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

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 ∼1 mS cm–1 at 25 °C is described. Complementary analyses using conductivity measurement by the AC impedance method, 23Na 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 employing 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.