In summary, our calculations performed on the tetragonal LLZO polymorph are in agreement with published experimental work on the cubic polymorph. This implies that our results on the tetragonal phase are good indications of what to expect of the cubic. We suggest the disappearance of the Li 16f sites as a possible explanation for the large difference in ionic conductivity between the tetragonal and cubic polymorphs. Doping with Y does not result in an increased V ′ Li concentration. Doping with Al, Ga, Nb, or Ta will increase the V ′ Li concentration and increase disorder on the Li sublattice, thus stabilising the cubic LLZO structure. Al and Ga are located on tetrahedrally coordinated Li sites and will as a result block Li paths and disrupt Li ion mobility. Doping LLZO with Ga has been shown experimentally to give the best ionic conductivities, but our calculations show that doping with Ga will result in some trapping and less stabilisation of V ′ Li than the other donor defects. Therefore, we suggest that the effect of dopants on the sintering process is the most important property of the dopant after the ability to stabilise the cubic LLZO structure.