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.

Li-Ion Transport and Solution Structure in Sulfolane-Based Localized High-Concentration Electrolytes

“…The development of high-energy-density and low-cost rechargeable batteries is important to realize a shift from fossil fuel sources to those of renewable energy. , Electrolyte materials play a crucial role in realizing the practical applications of next-generation rechargeable batteries. Among the proposed electrolyte materials, highly concentrated electrolytes (HCEs) exhibit improved thermal and electrochemical stabilities as well as stable cycling of high-voltage positive electrodes and lithium metal negative electrodes. − Moreover, we previously reported the unique exchange/hopping-like Li + -ion transport in sulfolane (SL)-based HCEs containing Li salts . In the SL-based HCEs, SL coordinates to two vicinal Li + ions forming an SL- and anion-bridged, chain-like Li + coordination structure, which contributed to the highly efficient Li + -ion transport; this has been evidenced by the rapid diffusion of Li + ions compared to that of SL and the counter anions as well as a high Li + -ion transference number (0.7–0.8) under anion-blocking conditions. − Furthermore, the SL-based HCEs demonstrated an improved rate performance in Li-ion batteries and Li–S batteries. ,, …”

Lithium Aluminate Nanoflakes as an Additive to Sulfur Cathodes for Enhanced Mass Transport in High-Energy-Density Lithium–Sulfur Pouch Cells Utilizing Sparingly Solvating Electrolytes

“…The significantly large current flux acting on the Li anode in a Li–S pouch cell induces the inhomogeneous current and facilitates the further growth of detrimental Li dendrites and therefore is considered the prime deteriorating factor . Additionally, the current electrolyte composition (Li­[TFSA] 0.9 [FSA] 0.1 /SL 2 /HFE 2 ) is still not perfect in terms of Li anode reversibility. , Due to the compatibility with the sulfur cathode, this composition is selected in this study . The evolution of Li dendrites increases the effective surface area of the Li metal anode which aggravates the undesired side reaction with the electrolyte solvents, and the nascent Li dendrites spontaneously react with the electrolyte to form new solid-electrolyte interphases.…”

“…[TFSA] 0.9 [FSA] 0.1 /SL 2 /HFE 2 ) is still not perfect in terms of Li anode reversibility. 37,62 Due to the compatibility with the sulfur cathode, this composition is selected in this study. 37 The evolution of Li dendrites increases the effective surface area of the Li metal anode which aggravates the undesired side reaction with the electrolyte solvents, and the nascent Li dendrites spontaneously react with the electrolyte to form new solid-electrolyte interphases.…”

Enhancing Li–S Battery Performance with Limiting Li[N(SO2F)2] Content in a Sulfolane-Based Sparingly Solvating Electrolyte

“…Such side reactions are greatly suppressed when Li­[FSA] is used as a salt, compared with Li­[TFSA] . Furthermore, reduction potential and current of imide anions (especially [FSA] anion) are greatly upshifted and enlarged, respectively, in an SL-based high-concentration-electrolyte (HCE) and the HCE diluted by HFE (localized high-concentration-electrolyte, LHCE) . The [FSA] anion reduction (SEI formation) in the first sweep was accelerated at 60 °C, compared with that at 30 °C, which was revealed by slow-scan voltammetry.…”