Cellulose Coating Altered the Electro-Chemo-Mechanical Evolution of Sodium Thioantimonate Electrolyte in Solid-state Sodium Batteries: An Operando Raman Study

All-solid-state batteries (ASSBs) attracted increasing attention due to their improved safety and energy densities; yet electrolyte decomposition and subsequent contact loss limited the interfacial stability of ASSBs. Herein, we report an operando Raman characterization that provides high voltage, time, and spatial resolutions, which enables simultaneous analysis of interfacial decomposition mechanism and morphological evolution. Using Na3SbS4 electrolyte (NSS) and its carboxymethyl-cellulose-encapsulated analogue (NSS-CMC) as exemplars, we precisely contrasted the subtle differences in the two-step reduction mechanism of the two electrolytes. In both systems, Na3SbS3 formed as an intermediate, and Na3Sb binary as one major final product; while the CMC coating altered the kinetics of Na3SbS3 formation and consumption, and extended the formation potential of Na3Sb from 1.35 V (seen in NSS) to 0.50 V (vs. Na/Na+). Oxidation of NSS and NSS-CMC both occur near 2.20 V, although CMC coating altered the crystallinity of the oxidative products. Simultaneously, we captured phenomena that are unique to solid-state electrochemical systems such as particle relocation, morphological change, and reversed reactions. We inferred CMC’s dual role as a voltage barrier and a mechanical buffer in suppressing the electro-chemo-mechanical decomposition of NSS electrolyte. The deep mechanistic insights unravel the exact modification needed for improved interfacial stability.