Investigation of Valence Mixing in Sodium-Ion Battery Cathode Material Prussian White by Mössbauer Spectroscopy

At a Glance

Metadata	Details
Publication Date	2022-07-04
Journal	Frontiers in Energy Research
Authors	Tore Ericsson, Lennart Häggström, Dickson O. Ojwang, William R. Brant
Institutions	Uppsala University
Citations	7
Analysis	Full AI Review Included

Executive Summary

This study utilizes low-temperature Mössbauer spectroscopy to provide a rigorous, quantitative characterization of valence distribution and vacancy content in Prussian White (PW) cathode materials for Sodium-Ion Batteries (NIBs).

Compositional Validation: Mössbauer analysis at 90 K successfully suppressed intervalence charge transfer (IVCT), allowing for the accurate quantification of Fe²⁺ and Fe³⁺ ratios at both the low-spin (Fe_C) and high-spin (Fe_N) iron sites.
Valence Mixing Quantified: Contrary to simplified models, significant valence mixing was observed across both iron sites, even in partially charged states (e.g., x=1.0). For Na_1.0PW, the Fe_N site was 66% Fe³⁺/34% Fe²⁺, and the Fe_C site was 27% Fe³⁺/73% Fe²⁺.
Compositional Consistency: The total iron valence sum derived from the 90 K Mössbauer data was found to be consistent (within two standard deviations) with the sodium content (x) determined by independent elemental analysis.
Vacancy Confirmation: The highly sodiated sample (x=1.8) showed only two distinct iron environments, confirming the absence of [Fe(CN)₆]^4- vacancies within the detection limits of the measurement.
Methodological Advancement: This approach establishes a critical quality control step for iron-based Prussian Blue Analogs (PBAs), resolving ambiguities caused by rapid IVCT at room temperature (295 K) which broadens spectral lines and obscures accurate valence assignment.

Technical Specifications

Parameter	Value	Unit	Context
Material System	Na_xFe[Fe(CN)₆]·nH₂O	N/A	Prussian White (PW) Cathode
Theoretical Capacity (Ideal)	~170	mAhg^-1	Ideal composition (x=2)
Mössbauer Measurement Temperatures	295 and 90	K	Used to suppress Intervalence Charge Transfer (IVCT)
Absorber Concentration	10	mg/cm²	Sample density for Mössbauer measurement
Fe_N (High-Spin) Quadrupole Splitting (90 K, x=1.0)	2.194 (Fe²⁺)	mm/s	Large splitting due to suppressed IVCT and d-orbital configuration
Fe_C (Low-Spin) Center Shift (90 K, x=1.0)	-0.034 (±0.005)	mm/s	Intermediate shift indicating Fe²⁺/Fe³⁺ mixing
Fe_N Valence Ratio (90 K, x=1.0)	0.66 Fe³⁺ / 0.34 Fe²⁺	N/A	Determined from spectral intensities
Fe_C Valence Ratio (90 K, x=1.0)	0.27 Fe³⁺ / 0.73 Fe²⁺	N/A	Determined from center shift interpolation
Valence Balance Deficit (x=1.0)	-0.34 (±0.33)	Electronic units	Difference between total Fe valence sum and Na content (x)

Key Methodologies

The accurate determination of valence mixing relied on low-temperature Mössbauer spectroscopy to freeze the electronic states, overcoming the limitations of room-temperature measurements affected by rapid intervalence charge transfer (IVCT).

Sample Preparation: Prussian White (PW) powders with varying sodium content (x = 1.8, 1.0, 0.5) were prepared and characterized previously (Ojwang et al., 2020, 2021).
Mössbauer Absorber Construction: PW powder was mixed with Boron Nitride (BN) and sealed into aluminum pockets to achieve a consistent absorber concentration of 10 mg/cm².
Spectrometer Setup: A constant acceleration Mössbauer spectrometer was used with a ⁵⁷CoRh source. Calibration was performed using natural Fe metal foil at 295 K.
Low-Temperature Measurement: Spectra were recorded at 295 K and 90 K using an Oxford gas flow cryostat. The 90 K measurement was crucial as the electron hopping time became longer than the Mössbauer observation time (~100 ns).
Data Fitting and Analysis: Spectra were fitted using the least-square Mössbauer fitting program Recoil to extract hyperfine parameters: Center Shift (CS), Quadrupole Splitting (|QS|), line width (Γ), and spectral intensities (I).
Valence Determination (Fe_N): For the high-spin, N-coordinated iron (Fe_N), the proportions of Fe²⁺ and Fe³⁺ were determined directly from the relative spectral areas (intensities, I) of the two distinct doublets observed at 90 K.
Valence Determination (Fe_C): For the low-spin, C-coordinated iron (Fe_C), which appeared as a broad single line, the valence mixture was calculated by interpolating the measured Center Shift (CS) between the known CS values for pure Fe²⁺ and Fe³⁺ states.

Commercial Applications

This rigorous characterization methodology is essential for the commercialization and optimization of sustainable energy storage solutions, particularly those relying on iron-based active materials.

Sodium-Ion Battery (NIB) Manufacturing: Provides a necessary quality control (QC) standard for PBA cathode production, ensuring synthesized materials meet precise compositional and valence requirements for optimal capacity and cycle stability.
Electrochemical R&D: Enables accurate tracking of valence changes during battery cycling, crucial for diagnosing degradation mechanisms, phase transitions, and structural instability in iron-based battery materials.
Sustainable Energy Storage: Supports the development and validation of low-cost, high-performance cathode materials (like PW) that leverage abundant elements (Fe, Na), reducing reliance on expensive or scarce resources (e.g., Co, Li).
Coordination Chemistry and Catalysis: The technique for suppressing IVCT and quantifying mixed valence states is broadly applicable to other iron-based coordination polymers used in electrocatalysis, magnetic materials, and chemical sensing where electronic structure control is paramount.
Material Property Optimization: Allows engineers to correlate specific synthesis parameters (which influence vacancy content and Na stoichiometry) directly with quantifiable valence distributions, leading to targeted material property enhancement.

View Original Abstract

Prussian white (PW), Na 2 Fe [Fe(CN) 6 ], is a highly attractive cathode material for sustainable sodium-ion batteries due to its high theoretical capacity of ∼170 mAhg −1 and low-cost synthesis. However, there exists significant variability in the reported electrochemical performance. This variability originates from compositional flexibility possible for all Prussian blue analogs (PBAs) and is exasperated by the difficulty of accurately quantifying the specific composition of PW. This work presents a means of accurately quantifying the vacancy content, valence distribution, and, consequently, the overall composition of PW via Mössbauer spectroscopy. PW cathode material with three different sodium contents was investigated at 295 and 90 K. The observation of only two iron environments for the fully sodiated compound indicated the absence of [Fe(CN) 6 ] 4- vacancies. Due to intervalence charge transfer between iron centers at 295 K, accurate determination of valences was not possible. However, by observing the trend of spectral intensities and center shift for the nitrogen-bound and carbon-bound iron, respectively, at 90 K, valence mixing between the iron sites could be quantified. By accounting for valence mixing, the sum of iron valences agreed with the sodium content determined from elemental analysis. Without an agreement between the total valence sum and the determined composition, there exists uncertainty around the accuracy of the elemental analysis and vacancy content determination. Thus, this study offers one more stepping stone toward a more rigorous characterization of composition in PW, which will enable further optimization of properties for battery applications. More broadly, the approach is valuable for characterizing iron-based PBAs in applications where precise composition, valence determination, and control are desired.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fenrg.2022.909549

References

2022 - Octahedral Tilting in Prussian Blue Analogues [Crossref]
2019 - Selective Control of Composition in Prussian White for Enhanced Material Properties [Crossref]
2018 - Synthesis of Low Vacancies PB with High Electrochemical Performance Using a Facile Method [Crossref]
2018 - Prussian Blue Analogs as Battery Materials [Crossref]
2008 - Magnetic and Optical Bistability Driven by Thermally and Photoinduced Intramolecular Electron Transfer in a Molecular Cobalt−Iron Prussian Blue Analogue [Crossref]
2019 - Novel Acetic Acid Induced Na-Rich Prussian Blue Nanocubes with Iron Defects as Cathodes for Sodium Ion Batteries [Crossref]
2020 - Performance and Degradation of LiFePO4/Graphite Cells: The Impact of Water Contamination and an Evaluation of Common Electrolyte Additives [Crossref]
2006 - Heat-induced Charge Transfer in Cobalt Iron Cyanide [Crossref]
2020 - Influence of Sodium Content on the Thermal Behavior of Low Vacancy Prussian White Cathode Material [Crossref]
2021 - Moisture-Driven Degradation Pathways in Prussian White Cathode Material for Sodium-Ion Batteries [Crossref]