Abstract
The anion chemistry of lithium salts plays a pivotal role in determining the physicochemical and electrochemical performances of ionic liquid (IL) electrolytes. This work explores the effects of anion size and symmetry on coordination and dynamics of lithium ions (Li+) through molecular dynamics simulations. Four types of ILs composed of the same cation N-N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium (DEME+) and different anions of symmetrical bis(fluorosulfonyl)imide (FSI−) and bis(trifluoromethanesulfonyl)imide (TFSI−), as well as asymmetric (fluorosulfonyl)(trifluoromethylsulfonyl)imide (FTFSI−) and (difluoromethylsulfonyl)-(trifluoromethylsulfonyl)imide (DFTFSI−) are studied for comparison purpose. The calculation results show that the formation of Li+ aggregates in concentrated IL electrolytes could enhance Li+ transport due to the Li+ transport mechanism gradually shifts from vehicle transport to Li+ hopping through the anion ligand layers. Furthermore, it can be seen that the asymmetric anion based electrolytes possess superior performance than that of symmetric anions. In addition, the dual-anion based electrolytes all exhibit faster Li+ diffusion coefficient than mono-anion based electrolytes, which arise from the synergistic effect between mixed anions. As such, this work highlights the role of the anion structures and synergism in the rational design of concentrated IL-based electrolytes.
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