Fluorination Molecular Engineering of Trimethyl Phosphite: A Multiscale Theoretical Study

Abstract

Fluorination strategies have emerged as a key approach for overcoming the performance bottlenecks of lithium-ion batteries, enabling their development toward high voltage, long cycling life, and enhanced safety. Trimethyl phosphite (TMPi) has been widely employed as a multifunctional phosphorus-based electrolyte additive in lithium-ion batteries, exhibiting significant advantages in flame retardancy, acid scavenging, and suppression of transition-metal dissolution and side reactions. However, it still suffers from insufficient oxidation stability and continuous electrolyte decomposition under high-voltage conditions. In this study, TMPi was selected as the parent molecule and systematically modified through fluorination with varying substitution positions and quantities. A series of fluorinated additives was theoretically investigated using quantum chemical calculations and molecular dynamics simulations. The results demonstrate that fluorine substitution regulates the electronic structure of phosphite additives through a strong electron-withdrawing inductive effect. With increasing fluorination quantities, the HOMO energy level decreases, and moderate fluorination simultaneously enhances oxidation stability while enabling interphase formation. Meanwhile, the decrease in LUMO energy and the optimization of electrostatic potential (ESP) reactive sites improve the reduction activity of the molecules. Among these, the single-chain highly fluorinated TMPi_300 molecule exhibits a stronger reduction tendency compared to TMPi_111. At high fluorination degrees, the response of the reduction potential becomes less sensitive. In addition, fluorination weakens the coordination capability of the molecules, thereby indirectly altering the local coordination environment and affecting the solvation structure. In the single-chain substitution series (from TMPi_100 to TMPi_300), increasing fluorination quantities lead to decreased binding energy, reduced viscosity, and suppressed ion pairing, facilitating Li+ desolvation. In contrast, in the multichain fluorinated systems, molecules containing −CF3 groups create a low-dielectric environment that enhances the electrostatic interaction between Li+ and PF6–. Coupled with factors such as local dielectric heterogeneity, molecular volume, and viscosity, this results in a nonlinear variation in ion transport properties. This study identifies TMPi_133 as the optimal functional interfacial additive for high-voltage lithium-ion battery systems, exhibiting excellent oxidation stability and film-forming capability without compromising ion transport performance, thereby achieving superior overall properties. This work aims to provide practical design strategies for phosphite-based additives with different fluorination quantities and substitution positions to improve the performance of lithium-ion batteries.