Facet-dependent Thermal and Electrochemical Degradation of Lithium-rich Layered Oxides

Lithium-rich layered oxides (LLOs) are promising candidate cathode materials for safe and inexpensive high-energy-density Li-ion batteries. However, oxygen dimers are formed from the cathode material through oxygen redox activity, which can result in morphological changes and structural transitions that cause performance deterioration and safety concerns. Herein, a flake-like LLO is prepared and aberration-corrected scanning transmission electron microscopy (STEM), in situ high-temperature X-ray diffraction (HT-XRD), and soft X-ray absorption spectrum (sXAS) are used to explore its crystal facet degradation behavior in terms of both thermal and electrochemical processes. Void-induced degradation behavior of LLO in different facet reveals significant anisotropy at high voltage. Particle degradation originates from side facets, such as the (010) facet, while the close (003) facet is stable. These results are further understood through ab initio molecular dynamics calculations, which show that oxygen atoms are lost from the {010} facets. Therefore, the facet degradation process is that oxygen molecular formed in the interlayer and accumulated in the ab plane during heating, which result in crevice-voids in the ab plane facets. The study reveals important aspects of the mechanism responsible for oxygen -anionic activity-based degradation of LLO cathode materials used in lithium-ion batteries. In particular, this study provides insight that enables precise and efficient measures to be taken to improve the thermal and electrochemical stability of an LLO.