Beyond Vehicular Transport: Fluorination-Induced Structural Diffusion in Ether Electrolytes

Abstract

Developing high-performance sodium-ion batteries requires stable electrolytes with high ionic conductivity and cation transference numbers. This study employs molecular dynamics simulations and analysis of Onsager transport coefficients to investigate the influence of fluorination on ion transport mechanisms in diethyl ether-based electrolytes. We demonstrate that fluorination significantly alters the static and dynamic characteristics of the solvation environment, leading to distinct trends in conductivity and transference number. Low-fluorinated bilaterally substituted ethers exhibit low conductivity due to long solvent lifetimes and a predominantly vehicular transport mechanism. In contrast, highly fluorinated ethers, particularly F3F0/F3F3, demonstrate enhanced transport properties due to a shift toward structural diffusion facilitated by fast anion and solvent exchange dynamics. Despite increased ion pairing, high conductivity is achieved through concerted ion motion and ultrafast relaxation modes, surpassing the ionic conductivity of the nonfluorinated parent solvent. Our analysis reveals the crucial role of distinct ion correlations in highly aggregated systems and highlights the continuous ion pair lifetime as a key descriptor for understanding cation-anion correlations. These molecular-level insights provide a framework for the rational design of electrolytes with tailored transport properties, advancing the development of high-performance sodium-ion batteries.