Boosting ultra-fast charging in lithium metal batteries through enhanced solvent-anion interaction via conjugation effect

In pursuit of higher energy density, adopting a lithium metal anode holds promise for the evolving battery technology. Nevertheless, practical obstacles persist, including explosion hazards, restricted fast charging (>5C), and lithium metal compatibility issues. To tackle these challenges, we devised a non-flammable deep eutectic electrolyte (DEE) comprising lithium bisfluorosulfonimide (LiFSI) and prop-1-ene-1,3-sultone (PES). This DEE demonstrates exceptional attributes, including near 98.76% Coulombic efficiency and a high ionic conductivity of 1.96 mS cm⁻¹. We found that the electron-absorbing effect of the double-bonded structure enhances the interactions between PES and FSI⁻, releasing more free lithium ions and boosting the Li⁺ transference number of 0.78. Our engineered DEE fosters the formation of a robust organic-inorganic gradient solid electrolyte interphase (SEI) with a surface layer rich in organic species and an inner layer rich in inorganic species. The high Li⁺ transfer number and stable SEI together enable ultra-fast charging and sustained cycling, with 81.32% capacity retention after 1000 cycles at 10C in the LiFePO₄‖DEE‖Li battery. Meanwhile, the mechanistic reasons behind fast charging performance are elaborated by theoretical calculations, and its practical applicability is underscored through successful implementation in pouch cells with high loading and 30 μm of Li metal.