Tuning the ion-dipole interactions between fluoro and carbonyl (EC) by electrolyte design for stable lithium metal batteries

Abstract

Ethylene carbonate (EC) is the conventional and promising solvent to achieve high energy lithium metal battery. However, the innate low energy level of lowest unoccupied molecular orbital (LUMO) in EC makes it incompatible with lithium metal, causing uncontrolled lithium growth and low Coulombic efficiency (CE). Herein, we introduced bis(2,2,2-trifluoroethyl) carbonate (TFEC), a carbonate with a strong electron-withdrawing effect (-CF₃), which enhances the stability of EC at electrode interface by reducing ion-dipole interactions between Li⁺ and EC. As the interaction between Li and EC weakens, TFEC and more PF_6⁻ anions coordinate with Li⁺, promoting the formation of contact ion pairs (CIPs) and aggregates (AGGs), thereby increasing the inorganic composition within the solid electrolyte interphase. Additionally, the distinct solvated sheath structure favors the decomposition of fluorinated solvents and PF₆₋ anions, forming inorganic-rich electrode-electrolyte interfaces (SEI and CEI), thereby ensuring high stability for both the Li anode and high-voltage cathode. Hence, when applied in the full-cell Li||LiMn_0.8Fe_0.2PO₄, it displays consistent cycling performance, exhibiting minimal capacity decay with a retention rate of 62.5% after 800 cycles, substantially surpassing that of cells using base electrolytes (29.8%).