Thermal, Physical, and Electrochemical Properties of Li[N(SO₂F)₂]-[1-Ethyl-3-methylimidazolium][N(SO₂F)₂] Ionic Liquid Electrolytes for Li Secondary Batteries Operated at Room and Intermediate Temperatures

Abstract

The Li[FSA]-[C 2 C 1 im][FSA] (FSA − : bis(fluorosulfonyl)amide and C 2 C 1 im + : 1-ethyl-3methylimidazolium) ionic liquids have been studied as electrolytes for Li secondary batteries, though their thermal, physical, and electrochemical properties have not been systematically characterized. In this study, the thermal and transport properties of Li[FSA]-[C 2 C 1 im][FSA] ionic liquids as a function of the Li[FSA] molar fraction and temperature, in view of their operation at both room and intermediate temperatures. Differential scanning calorimetric analysis revealed that this system has a wide liquidphase temperature range from Li[FSA] fractions of 0.0 to 0.4 and indicated the existence of the Li[C 2 C 1 im][FSA] 2 line compound. Single-crystal X-ray diffraction analysis was used to determine the crystal structure of Li[C 2 C 1 im][FSA] 2 , which consists of Li + octahedrally coordinated by six O atoms originating from four FSA − anions. The temperature dependences of the viscosity and ionic conductivity were fitted by the Vogel-Tammann-Fulcher equation, and the viscosity and molar ionic conductivity were connected by the fractional Walden rule. Lithium-metal deposition/dissolution efficiency decreased with increasing measurement temperature and decreasing Li[FSA] fraction. Aluminium corrosion at positive potentials was investigated by a potential step method, which revealed that the stability of an aluminium electrode was improved at high Li[FSA] fractions at 298 K and the corrosionlimit potential decreased at elevated temperatures. .

Advances in sodium secondary batteries utilizing ionic liquid electrolytes

“…However, Al corrosion is minimised in Li-based IL electrolytes. 124,[234][235][236] The same behaviour has also been observed in Na systems; Al corrosion also occurs in 1 mol dm À3 Na[TFSA]-PC solution, 152 but not in FSA-or TFSA-based ILs, 237,238 indicating that, in ILs, a stable passivation film is formed on Al electrodes at high potentials.…”

An Energy‐Dense Solvent‐Free Dual‐Ion Battery

“…[ 8 ] Notably, the Al current collector for the graphite cathode recovered from the Na/K[FSA]/graphite cell cycled over 100 cycles at 120 °C shows no signs of corrosion owing to the absence of solvent molecules that can solvate Al 3+ (Figure S15, Supporting Information); even if [Al(FSA) x ] 3− x complexes are formed, they hardly dissolve in the molten salt electrolyte and could serve as a protecting layer. [ 55–57 ] It is also valuable to point out that employing an inorganic molten salt electrolyte eliminates organic cation and solvent cointercalation issues. [ 8,12 ] Moreover, lingering concerns raised from highly flammable and volatile organic solvents [ 46 ] toward reactive alkali metals under abuse conditions (e.g., overcharge and short circuit) are expected to be eliminated by exploiting intrinsically noncombustible and nonvolatile molten salt electrolytes.…”

Electrolytes for Lithium (Sodium) Batteries Based on Ionic Liquids: Highlighting the Key Role Played by the Anion

“…This process is complex and the determination of the number of anions and their mode of interaction with the solute is further complicated by factors such as concentration and temperature dependence and by the unresolved debate around the transport mechanism of charge carrier, vehicular vs. structure diffusion mechanism . While the unambiguous identification of the true nature of the metal species in solution may be difficult to achieve, a small number of reports have described the isolation of crystalline materials suitable for single crystal X‐ray diffraction . It can be argued that these solids are not representative of the real solution speciation, but they are still accurately characterized compounds demonstrating what could be possible in solution; additionally, solid‐state structural studies are complemented by spectroscopic analyses.…”

“…Crystal structures for lithium salts with the [FSI] − anion crystallized from pyrrolidinium ILs have not been reported but a structure is available for Li[emim][FSI] 2 where Li is in an octahedral oxygen environment composed of two bidentate and two monodentate anions.…”