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

Abstract

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 (σion) of Na3-xZnGaS4-xIx, from ca. 10−3 mS·cm−1 in pristine Na3ZnGaS4 to 1.12 mS·cm−1 in Na2.9ZnGaS3.9I0.1. 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 σion, highlighting the effectiveness of disturbing the Na site. Cl- or Br-doped Na3ZnGaS4 similarly enhances σion, 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 Na2.9ZnGaS3.9I0.1. Finally, due to its electrochemical stability and high σion, we exemplify the practicality of Na2.9ZnGaS3.9I0.1 in all-solid-state batteries using Na2Sn ‖ TiS2 full cells.