Vacancies Introduced during the Crystallization Process of the Glass-Ceramics Superionic Conductor, Na₃PS₄, Investigated by Neutron Total Scattering and Reverse Monte Carlo Method

Sodium solid electrolytes are expected to be safer and more durable than lithium solid electrolytes, which affect the properties of all-solid-state lithium-ion batteries nearing commercialization. The heat treatment of Na₃PS₄glass ceramics, a strong candidate for sodium solid electrolytes, results in ionic conductivity exceeding 10^-4S cm^-1at ambient temperature, depending on the heat-treatment conditions. First-principles calculations have suggested that vacancies at Na sites promote ionic conduction; however, the existence of vacancies has not been confirmed through a conventional average structure analysis of X-ray and neutron diffraction profiles or pair distribution functions. In this study, local structure analysis of Na₃PS₄glass ceramics using reverse Monte Carlo modeling was performed on the pair distribution function obtained from neutron total scattering measurements during and after heat treatment to identify the vacancy locations that differ from the Na occupation sites. The vacancies in Na₃PS₄glass ceramics after heat treatment revealed a more disordered atomic arrangement than Na, suggesting that the vacancies connecting Na occupation sites are most likely responsible for the promotion of Na ion conduction. The control of vacancies in solid electrolytes is essential for the design of practical all-solid-state sodium batteries.