Ultralow Concentration Nonflammable Electrolytes Mediated by Intermolecular Interactions for Safer Potassium-Ion Sulfur Batteries

Abstract

Designing electrolytes that are compatible with graphite anodes and possess flame-retardant features is strongly demanded in potassium-ion batteries (PIBs) to inhibit solvent co-insertion and graphite exfoliation, and also stabilize the highly active potassium-based species (e.g., KC₈). Herein, a nonflammable electrolyte is designed by introducing the fluoroethers to stabilize graphite anodes, particularly at an ultralow concentration (<0.43 m) that is rarely reported before. It is discovered that intermolecular interactions can form between the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (i.e., HFE) diluent and trimethyl phosphate (TMP) flame-retardant by electronegative fluorine (δ⁻F) and electropositive hydrogen (δ⁺H). The intermolecular interactions can change the potassium ion (K⁺) solvation structure (e.g., weakening the K⁺-TMP interaction), and then determine the properties of the K⁺-solvent-anion complex at the electrode interface. a molecular interfacial model is presented with a new coordination mechanism involving the diluent to elucidate the relationship between the intermolecular interactions and electrode performance (i.e., K⁺-solvent co-insertion, or reversible K⁺ (de)intercalation) at the molecular scale, facilitating the design of high safety and high energy density potassium-ion sulfur batteries. This study sheds light on the importance of intermolecular interactions to tune electrolyte properties and also opens new avenues for designing electrolytes for safe and practical PIBs.