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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.

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