Impact of fluorination on Li⁺ solvation and dynamics in ionic liquid-hydrofluoroether locally concentrated electrolytes

Journal Article · 22 December 2023 · Journal of Power Sources

DOI: https://doi.org/10.1016/j.jpowsour.2023.233984 · OSTI ID:2329453

Penley, Drace  ^[1]; Gerber, Hazel  ^[1]; Garaga, Mounesha N. ^[2]; Wickramasinghe, Nalinda P.  ^[1]; Greenbaum, Steven G. ^[2]; Maginn, Edward J.  ^[3]; Zhang, Yong  ^[3]; Gurkan, Burcu  ^[1]

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1. Case Western Reserve University, Cleveland, OH (United States)
2. Hunter College, New York, NY (United States)
3. University of Notre Dame, IN (United States)

The thermal, physical, structural, and transport properties of ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI] and Li-salts of lithium (nonafluorobutane)(trifluoromethanesulfonyl)imide [Li][IM14] and [Li][TFSI] (0 ≤ x_Li ≤ 0.3), with addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) (0 ≤ x_TTE ≤ 0.45) were studied. While [IM14] increases glass transition, TTE improves conductivity over a wider liquidus range. Li⁺ is solvated primarily by [TFSI] with some contribution by [IM14]. However, both ⁷Li- ¹⁹F HOESY NMR and spatial distribution functions (SDFs) derived from molecular dynamics (MD) indicate short contacts between the fluorines of TTE and Li⁺. This is a result of tighter anion-solvated Li⁺ without aggregation of the Li⁺ solvates, consistent with Raman measurements, thus confirming the existence of fluorous domains in the bulk which leads to improved fluidity and a Li⁺ diffusivity of 1.26 x 10^–12 m²/s at 0 °C (x_Li = 0.20) with a Li⁺ transference of 0.16. Improved transport properties translated to a higher capacity of the TTE/IL electrolyte in a Li||LiFePO₄ half-cell with 95mAh/g at 0.1C, compared to the IL electrolyte without TTE (60 mAh/g). Furthermore, this study demonstrates the tunability of solvation and transport properties in IL electrolytes by asymmetric fluorinated anions and hydrofluoroether co-solvents for low temperature Li-ion batteries.

• Research Organization: Case Western Reserve Univ., Cleveland, OH (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0019409
• OSTI ID: 2329453
• Journal Information: Journal of Power Sources, Vol. 593; ISSN 0378-7753
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English