Unveiling Ultra-High Ionic Conductivity in W-Doped Na₃SbS₄: Grain Boundary Effects and Pure Bulk Transport

W-doped Na₃SbS₄ is a promising solid electrolyte for all-solid-state sodium batteries, exhibiting a sodium (Na⁺) ionic conductivity higher than 30 mS cm^-1 (A. Hayashi, N. Masuzawa, S. Yubuchi, F. Tsuji, C. Hotehama, A. Sakuda, M. Tatsumisago, Nat. Commun. 2019, 10, 5266). This exceptional conductivity arises primarily from the introduction of sodium ion vacancies (V'_Na) via supervalent substitution of Sb⁵⁺ with W⁶⁺. Using low-temperature impedance spectroscopy down to T = 113 K (-160 °C), we demonstrate that previously reported room temperature conductivities of Na_2.9Sb_0.9W_0.1S₄ 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^-1 (D_σ = 0.98 × 10^-10 m² s^-1) at room temperature, as extrapolated from accurately measured low-temperature data (1.8 mS cm^-1 at -130 °C). Our study suggests that further minimizing detrimental grain boundary effects enables extraordinarily fast long-range Na⁺ ion transport in this sulfide.