On the mechanism of the P2-Na0.70CoO2→O2-LiCoO2 exchange reaction—Part II: an in situ X-ray diffraction study

A model was proposed to describe the exchange reaction of sodium by lithium in P2 crystals, it was based first on the formation of nucleation centers and then on the growth of O2 domains in P2 crystals from these nucleation centers. This study has shown that depending on the ratio between the growing and nucleation speeds, O2, O6 or faulted structures are obtained and that this model allows a good analysis of the exchange process. XRD patterns simulation and their comparison with that of experimental O2-LiCoO₂ have shown that there was almost no defects in the O2-LiCoO₂ structure obtained by ion exchange in water. Therefore, this study has shown that the growth of the O2 domains in the P2-Na_0.7CoO₂ crystals is faster than the formation of nucleation centers.This P2-Na_0.7CoO₂→O2-LiCoO₂ exchange reaction was also studied in situ by X-ray diffraction; simulations of key XRD patterns by P2-O2 intergrowths were also achieved. It was shown, in good agreement with the simulations, that the growth of O2 domains was faster than the formation of the nucleation centers and kinetically activated by a P2-Na_0.70CoO₂→P2^*-Na_∼0.50CoO₂ phase transition. For those reasons, the P2-Na_0.70CoO₂→O2-LiCoO₂ exchange reaction in water leads to an O2 phase, with an almost ideal packing.