Typically, the size and charge of the migrating atom have been considered as determining factors as shown in Fig. for spinel materials. For example, Na has the same charge as Li but is larger leading to a larger barrier with respect to the energy minimum position along a path from tetrahedral through octahedral to the next tetrahedral site. On the other hand, Mg has almost the same size as Li, but twice the charge, also causing a larger migration barrier. However, these two factors are not sufficient to capture a complete picture. The concepts of soft and hard ions have also been invoked, but not in a quantitative way .