Density functional theory (DFT) calculations have been performed with the VASP code to investigate the effect of Al, Ga, Nb, Ta and Y doping on the defect chemistry of tetragonal LLZO. Handling partial occupancies with DFT is computationally demanding as a large number of possible configurations needs to be evaluated and as such, no calculations were performed on the cubic LLZO structure. Additionally, the cubic structure is not the ground state structure and thus relaxing different configurations of the cubic structure can result in the cells transforming into the tetragonal structure as symmetry constraints must be turned off. The PBEsol 57 (Perdew-Burke-Ernzerhof revised for solids) functional was used for the initial screening of dominant intrinsic defects to reduce computational cost which were then treated with the Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), with mixing parameter α = 0.25 and screening parameter ω = 0.11 bohr -1 (HSE06). Hybrid functionals have consistently been shown to offer a greater agreement to experiment, particularly with respect to defect chemistry. All defects cells (96-atom unit cell) and their respective charge states were subjected to a geometry relaxation, keeping the cell parameters fixed, using a plane wave cutoff energy of 600 eV and a Monhorst-Pack k-point of 2 × 2 × 2. The geometry was optimised until the forces on the ions were below 0.01 eV Å -1 . Li (1s 2 , 2s 1 ), La (4s 2 , 4p 6 , 5s 2 , 5d 1 ), Zr (3s 2 , 3p 6 , 4s 2 , 4d 2 ), O (2s 2 , 2p 4 ), Al (3s 2 , 3p 1 ), Ga (3s 2 , 3d 10 , 4p 1 ), Nb (3s 2 , 3p 6 , 4s 2 , 4d 3 ), Ta (4p 6 , 5s 2 , 5d 3 ), Y (3s 2 , 3p 6 , 4s 2 , 4d 1 ) were treated as valence electrons, and the interactions between valence electrons and cores were described by the projector-augmented wave method (PAW). Inequivalent interstitial sites were determined using a Voronoi tessellation approach as implemented within the pymatgen package.