Impact of Oxygen Deficiency on Structural and Electrochemical Properties of Single-Crystalline LiNi0.8Co0.1Mn0.1O2 (NCM 811)

Ni-rich transition metal oxides of the type LiNi1–x–yCoxMnyO2 (NCM) are among the most important cathode active materials (CAMs) for lithium-ion batteries (LIBs). The oxygen content and defect chemistry of these materials are largely determined during the high-temperature synthesis process. During synthesis, precursors need to be calcined and oxidized. Insufficient oxygen supply can lead to incomplete oxidation, and if overlooked, small concentrations of oxygen deficiency can remain. In this work, the electrochemical performance of single-crystalline NCM 811 with controlled oxygen deficiency (1 mol %, 2 mol %, and 5 mol %) is investigated and compared with a stoichiometric reference. High-temperature coulometric titration is used to precisely control the oxygen deficiency after synthesis. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to further characterize the samples. With increasing oxygen deficiency, a gradual deterioration in electrochemical performance is observed, including reduced first-cycle discharge capacity, capacity retention, and rate capability. We attribute this behavior to sluggish lithium kinetics, formation of rock-salt-type phases (RSP), bulk micropores, Li/Ni antisite defects, and other lattice defects. Overall, this study provides clear evidence of the detrimental effects of oxygen deficiency on both the morphology and electrochemical performance of layered NCM materials, thereby contributing to a more comprehensive understanding of oxygen defect chemistry in CAMs.