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-coated 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.