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 Li₂MnO₃ phase is known to play an essential role in providing superior capacity, the mechanism of activation of the Li₂MnO₃ phase in Li-rich cathode materials is still not fully understood. In this work, an interesting Li-rich cathode material Li_1.87Mn_0.94Ni_0.19O₃ is reported where the Li₂MnO₃ 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 Li₂MnO₃ 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 O₂ generation may feature a much faster kinetics than the transition metal diffusion during the Li₂MnO₃ activation process, indicating that the latter plays a crucial role in determining the Li₂MnO₃ activation rate and leading to the unusual stepwise capacity increase over charging cycles.