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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
Author(s) -
Dixon Ditty,
Mangold Stefan,
Knapp Michael,
Ehrenberg Helmut,
Bhaskar Aiswarya
Publication year - 2021
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.202100479
Subject(s) - xanes , materials science , cobalt , nickel , cathode , electrochemistry , absorption (acoustics) , x ray absorption spectroscopy , redox , absorption spectroscopy , metal , transition metal , oxygen , spectroscopy , inorganic chemistry , analytical chemistry (journal) , electrode , chemistry , catalysis , metallurgy , biochemistry , physics , organic chemistry , quantum mechanics , chromatography , composite material
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 Li 1.2 Mn 0.56 Ni 0.16 Fe 0.08 O 2 ) 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 Fe 3+ →Fe 4+ and Ni 2+ →Ni 4+ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe 4+ →Fe 3+ and Ni 4+ →Ni 3+ 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. Li 1.2 Mn 0.56 Ni 0.16 Fe 0.08 O 2 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.