Eutectic Electrolytes Composed of LiN(SO₂F)₂ and Sulfones for Li-Ion Batteries

Abstract

Sulfones are polar molecules that can be used as thermally stable electrolyte solvents for Li-ion batteries (LIBs). Li salts form stoichiometric solvates with sulfones in the electrolytes. The melting points of the solvates tend to be higher than room temperature, thereby limiting the operating temperature range of the batteries. In this study, the applicability of ternary eutectic mixtures of LiN­(SO2F)2 (LiFSA), sulfolane (SL), and dimethyl sulfone (DMS) as LIB electrolytes was assessed. Relative to the binary LiFSA–sulfone electrolytes, the ternary eutectic electrolytes remained liquid over a wide temperature range due to the increased entropy of mixing. Sulfone-bridged Li+–sulfone–Li+ and anion-bridged Li+–FSA––Li+ network structures were formed in the eutectic electrolyte with a composition of [LiFSA]/[SL]/[DMS] = 1/1.5/1.5. Pulsed-field gradient NMR measurements revealed that the Li+ ion dynamically exchanges sulfones and anions and diffuses more rapidly than these ligands, resulting in the relatively high Li+ transference number of the electrolyte. Highly reversible charge–discharge processes of the LiCoO2 and graphite electrodes were attained using the ternary eutectic electrolyte. The rate capability of the Li/LiCoO2 cell in the eutectic electrolyte was comparable to that of the cell in the conventional 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate solution despite its lower ionic conductivity.

Temperature Dependency of Ion Transport in Highly Concentrated Li Salt/Sulfolane Electrolyte Solutions

“…Li salts and SL form crystalline solvates, which have melting points higher than room temperature. ,, Therefore, precipitation of the crystalline solvates often occurs in SL-based electrolytes containing high concentration Li salts. Among various Li salt/SL binary mixtures, the LiBF 4 /SL mixtures were found to maintain their liquid state over a wide temperature range even at high salt concentrations .…”

“…In these networks, Li + ions exchange ligands (solvent and anion) dynamically, and Li + -ion-hopping conduction is assumed to occur through ligand exchange. Li + -ion-hopping conduction increases the Li + -ion transference number under anion-blocking conditions (0.6–0.8), which leads to the suppression of concentration polarization in Li batteries during discharging at high current densities. ,, However, the detailed mechanism of Li + -hopping conduction in these electrolytes remains unclear. In this study, we investigated the transport properties of highly concentrated Li-salt/SL electrolytes at various temperatures.…”

Study on Fundamental Properties of Solvate Electrolytes and Their Application in Batteries

“…10. 54 Further, this electrolyte shows Li + ion-hopping conduction, and the Li + transference number is 0.45. The ionic conductivity of this electrolyte is 2.28 mS cm -1 (at 30 °C), which is considerably lower than that of a carbonate-based electrolyte, 1 mol dm -3 LiPF6 in EC/DMC…”

Does Li-ion transport occur rapidly in localized high-concentration electrolytes?

“…Additionally, we recently found that certain HCEs exhibit a unique ion transport property, which involves conduction via a lithium-ion hopping mechanism. [13][14][15][16][17][18] The solvent-to-Li molar ratio decreases with increasing Li salt concentration in HCEs, because a greater fraction of the solvent participates in Li + solvation. Anions also are involved in Li + coordination, and contact ion pairs (CIPs) and ion aggregates (AGGs) form in HCEs.…”

“…Lithiumion hopping causes the Li + transference number in certain HCEs to become greater than 0.5, whereas this parameter is less than 0.3 in conventional 1 M electrolytes. 7,18 A large Li + transference number is advantageous in suppressing concentration polarization during high-rate charging and discharging in a battery. 19,20 One disadvantage of HCEs is their high viscosity.…”