Reversibility of (De)lithiation of Antiperovskite Li2FeOS through a Paracrystalline, Mixed-Valent Intermediate
8 affiliations
- 1Center for Cooperative Research on Alternative Energies (CIC energiGUNE)
- 2Basque Research and Technology Alliance (BRTA)
- 3ALISTORE - European Research Institute
- 4CNRS FR 3104
- 5Department of Radiation Science and Technology
- 6Delft University of Technology
- 7Inorganic Chemistry Laboratory, Department of Chemistry
- 8University of Oxford
Abstract
The antiperovskite Li 2 FeOS is simultaneously cation-disordered and heteroanionic, and delivers more than one lithium per formula unit — but the structural and redox pathway behind that capacity, and the origin of its fading, remain unclear. Combining galvanostatic intermittent titration, operando X-ray diffraction and ex situ 57 Fe Mössbauer spectroscopy across the electrochemically accessible compositional range, we separate three processes: a two-phase transformation at a flat equilibrium potential of 1.95 V vs. Li/Li + , a solid solution spanning 2.30–2.55 V, and an irreversible process above 2.6 V. Delithiation proceeds with severe loss of long-range order evident by extreme broadening and attenuation of diffracted intensity. Yet, if limited to the first two processes, accounting for ca. one lithium per formula unit, the pristine crystalline reflections are recovered exactly in position and profile on relithiation, excluding amorphization and particle-size reduction. We propose a description of the intermediate as a paracrystal, retaining the parent antiperovskite topology while losing periodicity through aperiodic displacive relaxation. 57 Fe Mössbauer isomer shifts evolve continuously through intermediate values rather than as coexisting Fe 2+ and Fe 3+ populations, indicating mixed-valent iron, and saturate near Fe 2.8+ placing the onset of anion redox earlier than the prevailing sequential cation-then-anion picture. Cells limited to 2.6 V retain >90 % of capacity over 150 cycles, against <70 % at 3.0 V. Reversible cycling of Li 2 FeOS therefore rests on a mixed-valent, paracrystalline intermediate, and capacity fade follows once sulfur oxidation is driven beyond it.
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BibTeX
@misc{kouoi2026reversibility,
title = {{Reversibility of (De)lithiation of Antiperovskite Li2FeOS through a Paracrystalline, Mixed-Valent Intermediate}},
author = {Xavier Kouoi and Iulian Dugulan and Marine Reynaud and Theodosios Famprikis},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15008493/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008493/v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Reversibility of (De)lithiation of Antiperovskite Li2FeOS through a Paracrystalline, Mixed-Valent Intermediate
AU - Xavier Kouoi
AU - Iulian Dugulan
AU - Marine Reynaud
AU - Theodosios Famprikis
PY - 2026
DA - 2026/09/08
PB - ChemRxiv
DO - 10.26434/chemrxiv.15008493/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008493/v1
AB - The antiperovskite Li 2 FeOS is simultaneously cation-disordered and heteroanionic, and delivers more than one lithium per formula unit — but the structural and redox pathway behind that capacity, and the origin of its fading, remain unclear. Combining galvanostatic intermittent titration, operando X-ray diffraction and ex situ 57 Fe Mössbauer spectroscopy across the electrochemically accessible compositional range, we separate three processes: a two-phase transformation at a flat equilibrium potential of 1.95 V vs. Li/Li + , a solid solution spanning 2.30–2.55 V, and an irreversible process above 2.6 V. Delithiation proceeds with severe loss of long-range order evident by extreme broadening and attenuation of diffracted intensity. Yet, if limited to the first two processes, accounting for ca. one lithium per formula unit, the pristine crystalline reflections are recovered exactly in position and profile on relithiation, excluding amorphization and particle-size reduction. We propose a description of the intermediate as a paracrystal, retaining the parent antiperovskite topology while losing periodicity through aperiodic displacive relaxation. 57 Fe Mössbauer isomer shifts evolve continuously through intermediate values rather than as coexisting Fe 2+ and Fe 3+ populations, indicating mixed-valent iron, and saturate near Fe 2.8+ placing the onset of anion redox earlier than the prevailing sequential cation-then-anion picture. Cells limited to 2.6 V retain >90 % of capacity over 150 cycles, against <70 % at 3.0 V. Reversible cycling of Li 2 FeOS therefore rests on a mixed-valent, paracrystalline intermediate, and capacity fade follows once sulfur oxidation is driven beyond it.
ER -