Modulating local electronic structure enhances superior electrochemical activity in Li-rich oxide cathodes

Li₂MnO₃ is the parent compound of Li-rich Mn-based cathode materials for high energy density Li-ion batteries. The existence of oxygen vacancies yields anomalous capacity and can significantly increase the electrochemical activity of Li₂MnO₃, attracting considerable interest. However, the mechanism behind oxygen vacancies in Li₂MnO₃ is still under debate, due to the challenges in directly observing O and following relevant changes upon electrochemical cycling. Herein, to address the poor electrochemical activity of Li₂MnO₃ and further reveal its reaction mechanism, a mechanical thermal activation engineering strategy was used to synthesize Li₂MnO₃ with different contents of oxygen vacancies. The electronic structure can be effectively modulated by introducing different contents of oxygen vacancies in the local atomic coordination around Mn and O, which induces the distortion of the interfacial structure with a higher lattice d-spacing and effectively stimulates the electrochemical activity. By combining in situ XRD, ex situ XPS and ex situ Raman to investigate the evolution of Mn and O in Li₂MnO₃ during cycling. It is found that the Mn-O hybridization shows strong correlations with the oxygen redox behaviors, and high electrochemical activity and excellent cycling stability cannot coexist. Thus, the electrochemical activity and stability of Li₂MnO₃ are affected synchronously by lattice vacancies and local coordination configuration. This work provides valuable insights into the origin of electrochemical activity in Li₂MnO₃, which may inspire the design of high energy density cathode materials.