Vacancy-induced MnO₆ distortion and its impacts on structural transition of Li₂MnO₃

As a new class of high-capacity cathode materials, the Li-rich Mn-based layer-structured xLi₂MnO₃·(1 - x)LiMO₂ (M = Ni, Co, Mn, etc.) is a promising candidate for constructing high energy-density Li-ion batteries. Unfortunately, drawbacks such as oxygen evolution, poor rate performance and potential fading during cycling hinder their commercial applications. Migration of the transition metal (Mn) into the Li layer of Li₂MnO₃ and the resultant irreversible structural transition are believed to be responsible for these issues. Therefore, it is essential to explore the driving force for the Mn migration. In this study, we show, starting from understanding the impact of O and Li vacancies on the migration of the Mn atoms by the first-principles molecular dynamics simulation, that Mn migration is closely involved in the breaking and forming of the Mn-O bonds of the MnO₆ octahedron and its continuous distortion (MnO_x, 4 ≤ x ≤ 6). In addition, Mn migration along with the generation of O vacancy lowers the delithiation potential. Inconsistent with conventional beliefs, Mn migration into the Li layer was found to promote, rather than block, Li diffusion in some cases. The mechanism for MnO₆ distortion provides new insight into understanding the micro mechanism of the layered-to-spinel structural transition and revealing the designing of superior cathode materials.