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

D Penley and XY Wang and YY Lee and MN Garaga and R Ghahremani and S Greenbaum and EJ Maginn and B Gurkan, JOURNAL OF CHEMICAL AND ENGINEERING DATA, 67, 1810-1823 (2022).

DOI: 10.1021/acs.jced.2c00294

The solvation structure and transport properties of Li+ in ionic liquid (IL) electrolytes based on n-methyl-nbutylpyrrolidinium 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(trifluoromethanesulfonyeimide (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 Litcyano 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.

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