Effect of Electrolyte Concentration on the Solvation Structure of Gold/LITFSI–DMSO Solution Interface
Lei Wang, Kohei Uosaki, Hidenori Noguchi
Abstract
The use of high-concentration electrolytes in lithium metal batteries enables the effective suppression of lithium dendrite growth at the lithium anode. The issue of the solvation structure at the electrolyte/electrode interface in low- and high-concentration electrolytes must be addressed to understand the electroreduction stability and solid–electrolyte interphase (SEI) formation. The concentration-dependent solvation structure of lithium bis(trifluoromethanesulfonyl)imide (LITFSI)–dimethyl sulfoxide (DMSO) on a gold model electrode has been investigated with various electrolyte concentrations using surface-enhanced Raman spectroscopy. At low concentrations, free DMSO and TFSI– were found to be chemisorbed on the gold surface to form an adsorption layer. Among these, TFSI– was chemisorbed via its oxygen atoms based on the observation of the ν(Au–O) bond. Besides, chemisorbed TFSI– at the gold surface did not coordinate with Li+ in the low-concentration electrolyte. As the concentration gradually increased up to 2.31 M, chemisorbed TFSI– started to coordinate with Li+. On further increase in LITFSI concentration above 2.31 M, chemisorbed DMSO was absent in the adsorption layer; instead, chemisorbed TFSI– became the only species. The chemisorbed TFSI– is proposed to be coordinated with Li+ by two of its oxygens in such high concentration, making its coordination saturated. The DMSO-free adsorption layer in electrolytes with concentration above 2.31 M might help to form a dense and uniform SEI once an electrical field is applied, which contributes to improve the long-term cyclability of batteries.