Impact of Fluorination Degree of Ether-Based Electrolyte Solvent on Li-metal Battery Performance

Electrolytes using fluorinated solvents have proven effective in improving the cycling life of Li-metal batteries, by forming a robust solid-electrolyte interphase through decomposition of anion and fluorinated solvent molecules. Herein, we modulated the fluorination degree of ether-based electrolyte solvents to investigate their performance in Li-metal batteries. We tuned the fluorination degree by installing a monofluorine substituent on one ethoxy group of 1,2-diethoxyethane (DEE) and varying the fluorination degree on the other one, providing three fluorinated DEE solvent molecules (i.e., F1F0, F1F1 and F1F2) with a relatively low fluorination degree. All the three electrolytes showed improved solvation strength and ionic conductivities compared with previous highly fluorinated DEE elec-trolytes, while retaining good oxidative stability. Full cell test using Li-metal anode and nickel-rich cathode revealed that a higher degree of fluorination is beneficial to the cycling performance, and the cycling stability follows F1F0 < F1F1 < F1F2. Specifically, F1F0 exhibited poor cycling stability due to its instability against both anode and cathode. While F1F1 and F1F2 both showed good stability against Li-metal anode, their relative long-term oxidative stability was responsive for the distinct performance, in which the cycle numbers at 80% capacity retention for F1F1 and F1F2 were ~20 and ~80, respectively. This work shows the importance to modulate the fluorination degree of elec-trolyte solvents, and this approach is suitable for various cathode materials.