The noble‐gas difluoride adducts, NgF2⋅CrOF4 and NgF2⋅2CrOF4 (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single‐crystal X‐ray diffraction. The low fluoride ion affinity of CrOF4 renders it incapable of inducing fluoride ion transfer from NgF2 (Ng=Kr and Xe) to form ion‐paired salts of the [NgF]+ cations having either the [CrOF5]− or [Cr2O2F9]− anions. The crystal structures show the NgF2⋅CrOF4 adducts are comprised of Ft−Ng−Fb‐ ‐ ‐Cr(O)F4 structural units in which NgF2 is weakly coordinated to CrOF4 by means of a fluorine bridge, Fb, in which Ng−Fb is elongated relative to the terminal Ng−Ft bond. In contrast with XeF2⋅2MOF4 (M=Mo or W) and KrF2⋅2MoOF4, in which the Lewis acidic, F4(O)M‐ ‐ ‐Fb‐ ‐ ‐M(O)F3 moiety coordinates to Ng through a single M‐ ‐ ‐Fb−Ng bridge, both fluorine ligands of NgF2 coordinate to CrOF4 molecules to form F4(O)Cr‐ ‐ ‐Fb−Ng−Fb‐ ‐ ‐Cr(O)F4 adducts in which both Ng−Fb bonds are only marginally elongated relative to the Ng−F bonds of free NgF2. Quantum‐chemical calculations show that the Cr−Fb bonds of NgF2⋅CrOF4 and NgF2⋅2CrOF4 are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr‐ ‐ ‐Fb−Ng bridge angles and staggered O−Cr‐ ‐ ‐Fb−Ng−Ft dihedral angles. The crystal structures and Raman spectra of two CrOF4 polymorphs have also been obtained. Both are comprised of fluorine‐bridged chains that are cis‐ and trans‐fluorine‐bridged with respect to oxygen.

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.

In this Review, we report and compare non‐fluorinated with fluorinated cathode active materials (CAMs) based on a nickel‐cobalt‐manganese (NCM) composition such as HE‐NCM, NCM, Ni‐rich, Co‐rich, and Mn‐rich CAMs. We evaluate the CAMs according to their fluoride concentration, the F sources used in the synthesis method, characterization techniques, and battery performance [(initial) capacity or coulombic efficiency at C‐rate before/after cycles and the temperature]. In detail, we address 33 publications with “single” fluorinated NCM CAMs, including F sources such as LiF, [NH4]F, [NH4]FHF, NaF, NiF2, and F2. Furthermore, we give a short overview of 52 articles for “multiple” (anion and cation) treated NCM CAMs. For all reported articles (single and multiple treated NCM CAMs), we give the CAMs, conductive additives, and binder materials, as well their electrolyte compositions.

The mild fluorination of Ni‐rich NCM CAMs (NCM=nickel‐cobalt‐manganese oxide; CAM=cathode active material) with a few hundred mbar of elementary fluorine gas (F2) at room temperature was systematically studied. The resulting fluorinated CAMs were fully analyzed and compared to the pristine ones. Fluorination at room temperature converts part of the soluble basic species on the CAM‐surface into a protecting thin and amorphous LiF film. No formation of a metal fluoride other than LiF was detected. SEM images revealed a smoothened CAM surface upon fluorination, possibly due to the LiF film formation. Apparently due to this protecting, but insulating LiF‐film, the fluorinated material has a reduced electrical conductivity in comparison to the pristine material. Yet, all fluorinated Ni‐rich NCM CAMs showed a considerably higher press density than the pristine material, which in addition increased with higher fluoride concentrations. In addition, fluorination of the Ni‐rich CAMs led to the chemically induced formation of small amounts of water, which according to TGA‐MS‐measurements can be removed by heating the material to 450 °C for a few hours. Overall, the tested fluorinated NCM 811 samples showed improved electrochemical performance over the pristine samples in full‐cells with graphite anodes at 30 °C and 45 °C after 500 cycles. Moreover, the fluorination apparently reduces Mn and Co cross talk from the CAM to the anode active material (AAM) through the electrolyte during charge/discharge.

The volatile alane (H2AlO t Bu)2 decomposes into amorphous HAlO or Al0@Al2O3 nanoparticles upon heating, depending on the time and temperature. By coating the Ni-rich cathode material NCM851005 with this compound, the NCM’s cycling stability and electric conductivity were increased. Thus, the coating not only yielded Al0@Al2O3 nanoparticles but also, by reaction with surface Li2O/LiOH/Li2CO3, a Li+ conductive LiAlO2 layer. The coatings with 0.3 and 0.1 wt% (H2AlO t Bu)2, respectively, significantly reduced the resistance build-up to 70/115% after 280 cycles at 1 C (351% without coating). Upon treatment of the 0.3 wt% Al-coating with two equivalents of anhydrous HF, the Al2O3 and LiAlO2 parts were transformed into a Li[AlO(OH)F] layer, which yielded better capacity retention, retaining the low impedance build-up of only +120% (280 cycles at 1 C). This treatment, however, proved to have the same effect as simply reducing the amount of (H2AlO t Bu)2 for the coating to 0.1%.