Vacancy-Driven Na+ Superionic Conduction in New Ca-Doped Na3PS4 for All-Solid-State Na-Ion Batteries

Abstract

Mechanically sinterable sulfide Na+ superionic conductors are key to enabling room-temperature-operable all-solid-state Na-ion batteries (ASNBs) for large-scale energy storage applications. To date, few candidates can fulfill the requirement of a high ionic conductivity of >= 1 mS cm(-1) using abundant, cost-effective, and nontoxic elements. Herein, the development of a new Na+ superionic conductor, Ca-doped cubic Na3PS4, showing a maximum conductivity of similar to 1 mS cm(-1) at 25 degrees C is described. Complementary analyses using conductivity measurement by the AC impedance method, Na-23 nuclear magnetic resonance spectroscopy, and density functional theory calculations reveal that the aliovalent substitution of Na' in Na3PS4 with Ca2+ renders a cubic phase with Na vacancies, which increases the activation barriers but drastically enhances Na-ion diffusion. It is demonstrated that TiS2/Na-Sn ASNBs emp oying Ca-doped Na3PS4 exhibit a high charge capacity of 200 mA h g(-1) at 0.06C, good cycling performance, and higher rate capability than those employing undoped cubic Na3PS4