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

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 (LiTFSA) and lithium bis(fluorosulfonyl)amide (LiFSA) 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 LiFSA solutions than in the LiTFSA solutions. Particularly, the CE of the LiFSA HCE and LHCE solutions reached 98–99%. The reduction potentials of the chemical species in these solutions followed the order ELi/Li+ < ESL < Eanion. Reflecting on the change in the solvation structures, ELi/Li+ and Eanion 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 LiTFSA and LiFSA 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 LiTFSA solutions was rich in anion fragments and organic compounds, whereas that for the LiFSA 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.