Building Stable Solid‐State Potassium Metal Batteries

Abstract Solid‐state potassium metal batteries (SPMBs) are promising candidates for the next generation of energy storage systems for their low cost, safety, and high energy density. However, full SPMBs are not yet reported due to the K dendrites, interfacial incompatibility, and limited availability of suitable solid‐state electrolytes. Here, stable SPMBs using a new iodinated solid polymer electrolyte (ISPE) are presented. The functional ions reconstruct ion transport channels, providing efficient potassium ion transport. ISPE shows a combination of high ionic conductivity, superior interfacial compatibility, and electrochemical stability. In situ alloying and iodinated interlayer increase K metal compatibility for prolonged cycling with low polarization. Moreover, the ISPE enables SPMBs with Prussian blue cathode stable operation at a high voltage of 4.5 V, a superior rate capability, and long‐term cycling over 3000 cycles (4.2 V vs K + /K) with an ultra‐high coulombic efficiency of 99.94%. More importantly, a classic solid‐state potassium metal pouch cell achieves 4.2 V stable cycling over 800 cycles with a high retention of 93.6%, presenting a new development strategy for secure and high‐performance rechargeable solid‐state potassium metal batteries.