Predicting Ion Association in Sodium Electrolytes: A Transferrable Model for Investigating Glymes

© 2017 American Chemical Society. Given the prior success in developing lithium batteries for similar purposes, many of the same types of solvent molecules and salt pairings have been investigated as electrolytes in sodium-ion and sodium-oxygen systems. Of these candidates, ether-containing electrolytes have emerged as a promising material as a result of their electrochemical stability and utility in tuning the pertinent electrochemistry. The ability for ethers to chelate metal ions provides a unique feature to ion solvation structure; however its role in changing the association of ions in solution has not been fully explored. By using computational simulations validated by FTIR and NMR spectroscopy, detailed descriptions of the changes to solvation structure as a result of chelation and concentration were investigated for a series of ethers (monoglyme to tetraglyme). From these simulations it can clearly be seen that with increasing chelation, ion association is diminished in a nonlinear fashion. For a monoglyme solvent, sodiums are entirely coordinated by triflates in solution, even at low concentrations. In contrast, tetraglymes retain a significant solvent separation of sodium cations from triflate anions even at high concentrations. The former implies that the utility of monoglyme and diglyme solvents for sodium-air batteries in specific is likely linked to their favoring ion association, while the poor performance of tetraglyme is a result of its excessive binding to sodium. Finally, triglyme was shown to produce anomalous behavior as a result of a mismatch between sodium coordination and steric interactions