Combined X-ray study of lithium (tin) cobalt oxide matrix negative electrodes for Li-ion batteries

Abstract

The lithium insertion behaviours of the oxides Co 3O 4 and Co 2SnO 4 were studied using a range of electrochemical, spectroscopic and diffraction techniques. Co K-edge EXAFS studies on the Co 3O 4 oxide showed that the reversible lithium insertion is coupled with changes in cobalt oxidation state. On lithium insertion, Co 3O 4 is reduced to yield Co(II) and yields only metallic cobalt species on complete reduction. On lithium removal an oxide of Co is formed, which from coulometry should be CoO, however, EXAFS indicates the short range structure is quite different to that of the rocksalt CoO. The long range structure of the matrix is amorphous according to XRD. The EXAFS and XRD data also revealed that both the metallic and oxide phases were disordered, having low co-ordination numbers and large shell spacings, and that there was an initial reduction to CoO before full reduction to metallic Co. The electrochemical behaviour of Co 2SnO 4 cells was more reminiscent of that of SnO 2 than that of Co 3O 4, but did exhibit significant differences due to the presence of cobalt. EXAFS on Co 2SnO 4 cells revealed that Co is reduced to metallic cobalt on the initial discharge, but that it does not convert back to an oxide on cycling even though the electrochemical treatment was the same as for Co 3O 4. Together the EXAFS and Mössbauer data show that the Co and Sn are reduced concurrently, and that some of the Sn remains in the oxidised form. In summary, we have a surprising result in that the presence of the tin dramatically changes the redox behaviour of the cobalt. In a matrix derived from a cobalt oxide spinel, cobalt undergoes redox cycling, whereas in a matrix derived from a cobalt tin oxide spinel, the cobalt does not cycle whilst the tin does.