Activation Mechanism of LiNi0.80Co0.15Al0.05O2: Surface and Bulk Operando Electrochemical, Differential Electrochemical Mass Spectrometry, and X-ray Diffraction Analyses

The lithium (de)-insertion mechanism from LiNi0.80Co0.15Al0.05O2 (NCA) has been investigated by means of combined electrochemical analysis, operando differential electrochemical mass spectrometry (DEMS) experiments, and in situ X-ray diffraction (XRD) experiments during the first three cycles. Qualitative analysis of cyclic voltammetry data illustrated a possible irreversible activation of the material. Operando DEMS and internal cell pressure measurements combined with ex situ XRD and electrochemical impedance spectroscopy demonstrated that Li2CO3 surface film on the NCA electrode degrades on oxidation and reforms on reduction, which has an effect on the lithium (de)-insertion reaction kinetics. In situ XRD studies clearly show mechanistic differences in the reaction pathways between the first and second cycle/following cycles. While the first charge reveals a combination of an irreversible two-phase transition plus a reversible solid solution reaction mechanism, the second charge is mainly dominated by a solid solution process. Such differences have been ascribed to changes in two factors, the electronic conductivity and the Li ion mobility of the NCA electrode.