Sodium-ion batteries (SIBs) are emerging as a cost-effective alternative to lithium-ion batteries for large-scale energy storage, leveraging the abundance and low cost of sodium. Various cathode materials are being explored, including layered oxides, polyanionic compounds, and Prussian blue analogs (PBAs). PBAs are particularly attractive due to their open framework structures, facilitating easy Na+ accommodation and fast transport. Their synthesis avoids high-temperature calcination, lowering manufacturing costs. The general formula for PBAs in SIBs is Na2-xM[Fe(CN)6]1-y·y·nH2O, where x ranges from 0–2, M represents transition metals (like Fe, Mn, Co, Ni), and y signifies vacancies filled with water. While Mn-based PBAs were initially explored, their performance suffers from poor electronic conductivity and structural distortion. Fe-based Prussian white shows excellent cycling and rate performance due to its rhombohedral structure, but its synthesis is challenging and produces toxic byproducts. Previous co-precipitation methods yielded cubic Fe-based PBAs with low sodium content, poor crystallinity, and limited cycle life. This study aimed to overcome these limitations by developing a controllable precipitation method to synthesize sodium-rich Na2xFeFe(CN)6, investigating its nucleation and growth, and characterizing its phase transitions during sodium-ion intercalation/deintercalation.

The literature extensively discusses the potential of Prussian blue analogs (PBAs) as cathode materials for sodium-ion batteries. Early research focused on Mn-based PBAs, highlighting the strong dependence of electrochemical performance on phase evolution during cycling. Studies demonstrated the conversion of a monoclinic phase to a rhombohedral structure, improving cycling stability due to higher reversibility. However, limitations persisted, including poor electronic conductivity and structural distortion from the Jahn-Teller effect of Mn3+. Subsequent work on Fe-based Prussian white showcased its excellent cycling and rate performance, attributable to a rhombohedral structure resulting from high sodium and low water content. Yet, the hydrothermal synthesis method used presented scalability challenges and generated toxic NaCN as a byproduct. Efforts to utilize environmentally friendly co-precipitation faced difficulties in achieving a sodium-rich rhombohedral structure, often resulting in cubic phases with low sodium content, poor crystallinity, and short cycle lives. The irreversible phase transitions of cubic Na2-xFeFe(CN)6 were previously observed via in-situ PXRD, hindering their practical application. This work builds upon these previous studies, aiming to address the challenges of scalable synthesis and to investigate the phase transitions of a sodium-rich rhombohedral structure.

A modified co-precipitation method at 25 °C was employed to synthesize a series of sodium-rich Na2-xFeFe(CN)6 samples. The synthesis involved controlling the concentration of sodium citrate, a chelating agent, to regulate the precipitation rate and increase sodium content. Four samples (PB-S1 to PB-S4) were selected to represent the evolution of the material's crystallinity. Powder X-ray diffraction (PXRD), both laboratory-based and synchrotron-based in situ analysis, was used to determine crystallographic information, including unit cell parameters, lattice parameters, and crystal symmetry. Rietveld refinement was conducted using TOPAS 5 software to analyze the crystal structures. Inductively coupled plasma (ICP) and thermogravimetric analysis (TGA) were used to determine the elemental concentrations and water content, respectively. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), and scanning transmission electron microscopy (STEM) were used to characterize particle morphology, size, and crystal structure. Electrochemical performance was evaluated using coin cells (2032 type) with sodium metal as the counter electrode, and a specific electrolyte solution containing 1 M NaClO4 in EC:PC (1:1) with 3% FEC additive. Electrochemical tests included galvanostatic charge-discharge cycling, rate capability tests, cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT). Synchrotron in-situ PXRD was used to investigate phase transitions during cycling. For large-scale synthesis, a 100 L reactor was utilized. Pouch full cells were assembled using the large-scale synthesized material as the cathode and commercial hard carbon as the anode. The pouch cells were tested using a Land battery system.

A controllable precipitation method successfully synthesized sodium-rich Na2-xFeFe(CN)6 with a rhombohedral structure. The use of sodium citrate as a chelating agent and sodium supplement was crucial in controlling the precipitation rate, resulting in the formation of highly crystalline microcubes. Rietveld refinement of PXRD data confirmed the rhombohedral structure (R-3 space group) for samples PB-S2, PB-S3, and PB-S4, while PB-S1 exhibited a cubic structure (Fm-3m space group). The chemical formula of the samples varied slightly due to differences in sodium and water content. SEM and TEM imaging revealed the evolution from irregular nanoparticles to aggregated sub-microcubes, finally to single microcubes as the sodium citrate concentration increased. Electrochemical tests showed that the rhombohedral PB-S3 (Na1.73Fe[Fe(CN)6]·3.8H2O) exhibited superior performance compared to the cubic PB-S1 (Na1.53Fe[Fe(CN)6]·4.2H2O). PB-S3 demonstrated a high initial Coulombic efficiency (97.4%), excellent rate capability (retaining 83% capacity at 500 mA g⁻¹), and stable cycling performance (71% capacity retention after 500 cycles). Synchrotron in-situ PXRD revealed highly reversible three-phase transitions (rhombohedral, cubic, and tetragonal) during sodium-ion intercalation/deintercalation for PB-S3. In contrast, the cubic PB-S1 showed irreversible lattice shrinkage. The large-scale synthesis using a 100 L reactor yielded a product with similar characteristics to PB-S3. The pouch full cell fabricated with the large-scale material exhibited stable cycling performance over 1000 cycles (78% capacity retention).

The findings directly address the research question of developing a scalable method for synthesizing high-performance sodium-rich rhombohedral Prussian blue for sodium-ion batteries. The controlled precipitation method using sodium citrate successfully yielded a material with improved electrochemical properties compared to previously reported methods. The observed reversible three-phase transitions explain the excellent performance of the rhombohedral structure. The high initial Coulombic efficiency is attributed to the stable sodium-rich rhombohedral framework. The superior rate capability stems from the open framework structure facilitating fast sodium-ion transport. The long cycle life is due to the reversible structural transformations upon sodiation/desodiation. The success in scaling up the synthesis demonstrates the potential for industrial applications. This work provides valuable insights into the structure-property relationships of PBAs and their phase transitions, guiding the rational design of advanced SIBs.

This study successfully synthesized sodium-rich rhombohedral Na2-xFeFe(CN)6 using a scalable co-precipitation method. The controlled nucleation and growth process led to highly crystalline microcubes with excellent electrochemical performance. The observed reversible three-phase transitions (rhombohedral, cubic, and tetragonal) during sodium-ion intercalation/deintercalation are key to the material's high initial Coulombic efficiency, superior rate capability, and long cycle life. The large-scale synthesis and successful pouch full cell demonstration highlight the potential of this material for practical applications in sodium-ion batteries. Future research could explore further optimization of the synthesis process, investigation of different transition metal substitutions, and the development of advanced electrode architectures to enhance battery performance.

While the study demonstrates excellent performance, some limitations exist. The capacity of the rhombohedral PB-S3 sample is still below the theoretical capacity of PBAs, potentially due to defects, coordinated water in the structure, and slight oxidation of the material. The study focused on a specific electrolyte; investigating the performance in other electrolytes would broaden the scope. Although the large-scale synthesis is demonstrated, further optimization is needed for full industrial scale-up. Long-term stability over thousands of cycles at higher current densities needs further investigation.