All-solid-state Na-ion batteries that operate at or close to room temperature are a promising next-generation battery technology with enhanced safety and reduced manufacturing cost. An indispensable component of this technology is the solid state electrolyte that allows rapid shuttling of the mobile cation (i.e., Na⁺) between the cathode and anode. However, there are very few fast Na-ion conductors with ionic conductivity approaching that of the liquid counterparts (i.e., 1 mS cm^-1).

Compared to oxides and sulfides, selenides form a huge unexplored chemical space for fast ionic conductor discovery. Based upon ionic size considerations, selenide-based compounds may provide a more suitable structural framework for the rapid diffusion of the much larger Na⁺ cations compared with Li⁺. This consideration led us to an extensive search of fast Na-ion conductors in the Se-based chemical space via high-throughput computational screening, and has identified cubic Na₃PSe₄ as one of the most promising candidates.

In this work, we present the synthesis and structural characterization of Na₃PS_xSe_4-x (x = 0, 1, 2, 3, 4). The structures of these phases are determined from high-resolution synchrotron X-ray diffraction data, which allow us to unambiguously identify a tetragonal-to-cubic phase transition with increasing Se concentration in Na₃PS_xSe_4-x. The Na-ion conductivity and activation energy for Na-ion diffusion were further characterized via variable temperature impedance spectroscopy, from which a negative correlation between the activation barrier and Se concentration can be established. Of particular note is that the x = 4 end member (i.e., cubic Na₃PSe₄) possesses a room-temperature ionic conductivity exceeding 0.1 mS cm^-1, and does not require high temperature sintering to minimize grain boundary resistance, making it a promising solid-state electrolyte candidate for all-solid-state Na-ion battery applications. These experimental results were complemented by ab initio computation through which we demonstrate an intriguing interplay between the tetragonal-to-cubic phase transition, defect formation, and Na⁺ migration.

This work has two important implications for future studies in related fields: (1) selenide based compounds represent a promising platform for the discovery of new fast Na-ion conductors; and (2) the current study opens up new opportunities for the exploration of Se and/or S based battery chemistry, such as the construction of all-solid-state Na/Se battery.