Y is commonly used as a sintering aid in LLZO helping with densification, which in turn results in higher ionic conductivities. Gai et al. have shown that co-doping with Nb and Y gives good ionic conductivity and improved air stability. The effect of Y-doping on the stabilisation of the cubic phase has not been explicitly studied despite Ydoped cubic LLZO having been reported. The site preference of Y and the resulting defects are also not clear, with Y Zr and Y La sites being reported in the literature. Here we use hybrid density functional theory (DFT) to investigate the role of Al, Ga, Nb, Ta, and Y doping in tetragonal LLZO. We calculate and compare defect formation energies, and investigate binding energies between dopants and charge carriers. Furthermore, we use calculated defect formation energies to predict changes in V ′ Li concentrations resulting from the addition of the different dopants.