Revealing the Structural Stability and Na-Ion Mobility of 3D Superionic Conductor Na₃SbS₄ at Extremely Low Temperatures

All-solid-state batteries that employ superionic solid conductor potentially enable the broadening of battery operation in harsh environments, such as under subzero temperatures and even lower. The solid electrolyte as the key component requires structural stability, high-efficiency of ion transportation channels, and low activation energy to maintain the fast-ionic conduction against temperature drop. Herein, we use 3D superionic conductor Na₃SbS₄ as a model to investigate the structure and conductive mechanism at extremely low temperature. Cryogenic in situ neutron and X-ray diffractions reveal that Na₃SbS₄ maintains a stable tetragonal crystal structure and the anisotropic lattice contraction upon cooling. The dimensions of the S-gate (represented by the S-S pair length) that Na ions hop through in the 3D transportation network is found to maintain open sizes in the xy-plane, contributing to the low activation energy and impressive ionic conductivity. The Na-ion transportation network is demonstrated to be directionally accessible at the extremely low temperature, which reveals the ion conductive mechanism at broadened temperature range in the view of structure. These findings provide valuable guidance in the search for materials as promising solid electrolyte in solid-state batteries to fulfill harsh environmental needs.