Hopping Rate and Migration Entropy as the Origin of Superionic Conduction within Solid-State Electrolytes

Inorganic solid-state electrolytes (SSEs) have gained significant attention for their potential use in high-energy solid-state batteries. However, there is a lack of understanding of the underlying mechanisms of fast ion conduction in SSEs. Here, we clarify the critical parameters that influence ion conductivity in SSEs through a combined analysis approach that examines several representative SSEs (Li₃YCl₆, Li₃HoCl₆, and Li₆PS₅Cl), which are further verified in the xLiCl-InCl₃ system. The scaling analysis on conductivity spectra allowed the decoupled influences of mobile carrier concentration and hopping rate on ionic conductivity. Although the carrier concentration varied with temperature, the change alone cannot lead to the several orders of magnitude difference in conductivity. Instead, the hopping rate and the ionic conductivity present the same trend with the temperature change. Migration entropy, which arises from lattice vibrations of the jumping atoms from the initial sites to the saddle sites, is also proven to play a significant role in fast Li⁺ migration. The findings suggest that the multiple dependent variables such as the Li⁺ hopping frequency and migration energy are also responsible for the ionic conduction behavior within SSEs.