Electrochemical reactions and cathode properties of Fe-doped Li₂O for the hermetically sealed lithium peroxide battery

Fe-doped Li₂O (FDL) is synthesized mechanochemically and is demonstrated as a new Co-free cathode material for use in sealed Li₂O₂ batteries, which have been proposed as high energy density batteries. Fe³⁺ ions are substitutionally doped into the Li sites in an antifluorite-type Li₂O structure to create FDL. The FDL consists of (Li_0.82Fe_0.06)₂O (d-FDL) and high-temperature form of Li₅FeO₄ (o-FDL), in which Fe³⁺ ions disorderly and orderly arranged, respectively. According to the Mössbauer spectra and quantitative peroxide species analysis, the FDL cathode operates principally based on the redox reaction between O₂^2- and O^2-. X-ray diffraction study reveals that the reversible formation of O₂^2- proceeds mainly in the d-FDL. An irreversible side reaction involving the evolution of oxygen gas occurs when the cathode is charged to more than 250 mAh g^-1. The FDL (Fe/(Li + Fe) = 10 at%) cathode exhibits a reversible capacity of 200 mAh g^-1 over 200 cycles at a current density of 22.5 mA g^-1.