Medium-fluorination carbonate electrolytes for stable 4.6 V lithium metal batteries

Fluorination of carbonate electrolytes is an effective strategy for improving the stability of high-voltage lithium metal batteries (LMBs). However, highly fluorinated carbonate solvents often suffer from limited lithium-salt solubility, increased cost, and potential environmental concerns. Herein, we develop medium-fluorination carbonate electrolytes (MFCE) based on monofluorinated fluoroethylene carbonate (FEC) and difluorinated bis(2-fluoroethyl) carbonate (BFC) as the main solvents, aiming to balance electrochemical performance, cost, and sustainability. We demonstrate that moderate fluorination weakens the ion-dipole interactions between Li⁺ and carbonate solvent molecules while maintaining sufficient salt-dissolution capability. This weak-solvation characteristic promotes anion participation in the primary solvation sheath and facilitates the formation of inorganic-rich interfacial protective layers on both electrodes. As a result, Li-Li symmetric cells using MFCE exhibit stable cycling for over 1200 h at 0.5 mA cm^-2 and 0.5 mAh cm^-2, while Li-Cu half cells maintain reversible Li plating/stripping for nearly 480 cycles under the same conditions. Moreover, Li-LiCoO₂ (LCO) full cells (14 mg cm^-2, 50 µm Li, N/P ratio of 3.7) using MFCE can stably cycle for over 150 cycles at a cutoff voltage of 4.6 V, significantly outperforming those using conventional carbonate electrolytes. These results demonstrate that electrolyte performance is governed by the rational regulation of fluorination degree rather than simply increasing fluorine content, providing a practical design principle for advanced carbonate electrolytes in high-voltage LMBs.