Considering the ionic radii of 4 and 6 coordinated Al (0.39 Å, 0.535 Å) and Ga (0.47 Å, 0.62 Å) ions, a preference for the smallest sites, Li 8a/24d (0.59 Å), is expected. However, despite what is reported on the site preference of Al and Ga ions in cubic LLZO, the low formation energy for Al(Ga) ′ Zr is likely due to the small charge difference between the Al(Ga) and Zr ions. Considering the similarity between the Zr sites in the tetragonal and the cubic structure, we do not expect the formation energies of dopants at Zr sites to be notably different in the cubic structure. However, the same argument cannot be made for the Li sites, and therefore, the formation energies of dopants at Li sites in the cubic structure could be slightly different to what is presented in Figure . There exists a high degree of ambiguity on the site preference of Al and Ga in cubic LLZO in the reported literature ; our results suggest a preference for the Li 24d sites, but do not discount the possibility of Al or Ga being located on Zr or Li 96h sites.