Mechanism of Ion Conduction and Dynamics in Tris(N, N-dimethylformamide) Perchloratosodium Solid Electrolytes

(DMF)<inf>3</inf>NaClO<inf>4</inf>is a soft-solid cocrystalline electrolyte with channels of Na⁺ions, which can be reversibly converted to a less conductive form (DMF)<inf>2</inf>NaClO<inf>4</inf>by the application of pressure or heat, leading to a melt- or press-castable electrolyte. Molecular dynamics simulations performed on the 3:1 stoichiometry suggest that Na⁺ions conduct via a one-dimensional channel, which is supported by van-Hove autocorrelation function analysis. The simulations show that the transference number for Na⁺ions is 0.43 at room temperature and exceeds 0.5 at higher temperatures in the molten mixture. The calculated activation energy for the diffusion of Na⁺ions from MD simulations is 45 kJ mol^-1. The minimum-energy path of Na⁺ion migration in a 3:1 crystal is assessed using periodic density functional theory calculations, which provides a barrier of 33 kJ mol^-1for Na⁺ion conduction, in reasonable agreement with the experimental value of 25 kJ mol^-1. The motion of Na⁺ions during conduction is vacancy-driven because the presence of a vacancy site enables jump events for Na⁺ions. The activation energy is the penalty for a sodium ion to leave the octahedrally coordinated DMF ligand field via a transition state where only three molecules of DMF form a 3-O-Na trigonal planar geometry, with no involvement of ClO<inf>4</inf>^-in the coordination sphere of the transition state. In contrast, the calculated activation energy barrier for the 2:1 stoichiometry is higher (E<inf>a,DFT</inf>= 43 kJ mol^-1, E<inf>a,exp</inf>= 49 kJ mol^-1) due at least in part to the partial coordination of strongly binding perchlorate anions with Na⁺ions in the transition state.