Distinct Differences in Li-Deposition/Dissolution Reversibility in Sulfolane-Based Electrolytes Depending on Li-Salt Species and Their Solvation Structures

Abstract

Herein, distinct differences in Li-deposition/dissolution reversibility were found in sulfolane (SL)-based electrolytes, depending on the Li-salt species and their solvation structures, owing to changes in the composition and nature of the solid-electrolyte interphase (SEI) and in the Li-deposit morphology. For this purpose, two lithium salts, lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and lithium bis(fluorosulfonyl)amide (Li[FSA]) were selected. Relatively low-concentration electrolytes (1 mol dm –3, LCEs), high-concentration electrolytes (∼3 mol dm –3, HCEs), and localized high-concentration electrolytes (HCEs diluted by a noncoordinating solvent at 1 mol dm –3, LHCEs) were prepared to alter the solvation structures. The Coulombic efficiency (CE) for Li deposition/dissolution was better in the Li[FSA] solutions than in the Li[TFSA] solutions. Particularly, the CE of the Li[FSA] HCE and LHCE solutions reached 98–99%. The reduction potentials of the chemical species in these solutions followed the order E Li/Li + < E SL < E anion . Reflecting on the change in the solvation structures, E Li/Li + and E anion increased in the following order: LCE < HCE < LHCE, which was established by both experiments and DFT-MD calculations. The anion reduction current for the formation of the SEI was much larger than the SL reduction current and was the largest in the LHCEs for both Li[TFSA] and Li[FSA] solutions. Thus, SEI formation may be mainly attributed to anion reduction, which was accelerated in the HCEs and LHCEs. However, the compositions of the formed SEI were different; the SEI for the Li[TFSA] solutions was rich in anion fragments and organic compounds, whereas that for the Li[FSA] solutions was rich in LiF and inorganic compounds. The difference in the SEI formation process was also supported by DFT-MD calculations. The Li-deposit morphology increased in the order LCE < HCE < LHCE in accordance with the increasing CE. However, the nature and composition of the SEI were the most critical factors for enhancing Li-deposition/dissolution reversibility.