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<p>Iron redox cycling between low-valent oxidation states of Fe<sup>II</sup> and Fe<sup>III</sup> drives crucial processes in nature. The Fe<sup>II/III</sup> redox couple charge compensates the cycling of lithium iron phosphate, a positive electrode (cathode) for lithium-ion batteries. High-valent iron redox couples, involving formal oxidation higher than Fe<sup>III</sup>, could deliver higher electrochemical potentials and energy densities. However, because of the instability of high-valent Fe electrodes, they have proven difficult to probe and exploit in intercalation systems. Here we report and characterize a formal Fe<sup>III/V</sup> redox couple by revisiting the charge compensation mechanism of (de)lithiation in Li<sub>4</sub>FeSbO<sub>6</sub>. Valence-sensitive experimental and computational core-level spectroscopy reveal a direct transition from Fe<sup>III</sup> (3<i>d</i><sup>5</sup>) to a negative-charge-transfer Fe<sup>V</sup> (3<i>d</i><sup>5</sup><Emphasis Type="Underline">L</Emphasis><sup>2</sup>) ground state on delithiation, without forming Fe<sup>IV</sup>, or oxygen dimers. We identify that the cation ordering in Li<sub>4</sub>FeSbO<sub>6</sub> drives a templated phase transition to stabilize the unique Fe<sup>V</sup> species and demonstrate that disrupting cation ordering suppresses the Fe<sup>III/V</sup> redox couple. Exhibiting resistance to calendar aging, high operating potential and low voltage hysteresis, the Fe<sup>III/V</sup> redox couple in Li<sub>4</sub>FeSbO<sub>6</sub> provides a framework for developing sustainable, Fe-based intercalation cathodes for high-voltage applications.</p>

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A formal FeIII/V redox couple in an intercalation electrode

  • 期刊论文

  • 发表日期：

    2025年10月15日

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  • Hari Ramachandran,
  • Edward W. Mu,
  • Eder G. Lomeli,
  • Augustin Braun,
  • Masato Goto,
  • Kuan H. Hsu,
  • Jue Liu,
  • Zhelong Jiang,
  • Kipil Lim,
  • Grace M. Busse,
  • Brian Moritz,
  • Joshua J. Kas,
  • John Vinson,
  • John J. Rehr,
  • Jungjin Park,
  • Iwnetim I. Abate,
  • Yuichi Shimakawa,
  • Edward I. Solomon,
  • Wanli Yang,
  • William E. Gent,
  • Thomas P. Devereaux,
  • William C. Chueh

摘要

Iron redox cycling between low-valent oxidation states of FeII and FeIII drives crucial processes in nature. The FeII/III redox couple charge compensates the cycling of lithium iron phosphate, a positive electrode (cathode) for lithium-ion batteries. High-valent iron redox couples, involving formal oxidation higher than FeIII, could deliver higher electrochemical potentials and energy densities. However, because of the instability of high-valent Fe electrodes, they have proven difficult to probe and exploit in intercalation systems. Here we report and characterize a formal FeIII/V redox couple by revisiting the charge compensation mechanism of (de)lithiation in Li4FeSbO6. Valence-sensitive experimental and computational core-level spectroscopy reveal a direct transition from FeIII (3d5) to a negative-charge-transfer FeV (3d5L2) ground state on delithiation, without forming FeIV, or oxygen dimers. We identify that the cation ordering in Li4FeSbO6 drives a templated phase transition to stabilize the unique FeV species and demonstrate that disrupting cation ordering suppresses the FeIII/V redox couple. Exhibiting resistance to calendar aging, high operating potential and low voltage hysteresis, the FeIII/V redox couple in Li4FeSbO6 provides a framework for developing sustainable, Fe-based intercalation cathodes for high-voltage applications.