Thermophysical Properties and Dynamics of Solvation Structures Formed in Fluorinated Lithium-Ion Electrolytes with Additives

Using the classical molecular dynamics simulation method, we explored the structure and dynamics of electrolytes for Li-ion batteries (LIB) with the effects of additives at different temperatures. We used fluoroethylene carbonate (FEC) as an electrolyte additive. The fluorinated electrolytes consist of 1 M LiPF₆ salt in trifluoropropylene carbonate (TFPC) and methyl 2, 2, 2-trifluoroethyl carbonate (FEMC) solvents. Using insights from structural analysis, we distinguished different solvation structures in bulk electrolytes. The potential of mean force between Li⁺ and PF₆^- reveals the presence of two major types of solvation structures-contact ion pair (CIP) and solvent-separated ion pair (SSIP), with SSIP being the most stable. The entropic contribution destabilizes and the enthalpy stabilizes the formed solvation structures in the electrolyte with the additive, while the reverse is observed for the electrolyte without the additive. The dynamic properties of the electrolytes were quantified by using diffusivity and conductivity calculations. The presence of additives showed enhanced dynamics. The ion-cage lifetime of the Li⁺-FEMC pair is higher than that of the others. Further, higher domain counts are observed around Li⁺-TFPC than in the case of Li⁺-FEMC in both electrolytes. The results reported here will be useful in gaining insights into fluorinated Li-ion battery electrolytes containing FEC as an additive.