How does hydrofluoroether affect the liquid structure, transport properties, and electrochemistry of localized high-concentration electrolytes?

Understanding the effects of non-coordinating diluents on the physicochemical properties of localized high-concentration electrolytes (LHCEs) is essential for the rational design of battery electrolytes. In this study, we examined the effect of a hydrofluoroether (HFE), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, on the liquid structure, transport properties, and electrochemical reaction kinetics of a model LHCE containing lithium bis(fluorosulfonyl)amide (LiFSA), 1,2-dimethoxyethane (DME), and HFE. Raman spectroscopy revealed that the Li⁺ solvation structure in the model LHCE remained largely unchanged upon dilution with HFE. The ion-pairing environment involving FSA⁻ was also preserved, consistent with the weak coordinating ability of HFE. Although HFE did not coordinate with Li⁺, molecular dynamics simulations indicated weak interactions between HFE protons and FSA⁻, supporting its miscibility with the concentrated [LiFSA]/[DME] = 1/2 electrolyte. With increasing HFE content, viscosity decreased, while ionic conductivity reached a maximum at an intermediate LiFSA concentration owing to the trade-off between ion concentration and mobility. The diffusion coefficients increased with dilution; however, the decreasing molar conductivity/diffusivity ratio indicated a dynamic domain structure and prolonged ion-pair lifetime in the LHCEs. Electrochemical impedance analysis revealed that the charge-transfer reaction resistance at the LiMn₂O₄ electrode reached a minimum at an intermediate concentration ([LiFSA]/[DME]/[HFE] = 1/2/1), while the activation energy remained nearly constant. This finding indicates that HFE lowers viscosity without affecting the energy barrier for Li⁺ desolvation at the electrode-electrolyte interface. These findings demonstrate that non-coordinating diluents modulate the liquid structure, ion transport, and interfacial properties of LHCEs.