We first discuss the situation with constrained dopant stoichiometry. Typical nominal dopant concentrations in LLZO are 0.25 dopant per formula unit. Assuming that all dopants are incorporated into the LLZO structure, this results in expected V ′ Li concentrations of ∼ 0.5 and ∼ 0.25 per formula unit for Al/Ga and Nb/Ta, respectively, dashed lines in Figure . This is typically what is assumed as the response to the addition of dopants when making LLZO by solid-state and sol-gel syntheses. However, as indicated by the differences in the defect formation energies, some dopants are less energetically favoured than others, and thus may result in the formation of secondary phases or segregation towards grain boundaries or surfaces. Strain fields associated with grain boundaries are known to often reduce defect formation energies, and lead to defects accumulation at or close to grain boundaries. Secondary phases at grain boundaries may be difficult to detect, but may still strongly affect the sintering properties, density and electrical properties of polycrystals.