Structural changes occurring upon electrochemical delithiation in LixCo1−yFeyO2 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 LixCoO2 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 LixCoO2: 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 Li0.5CoO2 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 LixCo0.6Fe0.4O2, 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.
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