Twinning mediated intralayer frustration governs structural degradation in layered Li-rich oxide cathode

Layered Li-rich Mn-based (LRM) oxides are promising cathode materials for next-generation high-energy batteries. However, their commercialization is hindered by intrinsic structural issues and subsequent degradation processes. In order to address the degradation mechanisms, we use operando neutron diffraction and scanning transmission electron microscopy to follow the microstructural degeneration of the LRM oxides in a prepared full cell with a graphite anode. The methods enable both real-time phase analysis and structural evolution mapping across a wide field of view. The LRM oxide is observed to initially have a partially ordered Li₂MnO₃-like structure with multiple planar defects. It transitions from an ordered monoclinic phase to a disordered rhombohedral phase as a result of irreversible Li⁺ migration and transition metal rearrangement during cycling. Especially after the first full charge, the interlayer (001) twining-like structures and local intralayer frustrations formed. Over cycling, the intralayer frustrations further develop into pore-like microstructures along the {012} twinning boundary in the bulk of the particles, which contributes significantly to performance reduction. The results clarify the link between microstructure degradation and performance loss and provide valuable insights into the optimization of high-performance cathodes.