Structural Transition and Charge Compensation during the Initial Delithiation and Lithiation of Li2MoO3

Li₂MnO₃ stabilizes the structure of LiMO₂ (M = Mn, Ni, Co, etc.) and makes Li-rich layer-structured xLi₂MnO₃·(1-x)LiMO₂ (0<x<1.0, M = Mn, Ni, Co, etc.) composites (or solid solutions) become potential cathode materials for high energy density lithium ion batteries. However, as Mn⁴⁺ is inactive in Li₂MnO₃, the charge compensation from O^2- ions during the initial delithiation and the irreversible layer-to-spinel structural transition in the subsequent lithiation makes the composite suffer from drawbacks such as low initial coulombic efficiency, discharge voltage and energy density falling, and poor rate performance during cycling as well as safety hazard in the initial cycle. Although surface modification, atomic substitution and optimization of synthesis strategies have been pursued to improve the performances of the xLi₂MnO₃·(1-x)LiMO₂ composites, the inherent drawbacks of Li₂MnO₃ component have not been, and cannot, overcome. Here we introduce Li₂MoO₃ with disordered NaFeO₂ structure (R-3m) as a prospective alternation of Li₂MnO₃ for designing novel Li-rich cathode materials xLi₂M´O₃·(1-x)LiMO₂.

    In this report, the structural transition and charge compensation of Li₂MoO₃ during the initial charge and discharge were investigated with STEM and synchrotron in situ XRD and XAS techniques. It is shown that, during the initial delithiation, solid-solution reaction and two-phase reaction (slipped O3 → faulted O1) go in series due to charge compensation from Mo⁴⁺ ions in both the Mo-O and Mo-Mo covalent bonds in the Mo₃O₁₃ cluster accompanied with the Mo ion migration from Li-2Mo layer to Li layer. In the subsequent lithiation, its structure is recovered to a Li-insufficient O3 type Li_2-xMoO₃ (x = 0.50) due to the incomplete reduction of Mo⁶⁺ ions and the nearly reversible migration of the Mo ions at the end of lithiation. Unlike the irreversible oxygen release in deeply delithiated Li₂MnO₃, the O K-edge soft XAS of Li₂MoO₃ illustrates that oxidation of O^2- to O^(2-σ)- is nearly reversible and is required dynamically rather than thermodynamically. These features make Li₂MoO₃ a promising superior alternate in constructing novel Li-rich cathode material with improved structural stability and easy charge compensation. In addition, the contribution of Mo-Mo covalent bond seems to help to maintain the framework of the electrode material by hindering the loss of oxygen. Therefore, the basic findings in this work will also bring new insight on understanding the performance decay and searching for new ways to improve the performance of the conventional xLi₂MnO₃·(1-x)LiMO₂materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation (No. 51372268) of China and the National 973 Program of China (2009CB 220100).

The work at Brookhaven National Lab. was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DEAC02-98CH10886.