Effect of ion structure on conductivity in lithium-doped ionic liquid electrolytes: A molecular dynamics study
HJ Liu and E Maginn, JOURNAL OF CHEMICAL PHYSICS, 139, 114508 (2013).
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)sulfonylimide (bmimTf2N) and 1-butyl-4-methylpyridinium pyrrolide (bmpyrpyl) doped with LiTf2N and Lipyl, respectively. The results have demonstrated that the Li+ doped IL containing the planar bmpyr cation paired with the planar pyl anion significantly outperformed the bmimTf2N 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 bmimTf2N, 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 bmpyrpyl, each Li+ is coordinated by two nitrogen atoms from pyl(-) anions. The original alternating structure cation vertical bar anion vertical bar cation in the neat bmpyrpyl is replaced by another alternating structure cation vertical bar anion vertical bar Li+vertical bar anion vertical bar cation in the doped bmpyrpyl. Increases of Li+ mole fraction in doped bmpyrpyl affects the dynamics to a much lesser extent compared with bmimTf2N 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 bmpyrpyl is comparable to that observed in organic liquid electrolytes and is about an order of magnitude higher than that of doped bmimTf2N. Such Li+ conductivity improvement suggests that this and related ILs may be promising candidates for use as electrolytes in lithium ion batteries and capacitors. (C) 2013 AIP Publishing LLC.
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