Structural assessment of the electrochemical performance of LixCoO2 membrane electrodes by X-ray diffraction and absorption refinements

Li_xCoO₂ samples with different nominal load x were prepared submitting HT-LiCoO₂ membrane electrodes to electrochemical polarisation, followed by aging in open circuit conditions. Care was taken to obtain samples for both positive and negative overcharge beyond the canonical regime (0.6≤x≤1) of the Li_xCoO₂ electrode. For 11 samples in the range 0.37≤x≤1.43, Rietveld refinements of X-ray diffraction data were performed, with Li at 3a, Co at 3b and O at 6c in the space group R-3m as starting model. In the range 0.37≤x≤1, results show a continuous variation of theac unit cell and structural parameters such as the lithium site occupancy and the Li-O and Co-O interatomic distances. For samples in the range 1≤x≤1.43, Li₂CO₃ was detected as an impurity which might be due to decomposition of the electrolyte, and, if the negative overcharge was high enough, an abnormally high electron density at the 3a site indicates some Li→Co substitution. Results from Co-K edge X-ray absorption spectroscopy for the samples with x=0.37, 0.89, 1 and 1.43 show a clear shift of the absorption maximum with the oxidation state of Co and support the conclusions from Rietveld refinement. GNXAS refinement of EXAFS spectra for the samples with x=0.37 and 1 suggest that cation disordering may occur also for deep lithium deintercalation (positive overcharge), a fact which was not detectable from diffraction data.