Structural Changes of Li2MoO3 As Cathode Material for Li-Ion Batteries during Initial Charge-Discharge Studied By Synchrotron-Based X-Ray Diffraction and Absorption
Authors: Zhou Yong-Ning , Ma Jun , Hu Enyuan , Yu Xiqian , Nam Kyung-Wan , Gu Lin , Wang Zhaoxiang , Yang Xiao-Qing , Chen Liquan 
Published: February 2020

Introduction
James and Goodenough first introduced layer-structured Li2MoO3 as a lithium-battery cathode material [1]. It has a disordered NaFeO2 structure (; a = 2.884 Å, c = 14.834 Å) consisting of a cubic close-packed oxygen cations with basal planes of octahedral sites alternatively filled with Li ions (3a sites) and a randomly distributed mixture of 1/3 Li (3b sites) and 2/3 Mo (3b sites) forming a Li-Mo layer(with a 1:2 Li:Mo ratio), in which the Mo ions forming disordered Mo3O13 clusters rather than isolated Mo ions.[1-5] The theoretical capacity of the Li2MoO3 reaches 339 mAh g-1 based on the Mo4+/Mo6+ redox reaction alone (i.e. without oxidation of the O2- anions) and the Mo4+/Mo6+ redox reaction potential is much lower than that of the oxygen release in Li2MnO3. Therefore, Li2MoO3 is expected to be a good component in building a new layer-structured xLi2MoO3·(1-x)Li
M
O2 system as high-capacity cathode materials. Here we report the structural studies and charge compensation of Li2MoO3 during the initial charge and discharge. The close to fully reversible structural changes and Mo ion migration, originated from the charge compensation of Mo ions in both the Mo-O and Mo-Mo covalent bonds in the Mo3O13 cluster, make the Li2MoO3 an appropriate alternative of Li2MnO3 in constructing new xLi2MoO3·(1-x)Li
M
O2 cathode materials, which have less irreversible transition metal migration and oxygen evolution. The findings in this work will also shed light on the fundamental understandings of the relationships between the performance and structure changes, as well as on the new approaches in developing lithium-rich cathode materials with both high energy density and long cycle life.

Results and discussion

To understand the structural changes of Li2MoO3 during lithium extraction, in situ x-ray diffraction (XRD) and X-ray absorption (XAS) spectroscopy at Mo K-edge were used to study the crystal structure and valence state as well as local structural changes of Mo ions in charging process. The X-ray absorption near edge structure (XANES) spectra of the Mo K-edge during charge show a continuous increase of the pre-edge peaks indicates the increased distortion of Mo-O6 octahedral. The white line of the K-edge shifted to the higher energy gradually, suggesting the increasing oxidation state of Mo ions upon charge. Compare with the Mo K-edge XANES data of the MoO2 and MoO3 references, it can be estimated that the Mo ions were oxidized from Mo4+ to average oxidation state close to Mo6+. More detailed results will be discussed in the presentation.

Acknowledgement

This work 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. DE-AC02-98CH10886. Use of the National Synchrotron Light Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The work at Institute of Physics, Chinese Academy of Sciences was supported by the National Natural Science Foundation (No. 51372268) of China and the National 973 Program of China (2009CB 220100).

References

[1] A. C. W. P. James and J. B. Goodenough, J. Solid State Chem., 1988, 76, 87..
[2] S. J. Hibble and I. D. Fawcett, Inorg. Chem., 1995, 34, 500.
[3] S. J. Hibble, I. D. Fawcett and A. C. Hannon, Acta Cryst., 1997, B53, 604.
[4] S. J. Hibble, A. C. Hannon and I. D. Fawcett, J. Phys.: Condens. Matter, 1999, 11, 9203
[5] W. H. McCarroll, L. Katz and R. Ward, J. Am. Chem. Soc., 1957, 79, 5410.