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 (xLi &gt; 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 xLi &gt; 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 xLi&gt; 0.05, the IL electrolyte with [CTFSI] is a homogenous liquid that turns into a clear gel at xLi=0.5 and further a paste at xLi=0.7. The Li-LiFePO4 half cells fabricated with IL electrolyte (xLi=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 xLi=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 xLi=0.7 demonstrated the best capacity retention and with superior rate capability compared to xLi=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.

Lithium solvation and mobility in ionic liquid electrolytes with asymmetric anion

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