Unraveling the Degradation Process of LiNi_0.8Co_0.15Al_0.05O₂ Electrodes in Commercial Lithium Ion Batteries by Electronic Structure Investigations

The degradation of LiNi0.8Co0.15Al0.05O2 (LNCAO) is reflected by the electrochemical performance in the fatigued state and correlated with the redox behavior of these cathodes. The detailed electrochemical performance of these samples is investigated by galvanostatic and voltammetric cycling as well as with the galvanostatic intermittent titration technique (GITT). Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to investigate the oxidation state of all three materials at the Ni L2,3, O K, and Co L2,3 edges at five different states of charge. Surface and more bulklike properties are distinguished by total electron yield (TEY) and fluorescence yield (FY) measurements. The electrochemical investigations revealed that the changes in the cell performance of the differently aged materials can be explained by considering the reaction kinetics of the intercalation/deintercalation process. The failure of the redox process of oxygen and nickel at low voltages leads to a significant decrease of the reaction rates in the fatigued cathodes. The accompanied cyclic voltammogram (CV) peaks appear as two peaks because of the local minimum of the reaction rate, although it is one peak in the CV of the calendarically aged LNCAO. The absence of the oxidation/reduction process at low voltages can be traced back to changes in the surface morphology (formation of a NiO-like structure). Further consequences of these material changes are overpotentials, which lead to capacity losses of up to 30% (cycled with a C/3 rate).