Mild Lithium‐Rich Manganese‐Based Cathodes with the Optimal Activation of Li2MnO3 for Stable and High Capacity Lithium‐Ion Batteries

The commercial application of lithium‐rich layered oxides still has many obstacles since the oxygen in Li2MnO3 has an unstable coordination and tends to be released when Li‐ion is extracted at the voltage higher than 4.5 V. In this work, a series of cobalt‐free lithium‐rich manganese‐based oxide cathodes (Li1+xTM1‐xO2, TM = Mn, Ni) are synthesized by gradually decreasing the Li/TM ratio. Among these cobalt‐free Li‐rich manganese‐based oxides (LRMO), LR‐1.2 (when Li/TM = 1.2) has an optimized dual‐phase (namely Li2MnO3 and LiTMO2‐like) structure, in which the coordination environment of part of oxygen is transformed from 4Li‐O‐2TM octahedra to 3Li‐O‐3TM octahedra due to the partial substitution of TM for Li at Li‐2b site. Thus, some of the original unstable Li–O–Li configurations change to Li–O–TM configurations, forming strong TM–O covalent bonding and enhancing the structural stability of the oxygen. Consequently, the LR‐1.2 achieved a high reversible capacity of 282.3 mAh g−1 (Coulombic efficiency of 90.9%) at 0.1 C, exhibiting outstanding cycling stability (capacity retention of 90.3% after 400 cycles at 2 C) and superior rate performance. This work establishes a correlation between the microstructure modulation tuned by the Li/TM ratio and their electrochemical performance, offering insights into the design of cathode materials for high‐performance lithium‐ion batteries. By controlling the Li/TM ratio, the distribution of Li2MnO3 structural domains and the local environment of oxygen coordination are regulated. As a result, the structural damage and oxygen loss of the sample LR‐1.2 with the optimal dual‐phase structure are greatly suppressed, and it shows excellent rate performance and cycling stability (87.8%, 400th, 1C; 90.3%, 400th, 2C).