Conduction channel creation by interstitial site engineering in otherwise insulating Na6ZnS4 for Na-conducting solid-state electrolytes

Na_5.6Zn_0.6Ga_0.4S₄, which retains the crystalline structure of its parent form Na₆ZnS₄, is described as a new class of Na-conducting solid-state electrolytes (SSEs) for all-solid-state batteries. We demonstrate that while Na₆ZnS₄ is ionically insulating (1.4 nS cm^-1), Ga-substitution results in an astonishing improvement of ionic conductivity (σ_ion) to 70.1 μS cm^-1, making Na_5.6Zn_0.6Ga_0.4S₄ a practical SSE. This dramatic increase in σ_ion (5 × 10⁴ fold) is associated with an increased Na⁺ occupancy in interstitial sites as ‘x’ increases in Na_6-xZn_1-xGa_xS₄, where interstitial Na ions facilitate long-range Na⁺ conduction, which is otherwise immobile. Ga-substitution also results in phase-pure Na_6-xZn_1-xGa_xS₄, contributing at least partially to the enhancement of σ_ion. Furthermore, Na_6-xZn_1-xGa_xS₄ exhibits neither releasing H₂S gas nor compromising its crystalline structure for several hours under ambient conditions. Ga-substitution also enhances electrochemical stability. While the anodic limit remains largely unchanged, the cathodic limit is significantly lowered from 0.99 V vs. Na₂Sn in Na₆ZnS₄ to 0.35 V in Na_5.6Zn_0.6Ga_0.4S₄, resulting in stable Na alloying/dealloying reactions in a symmetric Na₂Sn ‖ Na₂Sn cell. These findings are comprehensively supported by various experimental and theoretical methods. Finally, we construct a full cell (Na₂Sn ‖ TiS₂) and demonstrate the practicality of Na_5.6Zn_0.6Ga_0.4S₄ as a promising SSE in all solid-state Na ion batteries.