Terminally fluorinated ether as a solvent for high-performance lithium metal batteries electrolyte

Abstract

Terminally fluorinated ether 5FDEE shows exceptional compatibility with LiPF6, enabling high-performance Li-metal batteries. Li||NMC811 cells with a 1 M LiPF6 in 5FDEE:FEC (9:1 by vol.) electrolyte demonstrate remarkable cycling stability with an average Coulombic efficiency exceeding 99.9% and no capacity fading over 550 cycles at 2.7-4.3 V vs. Li+/Li. Lithium metal batteries (LMB) hold immense promise for high-energy density applications; however, achieving long-term cycling stability remains a major challenge. This challenge stems from the thirst for electrolytes that are stable towards both highly reactive lithium metal anode and high-voltage cathode materials. Ether solvents have shown potential due to their compatibility with lithium metal,1,2,3 but their limited oxidative stability restricts use in high-voltage systems.4,5,6 Furthermore, the prevalent electrolyte salt, lithium hexafluorophosphate (LiPF6), is typically insoluble or reacts with ether-based solvents due to its acidity, which promotes polymerization reactions.7,8 This incompatibility has necessitated the use of sulfonylimide salts, such as lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), as alternatives to LiPF6. However, the employment of sulfonylimide salts introduces a new obstacle: degradation of aluminum cell parts at high voltages,9,10 limiting the electrochemical window of the electrolyte. Efforts to improve the oxidative stability of ether solvents often focus on manipulating the solvation structure of lithium complexes. While high-concentration electrolytes enhance oxidative stability by increasing the ratio of lithium-coordinating solvents,11,12 they suffer from high viscosity, low ionic conductivity, poor wettability, and hindered reaction kinetics.13,14 Localized high-concentration electrolytes, containing diluting solvents such as perfluorinated ethers, address these challenges by reducing viscosity while preserving oxidative stability.15,16 However, these approaches often rely on the use of LiFSI salt, which unfortunately induces aluminum corrosion. Recent research has explored incorporating fluorine atoms directly into the molecular structure of ethers to enhance their oxidative stability.17,18,19 While this strategy shows promise for compatibility with high-voltage cathodes, it often still requires the use of sulfonylimide salts. Therefore, developing a new ether-based electrolyte that simultaneously exhibits decent ionic conductivity, excellent oxidative stability, and compatibility with LiPF6 is crucial for enabling practical high-voltage Li-metal batteries.