Effect of iron on delithiation in LixCo1−yFeyO2. Part 2:in-situXANES and EXAFS upon electrochemical cycling

Abstract

Li_xCo_1−yFe_yO₂ 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 cycling of three particular stoichiometries: Li_xCo_0.9Fe_0.1O₂, Li_xCo_0.8Fe_0.2O₂ and Li_xCo_0.6Fe_0.4O₂. While the cobalt environment shows little variation, a distortion of the FeO₆ 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 Fe⁴⁺ (a d⁴ system), that is formed upon delithiation. No distortion is observed for cobalt. Along the delithiation process a simultaneous increase in the edge energy occurs 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 Fe³⁺ 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 a 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.