Formation of Surface Impurities on Lithium–Nickel–Manganese–Cobalt Oxides in the Presence of CO2 and H2O

Surface impurities involving parasitic reactions and gas evolution contribute to the degradation of high Ni content LiNixMnyCozO2 (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 CO2 and H2O. CO2 reactivity on a clean surface as characterized by CO2 conversion rate in the TAP reactor follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111. The capacity of CO2 uptake follows a different order: NMC532 > NMC433 > NMC622 > NMC811 > NMC111. Moisture pretreatment slows down the direct CO2 adsorption process and creates additional active sites for CO2 adsorption. Electronic structure calculations predict that the (012) surface is more reactive than the (1014) surface for CO2 and H2O adsorption. CO2 adsorption leading to carbonate formation is exothermic with formation of ion pairs. The average CO2 binding energies on the different materials follow the CO2 reactivity order. Water hydroxylates the (012) surface and surface OH groups favor bicarbonate formation. Water creates more active sites for CO2 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.伴随副反应与气体析出的表面杂质，是高镍含量LiNₓMnᵧCo_zO₂（NMC）正极材料性能退化的关键诱因。本研究借助产物瞬态分析（Temporal Analysis of Products, TAP）这一瞬态动力学技术、密度泛函理论与红外光谱法，针对CO₂与H₂O氛围下不同镍含量的NMC材料（NMC811、NMC622、NMC532、NMC433、NMC111）的表面杂质生成过程展开了系统研究。以TAP反应器内CO₂转化率表征的清洁表面CO₂反应活性，遵循如下排序：NMC811 > NMC622 > NMC532 > NMC433 > NMC111。而CO₂吸附容量的排序则有所差异：NMC532 > NMC433 > NMC622 > NMC811 > NMC111。水分预处理会延缓CO₂的直接吸附进程，同时为CO₂吸附提供额外的活性位点。电子结构计算结果显示，针对CO₂与H₂O吸附过程，(012)晶面的反应活性高于(1014)晶面。生成碳酸盐的CO₂吸附过程为放热过程，且伴随离子对的形成。不同材料表面的平均CO₂结合能，与CO₂反应活性的排序保持一致。水分子会对(012)晶面进行羟基化修饰，而表面羟基基团则有利于碳酸氢盐的生成。得益于氢键作用，水分子可在(1014)晶面为CO₂吸附创造更多活性位点。暴露于环境空气中所生成的表面杂质的组成，取决于空气中的水分浓度与不同晶面的占比。不同晶面的反应活性差异表明，通过精准调控NMC材料的主导晶面，可有效缓解由表面杂质引发的电池性能退化。