Ga-doped LLZO is commonly reported with a higher ionic conductivities than the Al-doped variations. Our results, showing lower formation energies for Al defects, indicate that Al should be easier to incorporate into the structure, thus stabilising more V ′ Li and increasing the number of charge carriers. However, this enhanced incorporation of Al in the structure will, in addition to increase the V ′ Li concentration, result in more disruption of the Li diffusion paths than Ga. Furthermore, Al and Ga are known to segregate towards the grain boundaries forming competing phases with relatively low melting temperatures which beneficially act as sintering aids. In so, Al and Ga crucially affect the poor grain boundary conductivity in poly-crystalline LLZO. Based on Ga substitutional defects being less energetically favourable than Al defects in the structure, and thus Ga is expected to show a lower solid solubility, we expect this effect to be greater in Ga doped structures. The effect of Al and Ga acting as a sintering aid is likely a part of the explanation for why the reported ionic conductivities of both Al and Ga doped LLZO are similar to the Nb and Ta doped structures.