Understanding the Origin of Li₂MnO₃ Activation in Li‐Rich Cathode Materials for Lithium‐Ion Batteries

Li‐rich layered cathode materials have been considered as a family of promising high‐energy density cathode materials for next generation lithium‐ion batteries (LIBs). However, although activation of the Li 2 MnO 3 phase is known to play an essential role in providing superior capacity, the mechanism of activation of the Li 2 MnO 3 phase in Li‐rich cathode materials is still not fully understood. In this work, an interesting Li‐rich cathode material Li 1.87 Mn 0.94 Ni 0.19 O 3 is reported where the Li 2 MnO 3 phase activation process can be effectively controlled due to the relatively low level of Ni doping. Such a unique feature offers the possibility of investigating the detailed activation mechanism by examining the intermediate states and phases of the Li 2 MnO 3 during the controlled activation process. Combining powerful synchrotron in situ X‐ray diffraction analysis and observations using advanced scanning transmission electron microscopy equipped with a high angle annular dark field detector, it has been revealed that the subreaction of O 2 generation may feature a much faster kinetics than the transition metal diffusion during the Li 2 MnO 3 activation process, indicating that the latter plays a crucial role in determining the Li 2 MnO 3 activation rate and leading to the unusual stepwise capacity increase over charging cycles.