(Invited) The Complex Mechanisms That Create High Li-Ion Mobility in Oxides and Sulfides

The exciting development of solids with extremely high Li-ion conductivity, higher than that in traditional liquid electrolytes, in the last decade has forced us to rethink the processes that make cations move in solids. Super-ionic Li conductors typically have very high Li concentration so that their conductivity behavior cannot be explained with simple point defect mechanisms. Instead, ideas of concerted motion or highly correlated motion have been proposed. I will discuss our current understanding of the mechanisms that provide high Li-ion conductivity. We find that due to the high screening power of S 2- Li mobility in sulfides is mostly determined by local anion coordination and its variation along the migration pathway, making anion topology the main determinant of Li-ion conductivity. As a result, we find that there is little coordinated or concerted motion in sulfides. In contrast, due to the lower screening power of oxides and their lower Li-Li distances, Li-ion conductivity is much more determined by the electrostatic interaction with the other cations, setting the stage for possible concerted motion of multiple ions. I will show how this leads to very specific, but precise rules for selecting structures that may accommodate very fast Li-ion motion. While fast motion of divalent cations is much less likely, I will show computational results that indicate that even super-ionic motion of ions such as Mg 2+ and Ca 2+ may be possible in the right framework.