Perturbation of Na-ion distribution for enhancement of ionic conductivity to a practical level of over 1 mS·cm⁻¹ in Na₃ZnGaS₄

Na-ion conducting solid-state electrolytes (SSEs) are increasingly garnering attention as potential alternatives to Li-ion conducting SSEs in all-solid-state batteries. In this study, we demonstrate that replacing a small fraction of S²⁻ with I⁻ in Na₃ZnGaS₄ perturbs Na ion distributions, leading to an increase in vacancy concentration and a decrease in Na-Na distance along the conduction channel. This dual effect of I-substitution results in a remarkable increase in the ionic conductivity (σ_ion) of Na_3-xZnGaS_4-xI_x, from ca. 10⁻³ mS·cm⁻¹ in pristine Na₃ZnGaS₄ to 1.12 mS·cm⁻¹ in Na_2.9ZnGaS_3.9I_0.1. Importantly, this enhancement cannot be achieved through simple vacancy incorporation via aliovalent doping. The incorporation of less I⁻ (x = 0.05) or more I⁻ (x = 0.15 and 0.2) also leads to significant improvements in σ_ion, highlighting the effectiveness of disturbing the Na site. Cl- or Br-doped Na₃ZnGaS₄ similarly enhances σ_ion, but to a more limited extent, underscoring the pivotal role of dopant anion sizes in inducing Na disturbance. We provide evidence for the presence of iodides in a specific sulfur site (S3 site) and the consequential dispersion of Na ions to Na2 and Na3 sites through a combination of theoretical, crystallographic, and spectroscopic studies. Additionally, we demonstrate the electrochemical and moisture stability of Na_2.9ZnGaS_3.9I_0.1. Finally, due to its electrochemical stability and high σ_ion, we exemplify the practicality of Na_2.9ZnGaS_3.9I_0.1 in all-solid-state batteries using Na₂Sn ‖ TiS₂ full cells.