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.