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

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.