A formal Fe(III)/(V) redox couple in an intercalation electrode

Published

October 15, 2025

Author(s)

Hari Ramachandran, Edward Mu, Eder Lomeli, Augustin Braun, Masato Goto, Kuan Hsu, Jue Liu, Zhelong Jiang, Kipil Lim, Grace Busse, Brian Moritz, Joshua Kas, John Vinson, John Rehr, Jungjin Park, Iwnetim Abate, Yuichi Shimakawa, Edward Solomon, Wanli Yang, William Gent, Thomas Devereaux, William Chueh

Abstract

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.

Citation

Nature Materials

Pub Type

Journals

Download Paper

https://doi.org/10.1038/s41563-025-02356-x

Local Download

Citation

Ramachandran, H. , Mu, E. , Lomeli, E. , Braun, A. , Goto, M. , Hsu, K. , Liu, J. , Jiang, Z. , Lim, K. , Busse, G. , Moritz, B. , Kas, J. , Vinson, J. , Rehr, J. , Park, J. , Abate, I. , Shimakawa, Y. , Solomon, E. , Yang, W. , Gent, W. , Devereaux, T. and Chueh, W. (2025), A formal Fe(III)/(V) redox couple in an intercalation electrode, Nature Materials, [online], https://doi.org/10.1038/s41563-025-02356-x, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=957926 (Accessed November 21, 2025)