Effect of ion structure on conductivity in lithium-doped ionic liquid electrolytes: a molecular dynamics study

Molecular dynamics simulations were performed to examine the role cation and anion structure have on the performance of ionic liquid (IL) electrolytes for lithium conduction over the temperature range of 320-450 K. Two model ionic liquids were studied: 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([bmim][Tf2N]) and 1-butyl-4-methylpyridinium pyrrolide ([bmpyr][pyl]) doped with Li[Tf2N] and Li[pyl], respectively. The results have demonstrated that the Li(+) doped IL containing the planar [bmpyr] cation paired with the planar [pyl] anion significantly outperformed the [bmim][Tf2N] IL. The different coordination of Li(+) with the [Tf2N](-) or [pyl](-) anions produces a remarkable change in IL structure with a concomitant effect on the transport of all ions. For the doped [bmim][Tf2N], each Li(+) is coordinated by four oxygen atoms from [Tf2N](-) anions. Formation of a rigid structure between Li(+) and [Tf2N](-) induces a decrease in the mobility of all ions. In contrast, for the doped [bmpyr][pyl], each Li(+) is coordinated by two nitrogen atoms from [pyl](-) anions. The original alternating structure cation∣anion∣cation in the neat [bmpyr][pyl] is replaced by another alternating structure cation∣anion∣Li(+)∣anion∣cation in the doped [bmpyr][pyl]. Increases of Li(+) mole fraction in doped [bmpyr][pyl] affects the dynamics to a much lesser extent compared with [bmim][Tf2N] and leads to reduced diffusivities of cations and anions, but little change in the dynamics of Li(+). More importantly, the calculations predict that the Li(+) ion conductivity of doped [bmpyr][pyl] is comparable to that observed in organic liquid electrolytes and is about an order of magnitude higher than that of doped [bmim][Tf2N]. Such Li(+) conductivity improvement suggests that this and related ILs may be promising candidates for use as electrolytes in lithium ion batteries and capacitors.