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

This is the first report of a multifunctional separator for potassium-metal batteries (KMBs). Double-coated tape-cast micro-scale AlF3 on polypropylene (AlF3 @PP) yield state-of-the-art electrochemical performance: Symmetric cells are stable after 1,000 cycles (2,000 hours) 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 (14,400 minutes), with 0.138 V overpotential. Post-cycled plated surface is dendrite-free, while stripped surface contains smooth SEI. Conventional PP cells fail rapidly, with dendrites at plating, and "Dead Metal" and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF3 @PP display the enhanced capacity retention (91% at 100 cycles versus 58%). AlF3 partially reacts with K to form an artificial SEI containing KF, AlF3 and Al2 O3 phases. The AlF3 @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 AlF3 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 AlF3 prevents Fe crossover and cycling-induced cathode pulverization. This article is protected by copyright. All rights reserved.