Effects on the phases and crystalline structures of LiCoO₂ cathode under thermal treatments up to 400 °C

The performance of lithium-ion batteries is very much affected by the temperature under which they are running due to changes in the properties of the battery components. By employing techniques of thermogravimetric analysis (TGA), X-ray diffraction (XRD) followed by Rietveld refinements, infrared spectroscopy and scanning electron microscopy (SEM), we show that the LiCoO₂ cathode material of a lithium-ion battery is specially affected in their composition and structural properties when the entire cathode (cathode material + current collector) was heat-treated from room temperature till 400 °C. After 250 °C, the starting electroactive cathode material, Li_0.94CoO₂, decomposes thermally in the Li₁CoO₂, Co₃O₄ and O₂ reaction products, with their relative masses changing with the temperatures of heat treatments. Concomitantly, the treatments promoted changes in the lattice parameters of the pristine and decomposed cobalt oxides, as well as in the Li–O and Co–O interatomic distances, in the angles between the O–Co–O and O–Li–O bonds and in the intensity of infrared absorption of Co–O vibrational modes. Very surprisingly, all these parameters presented minimum or maximum values under the thermal treatment at 350 °C. This effect is believed to be related to the rate of Li_0.94CoO₂ mass decomposition that reaches its maximum value at 335 °C, close to the temperature of 350 °C. The α, β and γ crystalline phases of the polyvinylidene difluoride (PVDF) binder and the agglomeration states of the cathode particles are as well affected by the thermal treatment performed.