DFT investigation of Na+ diffusion on a giant NaSICON supercell

NaSICON is referred to a family of crystalline structures with the common property of an easy migration of sodium ions inside the crystal channels, a peculiarity that allows these materials to be employed in solid-state cells as electrolytes or even electrodes. Here we report the results of the investigation of the sodium migration process by means of density functional theory calculations on models of the non-stoichiometric NaSICON material Na3.31Zr1.67(SiO4)1.84(PO4)1.16. The size of the supercell used in this work should soften the spurious effects of periodicity on the final results, i.e. the activation energies corresponding to the migration of the Image 1001 ion between two topologically distinct sodium sites. Different local environments were investigated to highlight the dependence both on type and presence of atomic nuclei in the channel, thus obtaining the activation barriers of the migration process in the presence of single or multiple local Image 1002 substitutions, and in the presence of a Image 1003 ion vacancy. Concurrently, a structural analysis of the NaSICON cavities mainly involved in the process was carried out. The results obtained shed light on the effects of the alterations of the NaSICON local composition on the Image 1004 migration that, other than having an intrinsic interest per se, can be used as a starting point for statistical simulations.