The low-temperature tetragonal phase exhibits synchronous collective Li + ion hopping, causing a high activation energy barrier. In cubic LLZO, the Li + finds unoccupied sites, resulting in uncorrelated Li + motion that costs less activation energy. Hu et al. , and others reported that higher valent ion (Al 3+ ) substitution at Li sublattice improves the stability of cubic garnet of nominal composition in Li 7 La 3 Zr 2 O 12 . The Al 3+ substitution generates more Li sites, leading to more entropy and reducing the free energy. Furthermore, (e) Li + ion diffusion path indicated by iso-surface Li + ion density plot . This figure is taken from ref. (Copyright 2016 Springer Nature). (f) Various LISICON type materials and their conductivities .