Unveiling Ultra-High Ionic Conductivity in W-Doped NaSbS: Grain Boundary Effects and Pure Bulk Transport.

W-doped NaSbS is a promising solid electrolyte for all-solid-state sodium batteries, exhibiting a sodium (Na) ionic conductivity higher than 30 mS cm (A. Hayashi, N. Masuzawa, S. Yubuchi, F. Tsuji, C. Hotehama, A. Sakuda, M. Tatsumisago, 2019, 10, 5266). This exceptional conductivity arises primarily from the introduction of sodium ion vacancies (V') via supervalent substitution of Sb with W. Using low-temperature impedance spectroscopy down to = 113 K (-160 °C), we demonstrate that previously reported room temperature conductivities of NaSbWS are influenced by grain boundary resistances, which can only be effectively separated from the total conductivity at such low temperatures. Our results indicate that the pure Na bulk conductivity can reach 96 mS cm (D = 0.98 × 10 m s) at room temperature, as extrapolated from accurately measured low-temperature data (1.8 mS cm at -130 °C). Our study suggests that further minimizing detrimental grain boundary effects enables extraordinarily fast long-range Na ion transport in this sulfide.