Modeling and Simulation of Concentrated Aqueous Solutions of LiTFSI for Battery Applications

We propose a new nonpolarizable molecular mechanics force field for concentrated aqueous solutions of lithium bistriflylimide (LiTFSI), a promising candidate for battery applications. The model describes the TFSI anion by GAFF2-based Lennard-Jones parameters and new MP2-optimized intramolecular parameters. They are combined with existing models of Li+ and water (TIP4P-Ew). The charge transfer and electronic polarization effects between oppositely charged ions, depicted with ionic charge scaling by 0.8 in the present model, turn out to be crucial for the correct prediction of solution density and diffusivity of ions and water molecules over the concentration range from 1 to 21 m. Molecular dynamics simulations using this new model reveal that TFSI- interacts with Li+ predominantly through its sulfonyl oxygens (OT) and that OT can readily form hydrogen bonds (H-bonds) with water molecules. Moreover, a single Li+ is, on average, coordinated by approximately four oxygen atoms, either OT or OW, at all concentrations studied. These observations indicate that the extended and heterogeneous H-bond network formed by water and OT facilitates the solvation and ion conduction of Li+ in concentrated aqueous solutions of LiTFSI. The present modeling approach is applicable to a wide range of electrolyte solutions.