Thermal stability of LixCoO2 cathode materials

The thermal evolution of LiCoO 2 with conductive additive and binder to various states of charge (up to 4.2 V) is investigated using synchrotron X-ray diffraction and thermogravimetric analysis over a wide temperature range, from room temperature to 700 °C. Rietveld refinement of the structural models with XRD data revealed that lithium extraction led to lattice contraction along the a-axis and expansion along the c-axis. During heat treatment, the phase transition temperatures, decomposition products, and side reaction products are strongly dependent on the lithium content in . For (x > 0.79), decomposition to CoO occurred above 350 °C, followed by further reduction to metallic Co above 600 °C, whereas for (x < 0.79), Co 3 O 4 formed prior to CoO. The unique phase evolution of between 150 and 350 °C was also examined, revealing a trend of transformation from a hexagonal layered structure to a cubic spinel structure. This structural transition is considered a key factor in the formation of Co 3 O 4 . • Higher state-of-charge transitions to Co 3 O 4 before CoO and then metallic Co thermal evolution. • Lower state-of-charge transitions to CoO and then metallic Co. • Reducing environments favor Co reduction. • An unusual thermal feature is found above 650 °C.