In situ XRD studies of the structural changes of ZrO2-coated LiCoO2 during cycling and their effects on capacity retention in lithium batteries

Synchrotron based in situ X-ray diffraction was used to study the structural changes of a ZrO2-coated LiCoO2 cathode in comparison with the uncoated sample during multi-cycling in a wider voltage window from 2.5 to 4.8 V. It was found that the improved cycling performance of ZrO2-coated LiCoO2 is closely related to the larger lattice parameter c variation range, which is an indicator of how far the structural change has proceeded towards the two end members of the phase transition stream during charge-discharge cycling. At fifth charge, the lattice parameter variation ranges for both uncoated and ZrO2-Wated LiCoO2 were reduced compared with those for the first charge, reflecting the capacity fading caused by the high voltage cycling. However, this variation range reduction is smaller in ZrO2-coated LiCoO2 than that in the uncoated sample, and so is the capacity fading. These results point out an important direction for studying the fading mechanism and coating effects: the key issues are the surface protection, the interaction between-the cathode surface and the electrolyte and the electrolyte decomposition. In order to improve the capacity retention during cycling, the variation range of lattice parameter c of LiCoO2 should be preserved, not reduced. (c) 2006 Elsevier B.V. All rights reserved.