Investigation of the sodium-ion transport mechanism and elastic properties of double anti-perovskite Na3S0.5O0.5I

Abstract

Sodium-rich anti-perovskites have unique advantages in terms of composition tuning and electrochemical stability when used as solid-state electrolytes in sodium-ion batteries. However, their Na⁺ transport mechanism is not clear and Na⁺ conductivity needs to be improved. In this paper, we investigate the stability, elastic properties and Na⁺ transport mechanisms of both the double anti-perovskite Na₃S_0.5O_0.5I and anti-perovskite Na₃OI. The results indicate that the NaI Schottky defect is the most favorable intrinsic defect for Na⁺ transport and due to the substitution of S²⁻ for O²⁻, Na₃S_0.5O_0.5I has stronger ductility and higher Na⁺ conductivity compared to Na₃OI, despite the electrochemical window being slightly narrower. Divalent alkaline earth metal dopants can increase the Na⁺ vacancy concentration, while impeding Na⁺ migration. Among the dopants, Sr²⁺ and Ca²⁺ are the optimal dopants for Na₃S_0.5O_0.5I and Na₃OI, respectively. Notably, the Na⁺ conductivity of the non-stoichiometric Na₃S_0.5O_0.5I at room temperature is 1.2 × 10⁻³ S cm⁻¹, indicating its great potential as a solid-state electrolyte. Moreover, strain effect calculations show that biaxial tensile strain is beneficial for Na⁺ transport. Our work reveals the sodium-ion transport mechanism and elastic properties of double anti-perovskites, which is of great significance for the development of solid-state electrolytes.