Electronic structure of Li(Co0.7-xAl0.3)MgxO2 studied by electron energy-loss spectroscopy

Electronic structure of Li(Co0.7-xAl0.3)MgxO2 (x=0.01, 0.03, and 0.05) is studied via electron energy-loss spectroscopy. Oxygen K edge and Co L-2,L-3 edge were used to investigate the effect of Mg substitution on both the valence state of Co ion and the density of unoccupied states of O ion. A change in the ratio of the white lines of Co ion, L-3/L-2, is used to identify the change in the valence state in Co ion, which reveals that Mg substitution gives rise to an increase of the average valence of Co ion. This is a mixed valence state of Co3+ and Co4+ in Mg substitution materials. Relative lowering of the integrated intensity of prepeak in oxygen K edge with the Mg concentration connects with a lower covalency of the Co-O bond, which indicates a significant electron feeding back from Co 3d to O 2p. This makes the oxygen ions closer to the feature of closed shell, which helps electron hopping between Co3+ and Co4+ based on the double-exchange process. (C) 2003 American Institute of Physics.