Liquid structure in mixtures of imidazolium based ionic liquid/carbonate solvents for application in next generation lithium ion battery electrolytes: molecular dynamics simulation

We used molecular dynamics (MD) simulations based on classical force field to provide the liquid structure in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid in the presence of ethylene, dimethyl carbonate co-solvents and 1M LiTFSI mixtures which are currently being targeted for applications in next-generation Li-ion battery electrolytes. To show the dependence of the properties on the concentration, we simulated both pure IL and ternary mixtures of IL, EC and DMC with different mixing ratios. The solvation of lithium cations in pure and mixed IL/organic carbonate solvents has been investigated. The MD simulations have predicted a preference of Li+ ions to interact with DMC molecules within its first solvation shell rather than with the highly polar EC ones in the IL/carbonate mixtures, a phenomon which is attributed to the local tetrahedral packing of the solvent molecules in the first solvation shell of Li+ ions.The coordination of the O/N atoms of the bis(trifluoromethylsulfonyl)imide anion to Li+ ions as well as to the most acidic H atom of 1-ethyl-3-methylimidazolium cation has also been analysed, which showed a marked increase in the amplitude of RDF peaks with increase in carbonate concentration, indicating that the interaction between the ions was enhanced by the presence of high carbonate content. Furthermore, in the pure ionic liquid, adjacent cations are almost exclusively located on top and below the ring cation, whereas the anions mainly coordinate to the cation within the ring plane. The addition of large amount of carbonate co-solvents disturb the original near ordering which is found in the pure ionic liquid.