All phases were studied using periodic density functional theory, as implemented in the Vienna Ab initio Simulation Package, which has been shown to be well-suited to model the properties of batteries. The electron-core interactions are represented by the Projector Augmented Wave (PAW) method. Calculations were first performed for the rhombohedral unit cells of the two NASICON phases consisting of 6 formula units each with the Brillouin zone sampled using a 4×4×1 k-point grid. For adsorption, intercalation and ion diffusion, different supercells, which are specified in the respective sections, were used and the k-point grid was scaled accordingly in each lattice direction.