Fluorination Strategy in Designing Fluorinated Borate for a High-Voltage Lithium-Ion Battery and a Lithium Metal Battery: A Theoretical Study

Borate and its fluorination, as an electrolyte additive or solvent, have been proven to enhance electrochemical performance in high-voltage lithium-ion batteries and lithium metal batteries. However, there is a research gap in the comparison of borates with and without fluorination. In light of the above-mentioned research gap, quantum chemical calculations were employed to evaluate traditional solvents─ethylene carbonate (EC), methyl ethyl carbonate (EMC), trimethyl borate (TMB), and triethyl borate (TEB)─and 19 fluorinated borates designed from TMB and TEB (named TMB- and TEB-group fluorides) in this work, and the following results were obtained. First, as the number of F atoms increases, the oxidation resistance of the molecule is enhanced. Moreover, when the number of F atoms is the same, molecules with an evenly distributed fluorination across the three side chains exhibit greater oxidation resistance than those with fluorination concentrated in a single side chain. Meanwhile, we compared the oxidation potentials of each fluorinated molecule with that of the solvent EC (whose oxidation potential is higher than that of EMC). Molecules with lower oxidation potentials than EC were considered as additives, while those with higher oxidation potentials were regarded as solvents or diluents. Subsequently, we evaluated the application potential of each molecule across different dimensions. Second, we screened 13 molecules suitable as additives across six dimensions: binding energy with O₂^-, HF, F^-, Li⁺; LUMO (lowest unoccupied molecular orbital)-HOMO (highest occupied molecular orbital) energy gap; and the binding energy of decomposition products with Li⁺. TEB_23 (the structure is presented in Supporting Information) showed the greatest potential as an additive. Third, we evaluated and compared eight molecules as normal diluents or polarity diluents using four dimensions: the LUMO-HOMO energy gap, binding energy with Li⁺, dipole moment (only for polarity diluents), and minimum electrostatic potential. The result has showed that TMB_333 (the structure is presented in Supporting Information) showed the greatest potential as a normal diluent, while TMB_222 (the structure is presented in Supporting Information) showed the greatest potential as a polarity diluent. This work emphasizes the significance of the appropriate number and position of fluorine substitutions in borates and provides a practical strategy for improving the performance of various high-voltage lithium-ion batteries and lithium metal batteries.