Ni-rich nickel cobalt manganese oxide (NCM) cathode active materials are commercially used in high-energy lithium-ion batteries, e.g. in electric vehicles, due to their high capacity. However, structural and thermal degradation at high voltages prevent the usage of the total theoretical capacity. For targeted and resource-efficient material solutions, a deep understanding of the structural degradation phenomena is urgently needed. This work focuses on three specific aspects: (i) Time-dependent structural surface degradation at high potential holds, (ii) thermally induced structural degradation of protonated NCM, and (iii) Al2O3 coatings and their behavior during cycling. Scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy and in situ transmission electron microscopy were applied to investigate these structural degradation phenomena. The analysis of high-resolution STEM images and electron energy loss spectra in (i) confirmed an anisotropic surface degradation at high potential holds of 4.5 V, which deviates from the often proposed homogeneous core-shell model. A gradual transition boundary was observed between the intact layered structure and the rock salt structure, which formed epitaxially and coherently on the surface. Furthermore, we introduced a new image processing method that highlights the structural differences of the intact layered structure, the gradual transition boundary, and the rock salt layer immediately. The STEM investigations and energy-dispersive X-ray-spectroscopy analysis of protonated and heated Ni-rich NCM in (ii) showed strong rock salt formation on the surface. Furthermore, nanopore formation with oxygen depleted surfaces was observed at the particle surfaces, along defects, and in the core of particles, which was confirmed by in situ heating experiments. The observation of nanopore formation offers new insights into structural degradation caused by washing and subsequent heating. The STEM and energy-dispersive X-ray spectroscopy analysis in (iii) of Al2O3 coated polycrystalline NCM particles revealed a change in the coating layer from a homogeneously confined layer before to a more diffusive layer after cycling. Finally, we presented the first building block of in situ biasing transmission electron microscopy, which aims to study the dynamics of coating behavior during cycling. The results of the investigations in (i) and (ii) showed that the extent of structural degradation is more complex and pronounced than initially assumed. The results in (iii) highlight the need to investigate coatings at the nanoscale with high spatial resolution and provide ideas for an improved in situ biasing transmission electron microscopy approach to investigate the dynamics of Al2O3 coatings on polycrystalline NCM during cycling. The (high-resolution) STEM investigations have yielded a deeper understanding of the structural degradation behavior in Ni-rich NCM, which is essential for tailored material development and the optimization of commercial high-energy NCM cathodes.
Lara Ahrens (Wed,) studied this question.