Stimuli-responsive “solvate-sponge”-(DMF)3NaClO4 exhibits linear chains of DMF–Na+ ions with ClO4− anions in the interstitial space. At increased pressure or temperature, DMF is expelled (reversibly), resulting in a new stoichiometry-(DMF)2NaClO4.
Glyme-based sodium electrolytes show excellent electrochemical properties and good chemical and thermal stability compared with existing carbonate-based battery electrolytes. In this investigation, we perform classical molecular dynamics (MD) simulations to examine the effect of concentration and temperature on ion−ion interactions and ion− solvent interactions via radial distribution functions (RDFs), mean residence time, ion cluster analysis, diffusion coefficients, and ionic conductivity in sodium hexafluorophosphate (NaPF 6 ) salt in diglyme mixtures. The results from MD simulations show the following trends with concentration and temperature: The Na + ---O(diglyme) interactions increase with concentration and decrease with temperature, while the Na + ---F(PF 6 − ) interactions increase with concentration and temperature. The mean residence time suggests that Na + ---O(diglyme) are significantly longer lived compared with that of Na + ---F(PF 6 − ) and H (diglyme)---F(PF 6 − ), which shows the affinity of diglyme to the Na + ions. The ion cluster analysis suggests that the Na + ions largely exist as solvated ions (coordinated to diglyme molecules), whereas some fractions exist as contact-ion pairs, and negligible fractions as aggregated ion pairs, with the latter two increasing slightly with temperature and more with ion concentration. The magnitude of the diffusion coefficients of Na + and PF 6 − ions decreases with concentration and increases with temperature, where the Na + ion has slightly lower mobility compared with the PF 6 − anion. The simulated total ionic conductivities show qualitative trends comparable to experimental data and highlight the need for the inclusion of ion−ion correlations in the Nernst−Einstein equation, especially at higher concentrations and lower temperatures.
(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 (E a,DFT = 43 kJ mol–1, E a,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.
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