Ionic liquid electrolytes (ILEs) have become popular in various advanced Li-ion battery chemistries because of their high electrochemical and thermal stability, and low volatility.
In their paper 'Insights into the transport and thermodynamic properties of a bis(fluorosulfonyl)imide-based ionic liquid electrolyte for battery applications' (published in Journal of Physical Chemistry Letters) Jack Fawdon and his colleagues and collaborators considered the relatively high viscosity and poor Li+ diffusion, and the idea that the formation of large concentration gradients reduce rate capability.
For the first time, operando Raman microspectroscopy was utilized to visualize ILE concentration gradients. Using lithium bis(fluorosulfonyl)imide (LiFSI) in N-propyl-N-methylpyrrolidinium FSI, its 'apparent' diffusion coefficient, lithium transference number, thermodynamic factor, ionic conductivity and resistance of charge transfer against lithium metal were isolated.
The analysis of these concentration ingredients led to insights into the bulk structure of ILEs, and it is proposed that they are composed of large, ordered aggregates.