Fluorinated Thiophene-Based Electrolyte Additives Enable a Robust Cathode Electrolyte Interphase with Conformal Homogeneity for a High-Voltage LiNi_0.8Co_0.15Al_0.05O₂ Cathode

In view of the advantageous merits of LiNi_0.8Co_0.15Al_0.05O₂ (NCA), i.e., high voltage and decent specific capacity, an impressive electrolyte film-forming additive, methyl-3-fluorothiophene-2-carboxylate (MFC), is put forward to remarkably improve the cycle performance of NCA cathodes toward high energy density lithium-ion batteries (LIBs). Theoretical calculation and experimental investigation both reveal that MFC is more readily oxidized to construct a cathode electrolyte interphase (CEI) on the surface of NCA cathode. Spectroscopy and microscopy data display that a MFC-derived CEI layer is thin, homogeneous, and conformally covers on the NCA surface, maintains the NCA structural integrity without visible microcracks, and protects the NCA cathode from electrolyte corrosion with alleviated dissolution of Ni and Co. Electrochemical properties show that the NCA/Li half-cell with a 5 wt % MFC-containing electrolyte delivers a higher discharge capacity with a much superior capacity retention of 148.6 mA h g^–1/85.5% than that of base electrolyte (121.2 mA h g^–1/71.0%) after 150 cycles at 1 C. This work indicates that MFC can work as an effective film-forming additive for high-voltage NCA and provides precious insights and theoretical guidance to the development of advanced electrolyte additives for high energy density LIBs.