Li+ Solvation and Transport Properties in Ionic Liquid/Lithium Salt Mixtures: A Molecular Dynamics Simulation Study

Molecular dynamics simulations of N-methyl-N-propylpyrrolidinium (pyr13) bis(trifluoromethanesulfonyl)imide (Ntf2) ionic liquid [pyr13][Ntf2] mixed with [Li][Ntf2] salt have been conducted using a polarizable force field. Mixture simulations with lithium salt mole fractions between 0% and 33% at 363 and 423 K yield densities, ion self-diffusion coefficients, and ionic conductivities in very good agreement with available experimental data. In all investigated electrolytes, each Li+ cation was found to be coordinated, on average, by 4.1 oxygen atoms from surrounding anions. At lower concentrations (x ≤ 0.20), the Li+ cation was found to be, on average, coordinated by slightly more than three Ntf2 anions with two anions contributing a single oxygen atom and one anion contributing two oxygen atoms to Li+ coordination. At the highest [Li][Ntf2] concentration, however, there were, on average, 3.5 anions coordinating each Li+ cation, corresponding to fewer bidendate and more monodentate anions in the Li+ coordination sphere. This trend is due to increased sharing of anions by Li+ at higher salt concentrations. In the [pyr13][Ntf2]/[Li][Ntf2] electrolytes, the ion diffusivity is significantly smaller than that in organic liquid electrolytes due to not only the greater viscosity of the solvent but also the formation of clusters resulting from sharing of anions by Li+ cations. The ionic conductivity of the electrolytes was found to decrease with increasing salt concentration, with the effect being greater at the higher temperature. Finally, we found that the contribution of Li+ to ionic conductivity does not increase proportionally to Li+ concentration but saturates at higher doping levels.