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 S2- 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 Mg2+ and Ca2+ may be possible in the right framework.