Highly Crystallized Prussian Blue with Enhanced Kinetics for Highly Efficient Sodium Storage

Berlin Green with tunable iron content as ultra-high rate host for efficient aqueous ammonium ion storage

Prussian blue analogues (PBAs) are regarded as promising cathode materials for ammonium-ion batteries (AIBs) because of their low cost and superb theoretical capacity. However, its inherently poor conductivity and structural collapse can significantly limit the enhancement of rate property and cycling stability. In this work, Berlin Green (BG) electrode materials with similar wool-like clusters were constructed by direct precipitation method to accelerate the kinetic, which realizes outstanding cycling stability. Berlin Green with the appropriate amount of iron (BG-2) has a fast ion transport channel, enhanced structure stability, highly reversible insertion/extraction of NH4+, and fine electrochemical reaction activity. Benefiting from the unique architecture and component, the BG-2 electrode shows an excellent rate performance with a discharge/charge specific capacity of 60.1/59.3 mAh g-1 at 5 A g-1. Even at 5 A g-1, BG-2 exhibits remarkable cycling stability with an initial discharge capacity of 59.5 mAh g-1 and a capacity retention rate of approximately 76% after 30,000 cycles. The BG-2 reveals exceedingly good electrochemical reversibility during the process of NH4+ (de)insertion. BG materials indicate huge potential as a cathode material for the next generation of high-performance aqueous batteries.

High Capacity and Fast Kinetics Enabled by Metal-Doping in Prussian Blue Analogue Cathodes for Sodium-Ion Batteries

Prussian blue analogues (PBAs) have gained tremendous attention as promising low-cost electrochemically-tunable electrode materials, which can accommodate large Na+ ions with attractive specific capacity and charge-discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal-doping strategy has been demonstrated in FeNiHCF, Na1.40Fe0.90Ni0.10[Fe(CN)6]0.85 ⋅ 1.3H2O, delivering a capacity as high as 148 mAh g-1 at 10 mA g-1. At an exceptionally high rate of 25.6 A g-1, a reversible capacity of ~55 mAh g-1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal-doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long-term cyclability. In situ synchrotron X-ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low-spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe2+C6/Fe3+C6 redox couple is responsible for stable high-rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high-performance electrode materials for sustainable sodium-ion batteries.

Boosting sodium-ion battery performance with binary metal-doped Na3V2(PO4)2F3 cathodes

Na3V2(PO4)2F3 (NVPF), recognized for its Na superionic conductor architecture, emerges as a promising candidate among polyanion-type cathodes for sodium ion batteries (SIBs). However, its adoption in practical applications faces obstacles due to its inherently low electronic conductivity. To address this challenge, we employ a binary co-doped strategy to design Na3.3K0.2V1.5Mg0.5(PO4)2F3 cathode with nitrogen-doped carbon (NC) coating layer. This configuration enhances electronic conductivity and reduces diffusion barriers for sodium ion (Na+). The strategy of incorporating nitrogen-doped carbon coating not only facilitates the formation of a porous structure but also introduces additional defects and active sites. Such modifications accelerate the reaction kinetics and augment electrolyte interaction through an expanded specific surface area, thus streamlining the electrochemical process. Concurrently, strategic heteroatom substitution leads to a more efficient engagement of Na+ in the electrochemical activities, thereby bolstering the cathode's structural integrity. The vanadium fluorophosphate Na3.3K0.2V1.5Mg0.5(PO4)2F3@NC exhibits an electrochemical performance, including a high discharge specific capacity of 124.3 mA h g-1 at 0.1C, a long lifespan of 1000 cycles with a capacity retention of 93.1 % at 10C, and a rate property of 73.2 mA h g-1 at 20C. This research provides a method for preparing binary doped NVPF for energy storage electrochemistry.

Inhibiting the Jahn-Teller Effect of Manganese Hexacyanoferrate via Ni and Cu Codoping for Advanced Sodium-Ion Batteries

Manganese (Mn)-based Prussian blue analogs (PBAs) are of great interest as a prospective cathode material for sodium-ion batteries (SIBs) due to their high redox potential, easy synthesis, and low cost. However, the Jahn-Teller effect and low electrical conductivity of Mn-based PBA cause poor structure stability and unsatisfactory performance during the cycling. Herein, a novel nickel- and copper-codoped K2Mn[Fe(CN)6] cathode is developed via a simple coprecipitation strategy. The doping elements improve the electrical conductivity of Mn-based PBA by reducing the bandgap, as well as suppress the Jahn-Teller effect by stabilizing the framework, as verified by the density functional theory calculations. Simultaneously, the substitution of sodium with potassium in the lattice is beneficial for filling vacancies in the PBA framework, leading to higher average operating voltages and superior structural stability. As a result, the as-prepared Mn-based cathode exhibits excellent reversible capacity (116.0 mAh g-1 at 0.01 A g-1) and superior cycling stability (81.8% capacity retention over 500 cycles at 0.1 A g-1). This work provides a profitable doping strategy to inhibit the Jahn-Teller structural deformation for designing stable cathode material of SIBs.

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.

Buffer solution induced highly crystalline sodium-rich Prussian blue for sodium storage

In this work, we have developed an efficient method to synthesize Prussian blue by self-decomposition of sodium ferrocyanide in acetic acid-sodium acetate buffer solution. This buffer solution-based proton pool provides a relatively low and stable concentration of protons for the slow decomposition of sodium ferrocyanide to get highly crystalline and sodium rich Prussian blue, which can be used as the cathode for high-performance sodium-ion batteries.

Structural distortion-induced monoclinic sodium iron hexacyanoferrate as a high-performance electrode for rocking-chair desalination batteries

Sodium iron hexacyanoferrate (NaFeHCF) has been recognized as a promising Prussian blue analogue (PBA)-based electrode for electrochemical desalination; however, its application potential is limited by its unsatisfactory desalination capacity and cycling stability. Herein, the structurally distorted high-quality monoclinic NaFeHCF with fewer defects in the framework was synthesized by a crystal-controlled coprecipitation method via tuning the crystallization reaction conditions and applied to seawater desalination. Physicochemical characterization and desalination experiments show that the NFHFC-2 with minimized defects possesses enhanced electrochemical activity of Fe2+ and electrochemical kinetics, thus achieving higher desalination performance (specific capacity of 75.0 mA h g-1 and capacity retention of 85.3% after 50 cycles). Furthermore, a symmetrical NFHCF-2 RCDB is assembled, and the operation parameters (including various salinities and electrode spacing) are optimized to achieve a remarkable salt removal capacity (SRC) of 108.9 mg g-1 and a salt removal rate (SRR) of 2.22 mg g-1 min-1 with low energy consumption (0.056 kW h kg-1-NaCl) and outstanding cycling stability (almost no capacity attenuation in 150 cycles). Impressively, the RCDB further exhibits favorable technical feasibility in the simultaneous removal of univalent/bivalent ions from the natural seawater. This study inspires the design of high-quality PBA-based electrodes with optimized crystal structures for electrochemical desalination.

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.

Isostructural Synthesis of Iron-Based Prussian Blue Analogs for Sodium-Ion Batteries

Rechargeable sodium ion batteries (SIBs) have promising applications in large-scale energy storage systems. Iron-based Prussian blue analogs (PBAs) are considered as potential cathodes owing to their rigid open framework, low-cost, and simple synthesis. However, it is still a challenge to increase the sodium content in the structure of PBAs and thus suppress the generation of defects in the structure. Herein, a series of isostructural PBAs samples are synthesized and the isostructural evolution of PBAs from cubic to monoclinic after modifying the conditions is witnessed. Accompanied by, the increased sodium content and crystallinity are discovered in PBAs structure. The as-obtained sodium iron hexacyanoferrate (Na1.75 Fe[Fe(CN)6 ]0.9743 ·2.76H2 O) exhibits high charge capacity of 150 mAh g-1 at 0.1 C (17 mA g-1 ) and excellent rate performance (74 mAh g-1 at 50 C (8500 mA g-1 )). Moreover, their highly reversible Na+ ions intercalation/de-intercalation mechanism is verified by in situ Raman and Powder X-ray diffraction (PXRD) techniques. More importantly, the Na1.75 Fe[Fe(CN)6 ]0.9743 ·2.76H2 O sample can be directly assembled in a full cell with hard carbon (HC) anode and shows excellent electrochemical performances. Finally, the relationship between PBAs structure and electrochemical performance is summarized and prospected.

Prussian White Cathode Materials for All-Climate Sodium-Ion Batteries

Prussian white (PW) is considered one of the most promising cathode materials for sodium-ion batteries because of its large ion diffusion channels, low lattice strain, facile preparation, nontoxicity, and low cost. At present, research on PW mainly focuses on optimizing the material's structures for the ambient environment yet less on its practical application under extreme temperatures. In this Spotlight, we intend to offer progress we have made in developing PW cathode materials working over wide temperatures in terms of intrinsic feasibility and development prospects. These findings provide a direction to promote the practical viability of PW under extreme conditions.

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.

Engineering Crystal Growth and Surface Modification of Na3 V2 (PO4 )2 F3 Cathode for High-Energy-Density Sodium-Ion Batteries

Na₃ V₂ (PO₄ )₂ F₃ (NVPF) is a suitable cathode for sodium-ion batteries owing to its stable structure. However, the large radius of Na⁺ restricts diffusion kinetics during charging and discharging. Thus, in this study, a phosphomolybdic acid (PMA)-assisted hydrothermal method is proposed. In the hydrothermal process, the NVPF morphologies vary from bulk to cuboid with varying PMA contents. The optimal channel for accelerated Na⁺ transmission is obtained by cuboid NVPF. With nitrogen-doping of carbon, the conductivity of NVPF is further enhanced. Combined with crystal growth engineering and surface modification, the optimal nitrogen-doped carbon-covered NVPF cuboid (c-NVPF@NC) exhibits a high initial discharge capacity of 121 mAh g^-1 at 0.2 C. Coupled with a commercial hard carbon (CHC) anode, the c-NVPF@NC||CHC full battery delivers 118 mAh g^-1 at 0.2 C, thereby achieving a high energy density of 450 Wh kg^-1 . Therefore, this work provides a novel strategy for boosting electrochemical performance by crystal growth engineering and surface modification.

Highly Crystalline Prussian Blue for Kinetics Enhanced Potassium Storage

Prussian blue analogs (PBAs) are promising cathode materials for potassium-ion batteries (KIBs) owing to their large open framework structure. As the K⁺ migration rate and storage sites rely highly on the periodic lattice arrangement, it is rather important to guarantee the high crystallinity of PBAs. Herein, highly crystalline K₂ Fe[Fe(CN)₆ ] (KFeHCF-E) is synthesized by coprecipitation, adopting the ethylenediaminetetraacetic acid dipotassium salt as a chelating agent. As a result, an excellent rate capability and ultra-long lifespan (5000 cycles at 100 mA g^-1 with 61.3% capacity maintenance) are achieved when tested in KIBs. The highest K⁺ migration rate of 10^-9 cm² s^-1 in the bulk phase is determined by the galvanostatic intermittent titration technique. Remarkably, the robust lattice structure and reversible solid-phase K⁺ storage mechanism of KFeHCF-E are proved by in situ XRD. This work offers a simple crystallinity optimization method for developing high-performance PBAs cathode materials in advanced KIBs.

Charge Carriers for Aqueous Dual-Ion Batteries

Environmental and safety concerns of energy storage systems call for application of aqueous battery systems which have advantages of low cost, environmental benignity, safety, and easy assembling. Among the aqueous battery systems, aqueous dual-ion batteries (ADIBs) provide high possibility for achieving excellent battery performance. Compared with the "rocking chair" batteries with only one type of carrier involved in the charging and discharging, ADIBs with both cations and anions as charge carriers possess diverse selections of electrodes and electrolytes. Charge carriers are the basis of the configuration of ADIBs. In this Review, cations and anions that could be applied in ADIBs are demonstrated with corresponding electrode materials and favorable electrolytes. Some insertion mechanisms are emphasized to provide insights for the possibilities to enhance the practical performances of ADIBs.

Novel Dual-Signal SiO2-COOH@MIPs Electrochemical Sensor for Highly Sensitive Detection of Chloramphenicol in Milk

In view of the great threat of chloramphenicol (CAP) to human health and the fact that a few producers have illegally used CAP in the food production process to seek economic benefits in disregard of laws and regulations and consumer health, we urgently need a detection method with convenient operation, rapid response, and high sensitivity capabilities to detect CAP in food to ensure people's health. Herein, a molecularly imprinted polymer (MIP) electrochemical sensor based on a dual-signal strategy was designed for the highly sensitive analysis of CAP in milk. The NiFe Prussian blue analog (NiFe-PBA) and SnS₂ nanoflowers were modified successively on the electrode surface to obtain dual signals from [Fe(CN)₆]^3-/4- at 0.2 V and NiFe-PBA at 0.5 V. SiO₂-COOH@MIPs that could specifically recognize CAP were synthesized via thermal polymerization using carboxylated silica microspheres (SiO₂-COOH) as carriers. When the CAP was adsorbed by SiO₂-COOH@MIPs, the above two oxidation peak currents decreased at the same time, allowing the double-signal analysis. The SiO₂-COOH@MIPs/SnS₂/NiFe-PBA/GCE sensor used for determining CAP was successfully prepared. The sensor utilized the interactions of various nanomaterials to achieve high-sensitivity dual-signal detection, which had certain innovative significance. At the same time, the MIPs were synthesized using a surface molecular imprinting technology, which could omit the time of polymerization and elution and met the requirements for rapid detection. After optimizing the experimental conditions, the detection range of the sensor was 10^-8 g/L-10^-2 g/L and the limit of detection reached 3.3 × 10^-9 g/L (S/N = 3). The sensor had satisfactory specificity, reproducibility, and stability, and was successfully applied to the detection of real milk samples.

Fully 3D Modeling of Electrochemical Deionization

Electrochemical deionization devices are crucial for meeting global freshwater demands. One such is capacitive deionization (CDI), which is an emerging technology especially suited for brackish water desalination. In this work, we extend an electrolytic capacitor (ELC) model that exploits the similarities between CDI systems and supercapacitor/battery systems. Compared to the previous work, we introduce new implementational strategies for enhanced stability, a more detailed method of describing charge efficiency, layered integration of leakage reactions, and theory extensions to new material and operational conditions. Thanks to the stability and flexibility the approach brings, the current work can present the first fully coupled and spatiotemporal three-dimensional (3D) CDI model. We hope that this can pave the way toward generalized and full-scale modeling of CDI units under varying conditions. A 3D model can be beneficial for investigating asymmetric CDI device structures, and the work investigates a flow-through device structure with inlet and outlet pipes at the center and corners, respectively. The results show that dead (low-flow) areas can reduce desalination rates while also raising the total leakage. However, the ionic flux in this device is still enough under normal operating conditions to ensure reasonable performance. In conclusion, researchers will now have some flexibility in designing device structures that are not perfectly symmetric (real-life case), and hence we share the model files to facilitate future research with 3D modeling of these electrochemical deionization devices.

High Pseudocapacitance-Driven CoC2 O4 Electrodes Exhibiting Superior Electrochemical Kinetics and Reversible Capacities for Lithium-Ion and Lithium-Sulfur Batteries

In this study, cuboid-like anhydrous CoC₂ O₄ particles (CoC₂ O₄ -HK) are synthesized through a potassium citrate-assisted hydrothermal method, which possess well-crystallized structure for fast Li⁺ transportation and efficient Li⁺ intercalation pseudocapacitive behaviors. When being used in lithium-ion batteries, the as-prepared CoC₂ O₄ -HK delivers a high reversible capacity (≈1360 mAh g^-1 at 0.1 A g^-1 ), good rate capability (≈650 mAh g^-1 at 5 A g^-1 ) and outstanding cycling stability (835 mAh g^-1 after 1000 cycles at 1 A g^-1 ). Characterizations illustrate that the Li⁺ -intercalation pseudocapacitance dominates the charge storage of CoC₂ O₄ -HK electrode, together with the reversible reaction of CoC₂ O₄ +2Li⁺ +2e^- →Co+Li₂ C₂ O₄ on discharging and charging. In addition, CoC₂ O₄ -HK particles are also used together with carbon-sulfur composite materials as the electrocatalysts for lithium-sulfur (Li-S) battery, which displays a gratifying sulfur electrochemistry with a high reversibility of 1021.5 mAh g^-1 at 2 C and a low decay rate of 0.079% per cycle after 500 cycles. The density functional theory (DFT) calculations show that CoC₂ O₄ /C can regulate the adsorption-activation of reaction intermediates and therefore boost the catalytic conversion of polysulfides. Therefore, this work presents a new prospect of applying CoC₂ O₄ as the high-performance electrode materials for rechargeable Li-ion and Li-S batteries.

Controlled Self-Assembly of Hollow Core-Shell FeMn/CoNi Prussian Blue Analogs with Boosted Electrocatalytic Activity

Prussian blue analogs (PBAs) are considered as efficient catalysts for energy-related applications due to their porous nanoscale architectures containing finely disseminated active sites. Their catalytic capability can be greatly boosted by the rational design and construction of complex PBA hybrid nanostructures. However, present-day structure engineering inevitably involves additional etchant or procedure. Herein, a facile, yet controllable one-pot self-assembly strategy is introduced to prepare hierarchical core-shell polymetallic PBAs (featuring bimetallic FeMn PBAs cores and CoNi PBAs shells) with hollow nano-cages/solid nano-cube architectures. The detailed characterization of material morphology/composition, assisted with theoretical simulations, reveals the underlying formation mechanism where the key factor is the control of the nucleation rate via the use of chelating agent (citrates) and reaction kinetics. The resulting FeMn@CoNi-H compound is found to accelerate the oxygen evolution reaction activity with a low overpotential (236 mV at a current density 10 mA cm^-2 ) as well as a low Tafel slope (58.4 mV dec^-1 ). Such an impressive performance is endowed by the rational compositional and structural design with optimized electronic structures as well as an increase in exposed active sites. This work provides a robust, cost-effective pathway that enables chemical and morphological control in creating high-performance catalysts for water electrolysis.

Fluffy-Like Cation-Exchanged Prussian Blue Analogues for Sodium-Ion Battery Cathodes

Prussian blue (PB) and its analogues are considered as promising cathode materials for sodium-ion batteries (SIBs) owing to their low cost and high capacity. However, it is still a huge challenge to avoid obvious capacity decay during cycling due to the structural collapse. Herein, we design a method to replace parts of Fe ion sites in PB with Ni ions to prepare fluffy-like nickel PB (PB-Ni) by cationic solution immersion, which improves cycling stability for sodium storage. The content of Ni in PB-Ni is explored by regulating the soaking time in the Ni-containing solution, which results in different effects on the electrochemical performance as cathodes of SIBs. Especially, PB-Ni-1d (soaking in NiCl₂ solution for 1 day) exhibits an initial capacity of 114.2 mA h g^-1 at 50 mA g^-1 and a stable cycling performance of 800 cycles at 300 mA g^-1. Furthermore, the reversible phase transformation and small volume variation for PB-Ni-1d are revealed by in situ X-ray diffraction characterization. The nickel hexacyanoferrate in outer layer maintains the cubic phase to stabilize the crystal structure. The cation-exchange strategy provides a facile idea to fabricate high-quality PB cathodes with superior stability for high-performance SIBs.

Treatment dependent sodium-rich Prussian blue as a cathode material for sodium-ion batteries

In the preparation of Prussian blue analogs (PBAs), Na⁺ loss and Fe²⁺ oxidation take place when washing with water. Sodium-rich PBAs were prepared with sodium ascorbate aqueous solution as the washing solution, which can suppress the Na⁺ loss and Fe²⁺ oxidation. As the cathode of sodium-ion batteries, it exhibited excellent electrochemical performance.

Sodium to cesium ions: a general ladder mechanism of ion diffusion in prussian blue analogs

Prussian blue analogs (PBAs) form crystals with large lattice voids that are suitable for the capture, transport and storage of various interstitial ions. Recently, we introduced the concept of a ladder mechanism to describe how sodium ions inside a PBA crystal structure diffuse by climbing the frames formed by aligned cyanide groups in the host structure. The current work uses semi-empirical tight-binding density functional theory (DFTB) in a multiscale approach to investigate how differences in the size of the monovalent cation affect the qualitative and quantitative aspects of the diffusion process. The results show that the ladder mechanism represents a unified framework, from which both similarities and differences between cation types can be understood. Fundamental Coulombic interactions make all positive cations avoid the open vacant areas in the structure, while cavities surrounded by partially negatively charged cyanide groups form diffusion bottlenecks and traps for larger cations. These results provide a new and quantitative way of understanding the suppression of cesium adsorption that has previously been reported for PBAs characterized by a low vacancy density. In conclusion, this work provides a unified picture of the cation adsorption in PBAs based on the newly formulated ladder mechanism.

Prussian Blue Nanozymes with Enhanced Catalytic Activity: Size Tuning and Application in ELISA-like Immunoassay

Prussian blue nanozymes possessing peroxidase-like activity gather significant attention as alternatives to natural enzymes in therapy, biosensing, and environmental remediation. Recently, Prussian blue nanoparticles with enhanced catalytic activity prepared by reduction of FeCl₃/K₃[Fe(CN)₆] mixture have been reported. These nanoparticles were denoted as ‘artificial peroxidase’ nanozymes. Our study provides insights into the process of their synthesis. We studied how the size of nanozymes and synthesis yield can be controlled via adjustment of the synthesis conditions. Based on these results, we developed a reproducible and scalable method for the preparation of ‘artificial peroxidase’ with tunable sizes and enhanced catalytic activity. Nanozymes modified with gelatin shell and functionalized with affine molecules were applied as labels in colorimetric immunoassays of prostate-specific antigen and tetanus antibodies, enabling detection of these analytes in the range of clinically relevant concentrations. Protein coating provides excellent colloidal stability of nanozymes in physiological conditions and stability upon long-term storage.

Prussian Blue Analogues for Sodium-Ion Batteries: Past, Present, and Future

Prussian blue analogues (PBAs) have attracted wide attention for their application in the energy storage and conversion field due to their low cost, facile synthesis, and appreciable electrochemical performance. At the present stage, most research on PBAs is focused on their material-level optimization, whereas their properties in practical battery systems are seldom considered. This review aims to first provide an overview of the history and parameters of PBA materials and analyze the fundamental principles toward rational design of PBAs, and then evaluate the prospects and challenges for PBAs for practical sodium-ion batteries, hoping to bridge the gap between laboratory research and commercial reality.

Mg2+-Doping Constructed a Continuous and Homogeneous Cathode-Electrolyte Interphase Film on Na3.12Fe2.44(P2O7)2 with Superior and Stable High-Temperature Performance for Sodium-Ion Storage

Sodium-ion batteries (SIBs) are on the verge of achieving practical applications, and the key is to find suitable electrode materials. The polyanionic iron-based material Na_3.12Fe_2.44(P₂O₇)₂ (NFPO) possesses an open three-dimensional framework structure with good thermal stability and is regarded as an outstanding cathode material for SIBs. Nevertheless, its poor electrical conductivity, problems with erosion of electrolytes, and structural deterioration during cycling still need to be urgently addressed. Here, we first design a Mg²⁺-doped NFPO (NFPO-Mg) material with a dual-action effect. On the one hand, Mg²⁺ improves the intrinsic conductivity of the NFPO material, and on the other hand, Mg²⁺ promotes the formation of a homogeneous and stable cathode-electrolyte interphase film during the cycling process, which results in a superior rate performance and cycling stability. A capacity of 68.6 mAh g^-1 was achieved at 50C (1C = 117.4 mAh g^-1), and a capacity retention of 79.1% was maintained after 3000 cycles at 20C. More impressively, NFPO-Mg exhibits outstanding high-temperature electrochemical performance, with a capacity retention of 95.3% after 400 cycles at 10C at 60 °C (much higher than the 54.2% for the NFPO). This paper explores an effective method for improving the electrochemical performance of cathode materials, which may prove instrumental in guiding the design of more high-performance cathode materials in the future.

Interstitial Water Improves Structural Stability of Iron Hexacyanoferrate for High-Performance Sodium-Ion Batteries

Prussian blue analogues (PBAs) are considered one of the promising cathodes for sodium-ion batteries because of their low cost and tunable structure. As an intrinsic characteristic, the influence of structured water in PBAs on the electrochemical properties is still controversial. Herein, low-vacancy iron hexacyanoferrate with different interstitial water contents is synthesized through the citric acid-assisted single iron source method. Ex situ Fourier transform infrared and X-ray diffraction characterization reveals that the interstitial water can stably exist in the Prussian blue framework during repeated cycling. The long-standing interstitial water can reduce the volume change during the Na⁺ insertion/extraction process, resulting in improved cycling stability. Thanks to the low Fe(CN)₆^4- vacancies and pillar role of interstitial water in the crystal framework, the HW-PB exhibits a high reversible capacity of 117 mAh g^-1 and excellent long cycle performance with a capacity retention of 91% after 1380 cycles. This work broadens the understanding of the relationship between the interstitial water in PBAs and Na-storage performances, providing guidance for the precise synthesis of high-quality PBAs.

Ladder Mechanisms of Ion Transport in Prussian Blue Analogues

Prussian blue (PB) and its analogues (PBAs) are drawing attention as promising materials for sodium-ion batteries and other applications, such as desalination of water. Because of the possibilities to explore many analogous materials with engineered, defect-rich environments, computational optimization of ion-transport mechanisms that are key to the device performance could facilitate real-world applications. In this work, we have applied a multiscale approach involving quantum chemistry, self-consistent mean-field theory, and finite-element modeling to investigate ion transport in PBAs. We identify a cyanide-mediated ladder mechanism as the primary process of ion transport. Defects are found to be impermissible to diffusion, and a random distribution model accurately predicts the impact of defect concentrations. Notably, the inclusion of intermediary local minima in the models is key for predicting a realistic diffusion constant. Furthermore, the intermediary landscape is found to be an essential difference between both the intercalating species and the type of cation doping in PBAs. We also show that the ladder mechanism, when employed in multiscale computations, properly predicts the macroscopic charging performance based on atomistic results. In conclusion, the findings in this work may suggest the guiding principles for the design of new and effective PBAs for different applications.

Electrostatic interactions and physisorption: mechanisms of passive cesium adsorption on Prussian blue

The study finds atomic-level physisorption interactions that leads to electrostatic Langmuir adsorption.

Manipulating the Phase Compositions of Na3(VO1-xPO4)2F1+2x (0 ≤ x ≤ 1) and Their Synergistic Effects with Reduced Graphene Oxide toward High-Rate Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have aroused intense research and academic interest due to the natural abundance and cost-effectiveness of sodium resources. Presently, cathode materials based on the Na₃(VO_1-xPO₄)₂F_1+2x (0 ≤ x ≤ 1, NVPF_1+2x) polyanionic framework show intriguing electrochemical performances toward practical and advanced SIBs due to its high operating voltage (>3.9 V) and high energy density (>500 Wh kg^-1). Different from conventional approaches focusing on delicate morphology design, metal ion substitution, and the conductive matrix's incorporation to overcome the low intrinsic electrical conductivity, here we adopt a one-step microwave-assisted hydrothermal approach to optimize the electrochemical performances of NVPF_1+2x via manipulating its phase compositions with different vanadium sources and distinguishing the tetragonal (I4/mmm) symmetry of the Na₃(VOPO₄)₂F phase from the orthorhombic symmetry (Amam) of the Na₃V₂(PO₄)₂F₃ phase. The introduction of the conductive reduced graphene oxide (rGO) framework and its impacts on the phase compositions were systematically investigated. The rGO framework with different calcination temperature can alter the phase composition and the electrical conductivity of NVPF_1+2x cathodes significantly, thus having a great impact on their electrochemical performances. Galvanostatic charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy, and the galvanostatic intermittent titration technique are adopted to compare their electrode polarization and kinetics difference and show that NVPF@rGO-600 °C possesses a high rate, small polarization, and fast kinetics electrochemical properties. This work provides new insights into manipulating phase compositions of the NVPF_1+2x cathode by modulating the synthesis conditions and revealing their synergistic effect with a rGO conductive framework toward a superior rate capability and more realistic practical applications for SIBs.

Post-Synthetic and In Situ Vacancy Repairing of Iron Hexacyanoferrate Toward Highly Stable Cathodes for Sodium-Ion Batteries

Iron hexacyanoferrate (FeHCF) is a promising cathode material for sodium-ion batteries. However, FeHCF always suffers from a poor cycling stability, which is closely related to the abundant vacancy defects in its framework. Herein, post-synthetic and in-situ vacancy repairing strategies are proposed for the synthesis of high-quality FeHCF in a highly concentrated Na₄Fe(CN)₆ solution. Both the post-synthetic and in-situ vacancy repaired FeHCF products (FeHCF-P and FeHCF-I) show the significant decrease in the number of vacancy defects and the reinforced structure, which can suppress the side reactions and activate the capacity from low-spin Fe in FeHCF. In particular, FeHCF-P delivers a reversible discharge capacity of 131 mAh g^-1 at 1 C and remains 109 mAh g^-1 after 500 cycles, with a capacity retention of 83%. FeHCF-I can deliver a high discharge capacity of 158.5 mAh g^-1 at 1 C. Even at 10 C, the FeHCF-I electrode still maintains a discharge specific capacity of 103 mAh g^-1 and retains 75% after 800 cycles. This work provides a new vacancy repairing strategy for the solution synthesis of high-quality FeHCF.

In situ polyaniline coating of Prussian blue as cathode material for sodium-ion battery

Prussian blue (PB) has great potential for use as a sodium cathode material owing to its high working potential and cube frame structure. Herein, this work reports a two-step method to synthesize PB with ascorbic acid as the ball-milling additive, which improves the electrochemical rate performance of PB during the traditional co-precipitation method. The obtained PB sample exhibited a superior specific capability (113.3 mAh g-1 even at 20 C, 1 C = 170 mA g-1) and a specific capacity retention of 84.8% after 100 cycles at 1 C rate. In order to enhance the cycling performance of the PB, an in situ polyaniline coating strategy was employed in which aniline was added into the electrolyte and polymerized under electrochemical conditions. The coated anode exhibited a high specific capacity retention of 62.7% after 500 cycles, which is significantly higher than that of the non-coated sample, which only remains 40.1% after 500 cycles. This development has shown a great potential as a low-cost, high-performance and environment-friendly technology for large-scale industrial application of PB.