Structural Transition and Charge Compensation during the Initial Delithiation and Lithiation of Li2MoO3
Authors: Ma Jun , Zhou Yong-Ning , Gao Yurui , Yu Xiqian , Kong Qingyu , Gu Lin , Wang Zhaoxiang , Yang Xiao-Qing , Chen Liquan 
Published: February 2020

Li2MnO3 stabilizes the structure of Li
M
O2 (
M
= Mn, Ni, Co, etc.) and makes Li-rich layer-structured xLi2MnO3·(1-x)Li
M
O2 (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 O2- 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)Li
M
O2 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 xLi2

M
´O3·(1-x)Li
M
O2.
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 Li2-x
MoO3 (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 O2- 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)Li
M
O2materials.
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.