Synergistic Effects of Ni²⁺ and Mn³⁺ on the Electrochemical Activation of Li₂MnO₃ in Co-Free and Ni-Poor Li-Rich Layered Cathodes

Although a very high energy density can be stored in Co-free Li-rich layered oxide cathodes, it is difficult to fully exploit the reactivity of the dominant Li₂MnO₃ component. In this study, regulating the Ni²⁺ content and introducing trivalent Mn³⁺ are comprehensively investigated for developing a higher available capacity from the Li₂MnO₃ component. As the content of Ni²⁺ increases, the capacity property and the electrochemical stability are improved due to the decreased Li₂MnO₃-like domain and the enhanced layered structure. Under a proper oxygen partial pressure, the valence of partial manganese can be adjusted to trivalency without generating any impurity phase. Further analyzed by X-ray absorption fine spectroscopy and simulated by Monte Carlo calculation, we find that the local structure of the Li₂MnO₃-like domain is modulated to be more dispersed and uniform with the presence of Mn³⁺. With the assistance of Ni²⁺ ions, Mn³⁺ exhibits a greater effect on optimizing the local structure. As a consequence, under the synergy of Ni²⁺ and Mn³⁺, the optimized sample exhibits the available discharge capacity of over 280 mAh g^–1 after several cycles.