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

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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 n (BF 4 ) 4– m ) (4– m + n )–, 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 4 – anions and long ion pairing times, which are promoted upon lithium addition, are found to severely hinder the Li + transport properties.