High-voltage operation reveals surface reconstruction as primary contributor to impedance growth over CEI evolution in Ni-Rich layered oxides

Enhancing the energy density of lithium-ion batteries by increasing the nickel content in layered oxides as cathode materials is hindered by accelerated degradation at high potentials. Here, we resolve the degradation mechanism of single-crystalline LiNi$_{0.83}$Co$_{0.11}$Mn$_{0.06}$O$_2$ (NCM) by comparing different aging protocols, varying upper cutoff voltage, time exposed to high potential (4.5 V versus Li$^+$/Li) and number of cycles. The impedance growth due to these stressors is quantified by electrochemical analysis, including potentiostatic electrochemical impedance spectroscopy (PEIS). The structural and chemical degradation is investigated post mortem via transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). The analysis reveals that the organic components of the CEI are oxidized and removed during extended holds at 4.5 V. However, this CEI thinning occurs alongside an increase in charge transfer resistance R$_{CT}$. Furthermore, cells undergoing cycling versus hold protocols exhibited similar CEI compositions despite different R$_{CT}$. Therefore, we conclude that the CEI plays a minor role in the observed impedance increase. ... mehrIn contrast, the formation of a rock-salt-type SRL correlates with kinetic limitations and capacity fade. These findings provide a basis for distinguishing the effects of CEI evolution from surface reconstruction during long-term, high-voltage operation.