Solvation behavior of carbonate-based electrolytes in sodium ion batteries.

Sodium ion batteries are on the cusp of being a commercially available technology. Compared to lithium ion batteries, sodium ion batteries can potentially offer an attractive dollar-per-kilowatt-hour value, though at the penalty of reduced energy density. As a materials system, sodium ion batteries present a unique opportunity to apply lessons learned in the study of electrolytes for lithium ion batteries; specifically, the behavior of the sodium ion in an organic carbonate solution and the relationship of ion solvation with electrode surface passivation. In this work the Li and Na-based solvates were characterized using electrospray mass spectrometry, infrared and Raman spectroscopy, O, Na and pulse field gradient double-stimulated-echo pulse sequence nuclear magnetic resonance (NMR), and conductivity measurements. Spectroscopic evidence demonstrate that the Li and Na cations share a number of similar ion-solvent interaction trends, such as a preference in the gas and liquid phase for a solvation shell rich in cyclic carbonates over linear carbonates and fluorinated carbonates. However, quite different IR spectra due to the PF anion interactions with the Na and Li cations were observed and were rationalized with the help of density functional theory (DFT) calculations that were also used to examine the relative free energies of solvates using cluster - continuum models. Ion-solvent distances for Na were longer than Li, and Na had a greater tendency towards forming contact pairs compared to Li in linear carbonate solvents. In tests of hard carbon Na-ion batteries, performance was not well correlated to Na solvent preference, leading to the possibility that Na solvent preference may play a reduced role in the passivation of anode surfaces and overall Na-ion battery performance.