Abstract
Sodium-ion batteries (NIBs) have attracted considerable attention as a cost-effective and sustainable alternative to lithium-ion batteries (LIBs), owing to the abundance and low cost of sodium resources. Here, we investigate previously reported mixed-valent (Fe3+/2+) one-dimensional (1D) iron chalcogenides: Na1.5FeS2, its selenide analogue Na1.5FeSe2, and newly developed solid solution Na1.5FeSSe, focusing on their synthesis, structural characterization, and electrochemical performance in NIBs. While the full Fe2+sulfide version (NaFeS2) of the compound is explored in solid-state cells for its dual cation–anion redox mechanism, we reveal the effect of anion substitution on the electrochemical behavior of Na1.5FeS2–xSex(x = 0, 1, 2), demonstrating their potential for reversible cation and anion redox activity and its impact on structural changes in liquid cells. Cells with these mixed-valent phases show stable cycling in the cation redox regime and enable access to combined cation and anion redox, characterized by flat voltage plateaus and low polarization, features further enhanced by Se substitution. In a liquid electrolyte, stepwise cation redox from Na1.5FeS2to NaFeS2and Na1.5FeS2to Na2FeS2is clearly observed, resembling classic two-phase reactions. However, once anion redox is accessed, reversibility declines due to structural instability, likely from exfoliation caused by Na+extraction. Mössbauer and XPS analyses indicate deeper Fe oxidation and greater anion redox participation in liquid electrolyte cells compared to prior reports on solid-state batteries. This work provides mechanistic insights into polarization, structural transformations, and charge-compensation pathways that govern capacity retention and stability in mixed-valent Fe chalcogenides.
Recommended Citation
S. Sundaramoorthy et al., "Accessing Cation and Anion Redox in Mixed-Valent One-Dimensional Iron-Chalcogenides, Na1.5Fe1S2–xSex(x = 0, 1, and 2), for Na-Ion Batteries," Chemistry of Materials, vol. 37, no. 23, pp. 9544 - 9556, American Chemical Society, Dec 2025.
The definitive version is available at https://doi.org/10.1021/acs.chemmater.5c02316
Department(s)
Chemistry
International Standard Serial Number (ISSN)
1520-5002; 0897-4756
Document Type
Article - Journal
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 2026 American Chemical Society, All rights reserved.
Publication Date
09 Dec 2025
