Emerging solid electrolytes for solid-state sodium batteries: Synthesis and interface study.

Over the past decade, solid-state batteries have attracted significant attention as the next-generation energy storage technology due to their higher energy density and enhanced safety. Given the abundant sodium (Na) resources relative to lithium (Li), the development of sodium-based batteries has become increasingly compelling. Within a solid-state battery, the solid electrolytes (SEs) serve the dual function of (i) electronically insulating and physically separating the electrodes, and (ii) transporting ions between the anode and cathode. To meet these essential requirements, ideal SEs must process high ionic conductivity with negligible electronic conductivity, robust structural and chemical stability, a wide electrochemical window, and good interfacial compatibility with electrodes. Among different SE materials, sulfide-type solid electrolytes (e.g. Na3SbS4, Na3PS4) display advantages of high ionic conductivity at room temperature and easy densification. However, they also suffer from challenges such as the chemical and electrochemical instability toward Na metal anode and oxide cathode. This dissertation addressed each of these challenges through targeted materials design and synthesis strategies. Polymer-rich and ceramic-rich solid electrolytes were synthesized via solution casting and self-template approach, respectively. These composite electrolytes exhibit high ionic conductivity (> 0.1 mS cm-1) and excellent interfacial compatibility with the sodium metal anode. To elucidate the stabilization mechanism, the electrode/electrolyte interface was systematically characterized. To further enhance compatibility with high-voltage cathodes, two novel halide solid electrolytes were synthesized through a mechanochemical anion-substitution method. Both halide electrolytes demonstrate high ionic conductivity (> 0.5 mS cm-1) and extended electrochemical windows (> 4 V vs. Na+/Na). Electrochemical techniques, including in-situ electrochemical impedance spectroscopy (EIS) and the distribution of relaxation time (DRT), were employed to investigate the interface evolution during cycling. The compatibility and stability of halide electrolytes with cathodes were validated in all-solid-state sodium batteries, delivering ultra-long cycling performance (> 800 cycles). These advances further promote the development of composite and halide solid electrolytes and pave the way for the broader adoption of solid-state sodium batteries.