Lithium Solvation and Mobility in Ionic Liquid Electrolytes with Asymmetric Sulfonyl-Cyano Anion

• Published in: Journal of chemical and engineering data 2022-08, Vol.67 (8), p.1810-1823
• Main Authors: Penley, Drace, Wang, Xiaoyu, Lee, Yun-Yang, Garaga, Mounesha N., Ghahremani, Raziyeh, Greenbaum, Steve, Maginn, Edward J., Gurkan, Burcu
• Format: Article
• Language: English
• Subjects: Anions; Diffusion; Electrolytes; ENERGY STORAGE; Solvation; Thermophysical and Thermochemical Properties; Viscosity
• ISSN: 0021-9568; 1520-5134
• DOI: 10.1021/acs.jced.2c00294

The solvation structure and transport properties of Li+ in ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium cyano­(trifluoromethanesulfonyl)­imide [PYR14]­[CTFSI] and [Li]­[CTFSI] (0 ≤ x Li ≤ 0.7) were studied by Raman and Nuclear Magnetic Resonance (NMR) diffusometry, and molecular dynamics (MD) simulations. At x Li < 0.3, Li+ coordination is dominated by the cyano group. As x Li is increased, free cyano-sites become limited, resulting in increased coordination via the sulfonyl group. The 1:1 mixture of the symmetric anions bis­(trifluoromethanesulfonyl)­imide ([TFSI]) and dicyanamide ([DCA]) results in similar physical properties as the IL with [CTFSI]. However, anion asymmetry is shown to increase Li-salt solubility and promote Li+ transference. The lifetimes of Li+-cyano coordination for [CTFSI] are calculated to be shorter than those for [DCA], indicating that the competition from the sulfonyl group weakens its solvation with Li+. This resulted in higher Li+ transference for the electrolyte with [CTFSI]. In relation to the utility of these electrolytes in energy storage, the Li–LiFePO4 half cells assembled with IL electrolyte (x Li = 0.3, 0.5, and 0.7) demonstrated a nominal capacity of 140 mAh/g at 0.1C rate and 90 °C where the cell with x Li = 0.7 IL electrolyte demonstrated 61% capacity retention after 100 cycles and superior rate capability owing to increased electrochemical stability.