Safety concerns on conventional lithium ion batteries are attracting a lot of attention from both academia and industry with the rapid growth of the electric automobile market. Replacing flammable organic liquid electrolytes with nonflammable inorganic solid electrolytes can potentially eliminate the fire hazards. On the other hand, low-cost electrochemical energy storage technologies beyond lithium chemistry are also highly desired for large-scale energy storage applications. All-solid-state Na ion batteries are one of the promising candidates for grid-level energy storage owing to their low cost and outstanding safety properties. The key enabler of this technology is solid electrolyte with sufficiently high ionic conductivity at room temperature and good electrochemical stability. This work presents a group of novel sodium superionic conductors to address the foregoing issues, which is a topic currently receiving extensive attention in the field of electrochemistry and energy storage.

Through a rational design approach and with using facile solid-state reactions, a series of novel Na₄SnS₄-Na₃SbS₄ solid-solution-based compounds with cation and anion substitutions were obtained. These sulfides show room temperature conductivities of 0.1~1 mS/cm, which is among the highest values reported for sodium ion conductors to date. The underlying structural factors, including lattice parameters, concentration of defects, local ordering, etc., of these compounds are investigated with a variety of characterization methods, including synchrotron X-ray diffraction, neutron diffraction, and pair distribution function analysis. Interesting structure-property relationships in these compounds were revealed. All-solid-state Na-ion battery cells using these as-synthesized materials as electrolyte was successfully assembled and cycled, demonstrating the promise of this group of sulfide-based compounds as solid electrolytes and implying new opportunities for materials and device development in solid-state batteries.