure, the SPAN cathode inhibits the dissolution of KPS. These features generate synergy that the proposed K–S battery can provide a practical areal capacity of 2.5 mAh cm −2 and unprecedented lifetimes with high Coulombic efficiencies over 700 cycles.