A Cellulose Encapsulated Composite Electrolyte Design: Towards Chemically and Mechanically Enhanced Solid-sodium Batteries

Replacing liquid electrolytes with solid ionic conductors attracts increasing attention due to the potential of improved battery safety. Solid-state batteries show potential for further increased energy/power density by eliminating the use of packaging accessories for unit cells. Sulfide- and halide-based ceramic ionic conductors exhibit comparable ionic conductivity with liquid electrolytes. These materials, however, are inherently brittle, making them unfavorable for applications. Here, we report a mechanically enhanced composite Na+ conductor that contains 92.5 wt% of sodium thioantimonate (Na3SbS4, NSS) and 7.5 wt% of sodium carboxymethylcellulose (CMC); the latter serves as the binder and an electrochemically inert encapsulation layer. The ceramic and binder constituents were integrated at the particle level, providing ceramic NSS-level Na+ conductivity in the NSS-CMC composite, the more than five-fold decrease of electrolyte thickness obtained in NSS-CMC composite electrolytes provided a five-fold increase in Na+ conductance compared to NSS ceramic pellets. Resulting from the CMC encapsulation, this NSS-CMC composite shows increased moisture resistivity and electrochemical stability, which significantly promotes the cycling performance of NSS- based solid-state batteries, and improves ductility of the material. This work demonstrates a well-controlled, orthogonal process of ceramic-rich, composite electrolyte processing – independent streams for ceramic particle formation along and binder encapsulation in a solvent-assisted environment. This work provides insights into the interplay among the solvent, the polymeric binder, and the ceramic particles in composite electrolyte synthesis, and implies the critical importance of identifying the appropriate solvent/binder system for the precise control of this complicated process. Finally, the work also provides valuable insights into the potential of designing mechanically tailorable battery components via fundamental understanding of the effect of the constituents on the overall composite ductility.