Voltage-dependent charge compensation mechanism and cathode electrolyte interface stability of the lithium-ion battery cathode materials LiCoO₂ and LiNi_1/3Mn_1/3Co_1/3O₂ studied by photoelectron spectroscopy

Enhancing the high voltage limit of layered LiTMO2 (TM = 3d transition metal) cathode materials has motivated intensive research on their properties at high states of charge. This led to controversies regarding the origin of anionic and cationic redox reaction, the reversibility of oxygen redox reaction, the degree of covalency, and the intrinsic voltage limits of cathode materials. In this study, the commercially used cathode materials LiCoO2 (LCO) and LiNi1/3Mn1/3Co1/3O2 (NMC333) were investigated by core level photoelectron spectroscopy (XPS) after charging to different voltages. A large fraction of the cathode electrolyte interface (CEI) was removed after in vacuo scratching of the surface, enabling insights into the fundamental charge compensation mechanism in the bulk obtained by XPS. To gain insights into the oxidation state using XPS, the origin of the 2p satellite structure seen in the XPS spectra of transition metal oxides is reconsidered in comparison to reference materials. Additionally, both the CEI stability and the electrochemical behaviour are discussed and correlated to the charge compensation mechanism. For LCO, the XPS results confirm the intrinsic voltage limit of 4.2 V vs. Li+/Li with the Co3+/Co4+-redox couple at 4.0 V and subsequent oxygen-redox and CEI instability. For NMC333, the Ni3+/Ni4+ redox couple at 3.7 V, the Co3+/Co4+-redox couple at 4.0 V, and manganese in a stable formal oxidation state of 3.7+ were identified. The intrinsic voltage limit in NMC333 is enhanced to 4.5 V vs. Li+/Li.