Insight into the Tuning Fluorination of Carbonates for Lithium-Ion Batteries: A Theoretical Study

In the field of lithium-ion batteries, fluorinated solvents and additives have been proven to be crucial for enhancing battery performance, including cyclic stability, energy density, and high-low temperature adaptability. However, the impact of the number and position of fluorine substitutions on the properties of fluorinated solvents remains underexplored. To address this knowledge gap, we conducted quantum chemical calculations and molecular dynamics simulations on a series of fluorinated cyclic carbonates based on ethylene carbonate (EC) and propylene carbonate (PC). First, the findings revealed that higher fluorination degrees significantly widen the electrochemical stability window, thereby enhancing the electrochemical performance of carbonate-based electrolytes while promoting the desolvation process of Li⁺. Second, the influence of the fluorine substitution position is also quite significant. For ethylene carbonate (EC), geminal-difluorination exhibits a higher Li⁺ transference number and a wider electrochemical window compared to vicinal-difluorination. For propylene carbonate (PC), fluorination at the α-carbon on the methyl side provides the widest electrochemical window and the strongest Li⁺ coordination ability, followed by the α-carbon away from the methyl group, while fluorination on the methyl group shows the weakest performance. This work emphasizes the significance of the appropriate number and position of fluorine substitutions of cyclic carbonates and provides a practical strategy for the performance improvement of various high-voltage lithium metal batteries.