Na–K Co-deposition in Liquid-Alloy Batteries Revealed by Operando Visualization

Despite the great prospects of alkali metal batteries, safety concerns associated with dendrite growth still limit their commercial applications. An attractive alternative is to use the room temperature liquid sodium–potassium alloy as the anode, which inherently prevents dendrite growth. Currently, Na–K alloy anodes allow only Na⁺ or K⁺ to be cycled, depending on the choice of electrolyte and ion selectivity of the cathode, which results in reduced energy density of the battery. Herein, the possibility of concurrent use of both Na⁺ and K⁺ ions in Na–K alloy anodes is explored, and the working mechanism by operando optical microscopy is investigated. It is found that the type of deposited metal is dictated by both salt and solvent in electrolyte. Impressively, Na–K co-deposition is observed in KFSI-DME electrolyte for the first time, which strongly influences the dendrite morphology and evolution. Furthermore, the current density also has a great impact on the deposition pattern, which allows dendrite-free Na/K deposition on the liquid-alloy anode. These findings enrich our understanding of the intricate electrochemical behaviors of Na–K binary electroactive alloy systems and offer guidance for the sufficient use of Na and K while avoiding dendrite formation in such liquid-alloy batteries.