Fluorination of Ni‐Rich Lithium‐Ion Battery Cathode Materials by Fluorine Gas: Chemistry, Characterization, and Electrochemical Performance in Full‐cells

Abstract

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.

Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification

“…12 It is worthy to note that the lack of decomposed species observed for F-modified NMC811 in the IR spectra only indicates a more diminished interfacial reactivity compared to as-received NMC811 at the same scale rather than complete absence of interfacial reactions. The observed suppressed electrolyte decomposition for F-modified NMC811 presented in this work is also in agreement with improved capacity retention of Fdoped NMC811 upon charging to 4.3-4.4 V Li shown by Janek et al, 19 Xiong et al, 20 as well as Breddemann et al 25 through solidstate, sol-gel and gas-flow reactor synthesis.…”

Alleviating Anisotropic Volume Variation at Comparable Li Utilization during Cycling of Ni-Rich, Co-Free Layered Oxide Cathode Materials

“…Comparing the differential capacity curves (see Figure 5b), NiMn9010 exhibits a single peak at 3.7 V during charge, which corresponds to the Ni redox reaction, as Mn remains electrochemically inactive and retains a 4+ oxidation state throughout the measured potential window. 75,76 Meanwhile, the NiCo9010 curve shows additional features in this region, corresponding to the overlapping redox activities of Ni and Co, with a peak already at 3.6 V. 77,78 At higher potential, both materials show a peak that is usually attributed to the H2−H3 transition in LNO/LCO materials, which occurs at ≈4.17 V for NiCo9010 but is observed at higher potentials ≈4.23 V for NiMn9010. Even though a first-order phase transition is not observed by XRD, its signature remains in the differential capacity curve.…”

Mo–F Co-Doping LiNi0.83Co0.11Mn0.06O2 Stabilizes the Structure and Induces Compact Primary Particle To Improve the Electrochemical Performance

“…Eventually, the migration of Li + is hindered, and the capacity drops rapidly. 23 The slight TM dissolution in NCM83Mo/ F0.5% may benefit from the more stable surface structure, the thin amorphous LiF film produced by NH 4 F during sintering, 49 and the compact arrangement of primary particles.…”