Cellulose-Encapsulated Composite Electrolyte Design: Toward Chemically and Mechanically Enhanced Solid-Sodium Batteries

Sulfide- and halide-based ceramic ionic conductors exhibit comparable ionic conductivity with liquid electrolytes and are candidates for high-energy- and high-power-density all-solid-state batteries. 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 carboxymethyl cellulose (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 5-fold decrease of electrolyte thickness obtained in NSS–CMC composite provided a 5-fold increase in Na+ conductance compared to NSS ceramic pellets. As a result of 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. This work demonstrates a well-controlled, orthogonal process of ceramic-rich, composite electrolyte processing: independent streams for ceramic particle formation along with binder encapsulation in a solvent-assisted environment. This work also 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 precise control of this complicated process.