Charge compensation mechanisms and interphase chemistry of nickel-rich lithium-ion cathode materials studied by photoelectron spectroscopy

With the global transition to renewable energy and the increasing demand for energy storage solutions, lithium-ion batteries (LIBs) play a key role in electromobility and stationary energy storage. A central field of research is the development and characterization of cathode materials that not only enable high energy densities but are also thermally and electrochemically stable. The layered oxides investigated in this thesis – particularly LCO and the LiNi_xMn_yCo_zO₂ systems (NMC333, NMC622, NMC811) as well as LiNi_{0.8}Co_{0.16}Al_{0.04}O₂ (NCA) – belong to the most promising cathode materials. However, surface reactions and degradation mechanisms, in particular, are still poorly understood, hindering the optimization of these materials. The aim of this dissertation was therefore to investigate the electronic structure and interphase chemistry of these cathode materials using X-ray photoelectron spectroscopy (XPS). Central to this was the question of how the oxidation states of the transition metals and the chemical composition of the cathode surface change depending on the cell voltage, and what role the cathode electrolyte interphase (CEI) plays in the stability and performance of the materials. To overcome the surface sensitivity inherent to XPS, an innovative "in vacuo scratching" technique was developed. This method allows for the gentle removal of the CEI without – unlike conventional Ar⁺ sputtering – altering the chemical composition through reduction or selective removal of individual elements. This technique enabled the acquisition of high-resolution spectra for all cathode materials studied. A quantitative analysis of the XPS data enabled a comparative stoichiometry study of the bulk material, which – validated against ICP-OES and EDX results – demonstrated both the accuracy and limitations of the method. In the voltage-dependent XPS measurements, it was shown that changes in the spectral shape of Co and Ni 2p core spectra in these materials correlate clearly with the oxidation state changes with deintercalation. It was also observed that the CEI increasingly decomposes at high cell voltages, with LiF, organofluoride species, and carbonates being detected. Another methodological focus was the application of ligand field and charge transfer theory to the analysis of the Co and Ni 2p core level spectra. By using charge transfer multiplet (CTM) simulations, a direct comparison to the experimental spectra was performed, providing further information on the electronic structure. The combination of novel preparation techniques and detailed spectroscopic analysis thus provides a new methodological procedure for the characterization of commercial battery materials. This work therefore makes a substantial contribution to the improved understanding of electrochemically active transition metal oxides, their surface reactions, and their interactions with the electrolyte.