Fluorinated EC analogues as co-solvents in EMC-based electrolytes for Li-ion batteries: a computational study

The growing demand for high-energy-density lithium-ion batteries calls for electrolytes capable of stable operation at elevated voltages. Conventional carbonate-based systems, such as ethylene carbonate (EC) and ethyl methyl carbonate (EMC), suffer from limited oxidative stability and rapid degradation under such conditions. Fluorination of carbonates has emerged as an effective strategy to enhance electrochemical stability and cycling performance. In this study, we computationally investigate electrolyte formulations composed of EMC and fluorinated EC derivatives as co-solvents to elucidate how molecular fluorination influences solvation structure and ion transport. Quantum mechanical calculations reveal that among the studied species, trans-difluoroethylene carbonate (DFEC) exhibits the widest HOMO-LUMO gap (8.727 eV), indicating a higher electrochemical stability window, whereas non-fluorinated EC shows the strongest Li⁺ coordination, consistent with its superior ion-pair dissociation ability. Radial distribution functions and cluster analyses further confirm that electrolytes containing fluorinated solvents exhibit weaker ionic dissociation and higher ion aggregation compared to the EC-based system. While most fluorinated systems show a substantial increase in single-ion diffusion, true ionic conductivity is generally decreased upon fluorination with the exception of TFPC/EMC. This decrease is due to extensive clustering and absence of cyclic co-solvent in Li⁺ coordination, both of which lead to the formation of fewer charge carriers. The TFPC/EMC and FEC/EMC formulations exhibit comparable ionic conductivity values to EC/EMC (3.02 mS cm^-1 and 2.74 mS cm^-1 respectively, compared to 2.85 mS cm^-1) and are both the highest among the fluorinated systems. Finally, van Hove correlation analysis provides a detailed view of Li⁺ transport, revealing that the short-time peak of Li⁺ generally decays more prominently and displaces farther upon fluorination.