Part 2:In-situ XANES and EXAFS upon electrochemical cycling Effect of iron on delithiation in Li_xCo_(1-y)Fe_yO_2

摘要

Li_xCo_(1-y)Fe_yO_2 solid solutions were examined in an in-situ electrochemical X-ray absorption spectroscopy study using a plastic battery configuration.XANES and EXAFS were applied to elucidate the evolution of local symmetry and oxidation states of iron and cobalt upon electrochemical cycing of three particular stoichiometries:Li_xCo_(0.9)Fe_(0.1)O_2,Li_xCo_(0.8)Fe_(0.2)O_2 and Li_xCo_(0.6)Fe_(0.4)O_2.While the cobalt environment shows little variaton,a distortion of the FeO_6 octahedra occurs quite rapidly for all three samples with a decrease in the next neighbour numbers from six to four.This is ascribed to the Jahn-Teller effect which affects Fe~(4+) (a d~4 system),that is formed upon delithiaton.No distortion is observed for cobalt.Along the delithiation process a simultaneous increase in the edge energy occurrs for cobalt and iron in the case of y=0.1 and to a lesser extent for iron than for cobalt for y=0.4.This means that the oxidation of Fe~(3+) occurs together with the oxidation of the cobalt matrix in the cobalt-rich samples but becomes more difficult for iron-rich samples.In the case of the cobalt-rich samples,the edge-energy for both elements shifts back down at the end of charge,in spite of the continuing oxidation of the cations.This effect is probably related to the appearance of new lithium-poor phase,that is not formed for y=0.4,and confirms the in-situ X-ray diffraction results published in the first part of this series.