In Situ Structural and Electrochemical Study of Ni1-xCoxO2 Metastable Oxides Prepared by Soft Chemistry

Metastable Ni1-xCoxO2 phases were obtained from the electrochemical deintercalation of lithium from the precursor phases, LiNi1-xCoxO2 (x=0, 0.3, and 1), using plastic electrochemical cells. The structural changes induced during Li removal, together with the thermal stability of the end members Ni1-xCoxO2, were determined by in situ X-ray measurements using a high-resolution synchrotron beam line. The advantages of using a plastic battery configuration to perform such a study is emphasized. The structural/thermal stability results are discussed in relation to the electrochemical performance of these materials. Numerous changes in the unit cell symmetry were observed during deintercalation. The NiO2 end member is described with a single distorted CdI2-type structure, while two CdI2-type structures, both with reduced O–O interlayer distances, but differing by the presence of oxygen vacancies, are needed to describe the CoO2 end member. The reduction in O–O spacing is explained by partial polymerization within oxide frameworks. Such a behavior, already observed with layered chalcogenides, can be explained on the basis of the elegant electron-hole chemistry pioneered by J. Rouxel who directly evidenced the role of the anion during an intercalation process.