Mechanism of ion conduction and dynamics in tris(N,N-dimethylformamide) perchloratosodium solid electrolytes

Abstract

(DMF)3NaClO4 is a soft-solid cocrystalline electrolyte with channels of Na+ ions, which can be reversibly converted to a less conductive form (DMF)2NaClO4 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-1 for 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 ClO4- in the coordination sphere of the transition state. In contrast, the calculated activation energy barrier for the 2:1 stoichiometry is higher (Ea,DFT = 43 kJ mol-1, Ea,exp = 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.