Effect of iron on delithiation in LixCo1−yFeyO2. Part 1: in-situ electrochemical and X-ray diffraction study

Abstract

Structural changes occurring upon electrochemical delithiation in Li_xCo_1−yFe_yO₂ solid solutions have been examined using in-situ electrochemical X-ray diffraction on plastic batteries, for y = 0.1, 0.2, 0.4 and 0.7. Two key points are emphasised in this study: (i) the influence of partial iron substitution on the Li_xCoO₂ phase diagram, especially regarding the multiple phase transitions present for 0 ≤ x ≤ 1, (ii) the redox behaviour of iron along with cobalt during the electrochemical process. The y = 0.1 and y = 0.2 samples could be totally delithiated and resemble unsubstituted Li_xCoO₂: these systems are two-phase in the range 0.6 < x < 0.9, where two distinct rhombohedral phases coexist, and a specific phase exists in the highly delithiated region. However no ordered analog to Li_0.5CoO₂ is found. The symmetry of the completely delithiated phases is trigonal (O1, CdI₂-type, Pm1 space group) and monoclinic (O3, CdCl₂-type, C2/m space group) for y = 0.1 and 0.2, respectively. For Li_xCo_0.6Fe_0.4O₂, the two-phase regime is maintained down to x = 0.4, and no new phase is detected down to x = 0.18. The electrochemical capacities obtained show that iron is involved in the oxidation process. This will be confirmed by EXAFS and XANES measurements, the results of which are published in the second part of this series.