It has recently been shown, in the case of the bisfluorosulfonamide (FSI) based ionic liquids, that as the concentration of the alkali metal salt (LiFSI or NaFSI) is increased, the alkali metal cation transport number increases, along with an increase in viscosity and decrease in conductivity. At the same time, significant enhancements in electrochemical stability and rate performance of devices are also observed. Somewhat surprisingly, significant amounts of water can often be added to these electrolytes without a detrimental effect on Li and Na electrochemistry. The dynamics in these highly concentrated systems are different from the lower concentration analogues as observed from NMR diffusion measurements and also verified by MD simulations. Speciation in these systems is also changed since, with increasing alkali metal salt concentration towards a 1:1 mixture, the available FSI anions required to satisfy the coordination environment of the alkali metal cation (between 3 and 5) will become fewer and insufficient unless each anion begins to coordinate more than one metal cation, for example by bridging two or even three Li<sup>+</sup> or Na<sup>+</sup> ions. This would result in significant aggregation and structuring of the ionic liquid as observed via our MD simulations which also likely influences the electrochemical processes at the Li or Na metal electrodes leading to the improved performance.

Ion structure and dynamics in highly concentrated ionic liquid-alkali metal salt electrolytes

255th National Meeting (2018)

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