Solvation Structure and Concentration in Glyme-Based Sodium Electrolytes: A Combined Spectroscopic and Computational Study

The optimal salt concentration used in metal-ion energy storage devices has long focused heavily on 1 M electrolytes; however, recent evidence suggests taking a deeper look at electrolyte properties as a function of salt concentration. Toward that goal, the effect of concentration on solvation properties for a prototype sodium electrolyte is considered with potential applications for sodium-ion and sodium-air technologies. An empirical force field for sodium triflate in digylme, an electrolyte already in use with sodium air systems, was developed from ab initio molecular dynamics simulations in conjunction with the variational force-matching method. Atomistic simulations of this electrolyte along with Fourier transform infrared (FTIR) experimental studies validate the qualitative accuracy of the model and demonstrate its transferability across different concentrations. The solvation structure and the extent of ion pairing effects in the electrolyte were considered for concentrations ranging from 0.25 to 2.0 M in the sodium salt. Ion pairing effects are seen even at dilute concentrations of 0.5 M in both simulations and experiments, with a transition from solvent-separated species to direct contact ion pairs as the concentration increased to 1.5 M. With further increase in the concentration, evidence for ion aggregation is also presented.