(531a) Structure and Dynamics of Water-in-Salt Litfsi Electrolytes from First-Principles Molecular Dynamics Simulations

Concentrated aqueous solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have emerged as a safer alternative to the widely used organic-based electrolyte solutions for electrochemical energy storage in batteries. In the present work, we have performed first-principles molecular dynamics (FPMD) simulations to study the water network and ion solvation structures and their dynamics in aqueous LiTFSI at various water/salt ratios. The structure factor calculated from the simulations shows good agreement with the experimental results. Analysis of the ions’ solvation environments reveals that the lithium cation predominantly interacts with four oxygen atoms from different water molecules and TFSI anions, forming a near-tetrahedral geometry with lithium at the center. At lower salt concentrations, this first solvation shell is preferentially populated by three water molecules and one TFSI anion, but changes to two water molecules and two TFSI anions at higher salt concentrations.

Hydrogen bond analysis reveals a decrease in the number of intermolecular water-water hydrogen bonds per water molecule on increasing salt concentration. Network analysis suggests that as the salt concentration increases, the hydrogen bonding network of water molecules is disrupted, along with the concurrent formation of a TFSI network. These solvation environments computed from the FPMD simulations were compared with the solvation environments for the same system size calculated from simulations using two molecular mechanics force fields (FF), and a good agreement between the results was obtained.

Furthermore, to probe the dynamics of the system, diffusion coefficients of the species and the van Hove functions (from FF-based simulations) for the Li-water and Li-TFSI interactions are computed. Lithium diffusion coefficients at varying salt concentrations are compared with the experimental results. The structure decorrelation timescales obtained from van Hove functions are compared with the diffusion timescales to understand the lithium transport mechanism through the electrolyte. These insights into the structure and dynamics of the aqueous LiTFSI systems provide an opportunity to design electrolytes for Li-ion batteries with enhanced performance and safety.