Exploring 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as a high voltage electrolyte solvent for 5-V Li2CoPO4F cathode

1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) is investigated as a cosolvent for high voltage electrolytes of Li2CoPO4F. Compared with conventional carbonate-based electrolyte (1-M LiPF6 ethylene carbonate [EC]/dimethyl carbonate [DMC] [1:1, wt:wt]), 1 M LiPF6 F-EPE/DMC (1:2, wt:wt) exhibits significantly improved antioxidant ability in high voltage, thus greatly enhances the electrochemical performance of 5.0-V Li2CoPO4F/Li cells. Linear sweep voltammetry (LSV) and charging/discharging tests demonstrate that the F-EPE/DMC electrolyte possesses both a high oxidation voltage up to 6.2 V vs. Li+/Li on Pt electrode and superior oxidation stability on Li2CoPO4F cathode. Benefiting from its high antioxidant ability, the capacity retention of Li2CoPO4F cathode increases from 15% in EC/DMC electrolyte to 51% in F-EPE/DMC electrolyte after 100 cycles at 1 C between 3.0 and 5.4 V. Moreover, differential capacity (dQ/dV) analysis, electrochemical impedance spectroscopy, ex situ X-ray diffraction, and X-ray photoelectron spectroscopy are used to analyze the effects of F-EPE/DMC electrolyte on the improved electrochemical performance. It is illustrated that the high stability of F-EPE/DMC electrolyte effectively inhibits the oxidative decomposition of the electrolyte on Li2CoPO4F electrode above 5.0 V and suppresses the damage to the surface of Li2CoPO4F, thus alleviate the increase in electrode polarization and cell impedance.