Recent progress of Prussian blue analogues as cathode materials for nonaqueous sodium-ion batteries

Ligand Field-Induced Dual Active Sites Enhance Redox Potential of Nickel Hexacyanoferrate for Ammonium Ion Storage

Enhancing the redox potential of cathode materials is vital for increasing the energy density of aqueous ammonium ion batteries (AIBs). Prussian blue analogues (PBAs), with their inherently high redox potentials and open frameworks, are promising candidates. However, further boosting their redox potential and understanding their NH4 + ion storage mechanisms remain critical challenges. In this work, a novel ligand field-induced dual active sites mechanism is introduced by incorporating Ni into the PBA framework, activating Ni as an additional redox center for NH4 + ion storage. The electron transfer from Ni to Fe within the Ni─N≡C─Fe chain enhances the redox potential and electrochemical performance of nickel hexacyanoferrate (NiHCF). For the first time, the electrochemical activity of Ni is demonstrated in NiHCF during NH4 + ions intercalation and de-intercalation. NiHCF exhibits elevated redox potentials, superior rate performance, and robust cycling stability compared to iron hexacyanoferrate (FeHCF). Advanced characterization techniques and density functional theory calculations confirm the activation of Ni and the enhanced interaction between NH4 + ions and the framework. These findings provide new insights into the NH4 + ion storage mechanism of PBAs and offer a promising strategy for designing high-energy-density cathode materials for AIBs.

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.

Structural Modulation of Cu-Mn-Fe Prussian Blue Analogs for Practical Sodium Ion Cylinder Cells

High-performance, cost-effective cathodes are essential for grid-scale sodium-ion batteries (SIBs). Prussian blue analogs (PBAs) have shown great potential as SIB cathodes, but achieving both high capacity and long lifespan remains challenging. In this study, a series of low-cost ternary PBAs synthesized through structural regulation is presented to simultaneously achieve high capacity, stable cycling performance, and broad temperature adaptability. Among them, CuHCF-3 demonstrates a specific capacity of 132.4 mAh g-1 with 73.3% capacity retention over 1000 cycles. In-depth analyses, using in situ techniques and density functional theory calculations, reveal a highly reversible three-phase transition (monoclinic ↔ cubic ↔ tetragonal) in Na1.96Cu0.45Mn0.55[Fe(CN)6]0.91·□0.09·2.14H2O (CuHCF-3), which is driven by synergistic interactions between Mn and Cu. Mn enhances conductivity, increases the operating voltage, and introduces additional redox centers, while Cu mitigates the Jahn-Teller distortions associated with Mn and buffers volume changes during cycling. This structural synergy results in excellent temperature stability across a wide temperature range (-20 to 55 °C). 18650-type cylindrical cells based on CuHCF-3 with high loading density achieve 73.54% capacity retention over 850 cycles. This study offers valuable insights for designing durable, high-capacity electrode materials for SIB energy storage applications.

Synergistic effect of P2, O3 phase on biphasic layered oxide with enhanced electrochemical performance for sodium storage

Oxides with a layered structure are regarded as prospective candidates for use as cathodes in the next generation of sodium ion batteries. These materials, which exhibit a P2 structure and O3 structure, possess distinctive advantages that give rise to disparate electrochemical performance. Herein, a thermodynamically and kinetically stable P2/O3 biphasic layered oxide with a chemical formula of K0.05Na0.8Ni0.5Mn0.5O2 is synthesized using a simplistic high temperature solid-state method. P2 phase enhances the structural stability of the material, while O3 phase provides additional sodium storage sites, thereby increasing the material's capacity. The joint action of the two phases results in an improvement in the electrochemical characteristics. Moreover, the Ni-Mn-based layered oxide is enhanced by Fe-substitution, which effectively mitigates the Jahn-Teller effect caused by Mn3+, thereby improving the comprehensive electrochemical performance of the cathode. The Fe-doped specimen offers 102.8 mAh g-1 as the initial reversible discharge capacity under a high current density of 200 mA g-1, and a high capacity retention amounting to 83.17 % is attained after 200 cycles. In addition, the structural development of the P2/O3 biphasic sample during Na+ extraction/insertion was elucidated by in situ XRD. This paper employs the advantages of P2 and O3 phases to augment material electrochemical characteristics and verifies the possibility of the P2/O3 biphasic structure by density functional theory (DFT) calculations, providing new ideas for biphasic sodium ion battery cathode materials.

Understanding capacity fading from structural degradation in Prussian blue analogues for wide-temperature sodium-ion cylindrical battery

Low-cost Fe-based Prussian blue analogues often suffer from capacity degradation, resulting in continuous energy loss, impeding commercialization for practical sodium-ion batteries. The underlying cause of capacity decrease remains mysterious. Herein, we show that irreversible phase transitions, structural degradation, deactivation of surface redox centres, and dissolution of transition metal ions in Prussian blue analogues accumulate continuously during cycling. These undesirable changes are responsible for massive destruction of their morphology, leading to the capacity decay. A dual regulation strategy is applied to alleviate the above-mentioned problems of Prussian blue analogues. The designed 18650/33140 cylindrical cells using modified Prussian blue analogues and hard carbon display improved cycling stability and wide-temperature working performance (-40 oC-100 oC). The findings resolve the controversy over the origins of Prussian blue analogues cathode degradation, emphasize the necessity of eliminating irreversible crystal/structural changes to guarantee enhanced cycling stability, and demonstrate the potential of Prussian blue analogues cathode for commercial sodium-ion batteries.

Rational Design of Prussian Blue Analogues for Ultralong and Wide-Temperature-Range Sodium-Ion Batteries

Architecting Prussian blue analogue (PBA) cathodes with optimized synergistic bimetallic reaction centers is a paradigmatic strategy for devising high-energy sodium-ion batteries (SIBs); however, these cathodes usually suffer from fast capacity fading and sluggish reaction kinetics. To alleviate the above problems, herein, a series of early transition metal (ETM)-late transition metal (LTM)-based PBA (Fe-VO, Fe-TiO, Fe-ZrO, Co-VO, and Fe-Co-VO) cathode materials have been conveniently fabricated via an "acid-assisted synthesis" strategy. As a paradigm, the FeVO-PBA (FV) delivers a superb rate capability (148.9 and 56.1 mAh/g under 0.5 and 100 C, respectively), remarkable cycling stability over 30,000 cycles, high energy density (259.7 Wh/kg for the full cell), and a wide operation-temperature range (-60-80 °C). In situ/ex situ techniques and density functional theory calculations reveal the quasi-zero-strain and multielectron redox mechanisms of the FeVO-PBA cathode during cycling, supporting its higher specific capacity and stable cycling. It is considered that the d-d electron compensation effect between Fe and V enhanced the reversibility and kinetics of redox reactions and simultaneously improved the electronic conductivity and structural stability of the FeVO-PBA cathode. This work may pave a new way for the rational design of high-performance cathode materials with bimetallic reaction centers for SIBs.

In Situ Balanced Synthesis of High-Activity Low-Spin Iron Cathode Prussian Blue for Enhanced Sodium-Ion Storage

The growing market for sodium-ion batteries has stimulated interest in research on Prussian blue-type cathode materials. Iron hexacyanoferrate (FeHCF) is considered a desirable Prussian blue-type cathode, but the incomplete electrochemical property of its low-spin iron sites hinders its further practical application. In this paper, carboxymethyl cellulose is demonstrated to have an appropriate binding energy through DFT calculations, synthesize Prussian blue in situ, balance Fe3+ and water in FeHCF, and introduce FeIII vacancies to activate low-spin Fe sites. Thus, at a 1 C rate, it achieves an initial discharge capacity of 154.7 mAh g-1 with an energy density of 470.8 Wh kg-1. The capacity retention is 70.2% after 4000 cycles at a rate of 100 C. This work provides a simpler way to develop more cost-effective, faster, and more durable cathode materials for sodium-ion energy storage.

Progress of anion redox in Na-rich layered transition metal oxides (Na2MO3) as cathode materials for sodium-ion batteries

Under the background of surging global demand for batteries and scarcity of Li resources, sodium-ion batteries (SIBs) are attracting attention as a potential alternative with their unique advantages, and the layered transition metal (TM) oxides therein are considered to be one of the most promising cathode materials. In this paper, firstly, the diversity of cathode materials for SIBs is systematically introduced, as well as the layered oxide structures among them are categorized, and then it focuses on the O3-type sodium-rich Na2MO3, which is promising for large-scale commercial applications, illustrating the development and mechanism of anion redox. Excess Na transforms the TM layer into the mixed Na1/3M2/3O2layer, leading to the formation of localized configuration Na-O-Na. Thereby, isolated nonhybridized O 2p states are introduced, which participate in the charge compensation process (O2-/On-) under high-voltage conditions and provide the battery with additional capacity beyond the cation redox reaction. Therefore, the Na2MO3formed by its TM element located in different periods are classified, discussed and summarized in terms of structural change characteristics, electrochemical properties and anion-redox mechanism. However, this particular redox mechanism is also accompanied by the challenges such as voltage hysteresis, irreversible oxygen loss, TM migration, capacity decay and poor air stability. Therefore, to address these challenges, various improvement strategies have been proposed, including doping of large radius metal ions, light metal ions, TM ions with high covalency with O, nonmetal ions, formation of mixed phases, and surface modification. This work is expected to provide new ways to find and design novel high-capacity Na-rich layered oxide cathode materials.

Unconventional hexagonal open Prussian blue analog structures

Prussian blue analogs (PBAs), as a classical kind of microporous materials, have attracted substantial interests considering their well-defined framework structures, unique physicochemical properties and low cost. However, PBAs typically adopt cubic structure that features small pore size and low specific surface area, which greatly limits their practical applications in various fields ranging from gas adsorption/separation to energy conversion/storage and biomedical treatments. Here we report the facile and general synthesis of unconventional hexagonal open PBA structures. The obtained hexagonal copper hexacyanocobaltate PBA prisms (H-CuCo) demonstrate large pore size and specific surface area of 12.32 Å and 1273 m2 g-1, respectively, well exceeding those (5.48 Å and 443 m2 g-1) of traditional cubic CuCo PBA cubes (C-CuCo). Significantly, H-CuCo exhibits much superior gas uptake capacity over C-CuCo toward carbon dioxide and small hydrocarbon molecules. Mechanism studies reveal that unsaturated Cu sites with planar quadrilateral configurations in H-CuCo enhance the gas adsorption performance.

Building Robust Manganese Hexacyanoferrate Cathode for Long-Cycle-Life Sodium-Ion Batteries

Manganese Hexacyanoferrate (Mn─HCF) is a preferred cathode material for sodium-ion batteries used in large-scale energy storage. However, the inherent vacancies and the presence of H2O within the imperfect crystal structure of Mn─HCF lead to material failure and interface failure when used as a cathode. Addressing the challenge of constructing a stable cathode is an urgent scientific problem that needs to be solved to enhance the performance and lifespan of these batteries. In this review, the crystal structure of Mn─HCF is first introduced, explaining the formation mechanism of vacancies and exploring the various ways in which H2O molecules can be present within the crystal structure. Then comprehensively summarize the mechanisms of material and interfacial failure in Mn─HCF, highlighting the key factors contributing to these issues. Additionally, eight modification strategies designed to address these failure mechanisms are encapsulated, including vacancy regulation, transition metal substitution, high entropy, the pillar effect, interstitial H2O removal, surface coating, surface vacancy repair, and cathode electrolyte interphase reinforcement. This comprehensive review of the current research advances on Mn─HCF aims to provide valuable guidance and direction for addressing the existing challenges in their application within SIBs.

Mitigating Zinc Dendrite Formation and Parasitic Side Reactions in Aqueous Zn-Ion Batteries Via Laser-Assisted Carbonization of Cu-PANI Films on Zn Anodes

Aqueous zinc-ion batteries (ZIBs) are gaining attraction for large-scale energy storage systems due to their high safety, significant capacity, cost-effectiveness, and environmental friendliness. On the other hand, the development of aqueous ZIBs is restricted by the limited practical application of zinc (Zn) because of the high reactivity of Zn in aqueous electrolytes, which results in the severe dendrite growth and parasitic side reactions such as hydrogen evolution reaction (HER). In this study, heteroatom-doped carbon porous surface modification by laser-assisted carbonization of copper (Cu) doped polyaniline (PANI) is designed and fabricated on top of the Zn metal anode (c-Cu-PANI/Zn). The c-Cu-PANI surface-modified Zn anodes exhibit high electrochemical stability and performance during the Zn plating-stripping cycles and suppress the dendrite formation. The symmetrical cell and half-cell with the c-Cu-PANI/Zn anodes exhibit stable cycles for 6000 h and 100% Coulombic efficiency for 2500 cycles, respectively. Moreover, the c-Cu-PANI/Zn║V2O5 cell delivers a high specific capacity of 319 mAh g-1 at 0.2 A g-1, which is significantly higher than that of the bare Zn║V2O5 cell (240 mAh g-1). It is believed that applying c-Cu-PANI as a surface modification can enhance the stability and reversibility of the Zn anodes, therefore accelerating the commercialization of ZIBs.

Insights on Prussian Blue Analogue Cathode Material Engineered with Polypyrrole Surface Protection Layer for Aqueous Rechargeable Zinc Metal Battery

One of the key intricacies against using Prussian blue analogues (PBAs) in aqueous batteries is their gradual dissolution in aqueous electrolytes, resulting in inadequate cycling stability. Besides, the rate capability of PBAs is limited due to their poor electrical conductivity. To overcome these challenges, it is essential to tune the physical and chemical properties of PBAs at the nano regime without affecting the inherent charge storage properties, especially at high-voltage operating conditions. Through this work, a strategy is demonstrated to enhance the electrochemical performance of vanadium-based PBA (V-PBA) by surface engineering using a conducting polymer nano-skin (V-PBA/PPy) for aqueous zinc metal batteries. The polypyrrole (PPy) nano-skin over the V-PBA nanoparticles acts as an electron percolation path to ameliorate the poor electronic conductivity of the otherwise pristine V-PBA. Interestingly, the V-PBA with an optimized polypyrrole coating (V-PBA/PPy-2) exhibits an enhanced specific capacity (173 mAh g-1 at 0.10 A g-1) than the pristine V-PBA counterpart (80 mAh g-1) and 85% capacity retention up to 500 cycles. The DFT calculation confirms the synergistic interaction between PPy and V-PBA and the presence of PPy favors the adsorption of Zn.

Insights into the mechanism of a substituted metal center regulating the enzymatic activity of Prussian blue analogues for catalytic antioxidation

Atom engineering has been demonstrated to be an efficient strategy for optimizing the activities of Prussian blue analogue (PBA)-based nanozymes. Herein, using a series of metal atom-coordinated N PBAs as models, a mechanistic insight into the effect of substituted metal centers on the antioxidant activities of PBA-based nanozymes is provided for the first time. The PBAs exhibit substituted metal atom-dependent antioxidant activities and the optimal Cu-substituted PBAs can effectively protect cells from oxidative stress. Experimental characterization reveals that the effect of low redox potential significantly improves the catalytic antioxidant efficiency. Furthermore, theoretical calculations show that the catalytic activities are well related to the magnetic moment of the adjacent Fe site, which features a linear correlation with the energy barrier of the rate-determining step or adsorption energy of the substrate, respectively. This study may inspire further exploration of PBAs and shed light on the rational design of advanced antioxidant nanozymes.

Highly Crystalline Multivariate Prussian Blue Analogs via Equilibrium Chelation Strategy for Stable and Fast Charging Sodium-Ion Batteries

Prussian blue analogs (PBAs) have been widely recognized as superior cathode materials for sodium-ion batteries (SIBs) owing to numerous merits. However, originating from the rapid crystal growth, PBAs still suffer from considerable vacancy defects and interstitial water, making the preparation of long-cycle-life PBAs the greatest challenge for its practical application. Herein, a novel equilibrium chelation strategy is first proposed to synthesize a high crystallinity (94.7%) PBAs, which is realized by modulating the chelating potency of strong chelating agents via "acid effect" to achieve a moderate chelating effect, forcefully breaking through the bottleneck of poor cyclic stability for PBAs cathodes. Impressively, the as-prepared highly crystalline PBAs represent an unprecedented level of electrochemical performance including ultra-long lifespan (10000 cycles with 86.32% capacity maintenance at 6 A g-1), excellent rate capability (82.0 mAh g-1 at 6 A g-1). Meanwhile, by pairing with commercial hard carbon, the as-prepared PBAs-based SIBs exhibit high energy density (350 Wh kg-1) and excellent capacity retention (82.4% after 1500 cycles), highlighting its promising potential for large-scale energy storage applications.

Maximizing the Structural Stability of the Na-Rich Prussian Blue Analogue Electrode through Charge Cutoff Voltage Regulation

Prussian blue and its analogues (PB/PBAs) are promising sodium-ion battery cathode materials due to their easy synthesis and excellent discharge capacity. However, their low structural stability poses a challenge. One critical factor affecting stability is the charge cutoff voltage. Increasing this voltage can enhance lithium-ion battery capacity but may reduce cycle retention due to intensified electrode-electrolyte interface reactions. This study investigates whether this phenomenon also applies to sodium-ion batteries and proposes an optimal voltage to improve both the capacity and stability. By cycling between 3.8 and 4.2 V, the charge/discharge profiles and redox reaction intensity were analyzed. The correlation between structural stability and charge cutoff voltage was re-evaluated based on these findings. An optimal charge cutoff voltage of 4.1 V was identified, creating an additional plateau during initial charging and altering Na+ ion positioning to enhance capacity. Enhanced mobility of ions and electrons was observed through a stable solid-electrolyte interphase (SEI) formation. Ex situ X-ray diffraction (XRD) confirmed reversible phase transitions and excellent structural stability. This study underscores the strategic use of charge cutoff voltage to enhance both discharge capacity and structural stability, offering insights for advancing PB/PBA cathode materials with improved stability and capacity.

Confined Element Distribution with Structure-Driven Energy Coupling for Enhanced Prussian Blue Analogue Cathode

The structural failure of Na2Mn[Fe(CN)6] could not be alleviated with traditional modification strategies through the adjustable composition property of Prussian blue analogues (PBAs), considering that the accumulation and release of stress derived from the MnN6 octahedrons are unilaterally restrained. Herein, a novel application of adjustable composition property, through constructing a coordination competition relationship between chelators and [Fe(CN)6]4- to directionally tune the enrichment of elements, is proposed to restrain structural degradation and induce unconventional energy coupling phenomenon. The non-uniform distribution of elements at the M1 site of PBAs (NFM-PB) is manipulated by the sequentially precipitated Ni, Fe, and Mn according to the Irving-William order. Electrochemically active Fe is operated to accompany Mn, and zero-strain Ni is modulated to enrich at the surface, synergistically mitigating with the enrichment and release of stress and then significantly improving the structural stability. Furthermore, unconventional energy coupling effect, a fusion of the electrochemical behavior between FeLS and MnHS, is triggered by the confined element distribution, leading to the enhanced electrochemical stability and anti-polarization ability. Consequently, the NFM-PB demonstrates superior rate performance and cycling stability. These findings further exploit potentialities of the adjustable composition property and provide new insights into the component design engineering for advanced PBAs.

Prussian Blue Analogue-Templated Nanocomposites for Alkali-Ion Batteries: Progress and Perspective

Lithium-ion batteries (LIBs) have dominated the portable electronic and electrochemical energy markets since their commercialisation, whose high cost and lithium scarcity have prompted the development of other alkali-ion batteries (AIBs) including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Owing to larger ion sizes of Na+ and K+ compared with Li+, nanocomposites with excellent crystallinity orientation and well-developed porosity show unprecedented potential for advanced lithium/sodium/potassium storage. With enticing open rigid framework structures, Prussian blue analogues (PBAs) remain promising self-sacrificial templates for the preparation of various nanocomposites, whose appeal originates from the well-retained porous structures and exceptional electrochemical activities after thermal decomposition. This review focuses on the recent progress of PBA-derived nanocomposites from their fabrication, lithium/sodium/potassium storage mechanism, and applications in AIBs (LIBs, SIBs, and PIBs). To distinguish various PBA derivatives, the working mechanism and applications of PBA-templated metal oxides, metal chalcogenides, metal phosphides, and other nanocomposites are systematically evaluated, facilitating the establishment of a structure-activity correlation for these materials. Based on the fruitful achievements of PBA-derived nanocomposites, perspectives for their future development are envisioned, aiming to narrow down the gap between laboratory study and industrial reality.

High-Entropy and Component Stoichiometry Tuning Strategies Boost the Sodium-Ion Storage Performance of Cobalt-Free Prussian Blue Analogues Cathode Materials

Prussian blue analogs (PBAs) are appealing cathode materials for sodium-ion batteries because of their low material cost, facile synthesis methods, rigid open framework, and high theoretical capacity. However, the poor electrical conductivity, unavoidable presence of [Fe(CN)6] vacancies and crystalline water within the framework, and phase transition during charge-discharge result in inferior electrochemical performance, particularly in terms of rate capability and cycling stability. Here, cobalt-free PBAs are synthesized using a facile and economic co-precipitation method at room temperature, and their sodium-ion storage performance is boosted due to the reduced crystalline water content and improved electrical conductivity via the high-entropy and component stoichiometry tuning strategies, leading to enhanced initial Coulombic efficiency (ICE), specific capacity, cycling stability, and rate capability. The optimized HE-HCF of Fe0.60Mn0.10-hexacyanoferrate (referred to as Fe0.60Mn0.10-HCF), with the chemical formula Na1.156Fe0.599Mn0.095Ni0.092Cu0.109Zn0.105 [Fe(CN)6]0.724·3.11H2O, displays the most appealing electrochemical performance of an ICE of 100%, a specific capacity of around 115 and 90 mAh·g-1 at 0.1 and 1.0 A·g-1, with 66.7% capacity retention observed after 1000 cycles and around 61.4% capacity retention with a 40-fold increase in specific current. We expect that our findings could provide reference strategies for the design of SIB cathode materials with superior electrochemical performance.

Heterogeneous bimetallic selenides encapsulated within graphene aerogel as advanced anodes for sodium ion batteries

Metal selenides are promising anode candidates for sodium ion batteries (SIBs) because of their high theoretical capacity, low cost, and environmental friendship. However, the low rate capability at high current density due to its inherent low electrical conductivity and poor cycle stability caused by inevitable volume variations during cycling frustrate its practical applications. Herein, we have developed a simple metallic-organic frameworks (MOFs)-derived selenide strategy to synthesize a series of heterogeneous bimetallic selenides encapsulated within graphene aerogels (GA) as anodes for SIBs. The bimetallic selenides/GA composites have unique structural characteristics that can shorten the migration path for Na+/electrons and accommodate the volume variations via additional void space during cycling. The built-in electric fields induced at the heterointerfaces can greatly reduce the activation energy for rapid charge transfer kinetics and promote the diffusion of Na+/electrons. GA is also beneficial for accommodating the volume variations during cycling and improving conductivity. As an advanced anode for SIBs, the MoSe2-Cu1.82Se@GA with a special porous octahedron can deliver the highest capacity of 444.8 mAh/g at a high rate of 1 A/g even after 1000 cycles among the bimetallic selenides/GA composites.

Recent progress in high-voltage P2-Na x TMO2 materials and their future perspectives

P2-type layered materials (Na x TMO2) have become attractive cathode electrodes owing to their high theoretical energy density and simple preparation. However, they still face severe phase transition and low conductivity. Current research on Na x TMO2 is mostly focused on the modification of bulk materials, and the application performances have been infrequently addressed. This review summarizes the information on current common P2-Na x TMO2 materials and discusses their sodium-storage mechanisms. Furthermore, modification strategies to improve their performance are addressed for practical applications based on a range of key parameters (output voltage, specific capacity, and lifespan). We also discuss the future development trends and application prospects for P2 cathode materials.

Achieving High Rate and Long Cycle Performance of Na2FePO4F Cathode Through Co-Modification of Ti Doping and Carbon Coating

Layered Na2FePO4F (NFPF) cathode material has received widespread attention due to its green nontoxicity, abundant raw materials, and low cost. However, its poor inherent electronic conductivity and sluggish sodium ion transportation seriously impede its capacity delivery and cycling stability. In this work, NFPF by Ti doping and conformal carbon layer coating via solid-state reaction is modified. The results of experimental study and density functional theory calculations reveal that Ti doping enhances intrinsic conductivity, accelerates Na-ion transport, and generates more Na-ion storage sites, and pyrolytic carbon from polyvinylpyrrolidone (PVP) uniformly coated on the NFPF surface improves the surface/interface conductivity and suppresses the side reactions. Under the combined effect of Ti doping and carbon coating, the optimized NFPF (marked as 5T-NF@C) exhibits excellent electrochemical performance, with a high capacity of 108.4 mAh g-1 at 0.2C, a considerable capacity of 80.0 mAh g-1 even at high current density of 10C, and a high capacity retention rate of 81.8% after 2000 cycles at 10C. When assembled into a full cell with a hard carbon anode, 5T-NF@C also show good applicability. This work indicates that co-modification of Ti doping and carbon coating makes NFPF achieve high rate and long cycle performance for sodium-ion batteries.

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

Sodium-ion batteries (SIBs) are investigated as promising alternatives to lithium-ion batteries (LIBs) on account of the economical abundance and reliable availability of sodium, as well as its analogous chemical properties compared to lithium. Nevertheless, the performance of SIBs is severely restricted by the availability of satisfactory cathode nanomaterials with stable frameworks to accommodate the transportation of large-sized Na+ ions. These challenges can be effectively resolved when exploiting Prussian blue (PB) and its analogs (PBAs) as SIB cathodes. This is mainly because PB and PBAs have 3D open frameworks with large interstitial space, which are more favorable for fast insertion/extraction of Na+ ions during the charging/discharging process, thus enabling the improvement of integrated performance in SIB systems. This overview offers a comprehensive summarization of recent advancements in the electrochemical performance of PB and PBAs when employing them as cathodes in SIBs. For better understanding, the fabrication strategy, structural characterization, and electrochemical performance exposition are systematically organized and explained according to tuning PB and metal-based PBAs. Additionally, the current trajectories and prospective future directions pertaining to the utilization of PB and PBA cathodes in the SIB system are thoroughly examined and deliberated upon.

From lab to field: Prussian blue frameworks as sustainable cathode materials

Prussian blue and Prussian blue analogues have attracted increasing attention as versatile framework materials with a wide range of applications in catalysis, energy conversion and storage, and biomedical and environmental fields. In terms of energy storage and conversion, Prussian blue-based materials have emerged as suitable candidates of growing interest for the fabrication of batteries and supercapacitors. Their outstanding electrochemical features such as fast charge-discharge rates, high capacity and prolonged cycling life make them favorable for energy storage application. Furthermore, Prussian blue and its analogues as rechargeable battery anodes can advance significantly by the precise control of their structure, morphology, and composition at the nanoscale. Their tunable structural and electronic properties enable the detection of many types of analytes with high sensitivity and specificity, and thus, they are ideal materials for the development of sensors for environmental detection, disease trend monitoring, and industrial safety. Additionally, Prussian blue-based catalysts display excellent photocatalytic performance for the degradation of pollutants and generation of hydrogen. Specifically, their excellent light capturing and charge separation capabilities make them stand out in photocatalytic processes, providing a sustainable option for environmental remediation and renewable energy production. Besides, Prussian blue coatings have been studied particularly for corrosion protection, forming stable and protective layers on metal surfaces, which extend the lifespan of infrastructural materials in harsh environments. Prussian blue and its analogues are highly valuable materials in healthcare fields such as imaging, drug delivery and theranostics because they are biocompatible and their further functionalization is possible. Overall, this review demonstrates that Prussian blue and related framework materials are versatile and capable of addressing many technical challenges in various fields ranging from power generation to healthcare and environmental management.

Rational Design of Flexible, Self-Supporting, and Binder-Free Prussian White/KetjenBlack/MXene Composite Electrode for Sodium-Ion Batteries with Boosted Electrochemical Performance

Due to their cost-effectiveness, abundant resources, and suitable working potential, sodium-ion batteries are anticipated to establish themselves as a leading technology in the realm of grid energy storage. However, sodium-ion batteries still encounter challenges, including issues related to low energy density and constrained cycling performance. In this study, a self-supported electrode composed of Prussian white/KetjenBlack/MXene (TK-PW) is proposed. In the TK-PW electrode, the MXene layer is coated with Prussian white nanoparticles and KetjenBlack with high conductivity, which is conducive to rapid Na+ dynamics and effectively alleviates the expansion of the electrode. Notably, the electrode preparation method is uncomplicated and economically efficient, enabling large-scale production. Electrochemical testing demonstrates that the TK-PW electrode retains 74.9% of capacity after 200 cycles, with a discharge capacity of 69.7 mAh·g-1 at 1000 mA·g-1. Furthermore, a full cell is constructed, employing a hard carbon anode and TK-PW cathode to validate the practical application potential of the TK-PW electrode.

A collaborative manipulation strategy to enhance the sodium ion storage capability of Prussian white cathodes

A collaborative manipulation strategy of proper heat treatment and self-customized hydrofluoroether-based electrolyte design has been proposed for boosting the sodium-ion storage kinetics of Prussian white cathodes. Improved monoclinic phase stability and electrolyte-cathode compatibility are responsible for an impressive discharge capacity of 148.4 mA h g-1 and excellent electrode reversibility.

Hollow Layered Iron-Based Prussian Blue Cathode with Reduced Defects for High-Performance Sodium-Ion Batteries

Fe-based Prussian blue (Fe-PB) analogues have emerged as promising cathode materials for sodium-ion batteries, owing to their cost-effectiveness, high theoretical capacity, and environmental friendliness. However, their practical application is hindered by [Fe(CN)6] defects, negatively impacting capacity and cycle stability. This work reports a hollow layered Fe-PB composite material using 1,3,5-benzenetricarboxylic acid (BTA) as a chelating and etching agent by the hydrothermal method. Compared to benzoic acid, our approach significantly reduces defects and enhances the yield of Fe-PB. Notably, the hollow layered structure shortens the diffusion path of sodium ions, enhances the activity of low-spin Fe in the Fe-PB lattice, and mitigates volume changes during Na-ion insertion/extraction into/from Fe-PB. As a sodium-ion battery cathode, this hollow layered Fe-PB exhibits an impressive initial capacity of 95.9 mAh g-1 at a high current density of 1 A g-1. Even after 500 cycles, it still maintains a considerable discharge capacity of 73.1 mAh g-1, showing a significantly lower capacity decay rate (0.048%) compared to the control sample (0.089%). Moreover, the full cell with BTA-PB-1.6 as the cathode and HC as the anode provides a considerable energy density of 312.2 Wh kg-1 at a power density of 291.0 W kg-1. This research not only enhances the Na storage performance of Fe-PB but also increases the yield of products obtained by hydrothermal methods, providing some technical reference for the production of PB materials using the low-yield hydrothermal method.

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.

Metal-Organic Framework-Based Materials for Advanced Sodium Storage: Development and Anticipation

As a pioneering battery technology, even though sodium-ion batteries (SIBs) are safe, non-flammable, and capable of exhibiting better temperature endurance performance than lithium-ion batteries (LIBs), because of lower energy density and larger ionic size, they are not amicable for large-scale applications. Generally, the electrochemical storage performance of a secondary battery can be improved by monitoring the composition and morphology of electrode materials. Because more is the intricacy of a nanostructured composite electrode material, more electrochemical storage applications would be expected. Despite the conventional methods suitable for practical production, the synthesis of metal-organic frameworks (MOFs) would offer enormous opportunities for next-generation battery applications by delicately systematizing the structure and composition at the molecular level to store sodium ions with larger sizes compared with lithium ions. Here, the review comprehensively discusses the progress of nanostructured MOFs and their derivatives applied as negative and positive electrode materials for effective sodium storage in SIBs. The commercialization goal has prompted the development of MOFs and their derivatives as electrode materials, before which the synthesis and mechanism for MOF-based SIB electrodes with improved sodium storage performance are systematically discussed. Finally, the existing challenges, possible perspectives, and future opportunities will be anticipated.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

An Ultra-Flexible Sodium-Ion Full Cell with High Energy/Power Density and Unprecedented Structural Stability

Futuristic wearable electronics desperately need power sources with similar flexibility and durability. In this regard, the authors, therefore, propose a scalable PAN-PMMA blend-derived electrospinning protocol to fabricate free-standing electrodes comprised of cobalt hexacyanoferrate nanocube cathode and tin metal organic framework-derived nanosphere anode, respectively, for flexible sodium-ion batteries. The resulting unique inter-networked nanofiber mesh offers several advantages such as robust structural stability towards repeated bending and twisting stresses along with appreciable electronic/ionic conductivity retention without any additional post-synthesis processing. The fabricated flexible sodium ion full cells deliver a high working voltage of 3.0 V, an energy density of 273 Wh·kg-1 , and a power density of 2.36 kW·kg-1 . The full cells retain up to 86.73% of the initial capacity after 1000 cycles at a 1.0 C rate. After intensive flexibility tests, the full cells also retain 78.26% and 90.78% of the initial capacity after 1000 bending and twisting cycles (5 mm radius bending and 40o axial twisting), respectively. This work proves that the proposed approach can also be employed to construct similar robust, free-standing nanofiber mesh-based electrodes for mass-producible, ultra-flexible, and durable sodium ion full cells with commercial viability.

Elaborating the Crystal Water of Prussian Blue for Outstanding Performance of Sodium Ion Batteries

Prussian blue (PB) is one of the main cathode materials with industrial prospects for the sodium ion battery. The structural stability of PB materials is directly associated with the presence of crystal water within the open 3D framework. However, there remains a lack of consensus regarding whether all forms of crystal water have detrimental effects on the structural stability of the PB materials. Currently, it is widely accepted that interstitial water is the stability troublemaker, whereas the role of coordination water remains elusive. In this work, the dynamic evolution of PB structures is investigated during the crystal water (in all forms) removal process through a variety of online monitoring techniques. It can be inferred that the PB-130 °C retains trace coordination water (1.3%) and original structural integrity, whereas PB-180 °C eliminates almost all of crystal water (∼12.1%, including both interstitial and coordinated water), but inevitably suffers from structural collapse. This is mainly because the coordinated water within the PB material plays a crucial role in maintaining structural stability via forming the -N≡C-FeLS-C≡N- conjugate bridge. Consequently, PB-130 °C with trace coordination water delivers superior reversible capacity (113.6 mAh g-1), high rate capability (charge to >80% capacity in 3 min), and long cycling stability (only 0.012% fading per cycle), demonstrating its promising prospect in practical applications.

A high-energy-density NASICON-type Na3V1.25Ga0.75(PO4)3 cathode with reversible V4+/V5+ redox couple for sodium ion batteries

The stable three-dimensional framework and high operating voltage of sodium superionic conductor (NASICON)-type Na3V2(PO4)3 has the potential to work with long cycle life and high-rate performance; however, it suffers from the poor intrinsic electronic conductivity and low energy density. Herein, Ga3+ is introduced into Na3V2(PO4)3 to activate the V4+/V5+ redox couple at a high potential of 4.0 V for enhancing energy density of the materials (Na3V2-xGax(PO4)3). After the partial substitution of Ga3+ for V3+, three redox couples (V2+/V3+, V3+/V4+ and V4+/V5+) of V are reversibly converted in the voltage range of 1.4-4.2 V, suggesting multi-electrons (>2e-) involved in the reversible reaction, and simultaneously the electronic conductivity of the materials is effectively enhanced. As a result, the cathode with x = 0.75 exhibits excellent electrochemical properties: in the voltage range of 2.2-4.2 V, delivering an initial capacity of 105 mAh/g at 1C with a capacity retention rate of 92.3% after 400 cycles, and providing a stable reversible capacity of 88.3 mAh/g at 40C; and in the voltage range of 1.4-4.2 V, presenting the reversible capacity 152.3 mAh/g at 1C (497.6 Wh kg-1), and cycling stably for 1000 cycles at 20C with a capacity decay of 0.02375% per cycle. It is found that the Na3V2-xGax(PO4)3 cathodes possess the sodium storage mechanism of single-phase and bi-phase reactions. This investigation presents a useful strategy to enhance the energy density and cycling life of NASICON-structured polyanionic phosphates by activating high-potential V4+/V5+ redox couple.

Switchable NaCl cages via a MWCNTs/Ni[Fe(CN)6]2 nanocomposite for high performance desalination

With the application of nanomaterials in seawater desalination technology increasing, the adjustable characteristics of carbon-based nanomaterials make it possible to use multiwalled carbon nanotube (MWCNT) materials in seawater desalination technology. In this study, Ni[Fe(CN)6]2 is loaded onto the inner wall of MWCNTs by the co-precipitation method to prepare MWCNTs with variable pore size, making it a switchable cage for NaCl. During the procedure, most of the Ni[Fe(CN)6]2 is transferred to the outer surface of the MWCNTs after adsorption, and NaCl is stored inside the MWCNTs (which have been proved by characterization); at the same time, Ni can improve the cell stability of Ni[Fe(CN)6]2. The effect of adsorbent reaction time and addition amount on the desalination performance of MWCNTs/Ni[Fe(CN)6]2 has been tested. According to the results, the best desalination performance of MWCNTs/Ni[Fe(CN)6]2 is 1354.6 mg g-1 when the reaction time is 0.5 h and the addition amount is 20 mg. After 3 cycles of adsorption and desorption, its desalting performance decreased to 242.3 mg g-1.

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.

Promising Cathode Materials for Sodium-Ion Batteries from Lab to Application

Sodium-ion batteries (SIBs) are seen as an emerging force for future large-scale energy storage due to their cost-effective nature and high safety. Compared with lithium-ion batteries (LIBs), the energy density of SIBs is insufficient at present. Thus, the development of high-energy SIBs for realizing large-scale energy storage is extremely vital. The key factor determining the energy density in SIBs is the selection of cathodic materials, and the mainstream cathodic materials nowadays include transition metal oxides, polyanionic compounds, and Prussian blue analogs (PBAs). The cathodic materials would greatly improve after targeted modulations that eliminate their shortcomings and step from the laboratory to practical applications. Before that, some remaining challenges in the application of cathode materials for large-scale energy storage SIBs need to be addressed, which are summarized at the end of this Outlook.

High-Voltage Potassium Hexacyanoferrate Cathode via High-Entropy and Potassium Incorporation for Stable Sodium-Ion Batteries

Prussian blue analogues (PBAs) used as sodium ion battery (SIB) cathodes are usually the focus of attention due to their three-dimensional open frame and high theoretical capacity. Nonetheless, the disadvantages of a low working voltage and inferior structural stability of PBAs prevent their further applications. Herein, we propose constructing the Kx(MnFeCoNiCu)[Fe(CN)6] (HE-K-PBA) cathode by high-entropy and potassium incorporation strategy to simultaneously realize high working voltage and cycling stability. The reaction mechanism of metal cations in HE-K-PBA are revealed by synchrotron radiation X-ray absorption spectroscopy (XAS), ex situ X-ray photoelectron spectroscopy (XPS), and in situ Raman spectra. We also investigate the entropy stabilization mechanism via finite element simulation, demonstrating that HE-K-PBA with small von Mises stress and weak structure strain can significantly mitigate the structural distortion. Benefit from the stable structure and everlasting K+ (de)intercalation, the HE-K-PBA delivers high output voltage (3.46 V), good reversible capacity (120.5 mAh g-1 at 0.01 A g-1), and capacity retention of 90.4% after 1700 cycles at 1.0 A g-1. Moreover, the assembled full cell and all-solid-state batteries with a stable median voltage of 3.29 V over 3000 cycles further demonstrate the application prospects of the HE-K-PBA cathode.

Recent Advances in Sodium-Ion Batteries: Cathode Materials

Emerging energy storage systems have received significant attention along with the development of renewable energy, thereby creating a green energy platform for humans. Lithium-ion batteries (LIBs) are commonly used, such as in smartphones, tablets, earphones, and electric vehicles. However, lithium has certain limitations including safety, cost-effectiveness, and environmental issues. Sodium is believed to be an ideal replacement for lithium owing to its infinite abundance, safety, low cost, environmental friendliness, and energy storage behavior similar to that of lithium. Inhered in the achievement in the development of LIBs, sodium-ion batteries (SIBs) have rapidly evolved to be commercialized. Among the cathode, anode, and electrolyte, the cathode remains a significant challenge for achieving a stable, high-rate, and high-capacity device. In this review, recent advances in the development and optimization of cathode materials, including inorganic, organometallic, and organic materials, are discussed for SIBs. In addition, the challenges and strategies for enhancing the stability and performance of SIBs are highlighted.

Layered porous Mn0.18V2O5@C with manganese and carbon provided by a metal-organic framework precursor as a cathode material for aqueous zinc-ion batteries

At present, vanadium-based cathodes for aqueous zinc-ion batteries (AZIBs) are limited by their slow reaction kinetics, poor electrical conductivity, and low capacity retention. To overcome these problems, here, we design a layered porous Mn0.18V2O5@C as the cathode material for AZIBs using a manganese-containing metal-organic framework as a template through a simple solvothermal method. Such an electrode delivers an excellent specific capacity (380 mA h g-1 at 0.1 A g-1) accompanied by superior cycling stability (about 85% capacity retention for 2000 cycles at 6 A g-1). The excellent electrochemical performance of Mn0.18V2O5@C is ascribed to the improved interface activity including smooth zinc ion transport, abundant ion reaction active sites and accelerated charge transfer resulting from the coordination of the porous structure, doped conductive carbon, and the stable channel structure derived from the pillar effect of doping manganese ions, preventing a premature collapse of the electrode structure. It is also revealed by structural evolution analysis that the residual zinc ions also combine with the original Mn0.18V2O5 to form a ZnxMnyV2O5 phase, which serves as an additional structural pillar and in the meantime, also participates in the following cycles. These favorable electrochemical results suggest that Mn0.18V2O5@C is a suitable cathode material for AZIBs.

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.

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention being the most promising sustainable energy technology owing to their competitive energy density, great safety and considerable low-cost merits. Nevertheless, the commercialization process of SIBs is still sluggish because of the difficulty in developing high-performance battery materials, especially the cathode materials. The discovery of layered transition metal oxides as the cathode materials of SIBs brings infinite possibilities for practical battery production. Thereinto, the O3-type layered transition metal oxides exhibit attractive advantages in terms of energy density benefiting from their higher sodium content compared to other kinds of layered transition metal oxides. Enormous research studies have largely put forward their progress and explored a wide range of performance improvement approaches from the morphology, coating, doping, phase structure and redox aspects. However, the progress is scattered and has not logically evolved, which is not beneficial for the further development of more advanced cathode materials. Therefore, our work aims to comprehensively review, classify and highlight the most recent advances in O3-type layered transition metal oxides for SIBs, so as to scientifically cognize their progress and remaining challenges and provide reasonable improvement ideas and routes for next-generation high-performance cathode materials.

Advances in Mn-Based Electrode Materials for Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries have attracted extensive attention for large-scale energy storage applications, due to abundant sodium resources, low cost, intrinsic safety of aqueous electrolytes and eco-friendliness. The electrochemical performance of aqueous sodium-ion batteries is affected by the properties of electrode materials and electrolytes. Among various electrode materials, Mn-based electrode materials have attracted tremendous attention because of the abundance of Mn, low cost, nontoxicity, eco-friendliness and interesting electrochemical performance. Aqueous electrolytes having narrow electrochemical window also affect the electrochemical performance of Mn-based electrode materials. In this review, we introduce systematically Mn-based electrode materials for aqueous sodium-ion batteries from cathode and anode materials and offer a comprehensive overview about their recent development. These Mn-based materials include oxides, Prussian blue analogues and polyanion compounds. We summarize and discuss the composition, crystal structure, morphology and electrochemical properties of Mn-based electrode materials. The improvement methods based on electrolyte optimization, element doping or substitution, optimization of morphology and carbon modification are highlighted. The perspectives of Mn-based electrode materials for future studies are also provided. We believe this review is important and helpful to explore and apply Mn-based electrode materials in aqueous sodium-ion batteries.

Tailoring the p-Band Center of NS Pair for Accelerating High-Performance Lithium-Oxygen Battery

The local coordination environment of catalytical moieties directly determines the performance of electrochemical energy storage and conversion devices, such as Li-O₂ batteries (LOBs) cathode. However, understanding how the coordinative structure affects the performance, especially for non-metal system, is still insufficient. Herein, a strategy that introduces S-anion to tailor the electronic structure of nitrogen-carbon catalyst (SNC) is proposed to improve the LOBs performance. This study unveils that the introduced S-anion effectively manipulates the p-band center of pyridinic-N moiety, substantially reducing the battery overpotential by accelerating the generation and decomposition of intermediate products Li_1-3 O₄ . The lower adsorption energy of discharging product Li₂ O₂ on NS pair accounts for the long-term cyclic stability by exposing the high active area under operation condition. This work demonstrates an encouraging strategy to enhance LOBs performance by modulating the p-band center on non-metal active sites.

Metal-Organic Assembly Strategy for the Synthesis of Layered Metal Chalcogenide Anodes for Na+ /K+ -Ion Batteries

Layered transition metal chalcogenides (MX, M=Mo, W, Sn, V; X=S, Se, Te) have large ion transport channels and high specific capacity, making them promising for large-sized Na⁺ /K⁺ energy-storage technologies. Nevertheless, slow reaction kinetics and huge volume expansion will induce an undesirable electrochemical performance. Numerous efforts have been devoted to designing MX anodes and enhancing their electrochemical performance. Based on the metal-organic assembly strategy, nanostructural engineering, combination with carbon materials, and component regulation can be easily realized, which effectively boost the performance of MX anodes. In this Review, we present a comprehensive overview on the synthesis of MX nanostructure using the metal-organic assembly strategy, which can realize the design of MX nanostructures, based on self-sacrificial templates, host@guest tailored templates, post-modified layer and derivative templates. The preparation routes and structure evolution are mainly discussed. Then, Mo-, W-, Sn-, V-based chalcogenides used for Na⁺ /K⁺ energy storage are reviewed, and the relationship between the structure and the electrochemical performance, as well as the energy storage mechanism are emphasized. In addition, existing challenges and future perspectives are also presented.

Advance of Prussian Blue-Derived Nanohybrids in Energy Storage: Current Status and Perspective

Great changes have occurred in the energy storage area in recent years as a result of rapid economic expansion. People have conducted substantial research on sustainable energy conversion and storage systems in order to mitigate the looming energy crisis. As a result, developing energy storage materials is critical. Materials with an open frame structure are known as Prussian blue analogs (PBAs). Anode materials for oxides, sulfides, selenides, phosphides, borides, and carbides have been extensively explored as anode materials in the field of energy conversion and storage in recent years. The advantages and disadvantages of oxides, sulfides, selenides, phosphides, borides, carbides, and other elements, as well as experimental methodologies and electrochemical properties, are discussed in this work. The findings reveal that employing oxides, sulfides, selenides, phosphides, borides, and other electrode materials to overcome the problems of low conductivity, excessive material loss, and low specific volume is ineffective. Therefore, this review intends to address the issues of diverse energy storage materials by combining multiple technologies to manufacture battery materials with low cost, large capacity, and extended service life.

Nanodesigns for Na3V2(PO4)3-based cathode in sodium-ion batteries: a topical review

With the rapid development of sodium-ion batteries (SIBs), it is urgent to exploit the cathode materials with good rate capability, attractive high energy density and considerable long cycle performance. Na₃V₂(PO₄)₃(NVP), as a NASICON-type electrode material, is one of the cathode materials with great potential for application because of its good thermal stability and stable. However, NVP has the inherent problem of low electronic conductivity, and various strategies are proposed to improve it, moreover, nanotechnology or nanostructure are involved in these strategies, the construction of nanostructured active particles and nanocomposites with conductive carbon networks have been shown to be effective in improving the electrical conductivity of NVP. Herein, we review the research progress of NVP performance improvement strategies from the perspective of nanostructures and classifies the prepared nanomaterials according to their different nano-dimension. In addition, NVP nanocomposites are reviewed in terms of both preparation methods and promotion effects, and examples of NVP nanocomposites at different nano-dimension are given. Finally, some personal views are presented to provide reasonable guidance for the research and design of high-performance polyanionic cathode materials of SIBs.

Iron-Vanadium Incorporated Ferrocyanides as Potential Cathode Materials for Application in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are potential replacements for lithium-ion batteries owing to their comparable energy density and the abundance of sodium. However, the low potential and low stability of their cathode materials have prevented their commercialization. Prussian blue analogs are ideal cathode materials for SIBs owing to the numerous diffusion channels in their 3D structure and their high potential vs. Na/Na⁺. In this study, we fabricated various Fe-V-incorporated hexacyanoferrates, which are Prussian blue analogs, via a one-step synthesis. These compounds changed their colors from blue to green to yellow with increasing amounts of incorporated V ions. The X-ray photoelectron spectroscopy spectrum revealed that V³⁺ was oxidized to V⁴⁺ in the cubic Prussian blue structure, which enhanced the electrochemical stability and increased the voltage platform. The vanadium ferrocyanide Prussian blue (VFPB1) electrode, which contains V⁴⁺ and Fe²⁺ in the Prussian blue structure, showed Na insertion/extraction potential of 3.26/3.65 V vs. Na/Na⁺. The cycling test revealed a stable capacity of ~70 mAh g^-1 at a rate of 50 mA g^-1 and a capacity retention of 82.5% after 100 cycles. We believe that this Fe-V-incorporated Prussian green cathode material is a promising candidate for stable and high-voltage cathodes for SIBs.

Low-Cost and Scalable Ni-Prussian Blue Analogue//Functionalized Carbon Based Na-Ion Systems for all Climate Operations

Herein, we have developed a sodium ion based aqueous energy storage device with nickel prussian-blue-analogue (Ni-PBA) positive and functionalized carbon-black negative electrodes in 1 M Na₂ SO₄ electrolyte solution. The components required to develop the device, i. e., stainless steel (SS) current-collectors, absorbent-glass-mat separator, electrolyte, carbon-black, and precursors of Ni-PBA, are all environmentally benign and inexpensive. To minimize the corrosion of pristine-SS, polyaniline coating on the SS surface is applied by in situ electrodeposition method. The full cell exhibits a specific capacity of 28 mAh g^-1 with 90 % Coulomb efficiency (@0.2C), an energy density of 34 Wh kg^-1 (@20 W kg^-1 ), a power density of 100 W kg^-1 (@18 Wh kg^-1 ) and a good life cycle (70 % capacity-retention over 500 cycles @1.0C rate) within the 0-1.2 V window. The cell performance is further tested under variable temperatures, and 0-50 °C range is reported to be the working window for this cell.

Vanadium Ferrocyanides as a Highly Stable Cathode for Lithium-Ion Batteries

Owing to their high redox potential and availability of numerous diffusion channels in metal-organic frameworks, Prussian blue analogs (PBAs) are attractive for metal ion storage applications. Recently, vanadium ferrocyanides (VFCN) have received a great deal of attention for application in sodium-ion batteries, as they demonstrate a stable capacity with high redox potential of ~3.3 V vs. Na/Na⁺. Nevertheless, there have been no reports on the application of VFCN in lithium-ion batteries (LIBs). In this work, a facile synthesis of VFCN was performed using a simple solvothermal method under ambient air conditions through the redox reaction of VCl₃ with K₃[Fe(CN)₆]. VFCN exhibited a high redox potential of ~3.7 V vs. Li/Li⁺ and a reversible capacity of ~50 mAh g^-1. The differential capacity plots revealed changes in the electrochemical properties of VFCN after 50 cycles, in which the low spin of Fe ions was partially converted to high spin. Ex situ X-ray diffraction measurements confirmed the unchanged VFCN structure during cycling. This demonstrated the high structural stability of VFCN. The low cost of precursors, simplicity of the process, high stability, and reversibility of VFCN suggest that it can be a candidate for large-scale production of cathode materials for LIBs.