Research Progress of Prussian Blue and Its Analogs as High-Performance Cathode Nanomaterials for Sodium-Ion Batteries

Synergy from the Stepped Hollow FeHCFe Nanocubes and the Dimorphic Polypyrrole for High-Performance Electrochemical Water Desalination

Saline water desalination serves as an important route to increase freshwater supply, while capacitive deionization (CDI) has emerged as a promising technique to tackle this issue. To boost the successful application of CDI, development of advanced electrode materials is vital. Herein, we innovatively designed and synthesized a sophisticated Prussian blue/dimorphic polypyrrole composite for the hybrid CDI (HCDI). Specifically, the nanoparticle-like polypyrrole (PPy) in situ distributed on the surface of stepped hollow FeHCFe nanocubes, while the coexisting nanotube-like PPy interconnected the discrete stepped hollow FeHCFe nanocubes. The introduction of PPy nanoparticles/nanotubes promoted both electron and ion dynamics, and improved the electrochemical activity of FeHCFe. Meanwhile, the FeHCFe nanocubes with concave stepwise architecture on each side offered large accessible contact area for electrolyte, reduced Na+ migration path, improved tolerance for lattice expansion, and optimized redox sites for Na+ storage. Through such unique structural design and synergistic combination, the FeHCFe/PPy with appropriate component ratio achieved a remarkable desalination capacity and a fast desalination rate along with excellent cycling performance, outperforming other related materials. Moreover, the treated solution can meet the drinking water standard within 6 min via the use of six tandem HCDI cells. Density functional theory (DFT) revealed the mechanism of Na+ capture process and provided a fundamental understanding of the rapid Na+ migration and large Na+ storage capability. This study provides insights into ration design of superior electrode materials for high-performance electrochemical water desalination.

Synthesis of Low-Defect Iron-Based Prussian Blue with Low Water Content for High-Stability Sodium-Ion Batteries

This study proposes an innovative two-step synthesis strategy to significantly enhance the performance of sodium-ion batteries by developing low-defect, low water content iron-based Prussian blue (PB) materials. Addressing the limitations of traditional co-precipitation methods-such as rapid reaction rates leading to excessive crystal defects and interstitial water content-the research team introduced a synergistic approach combining non-aqueous phase precursor synthesis and controlled water-concentration secondary crystallization. The process involves preparing a PB precursor in a glycerol system, followed by secondary crystallization in a water-/ethanol-mixed solvent with a precisely regulated water content, achieving the dual objectives of water content reduction and crystal morphology optimization. Systematic characterization revealed that water concentration during secondary synthesis critically influences the material's crystal structure, morphological features, and water content. The optimized PB50-24 material exhibited a highly regular cubic morphology with a sodium content of 9.2% and a remarkably low interstitial water content of 2.1%. Electrochemical tests demonstrated outstanding performance-an initial charge-discharge capacity of 120 mAh g-1 at a 1C rate, the retention of 105 mAh g-1 after 100 cycles, and a high rate capability of 86 mAh g-1 at 10C, representing significant improvements in cycling stability and rate performance over conventional methods. This work not only establishes a cost-effective, scalable synthesis pathway for Prussian blue materials but also provides theoretical guidance for developing other metal-based Prussian blue analogs, offering substantial value for advancing the industrial application of sodium-ion batteries in next-generation energy storage systems.

A Comprehensive Review on Iron-Based Sulfate Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are advantageous for large-scale energy storage due to the plentiful and ubiquitous nature of sodium resources, coupled with their lower cost relative to alternative technologies. To expedite the market adoption of SIBs, enhancing the energy density of SIBs is essential. Raising the operational voltage of the SIBs cathode is regarded as an effective strategy for achieving this goal, but it requires stable high-voltage cathode materials. Sodium iron sulfate (NFSO) is considered to be a promising cathode material due to its stable framework, adjustable structure, operational safety, and the high electronegativity of SO4-. This paper reviews the research progress of NFSO, discusses its structure and sodium storage mechanism on this basis, and summarizes the advantages and disadvantages of NFSO cathode materials. This study also evaluates the advancements in enhancing the electrochemical characteristics and structural reliability of SIBs, drawing on both domestic and international research. The findings of this paper offer valuable insights into the engineering and innovation of robust and viable SIB cathodes based on NFSO at ambient temperatures, contributing to their commercial viability.

Synthesis of Iron-Based Prussian Blue Analogues with Ultralong Cycle Performance in a Novel T-Shaped Collision Microreactor

Iron based Prussian blue analogues (Fe-PBA) hold significant promise cathode materials for sodium ion batteries due to their low cost and desirable electrochemical properties. However, their practical application is often hindered by the presence of vacancy defects and issues of oxidation that arise during the material preparation process. To overcome these challenges, this study introduces an innovative T-shaped collision microreactor aimed at promoting a uniform concentration field, narrowing the gap between material mixing rate and reaction rate, and providing an oxygen-free environment for the synthesis of Fe-PBA. Employing this microreactor, Fe-PBAs were synthesized with a meticulous approach that led to a material possessing a well-defined morphology and a stoichiometry of Na1.54Fe[Fe(CN)6]0.93. The material exhibited a notable reduction in vacancy defects and minimized oxidation levels. Upon electrochemical evaluation, the Fe-PBA crafted within the microreactor demonstrated an impressive specific capacity of 94.1 mAh/g at a current density of 0.5 C and showcased a long-term cyclability with 72.6% capacity retention over 2000 cycles. Comparative analysis revealed that the structural and electrochemical properties of Fe-PBA prepared in the microreactor outperformed those of samples prepared using traditional reactors. This work provides a new approach for the continuous and stable fabrication of high-performance battery cathode materials.

Prussian White/Reduced Graphene Oxide Composite as Cathode Material to Enhance the Electrochemical Performance of Sodium-Ion Battery

Prussian white (PW) is considered a promising cathode material for sodium-ion batteries. However, challenges, such as lattice defects and poor conductivity limit its application. Herein, the composite materials of manganese-iron based Prussian white and reduced graphene oxide (PW/rGO) were synthesized via a one-step in situ synthesis method with sodium citrate, which was employed both as a chelating agent to control the reaction rate during the coprecipitation process of PW synthesis and as a reducing agent for GO. The low precipitation speed helps minimize lattice defects, while rGO enhances electrical conductivity. Furthermore, the one-step in situ synthesis method is simpler and more efficient than the traditional synthesis method. Compared with pure PW, the PW/rGO composites exhibit significantly improved electrochemical properties. Cycling performance tests indicated that the PW/rGO-10 sample exhibited the highest initial discharge capacity and the best cyclic stability. The PW/rGO-10 has an initial discharge capacity of 128 mAh g-1 at 0.1 C (1 C = 170 mA g-1), and retains 49.53% capacity retention after 100 cycles, while the PW only delivers 112 mAh g-1 with a capacity retention of 17.79% after 100 cycles. Moreover, PW/rGO-10 also shows better rate performance and higher sodium ion diffusion coefficient (DNa+) than the PW sample. Therefore, the incorporation of rGO not only enhances the electrical conductivity but also promotes the rapid diffusion of sodium ions, effectively improving the electrochemical performance of the composite as a cathode material for sodium-ion batteries.

Prussian Blue Analogues for Sodium-Ion Battery Cathodes: A Review of Mechanistic Insights, Current Challenges, and Future Pathways

Prussian blue analogues (PBAs) have emerged as highly promising cathode materials for sodium-ion batteries (SIBs) due to their affordability, facile synthesis, porous framework, and high theoretical capacity. Despite their considerable potential, practical applications of PBAs face significant challenges that limit their performance. This review offers a comprehensive retrospective analysis of PBAs' development history as cathode materials, delving into their reaction mechanisms, including charge compensation and ion diffusion mechanisms. Furthermore, to overcome these challenges, a range of improvement strategies are proposed, encompassing modifications in synthesis techniques and enhancements in structural stability. Finally, the commercial viability of PBAs is examined, alongside discussions on advanced synthesis methods and existing concerns regarding cost and safety, aiming to foster ongoing advancements of PBAs for practical SIBs.

A quasi-solid-state self-healing flexible zinc-ion battery using a dual-crosslinked hybrid hydrogel as the electrolyte and Prussian blue analogue as the cathode material

Due to their excellent safety and lower cost, aqueous Zn-ion batteries (AZIBs) have garnered extensive interest among various energy-storage systems. Here we report a quasi-solid-state self-healing AZIB by using a hybrid hydrogel which consists of dual-crosslinked polyacrylamide and polyvinyl alcohol as a flexible electrolyte and a cobalt hexacyanoferrate (K3.24Co3[Fe(CN)6]2·12.6H2O) Prussian blue analogue as the cathode material. The obtained hybrid hydrogel showed a superhigh fracture strain of up to 1490%, which was almost 15 times higher than that of the original size. Due to the fast formation of hydrogen bonds, the self-healed hydrogel from two pieces still displayed 1165% strain upon failure. As a result, the self-healed battery delivered stable capacities of 119.1, 108.6 and 103.0 mA h g-1 even after being completely cut into 2, 3 and 4 pieces, respectively. The battery capacity recovery rates for each bending cycle exceeded 99.5%, 99.8%, 98.6% and 98.9% during four continuous bending cycles (30 times bending at 90° for each cycle), which indicates outstanding flexibility and self-healing capability. In parallel, the hydrogel electrolyte displayed a broader electrochemically stable window of 3.37 V due to the suppression of water splitting and low overvoltage during the 500 h cycling in a symmetric cell. Zinc dendrites were also suppressed as evidenced in symmetric cell measurements. The assembled AZIB exhibited an initial capacity of 176 mA h g-1 upon vertical bending. The battery showed a reliable capacity of 140.7 mA h g-1 at 0.2 A g-1 after 100 cycles along with a coulombic efficiency of >99%. A reliable capacity of nearly 100 mA h g-1 was retained after 300 cycles at 1.0 A g-1. The highly flexible and self-healing AZIB demonstrates great potential in various wearable electronic devices.

Y-tube assisted coprecipitation synthesis of iron-based Prussian blue analogues cathode materials for sodium-ion batteries

Prussian blue analogues possess numerous advantages as cathode materials for sodium-ion batteries, including high energy density, low cost, sustainability, and straightforward synthesis processes, making them highly promising for practical applications. However, during the synthesis, crystal defects such as vacancies and the incorporation of crystal water can lead to issues such as diminished capacity and suboptimal cycling stability. In the current study, a Y-tube assisted coprecipitation method was used to synthesize iron-based Prussian blue analogues, and the optimized feed flow rate during synthesis contributed to the successful preparation of the material with a formula of Na1.56Fe[Fe(CN)6]0.90□0.10·2.42H2O, representing a low-defect cathode material. This approach cleverly utilizes the Y-tube component to enhance the micro-mixing of materials in the co-precipitation reaction, featuring simplicity, low cost, user-friendly, and the ability to be used in continuous production. Electrochemical performance tests show that the sample retains 69.8% of its capacity after 200 cycles at a current density of 0.5C (1C = 140 mA g-1) and delivers a capacity of 71.9 mA h g-1 at a high rate of 10C. The findings of this research provide important insights for the development of high-performance Prussian blue analogues cathode materials for sodium-ion batteries.