Design of Fluorine-Free Weakly Coordinating Electrolyte Solvents with Enhanced Oxidative Stability.

High concentrations of conducting salt in electrolyte formulations enhance the agglomeration of ionic species, which has been demonstrated to yield anion-derived electrode-electrolyte interphases and improved reversibility in several battery configurations. However, industrial application of these electrolytes may be limited due to high costs of electrolyte conducting salts. Here, weakly solvating electrolyte solvents with tailored coordination strength have been established as an approach to achieve ionic agglomeration at moderate conducting salt concentrations and without per-fluorinated diluents. However, the inevitable presence of uncoordinated solvent molecules in this electrolyte concept renders them susceptible to oxidative decomposition. Although previous efforts demonstrated fluorination as an effective design strategy to tailor the oxidative stability of weakly solvating electrolytes, the per-fluorinated solvents are toxic and harmful to the environment. Herein, the incorporation of silicon is evaluated as an eco-friendly approach to dispel electron density of the oxygen lone pair. Though steric demand of substituents is already sufficient to tailor the coordination strength, negative hyperconjugation effectively expands the oxidative stability limit of weakly solvating electrolytes. Combining ion agglomeration and intrinsic oxidative stability, the herein introduced weakly solvating electrolyte enables a notable improvement of reversibility under eco-friendly conditions, presenting a valid alternative to fluorinated electrolyte solvents.