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

Abstract

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.

Review on Synthesis, Characterization, and Electrochemical Properties of Fluorinated Nickel‐Cobalt‐Manganese Cathode Active Materials for Lithium‐Ion Batteries

“…For HE‐NCM treated with F 2 gas, 7 L‐i and 19 F‐MAS‐NMR verified the presence as LiF on the surface of the material . Evidence for transition metal fluoride formation was weak and only indirect ( XPS ) for the highest applied F 2 pressure …”

“…It is noteworthy that the intensity of the absorption peaks gradually weakens with increasing fluoride amount, which suggests that the surface Li 2 CO 3 residues are decreased by the F‐modification . However, in HE‐NCM treated with F 2 , all obtained IR spectra appeared to be identical and showed the typical characteristic bands for HE‐NCM material …”

“…7 Li‐ and 19 F‐MAS‐ NMR revealed that there was no fluoride substitution for oxygen, but a formation of LiF coating . For HE‐NCM treated with F 2 gas, 7 L‐i and 19 F‐MAS‐NMR verified the presence as LiF on the surface of the material . Evidence for transition metal fluoride formation was weak and only indirect ( XPS ) for the highest applied F 2 pressure …”

“…Raman spectra of the cycled electrodes ( Li‐rich NCM CAM ; F‐concentration: 0.17 to 1.34 wt %; F‐source: F 2 ) indicated no pronounced differences between the fluorinated and untreated samples …”

Fluorine-Doped LiNi0.8Mn0.1Co0.1O2 Cathode for High-Performance Lithium-Ion Batteries

“…However, serious problems with high-Ni cathodes have been observed during charging and discharging, such as destruction of the layered structure, cation mixing, oxygen evolution, and transition metal dissolution; these phenomena promote the irreversible consumption of Li + ions and thus degrade the LIB performance [11][12][13]. Various methods, such as coating, composite structure formation, cation/anion doping, and structure control, have been extensively studied to address these limitations of high-Ni cathodes [14][15][16][17][18]. Furthermore, the battery management system is also important for the improvement of Li-ion battery [19][20][21].…”