Improved Electron/Li-Ion Transport and Oxygen Stability of Mo-Doped Li2MnO3

Li₂MnO₃ plays significant roles in stabilizing the structure and ensuring a high specific lithium storage capacity of the Li-rich layered xLi₂MnO₃·(1-x)LiMO₂ (M = Ni, Co, Mn, etc.) composites (or solid solutions) as cathode materials for lithium ion batteries (LIBs)¹. Nevertheless, it meets challenges in irreversible structural transition and low coulombic efficiency due to the loss of oxygen in the initial charge process. In addition, its insulating property deteriorates the rate performance of the xLi₂MnO₃·(1-x)LiMO₂. These drawbacks hinder the commercialization and application of the composites as cathode materials for LIBs.

         Elemental substitution or atom doping has been commonly applied to improve the electrochemical performances of Li₂MnO₃ ^2-4. In an electrochemical viewpoint, as Mn⁴⁺ cannot be further oxidized in octahedral coordination in the pristine Li₂MnO₃, the charge compensation has to be conducted by the oxidation of the O^2- ions alone upon Li-ion extraction, leading to irreversible structural change and safety issues. In contrast, as Mo⁴⁺ is potentially oxidized to Mo⁶⁺, the charge compensation will be largely declined from oxygen if multi-electron transfer occurs on the doped transition metal atoms in Li₂Mn_1-xMo_xO₃. Therefore, substituting Mn with Mo is expected to reduce the loss of oxygen and improve the stability of the structure. In addition, considering the difference of Li₂MnO₃ (red) and its iso-structured Li₂MoO₃ (black) in color, Mo substitution for Mn might also improve the conductivity and rate performance of Li₂MnO₃and its related cathode materials.

         In this work, first-principles calculations are conducted to predict the impacts of molybdenum (Mo) doping on the physical and electrochemical properties of Li₂MnO₃. It demonstrates that Mo doping is indeed beneficial in improving both the dynamic and thermodynamic properties of Li₂MnO₃: (1) It decreases the band gap and increases the number of states around the Fermi level; (2) It enhances Li-ion diffusion especially between the lithium layer and the transition-metal layer; (3) Extra charge is transferred from Mo to O, accompanied with the reduction of the delithiation potential; (4) The charge of the removed Li-ion is compensated by both Mo and O in the Mo-doped Li₂MnO₃ during delithiation, and (5) Mo doping delays oxygen release and reinforces the stability of the structural oxygen based on the calculated reaction enthalpy. Therefore, Mo doping is expected to be an effective way in improving the structural stability and rate performance of Li₂MnO₃ and xLi₂MnO₃·(1-x)LiMO₂ cathode materials. These findings will facilitate the investigation and promote the development of Li₂MnO₃ and xLi₂MnO₃·(1-x)LiMO₂ cathode materials. 

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