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Abstract

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 0less
than or equal toxless than or equal to1, (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, P (3) over bar m1 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.