Ionic liquid (IL) electrolytes have been studied for Li-metal and Li-ion batteries (LIBs) owing to their large electrochemical windows, negligible volatility, and non-flammability. However, ion aggregates form as a result of strong interactions between the IL anion and Li⁺, thus leading to small Li⁺ transference. Such ion aggregates slow the vehicular motion of Li⁺where Li⁺ moves with its solvation shell. However, the structural diffusion, where Li⁺ jumps out of its solvation shell due to structural fluctuations or anion exchanges, is reported to gain significance towards higher Li-salt concentrations (x_Li> 0.3). While the addition of excess Li-salt to IL electrolytes would intuitively lead to more cluster formation, several recent studies reported improved Li⁺ transport, interfacial stability, and cycling with concentrated IL electrolytes in batteries.

In this study, the solvation of Li⁺ by different coordination sites of cyano(trifluoromethanesulfonyl)imide ([CTFSI]) anion in pyrrolidinium IL is examined by Raman Spectroscopy and Molecular Dynamics (MD) simulations. It is seen that at low Li-salt concentrations Li⁺-CN coordination is preferred and at x_Li> 0.3 the Li⁺ solvation environment can be represented by a mixture of sulfonyl and cyano coordination. Li⁺ transference is estimated from the measured NMR diffusivities as well as MD simulations and it is found to increase at high Li-salt concentrations. Overall, the anion asymmetry is found to significantly influence the solvation structure and Li⁺ transport. While the simple mixture of the symmetric anion analogues of [TFSI] and dicayanamide present phase separation above x_Li> 0.05, the IL electrolyte with [CTFSI] is a homogenous liquid that turns into a clear gel at x_Li=0.5 and further a paste at x_Li=0.7. The Li-LiFePO₄ half cells fabricated with IL electrolyte (x_Li=0.3, 0.5, and 0.7) demonstrated a nominal capacity of at least 140 mAh/g at 0.1C rate and 90 °C. As the C-rate is increased, the cell with x_Li=0.3 demonstrated the best overall rate capability due to its lower resistance. Interestingly, as the C-rate is increased to 1C and returned to 0.2C, the cell with x_Li=0.7 demonstrated the best capacity retention and with superior rate capability compared to x_Li=0.5. These results show the tunability of Li⁺ solvation and mobility by asymmetric anions, and superior electrochemical stability achieved with high Li-salt concentrations that are not achievable otherwise with symmetric anions.

Drace Penley

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