Mixed1-ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide/lithium bis(fluoromethylsulfonyl)imide based ionic liquid and carbonate co-solvents for Li-ion battery applications– analysis of transport properties using MD simulation

In this work, we examined the transport properties for different mixing ratios of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/lithium bis(fluoromethylsulfonyl)imide based ionic liquid and carbonate co-solvents using OPLS/AA force field. No single-solvent electrolyte can satisfy all the requirements of LIB electrolytes. For this reason, therefore, LIB electrolytes are often complex mixtures of multiple co-solvents with optimized properties. Our study finds that the addition of EC/DMC carbonate co-solvent mixtures into [EMI][TFSI]/1M LiFSI results in reduction in the values of the density of the mixture. By comparing the organic solvents EC/DMC with the IL solvents, the computed density of the pure IL/1M LiFSI system (no EC/DMC content) is higher than that of the EC/DMC/1M LiFSI system (no [EMI][TFSI] content). The relative increase in density with higher contents of IL/1M LiFSI system is suggestive of arising from the stronger interactions of [TFSI]- and [FSI]- anions with the Li+ ions as compared to the interaction of EC/DMC with Li+ ions. The behavior of the MSDs for the center-of-mass of the ions as a function of IL/carbonate co-solvent mixtures indicated that the ions exhibited slow dynamics (diffusivity) with higher carbonate content. Our study on the diffusion coefficient analysis of Li+, [FSI]- and [TFSI]- ions have revealed that the organic solvents restrict the free motion of the ions, reducing the dynamics (diffusivity) of the electrolytes. The simulation results also revealed that the total molar ionic conductivity for the different mixing ratios of IL/carbonate blends decrease with higher contents of EC/DMC co-solvents. Furthermore, the relative ionic contribution to the total molar conductivity of each individual cationic and anionic species increase with higher contents of IL in the IL/carbonate mixture, showing that higher carbonate co-solvents have the effects of reducing the molar ionic conductivity. Key words: 1-ethyl-3-methylimidazolium, bis(trifluoromethylsulfonyl)imide, bis(fluoromethylsulfonyl)imide, DFT, MD, OPLS/AA.