Multifunctional Separator Allows Stable Cycling of Potassium Metal Anodes and of Potassium Metal Batteries

Abstract This is the first report of a multifunctional separator for potassium‐metal batteries (KMBs). Double‐coated tape‐cast microscale AlF 3 on polypropylene (AlF 3 @PP) yields state‐of‐the‐art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm −2 and 0.5 mAh cm −2 , with 0.042 V overpotential. Stability is maintained at 5.0 mA cm −2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite‐free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and “dead metal” and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF 3 @PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF 3 partially reacts with K to form an artificial SEI containing KF, AlF 3 , and Al 2 O 3 phases. The AlF 3 @PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K + transference number is ascribed to the strong interaction between AlF 3 and FSI − anions, as revealed through 19 F NMR. The enhancement in wetting and performance is general, being demonstrated with ester‐ and ether‐based solvents, with K‐, Na‐, or Li‐ salts, and with different commercial separators. In full batteries, AlF 3 prevents Fe crossover and cycling‐induced cathode pulverization.