Phase separation dynamics in sodium solid-state batteries with Na-K liquid anodes

Alkali metal anodes hold great promise for high-energy-density batteries for grid-scale applications. However, challenges such as void formation at the solid electrolyte interface during electrodissolution and dendrite or filament growth during plating hinder their high-rate and high-capacity performance. Contact between an alkali anode can be improved either via increasing external stack-pressure or via introducing materials that are flexible, compliant, and conformable. Herein, we investigate a conformable sodium-potassium (NaK) liquid alloy anodes with a sodium solid electrolyte. Our results reveal that the liquid-solid phase separation in NaK anodes affects their long-term stability and performance. Critical stripping capacity is correlated with the liquid to solid phase transition upon sodium depletion upon during electrodissolution. Using a combination of electrochemical characterization and synchrotron 3D X-ray computed tomography, it is shown that the interaction between the NaK liquid alloy and its carbon substrate plays a crucial role in anode behavior. This effect is particularly pronounced at higher potassium concentrations, where decreases in viscosity and surface tension further restrict liquid mobility. These findings highlight the potential of NaK anodes for improving the durability of solid-state batteries while emphasizing the need to optimize interfacial dynamics and mitigate phase separation for reversible operation.