Decorating β′′-alumina solid-state electrolytes with micron Pb spherical particles for improving Na wettability at lower temperatures†
Overcoming poor physical contact is one of the most critical hurdles for batteries using solid-state electrolytes. In particular, overpotential from the liquid–solid interface between molten sodium and a β′′-alumina solid-state electrolyte (BASE) in a sodium–metal halide (Na–MH) battery could be enormous at lower operating temperatures (<200 °C) due to intrinsically poor Na wetting on the BASE surface. In this work, we describe how surface modification with lead acetate trihydrate (LAT) at different temperatures affects Na wetting on BASEs. LAT treatment conducted at a temperature of 400 °C (under a nitrogen gas atmosphere) shows significantly better Na wettability and battery performance than treatments at lower temperatures. The formation of a unique morphology—micron-sized Pb spherical particles—is observed on the surface of the BASE LAT treated at 400 °C. We also observed evolution of the Na wetting configuration from a Cassie drop, to a Wenzel drop, and finally to a sunny-side-up drop, which is clearly different from the Young–Dupré relation, with increasing the contact-angle measurement temperature. We conclude that formation of a thin Na penetrating film (sunny-side-up shape) on Pb-decorated BASEs is crucial for achieving good battery performance at lower operating temperatures. The new observations and fundamental understanding of Na wetting reported here will provide excellent guidance for improving cell performance in general and will further promote development of practical Na–MH battery technologies for large-scale energy storage applications.