Nominally stoichiometric Na3(WxSixSb1−2x)S4 as a superionic solid electrolyte

Abstract

Na₃MX₄ (M = P, Sb and X = S, Se) and its doped analogues are considered as a promising material in room-temperature (RT) Na⁺-conducting solid electrolytes. Herein, we first report that stoichiometric Na₃(W_xSi_xSb_1−2x)S₄ with no nominal vacancies shows significantly high ionic conductivity at RT (σ^RT) when compared with Na₃SbS₄. The σ^RT increases continuously with increases in ‘x’, revealing the highest σ^RT of 13.2 mS cm⁻¹ and the lowest activation of 0.16 eV in cubic Na₃(W_0.2Si_0.2Sb_0.6)S₄. Further increases in ‘x’ result in the formation of a glassy phase and a reduction in σ^RT. The σ^RT of Na₃(W_0.2Si_0.2Sb_0.6)S₄ is the highest in stoichiometric Na₃MX₄ known to date and suggests that the Na⁺ diffusion is influenced by the dopant types as well as structural defects. Ab initio molecular dynamics also reveal the improvement of σ^RT with increases in ‘x’, but the presence of naturally formed vacancies that are commonly observed in Na₃MX₄. The electronic conductivity of Na₃(W_0.2Si_0.2Sb_0.6)S₄ is also low (ca. 10⁻⁶ mS cm⁻¹). However, the cathodic stability is insufficient when W⁶⁺ and/or Si⁴⁺ are doped. Therefore, a solid-state cell (Na₁₅Sn₄∥TiS₂) is fabricated with an interlayer of Na₃SbS₄ between Na₁₅Sn₄ and Na₃(W_0.2Si_0.2Sb_0.6)S₄, and its excellent compatibility with a cathode is demonstrated.