Promoting Fast Ion Conduction in Li-Argyrodite through Lithium Sublattice Engineering

Fundamental understanding of ionic transport plays a pivotal role in designing and optimizing fast ionic conductors. Here, through a systematic neutron scattering and theoretical investigation, we discovered new insights about how anion sublattice affects Li+ distribution and transport in Li-argyrodite. We found that the promotion of Li+ conductivities is strongly correlated with a previously overlooked Li+ interstitial site (16e), which is critical for realizing intercage Li+ migration. More isotropic Li+ migration pathways with higher Li+ occupancies on the interstitial 16e site are found to be the underlying reason for the much higher Li+ conductivity in Li6PS5Cl relative to the Br- and I-based analogues. We further confirm that they are also the universal driving force for the ultrahigh Li+ conductivities in both anion-substituted Li-poor (Li6-aPS5-aXa, X = Cl and Br) and cation-substituted Li-rich argyrodite (e.g., Li6+aGeaP1-aS5I and Li6+aSiaSb1-aS5I). It is expected this strategy can be generally adopted to improve the ionic conductivity of the broad family of Li-rich argyrodite and beyond.