Interplay of Structure and Dynamics in Lithium/Ionic Liquid Electrolytes: Experiment and Molecular Simulation.

Despite their promising use in electrochemical and electrokinetic devices, ionic-liquid-based electrolytes often exhibit complex behavior arising from a subtle interplay of their structure and dynamics. Here, we report a joint experimental and molecular simulation study of such electrolytes obtained by mixing 1-butyl 3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate. More in detail, experiments consisting of X-ray scattering, pulsed field gradient NMR, and complex impedance spectroscopy are analyzed in the light of molecular dynamics simulations to probe the structural, dynamical, and electrochemical properties of this ionic-liquid-based electrolyte. Lithium addition promotes the nanostructuration of the liquid as evidenced from the appearance of a scattering prepeak that becomes more pronounced. Microscopically, using the partial structure factors determined from molecular dynamics, this prepeak is shown to correspond to the formation of well-ordered positive/negative charge series and also large aggregates (Li(BF)), which develop upon lithium addition. Such nanoscale ordering entails a drastic decrease in both the molecular mobility and ionic conductivity. In particular, the marked association of Li cations with four BF anions and long ion pairing times, which are promoted upon lithium addition, are found to severely hinder the Li transport properties.