Studying the Reversibility of Multielectron Charge Transfer in Fe(VI) Cathodes Utilizing X‑ray Absorption Spectroscopy

• Published in: Journal of physical chemistry. C Vol. 117; no. 39; pp. 19875 - 19884
• Main Authors: Farmand, Maryam, Licht, Stuart, Ramaker, David
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 03.10.2013
• Subjects: Chemistry; Condensed matter: structure, mechanical and thermal properties; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; Electrochemistry; Exact sciences and technology; General and physical chemistry; Materials science; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Properties of electrolytes: conductivity; Structure of solids and liquids; crystallography; Theory of crystal structure, crystal symmetry; calculations and modeling; Particle size; Reversible processes; Symmetry property; Phase transformations; Electron charge; Electron transfer; X-ray absorption; Physical properties; EXAFS; Interfaces; Solid state; Electrochemical properties; Composite materials; Valence; X-ray absorption spectra; Battery; Stress effects; Electrolytes; Crystal-phase transformations; Electron density; Charge transfer; Cathodes
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp406626x

The superiron salts BaFeO4 and K2FeO4 when utilized as battery cathodes both undergo a three electron charge transfer; however, they exhibit significantly different physical and electrochemical properties. K2FeO4 exhibits higher solid-state stability and higher intrinsic 3e– capacity (406 mAh/g) than BaFeO4 (313 mAh/g); however, the rate of cathodic charge transfer is considerably higher for BaFeO4. To understand these differences, primary coin cells of alkaline batteries containing either μm-BaFeO4, μm-K2FeO4, or nm-K2FeO4 (nm = nanometer, or μm = micrometer size particles) were constructed and discharged to various depths under a constant load. Discharged cathode composite were studied by ex-situ X-ray absorption measurements. The oxidation state of discharge product of the Fe local symmetry was followed by the magnitude of K-edge and pre-edge Fe 1s to 3d peak. To track structural changes, the extended X-ray absorption fine structure (EXAFS) χ functions of the partially discharged cathodes were subject to linear combination fitting. The expanded BaFeO4 lattice, or the much larger surface-electrolyte interface in the nm-K2FeO4 materials, significantly increased their capacities compared to μm-K2FeO4. In the case of nm-K2FeO4, electron density is more distributed by water intercalation about the Fe hydrous environment, which relieves the “stress” of full Fe6+ to Fe3+ reduction. The stronger Ba–FeO4 anion–cation interaction and increased lattice size apparently slows the rate of lattice rearrangement into the discharge product.