Elucidation of the Reaction Mechanisms in Antifluorite-Type Li5+xFe1-xCoxO4 Positive Electrodes for Li-Ion Batteries

The Li-rich antifluorite-type oxides Li₅FeO₄, Li_5.5Fe_0.5Co_0.5O₄ and Li₆CoO₄ have been investigated as positive electrode materials for Li-ion batteries in a combined operando XANES and XRD experiment. All materials show a similar two-step behaviour upon initial charge (termed Stage I and Stage II), and reversibility of subsequent cycling depends upon whether the initial charge cycle is terminated following Stage I or allowed to proceed through Stage II. By tracking the energetic evolution of the XANES pre-edge feature present in both Fe and Co K-edge spectra, as well as the evolution of X-ray diffractograms during charge and discharge, we correlate the changes in chemical coordination and oxidation states in both species and the structural changes to the electrochemical potential profile, and infer the role of anionic redox processes.