Neither Nb nor Ta are located on the Li sublattice, and thus do not give strong V ′ Li trapping, and have very low formation energies. However, both Al and Ga are reported with similar ionic conductivities despite being located on the Li sublattice, and showing relatively high binding energies. Doping with Al, Ga, Nb or Ta increases disorder within the structure, in particular on the Li sublattice, and therefore helps stabilise the cubic phase. We can therefore conclude that the stabilisation of the cubic structure is the main contribution to the high ionic conductivity and that the differences in the V ′ Li concentrations are less important. Such a conclusion is in good agreement with the work carried out by Allen et al. wherein they showed that codop- ing Ta-stabilised cubic LLZO with Al and Ga resulted in a reduction in the ionic conductivity from the monodoped Ta:LLZO (cubic phase). The structure codoped with Ta and Ga displayed a decreased reduction in ionic conductivity compared to the Ta and Al-doped structure, furthermore indicating that Al and Ga doping into the Li sublattice disrupts Li-ion mobility.