Direct Observation of Reductive Coupling Mechanism between Oxygen and Iron/Nickel in Cobalt-Free Li-Rich Cathode Material: An in Operando X-Ray Absorption Spectroscopy Study

Abstract

Li-rich cathodes possess high capacity and are promising candidates in next-generation high-energy density Li-ion batteries. This high capacity is partly attributed to its poorly understood oxygen-redox activity. The present Li-rich cathodes contain expensive and environmentally-incompatible cobalt as a main transition metal. In this work, cobalt-free, iron-containing Li-rich cathode material (nominal composition Li1.2Mn0.56Ni0.16Fe0.08O2) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g−1) and cycling stability. In operando, X-ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its electrochemical mechanism. X-ray absorption near edge structure (XANES) features of Fe and Ni K edges show unusual behavior: when an electrode is charged to 4.5 V, Fe and Ni K edges’ XANES features shift to higher energies, evidence for Fe3+→Fe4+ and Ni2+→Ni4+ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe4+→Fe3+ and Ni4+→Ni3+ reduction. This behavior can be linked to a reductive coupling mechanism between oxygen and Fe/Ni. Though this mechanism is observed in Fe-containing Li-rich materials, the only electrochemically active metal in such cases is Fe. Li1.2Mn0.56Ni0.16Fe0.08O2 has multiple electrochemically active metal ions; Fe and Ni, which are investigated simultaneously and the obtained results will assist tailoring of cost-effective Li-rich materials.