Formation of Surface Impurities on Lithium-Nickel-Manganese-Cobalt Oxides in the Presence of CO and HO.

Surface impurities involving parasitic reactions and gas evolution contribute to the degradation of high Ni content LiNiMnCoO (NMC) cathode materials. The transient kinetic technique of temporal analysis of products (TAP), density functional theory, and infrared spectroscopy have been used to study the formation of surface impurities on varying nickel content NMC materials (NMC811, NMC622, NMC532, NMC433, NMC111) in the presence of CO and HO. CO reactivity on a clean surface as characterized by CO conversion rate in the TAP reactor follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111. The capacity of CO uptake follows a different order: NMC532 > NMC433 > NMC622 > NMC811 > NMC111. Moisture pretreatment slows down the direct CO adsorption process and creates additional active sites for CO adsorption. Electronic structure calculations predict that the (012) surface is more reactive than the (1014) surface for CO and HO adsorption. CO adsorption leading to carbonate formation is exothermic with formation of ion pairs. The average CO binding energies on the different materials follow the CO reactivity order. Water hydroxylates the (012) surface and surface OH groups favor bicarbonate formation. Water creates more active sites for CO adsorption on the (1014) surface due to hydrogen bonding. The composition of surface impurities formed in ambient air exposure is dependent on water concentration and the percentage of different crystal planes. Different surface reactivities suggest that battery performance degradation due to surface impurities can be mitigated by precise control of the dominant surfaces in NMC materials.