NEW FINDINGS ON THE PHASE TRANSITIONS IN Li_(1-x) CoO_2 AND Li_(1-x)NiO_2 CATHODE MATERIALS DURING CYCLING: IN SITU SYNCHROTRON X-RAY DIFFRACTION STUDIES

摘要

We have utilized synchrotron x-ray radiation to perform "in situ" x-ray diffraction studies on Li_(1-x)CoO_2 and Li_(1-x)CoO_2 cathodes. A C/10 charging rate was used for a Li/Li_(1-x)CoO_2 cell. For the Li/Li_(1-x)CoO_2 cells, C/13 and C/84 rates were applied. The in situ XRD data were collected during the first charge from 3.5 to 5.2 V. For the Li_(1-x)CoO_2 cathode, in the composition range of x=0 to x=0.5, a new intermediate phase H2a was observed in addition to the two expected hexagonal phases H1 and H2. In the region very close to x=0.5, some spectral signatures for the formation of a monoclinic phase Ml were also observed. Further, in the x=0.8 to x=l region, the formation of a CdI_2 type hexagonal phase has been confirmed. However, this new phase is transformed from a CdCl_2 type hexagonal phase, rather than from a monoclinic phase M2 as previously reported in the literature. For the Li_(1-x)CoO_2 system, by taking the advantage of the high resolution in 2θ angles through the synchrotron based XRD technique, we were able to identify a two-phase coexistence region of hexagonal phase H1 and H2, which has been mistakenly indexed as a single phase region for monoclinic phase Ml. Interesting similarities and differences between these two systems are also discussed.