Since the demand for Li-ion batteries (LIBs) in large-scale electric devices, such as EVs and ESS, as well as various portable devices, has been dramatically increased, numerous investigations have focused on the enhancement of the performances (e.g. high specific capacity and energy density, etc.) of active materials in LIBs. Among them, many researchers have concentrated on the Li and Mn-rich layered oxides for the cathode material in LIBs due to their intrinsic high specific capacity and energy density. However, their superior electrochemical performances suffer from drastic degradations (e.g. voltage fading, capacity loss, large voltage hysteresis, etc.), mainly originating from the sluggish anionic redox reactions during the operation. Herein, we proposed an ultra-simple 1 step LiF coating method for the NMC-LLC system (Li_1.2Mn_0.54Ni_0.13Co_0.13O₂), enhancing their performance and cyclability.

During the post-heating, the LiF layer was formed through the reaction between Li residues on the surface of as-synthesized LLC active material particles and fluorine inside PVDF binder molecules. Since the Li residues, such as Li₂O, Li₂CO_3, and LiOH, can induce the continuous creation of corrosive HF molecules, resulting in the various vigorous side reactions (e.g. TM migration, TM dissolution, electrolyte decomposition, etc.). Therefore, the artificial SEI layer (LiF layer), passivating the autocatalytic HF-attack and the direct contact between active material and electrolyte, led to the increased cyclability of the cell. Our LiF-coated LLC (FLLC) samples exhibited the high capacity retentions of 95.9% after 100 cycles at 0.2 C and 92.5% after 150 cycles at 1 C. XPS and HAADF-STEM measurements were conducted to manifest not only the successful formation of LiF layer above the surface of LLC active material particles but also the maintained layer after the 100 cycles. In addition, electrochemical measurements, such as EIS and GITT analysis, revealed that the increased performance of FLLC was attributed to the alleviation of a decrease of Li-ion diffusivity over the cycle, particularly at the end of the discharge step. The superior Li-ion diffusivity in FLLC was due to the LiF coating layer, suppressing the phase transformations from the original layered structure (R-3m or C2/m space group) to the denser and insulating spinel-like (Fd-3m space group) or rock-salt (Fm-3m space group) structure at the interphase between the active material and carbonate electrolyte.