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.

Air-Stable Prussian White Cathode Materials for Sodium-Ion Batteries Enabled by ZnO Surface Modification

Iron-based Prussian white (PW) is one of the promising cathodes for sodium-ion batteries, owing to its high capacity and low cost. However, the practical application of PW is hindered by its poor air stability. The metal-oxide coating has been proven to be an effective way to improve the air stability of electrode materials. Whereas, the target electrode materials conventionally need to be dissolved in the aqueous solution to obtain precursor composites and subsequently calcined at a high temperature during the metal-oxide coating process, which could destroy the phase structure of PW as a result of the sodium leaching into the water and thermal decomposition at the high temperature. In this work, we propose a facile method to construct a ZnO surface layer on PW by utilizing ethanol as a solvent and a mild post-treatment temperature. The ZnO coating layer effectively enhances the air stability of PW and induces the formation of the stable interface on PW. The PW-5 wt % ZnO-E (exposed in 60% humidity air after 30 days) cathode demonstrates a much higher capacity retention (94.1%) at 1 C after 200 cycles than that of PW-E (54%). This work lays a solid foundation for further application of PW.

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.

Exploring the Mechanism of Single-Crystal MnO2 as Cathodes for Zinc Ion Batteries

MnO2 has the advantages of low cost and abundant resources, so it is considered to be an important electrode material in zinc ion batteries. However, its practical application is still challenged by easy collapse and capacity loss. In this paper, a stable single crystal β-MnO2 nanorod cathode material was prepared. When used as ZIBs cathode material, single crystal β-MnO2 has high ionic diffusion kinetics and calculability. In this paper, we prepared single-crystal MnO2 through hydrothermal nanotechnology. By leveraging the benefits of the single-crystal structure, we optimized the structural stability, ion conductivity, surface reactions, and phase control of the cathode material, resulting in improved battery performance and cycle life. In the fabricated single-crystal MnO2 aqueous zinc-ion battery, the elimination of internal crystal faces in MnO2 leads to ordered lattice arrangement, enabling a more direct and unobstructed diffusion path for H+ ions within the lattice. This significantly enhances the ion conductivity of the cathode material, promoting the rate and efficiency of the battery's charge and discharge processes. Therefore, single-crystal MnO2 exhibits excellent cycling performance for zinc-ion storage in ZIBs, achieving a high specific capacity of 224.7 mA h g-1 after 250 cycles under a current density of 0.3 A g-1 , while maintaining a Coulombic efficiency of 99.58 %.

PVDF-HFP/PAN/PDA@LLZTO Composite Solid Electrolyte Enabling Reinforced Safety and Outstanding Low-Temperature Performance for Quasi-Solid-State Lithium Metal Batteries

Lithium-ion batteries (LIBs) have achieved a triumph in the market of portable electronic devices since their commercialization in the 1990s due to their high energy density. However, safety issue originating from the flammable, volatile, and toxic organic liquid electrolytes remains a long-standing problem to be solved. Alternatively, composite solid electrolytes (CSEs) have gradually become one of the most promising candidates due to their higher safety and stable electrochemical performance. However, the uniform dispersity of ceramic filler within the polymer matrix remains to be addressed. Generally, all-solid-state lithium metal batteries without any liquid components suffer from poor interfacial contact and low ionic conductivity, which seriously affect the electrochemical performance. Here we report a CSE consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), polydopamine (PDA) coated Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) (denoted as PDA@LLZTO) microfiller, polyacrylonitrile (PAN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Introducing only 4 μL of liquid electrolyte at the electrode|electrolyte interface, the CSE-based cells exhibit high ionic conductivity (0.4 × 10^-3 S cm^-1 at 25 °C), superior cycle stability, and excellent thermal stability. Even under low temperatures, the impressive electrochemical performance (78.8% of capacity retention after 400 cycles at 1 C, 0 °C, and decent capacities delivered even at low temperature of -20 °C) highlights the potential of such quasi-solid-state lithium metal batteries as a viable solution for the next-generation high-performance lithium metal batteries.

Synergistic Modification of Fe-Based Prussian Blue Cathode Material Based on Structural Regulation and Surface Engineering

The Fe-based Prussian blue (Fe-PB) composite is considered as one of the most potential cathode materials for sodium-ion batteries because of its abundant iron resources and high theoretical capacity. However, the crystal water and vacancy in the Fe-PB structure will lead to poor capacity and cycle stability. In this work, a Cu-modified Fe-PB composite (FeCu-PB@CuO) is successfully prepared through regulating the Fe-PB structure by Cu doping and engineering the surface by CuO coating. The density functional theory calculation results confirm that Cu preferentially replaces Fe^HS in the Fe-PB lattice and Cu doping reduces the bandgap. Our experiment results reveal that CuO coating can provide more active sites, inhibit side reactions, and potentially enhance the activity of Fe^HS. Due to the synergistic effect of Cu doping and CuO coating, FeCu-PB@CuO has a considerable initial discharge capacity of 123.5 mAh g^-1 at 0.1 A g^-1. In particular, at 2 A g^-1, it delivers an impressive initial capacity of 84.3 mAh g^-1, and the capacity decreasing rate of each cycle is only 0.02% over 1500 cycles. Therefore, the synergistic modification strategy of metal ion doping and metal oxide coating has tremendous application potential and can be extended to other electrode materials.

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

Prussian blue analogues (PBAs) have attracted extensive attention as cathode materials in sodium-ion batteries (SIBs) due to their low cost, high theoretical capacity, and facile synthesis process. However, it is of great challenge to control the crystal vacancies and interstitial water formed during the aqueous co-precipitation method, which are also the key factors in determining the electrochemical performance. Herein, an antioxidant and chelating agent co-assisted non-aqueous ball-milling method to generate highly-crystallized Na_{2- x} Fe[Fe(CN)₆ ]_y with hollow structure is proposed by suppressing the speed and space of crystal growth. The as-prepared Na_{2- x} Fe[Fe(CN)₆ ]_y hollow nanospheres show low vacancies and interstitial water content, leading to a high sodium content. As a result, the Na-rich Na_1.51 Fe[Fe(CN)₆ ]_0.87 ·1.83H₂ O hollow nanospheres exhibit a high initial Coulombic efficiency, excellent cycling stability, and rate performance via a highly reversible two-phase transition reaction confirmed by in situ X-ray diffraction. It delivers a specific capacity of 124.2 mAh g^-1 at 17 mA g^-1 , presenting ultra-high rate capability (84.1 mAh g^-1 at 3400 mA g^-1 ) and cycling stability (65.3% capacity retention after 1000 cycles at 170 mA g^-1 ). Furthermore, the as-reported non-aqueous ball-milling method could be regarded as a promising method for the scalable production of PBAs as cathode materials for high-performance SIBs.

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.

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.

Metal-Organic Frameworks and Their Derivatives: Designing Principles and Advances toward Advanced Cathode Materials for Alkali Metal Ion Batteries

Metal-organic frameworks (MOFs) and their derivatives have attracted enormous attention in the field of energy storage, due to their high specific surface area, tunable structure, highly ordered pores, and uniform metal sites. Compared with the wide research of MOFs and their related materials on anode materials for alkali metal ion batteries, few works are on cathode materials. In this review, design principles for promoting the electrochemical performance of MOF-related materials in terms of component/structure design, composite fabrication, and morphology engineering are presented. By summarizing the advancement of MOFs and their derivatives, Prussian blue and its analogs, and MOF surface coating, challenges and opportunities for future outlooks of MOF-related cathode materials are discussed.

Introducing a cell moisturizer: organogel nano-beads with rapid response to electrolytes for Prussian white analogue based non-aqueous potassium ion battery

Prussian white analogue nanoparticles were connected internally by a composite consisting of poly(butyl methacrylate) (PBMA) nano-gel and a conducting polymer layer via a one-step route. The powder falling problems have been mitigated by the intrinsic good binding strength of PBMA organogel; meanwhile, the conducting polymer provides extra transfer paths for electrons.

A Heterostructure Coupling of Bioinspired, Adhesive Polydopamine, and Porous Prussian Blue Nanocubics as Cathode for High-Performance Sodium-Ion Battery

Prussian blue (PB) and its analogues are recognized as promising cathodes for rechargeable batteries intended for application in low-cost and large-scale electric energy storage. With respect to PB cathodes, however, their intrinsic crystal regularity, vacancies, and coordinated water will lead to low specific capacity and poor rate performance, impeding their application. Herein, nanocubic porous Na_x FeFe(CN)₆ coated with polydopamine (PDA) as a coupling layer to improve its electrochemical performance is reported, inspired by the excellent adhesive property of PDA. As a cathode for sodium-ion batteries, the Na_x FeFe(CN)₆ electrode coupled with PDA delivers a reversible capacity of 93.8 mA h g^-1 after 500 cycles at 0.2 A g^-1 , and a discharge capacity of 72.6 mA h g^-1 at 5.0 A g^-1 . The sodium storage mechanism of this Na_x FeFe(CN)₆ coupled with PDA is revealed via in situ Raman spectroscopy. The first-principles computational results indicate that Fe^II sites in PB prefer to couple with the robust PDA layer to stabilize the PB structure. Moreover, the sodium-ion migration in the PB structure is enhanced after coating with PDA, thus improving the sodium storage properties. Both experiments and computational simulations present guidelines for the rational design of nanomaterials as electrodes for energy storage devices.

Engineering 3D Well-Interconnected Na4MnV(PO4)3 Facilitates Ultrafast and Ultrastable Sodium Storage

Na₄MnV(PO₄)₃ (denoted as NMVP) has drawn increasing attention owing to the three-dimensional framework and high theoretical capacity. Nevertheless, the inherent low electronic conductivity of NMVP impedes the scale-up commercial applications. In this work, the feasibility to achieve ultrahigh-rate capability and long lifespan by in situ embedding the intertwined carbon nanotube (CNT) matrix into the bulk of Na₄MnV(PO₄)₃@C composites through a facile wet-chemical approach is reported. The elaborately prepared Na₄MnV(PO₄)₃@C@CNTs cathode delivers a discharge capacity of 109.9 mA h g^-1 at C/5 with an impressive rate capability of 68.9 mA h g^-1 at an ultrahigh current rate of 90 C as well as a fascinating cycling performance of 68.3% capacity retention at 40 C after 4000 cycles. The optimum design of the 3D well-interconnected NMVP permitting fast kinetics for transported Na⁺/e^- is beneficial to the excellent electrochemical performance, which is further studied by the galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectra measurements. The pseudocapacitance contributions are also investigated. The research demonstrates that the dual-nanocarbon synergistically modified NMVP composite is expected to facilitate the commercialization of sodium-ion batteries.