Fluorinated-diluent-driven modulation of Li+ solvation structure and ion transport in localized high concentration electrolytes by molecular dynamics simulation

Abstract

Localized high concentration electrolytes (LHCEs), introduced as a solution by adding inert diluents, show excellent ionic conductivity, stable electrode/electrolyte interface, and cost-effectiveness, which can mitigate serious challenges of lithium metal batteries (LMBs), such as lithium dendrite growth, unstable solid electrolyte interphase films, and high reactivity of lithium metal, by forming anion-dominated solvation structures with the balance between the performance and cost of electrolytes. However, the influence of diluent molecular structures, especially fluorine substitution patterns, on the solvation structure and diffusion of LHCEs at a molecular scale remain unclear. Herein, the solvation structure and ion transport behaviors of LHCEs in lithium bis(trifluoromethanesulfonyl)imide–propylene carbonate (LiTFSI–PC) electrolytes diluted by the fluorinated-tetrahydrofuran derivatives are systematically investigated via molecular dynamics (MD) simulations. The results indicate that in the LiTFSI–PC electrolyte system, low-fluorinated diluents engage in Li⁺ solvation (by entering the Li⁺ first solvation shell), which weakens the direct coordination between Li⁺ and TFSI⁻. In contrast, high-fluorinated diluents have higher first peak of Radial Distribution Function (RDF) with PC than Li⁺ and TFSI⁻, and enhance the interaction between Li⁺ and TFSI⁻, forming the localized high-concentration regions, which further confirmed by coordination types in the solvation shell of Li⁺. Relative to low-fluorinated diluents, high-fluorinated diluents show an increased molecular polarity and minimum/maximum of the electrostatic potential, leading to a significantly larger interaction energy with PC than Li⁺. Additionally, in the LHCEs with high-fluorinated diluents, Li⁺ coordinated by PC exhibit a vehicular diffusion mechanism, whereas the diffusion mechanism of Li⁺ coordinated by TFSI⁻ is the structural diffusion or mixed diffusion mechanism. These findings provide a deeper insight into the modulation of coordination and ion transport of Li⁺ and offer a cost-effective pathway for developing high-performance diluents for LHCEs.