Fluorination of Li-Rich Lithium-Ion-Battery Cathode Materials by Fluorine Gas: Chemistry, Characterization, and Electrochemical Performance in Half Cells

Mild fluorination of high-energy nickel-cobalt-manganese (HE-NCM) materials with low pressures of elementary fluorine gas (F₂) at room temperature was systematically studied. The fluorinated HE-NCM samples were analysed by ion chromatography, inductively coupled plasma mass spectrometry, FT-IR spectroscopy, powder X-ray diffraction, magic angle spinning NMR spectroscopy, scanning electron microscopy, thermo-gravimetric analysis, differential thermal analysis, electrochemical testing, and X-ray photoelectron spectroscopy. The treatment of the cathode materials with low pressures (a few hundred mbar) of elementary fluorine gas at room temperature led to the elimination of the basic surface film (LiOH, Li₂CO₃, Li₂O, etc.), and the resulting thin amorphous LiF film led to increased capacity and long-term stability of the battery. Impedance built-up was greatly reduced for these systems throughout cycling. Fluorination with F₂ only causes the formation of O−Me−F bonds (Me=Transition Metal), when treated with F₂ at higher pressures. If O−Me−F bonds are formed, it may be detrimental to the electrode surface film resistance and cycle stability of the electrodes. However, it may be that the LiF surface content, which can expand as long as the LiMeO₂ structure can be oxidized and Li⁺ can be extracted, has become too large and thus detrimental. Considering the evolution of differential capacity plots and taking into account the thermodynamic driving force of the F₂ treatment, it is likely that the same activation processes that occur electrochemically in Li-rich materials also occur chemically, when the material is exposed to F₂. Differential capacity plots show enhanced Mn⁴⁺ reduction peaks upon lithiation, when the material was exposed to F₂, only possible after activation of the Li₂MnO₃ phase. For this reason, we believe fluorination promotes to some extent an activation of this phase.