Na1.51 Fe[Fe(CN)6 ]0.87 ·1.83H2 O Hollow Nanospheres via Non-Aqueous Ball-Milling Route to Achieve High Initial Coulombic Efficiency and High Rate Capability in Sodium-Ion Batteries

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.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

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.

Prussian blue analogue-derived Co3O4/Fe2O3 with a partially hollow and octahedral structure for high-performance supercapacitors

A supercapacitor (SC) is considered as a promising energy storage device because of its high power density, fast charging/discharging speed and long cycle life. The transition metal oxides prepared by traditional methods face some challenges, such as low conductivity and uncontrollable pore size distribution. Therefore, we have prepared Prussian blue analogues (PBAs) using a coprecipitation method. By adjusting additives in the experimental process, uniform PBAs with a series of regular morphologies and structures are successfully prepared. Then the corresponding metal oxides are obtained by calcining precursors. We systematically study the influence of the morphology and structure of metal oxides Co3O4/Fe2O3 derived from PBAs on their electrochemical performance. The metal oxide with a partially hollow and octahedral structure shows excellent electrochemical performance. In a neutral electrolyte, the specific capacitance is 659.7 F g-1 at a current density of 0.5 A g-1. After 6000 cycles, the capacitance retention rate is 63.7%. An asymmetric supercapacitor (ASC) is constructed using Co3O4/Fe2O3 with an octahedral structure (CFMO-PVP-2) as the positive electrode and YP-50F as the negative electrode. The maximum energy density is 31.4 W h kg-1 at a power density of 1921 W kg-1. The maximum power density is 8421 W kg-1 at an energy density of 23.5 W h kg-1. The excellent electrochemical performance is attributed to the low resistance (Rw and Rct) and high DOH- derived from the oxide particles on the surface and within the inner parts of the octahedron, which are available for electron transport. Meanwhile, the open void between adjacent nanoparticles allows the electrolyte ions to diffuse more efficiently and ensures a much more effective area for participating in a reaction. The strategy will give new insights into designing high-performance SCs based on PBAs.

Resolving Deactivation of Low-Spin Fe Sites by Redistributing Electron Density toward High-Energy Sodium Storage

Prussian blue (PB) has been an emerging class of cathode material for sodium-ion batteries due to its low cost and high theoretical capacity. However, their working voltage and capacity are substantially restricted due to the deactivation of low-spin Fe sites. Herein, we demonstrate a universal strategy to activate the low-spin Fe sites of PB by hybridizing them with the π-π conjugated electronic conductors. The redistribution of electron density between π-π conjugated conductors and PB effectively promotes the participation of low-spin Fe sites in sodium storage. Consequently, the low-spin Fe-induced plateau is greatly aroused, resulting in a high specific capacity of 148.4 mAh g-1 and remarkable energy density of 444.2 Wh kg-1. In addition, the excellent structural stability enables superior cycling stability over 2500 cycles and outstanding rate performance. The work will provide fundamental insight into activating the low-spin Fe sites of PB for advanced battery technologies.

Defect-Healing Induced Monoclinic Iron-Based Prussian Blue Analogs as High-Performance Cathode Materials for Sodium-Ion Batteries

Prussian blue analogs (PBAs) have attracted wide interest as a class of ideal cathodes for rechargeable sodium-ion batteries due to their low cost, high theoretical capacity, and facile synthesis. Herein, a series of highly crystalline Fe-based PBAs (FeHCF) cubes, where HCF stands for the hexacyanoferrate, is synthesized via a one-step pyrophosphate-assisted co-precipitation method. By applying this proposed facile crystallization-controlled method to slow down the crystallization process and suppress the defect content of the crystal framework of the PBAs, the as-prepared materials demonstrate high crystallization and a sodium-rich induced rhombohedral phase. As a result, the as prepared FeHCF can deliver a high specific capacity of up to 152.0 mA h g-1 (achieving ≈90% of its theoretical value) and an excellent rate capability with a high-capacity retention ratio of 88% at 10 C, which makes it one of the most competitive candidates among the cathodes reported regarding both capacity and rate performance. A highly reversible three-phase-transition sodium-ion storage mechanism has been revealed via multiple in situ techniques. Furthermore, the full cells fabricated with as-prepared cathode and commercial hard carbon anode exhibit excellent compatibility which shows great prospects for application in the large-scale energy storage systems.

Composite cathode with low-defect NiFe Prussian blue analogue on reduced graphene oxide for aqueous sodium-ion hybrid supercapacitors

Although sodium-ion hybrid supercapacitor (Na-ion HSC) has attracted great interest, exploitation of suitable cathode materials for reversible Na⁺ insertion reaction remains a challenge. Herein, a novel binder-free composite cathode with highly crystallized NiFe Prussian blue analogue (NiFePBA) nanocubes in-situ grown on reduced graphene oxide (rGO) was fabricated via sodium pyrophosphate (Na₄P₂O₇)-assisted co-precipitation and the subsequent ultrasonic spraying and chemical reduction. Profiting from the low-defect PBA framework and close interface contact of PBA and conductive rGO, the NiFePBA/rGO/carbon cloth composite electrode exhibits a high specific capacitance of 451F g^-1, remarkable rate performance and satisfactory cycling stability in aqueous Na₂SO₄ electrolyte. Impressively, the aqueous Na-ion HSC assembled with the composite cathode and activated carbon (AC) anode manifests a high energy density of 51.11 Wh kg^-1, superb power density of 10 kW kg^-1 and the intriguing cycling stability. This work may open a door for scalable fabrication of binder-free PBA cathode material for aqueous Na-ion storage.

Recent Progress on Honeycomb Layered Oxides as a Durable Cathode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are becoming promising candidates for energy storage devices due to the low cost, abundant reserves, and excellent electrochemical performance. As the most important unit, layered cathodes attract much attention, where honeycomb-layered-oxides (HLOs) manifest outstanding structural stability, high redox potential, and long-life electrochemistry. Here, recent progress on HLOs as well as Na₃ Ni₂ SbO₆ and Na₃ Ni₂ BiO₆ as two representative materials are introduced, and the crystal and electronic structure, electrochemical performance, and modification strategies are summarized. The advanced high nickel HLOs are highlighted toward development of state-of-the-art sodium-ion batteries. This review would deepen the understanding of superstructure in layered oxides, as well as structure-property relationship, and inspire more interest in high output voltage, long lifespan sodium-ion batteries.

Reforming Magnet Waste to Prussian Blue for Sustainable Sodium-Ion Batteries

Increasing generation of permanent magnet waste has resulted in an urgent need to preserve finite resources. Reforming these wastes as feedstock to produce renewables is an ideal strategy for addressing waste and energy challenges. Herein, our work reports a smart and sustainable strategy to convert iron in magnet wastes into Prussian blue analogues that can serve as cathode materials for sodium-ion batteries. Moreover, a method to control feed rates is proposed to generate high-quality materials with fewer [Fe(CN)₆] vacancies at a feed rate of 3 mL min^-1. The recycled Na_1.46Fe[Fe(CN)₆]_0.85·□_0.15 shows low vacancies and excellent cycling stability over 300 cycles (89% capacity retention at 50 mA g^-1). In operando, evidence indicates that high-quality Prussian blue allows fast sodium-ion mobility and a high degree of reversibility over the course of cycling, although with a three-phase-transition mechanism. This study opens up a future direction for magnet waste created with the expectation of being environmentally reused.