计算机模拟咪唑型离子液体电解质:C2位甲基化的作用

Molecular dynamics simulation of imidazolium-based ionic liquids doped with lithium salt:The effects of C2-methylation

使用极化分子力场对两种锂盐浓度均为0.32 mol/kg的咪唑型离子液体电解质体系,LiFSI-[EMIM][FSI]和LiFSI-[EMMI][FSI],进行了分子动力学模拟.通过分析Li+离子和emim+/emmi+阳离子的溶剂化层结构,研究发现Li+与阴离子FSI较强的静电作用受咪唑阳离子结构变化的扰动微小,C2位甲基化明显改变的是离子液体中离子间的作用结构.结合自相关函数的计算,我们推测C2-H的消失增大了阴离子FSI同时与更多emmi+阳离子作用的倾向,增强了LiFSI-[EMMI][FSI]体系的整体网络结构.这是C2位甲基化造成体系离子电导率降低的主要原因.最后,从Li+的两种传输方式出发,着重分析了微观结构与Li+传输性能间的相互关联.

Molecular dynamics simulations with a polarized force field were performed on two classes of imidazolium-based ionic liquid electrolytes, LiFSI-[EMIM][FSI] and LiFSI-[EMMI][FSI], with the lithium salt concentration of 0.32 mol/kg. Study on the solvation shell of Li＋ and emim＋/emmi＋ shows that the strong electrostatic interaction between Li＋ and FSI- could be marginally disturbed by the C2-methylation of imidazolium cation, which evidently affects the interaction among ions of ionic liquid. The investigation of structural and dynamical properties found that the disappearance of C2-H makes FSK anions to interact with several imidazolium cations simultaneously, and thus favors the formation of the interaction network in LiFSI-[EMMI][FSI] system. The above reason is the primarily responsible for slower ionic conductivity of LiFSI-[EMMI][FSI] than that of LiFSI-[EMIM][FSI]. Finally, the connection between microstructure and Li＋ transport performance is emphasized in terms of two different transport mechanism.