Potassium Prussian Blue Nanoparticles: A Low‐Cost Cathode Material for Potassium‐Ion Batteries

Potassium‐ion batteries (KIBs) in organic electrolytes hold great promise as an electrochemical energy storage technology owing to the abundance of potassium, close redox potential to lithium, and similar electrochemistry with lithium system. Although carbon materials have been studied as KIB anodes, investigations on KIB cathodes have been scarcely reported. A comprehensive study on potassium Prussian blue K_0.220Fe[Fe(CN)₆]_0.805?4.01H₂O nanoparticles as a potential cathode material is for the first time reported. The cathode exhibits a high discharge voltage of 3.1–3.4 V, a high reversible capacity of 73.2 mAh g^?1, and great cyclability at both low and high rates with a very small capacity decay rate of ≈0.09% per cycle. Electrochemical reaction mechanism analysis identifies the carbon‐coordinated Fe^III/Fe^II couple as redox‐active site and proves structural stability of the cathode during charge/discharge. Furthermore, for the first time, a KIB full‐cell is presented by coupling the nanoparticles with commercial carbon materials. The full‐cell delivers a capacity of 68.5 mAh g^?1 at 100 mA g^?1 and retains 93.4% of the capacity after 50 cycles. Considering the low cost and material sustainability as well as the great electrochemical performances, this work may pave the way toward more studies on KIB cathodes and trigger future attention on rechargeable KIBs.     

Layered Oxide Cathodes Promoted by Structure Modulation Technology for Sodium‐Ion Batteries

Considering the ever‐growing climatic degeneration, sustainable and renewable energy sources are needed to be effectively integrated into the grid through large‐scale electrochemical energy storage and conversion (EESC) technologies. With regard to their competent benefit in cost and sustainable supply of resource, room‐temperature sodium‐ion batteries (SIBs) have shown great promise in EESC, triumphing over other battery systems on the market. As one of the most fascinating cathode materials due to the simple synthesis process, large specific capacity, and high ionic conductivity, Na‐based layered transition metal oxide cathodes commonly suffer from the sluggish kinetics, multiphase evolution, poor air stability, and insufficient comprehensive performance, restricting their commercialization application. Here, this review summarizes the recent advances in layered oxide cathode materials for SIBs through different optimal structure modulation technologies, with an emphasis placed on strategies to boost Na⁺ kinetics and reduce the irreversible phase transition as well as enhance the store stability. Meanwhile, a thorough and in‐depth systematical investigation of the structure–function–property relationship is also discussed, and the challenges as well as opportunities for practical application electrode materials are sketched. The insights brought forward in this review can be considered as a guide for SIBs in next‐generation EESC.     

Prussian White Hierarchical Nanotubes with Surface‐Controlled Charge Storage for Sodium‐Ion Batteries

Coordination compounds such as Prussian blue and its analogues are acknowledged as promising candidates for electrochemical sodium storage owing to their tailorable and open frameworks. However, a key challenge for these electrode materials is the trade‐off between energy and power. Here, it is demonstrate that Prussian white (Na_3.1Fe₄[Fe(CN)₆]₃) hierarchical nanotubes with fully open configurations render extrinsic Na⁺ intercalation pseudocapacitance. The cathode exhibits a capacity up to 83 mA h g^?1 at an ultrahigh rate of 50 C and an unprecedented cycle life over 10 000 times for sodium storage. In situ Raman spectroscopy together with in situ X‐ray diffraction analysis reveal that intercalation pseudocapacitance enables full reaction of N‐Fe^III/Fe^II sites in Prussian white with a negligible volume expansion (<2.1%). The discovery of surface‐controlled charge storage occurring inside the entire bulk of intercalation cathodes paves a new way for developing high power, high energy, and long life‐span sodium‐ion batteries.     

Low-Cost Zinc Substitution of Iron-Based Prussian Blue Analogs as Long Lifespan Cathode Materials for Fast Charging Sodium-Ion Batteries

Iron-based Prussian blue analogs (Fe-PBAs) are extensively studied as promising cathode materials for rechargeable sodium-ion batteries owing to their high theoretical capacity, low-cost and facile synthesis method. However, Fe-PBAs suffer poor cycle stability and low specific capacity due to the low crystallinity and irreversible phase transition during excess sodium-ion storage. Herein, a modified co-precipitation method to prepare highly crystallized PBAs is reported. By introducing an electrochemical inert element (Zn) to substitute the high-spin Fe in the Fe-PBAs (ZnFeHCF-2), the depth of charge/discharge is rationally controlled to form a highly reversible phase transition process for sustainable sodium-ion storage. Minor lattice distortion and highly reversible phase transition process of ZnFeHCF-2 during the sodium-ions insertion and extraction are proved by in-situ tests, which have significantly impacted the cycling stability. The ZnFeHCF-2 shows a remarkably enhanced cycling performance with capacity retention of 58.5% over 2000 cycles at 150 mA g⁻¹ as well as superior rate performance up to 6000 mA g⁻¹ (fast kinetics). Furthermore, the successful fabrication of the full cell on the as-prepared cathode and commercial hard carbon anode demonstrates their potential as high-performance electrode materials for large-scale energy storage systems.     

Engineering of Polyanion Type Cathode Materials for Sodium‐Ion Batteries: Toward Higher Energy/Power Density

The development of high energy/power density sodium‐ion batteries (SIBs) is still challenged by the high redox potential of Na/Na⁺ and large radius of Na⁺ ions, thus requiring extensive further improvement to, in particular, enhance the capacity and voltage of cathode materials. Among the various types of cathodes, the polyanion cathodes have emerged as the most pragmatic option due to their outstanding thermostability, unique inductive effect, and flexible structures. In this Review, a critical overview of the design principles and engineering strategies of polyanion cathodes that could have a pivotal role in developing high energy/power density SIBs are presented. Specifically, the engineering of polyanion cathode materials for higher voltage and specific capacity to increase energy density is discussed. The way in which morphology control, architectural design, and electrode processing have been developed to increase power density for SIBs is also analyzed. Finally, the remaining challenges and the future research direction of this field are presented.     

Prussian Blue@C Composite as an Ultrahigh‐Rate and Long‐Life Sodium‐Ion Battery Cathode

Rechargeable sodium ion batteries (SIBs) are surfacing as promising candidates for applications in large‐scale energy‐storage systems. Prussian blue (PB) and its analogues (PBAs) have been considered as potential cathodes because of their rigid open framework and low‐cost synthesis. Nevertheless, PBAs suffer from inferior rate capability and poor cycling stability resulting from the low electronic conductivity and deficiencies in the PBAs framework. Herein, to understand the vacancy‐impacted sodium storage and Na‐insertion reaction kinetics, we report on an in‐situ synthesized PB@C composite as a high‐performance SIB cathode. Perfectly shaped, nanosized PB cubes were grown directly on carbon chains, assuring fast charge transfer and Na‐ion diffusion. The existence of [Fe(CN)₆] vacancies in the PB crystal is found to greatly degrade the electrochemical activity of the Fe^LS(C) redox couple via first‐principles computation. Superior reaction kinetics are demonstrated for the redox reactions of the Fe^HS(N) couple, which rely on the partial insertion of Na ions to enhance the electron conduction. The synergistic effects of the structure and morphology results in the PB@C composite achieving an unprecedented rate capability and outstanding cycling stability (77.5 mAh g^?1 at 90 C, 90 mAh g?¹ after 2000 cycles at 20 C with 90% capacity retention).     

Approaching the Downsizing Limit of Maricite NaFePO4 toward High‐Performance Cathode for Sodium‐Ion Batteries

Maricite NaFePO₄ nanodots with minimized sizes (≈1.6 nm) uniformly embedded in porous N‐doped carbon nanofibers (designated as NaFePO₄@C) are first prepared by electrospinning for maximized Na‐storage performance. The obtained flexible NaFePO₄@C fiber membrane adherent on aluminum foil is directly used as binder‐free cathode for sodium‐ion batteries, revealing that the ultrasmall nanosize effect as well as a high‐potential desodiation process can transform the generally perceived electrochemically inactive maricite NaFePO₄ into a highly active amorphous phase; meanwhile, remarkable electrochemical performance in terms of high reversible capacity (145 mA h g^?1 at 0.2 C), high rate capability (61 mA h g^?1 at 50 C), and unprecedentedly high cyclic stability (≈89% capacity retention over 6300 cycles) is achieved. Furthermore, the soft package Na‐ion full battery constructed by the NaFePO₄@C nanofibers cathode and the pure carbon nanofibers anode displays a promising energy density of 168.1 Wh kg^?1 and a notable capacity retention of 87% after 200 cycles. The distinctive 3D network structure of very fine NaFePO₄ nanoparticles homogeneously encapsulated in interconnected porous N‐doped carbon nanofibers, can effectively improve the active materials' utilization rate, facilitate the electrons/Na⁺ ions transport, and strengthen the electrode stability upon prolonged cycling, leading to the fascinating Na‐storage performance.     

Understanding the Structural Evolution and Redox Mechanism of a NaFeO2–NaCoO2 Solid Solution for Sodium‐Ion Batteries

Na‐ion batteries have become promising candidates for large‐scale energy‐storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe³⁺/Fe⁴⁺ redox couple. The high‐rate performance derived from the low Lewis‐acidity of the Na⁺ ions is another advantage of Na‐ion batteries and has been demonstrated in NaFe_1/2Co_1/2O₂ solutions. Here, a solid solution of NaFeO₂‐NaCoO₂ is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end‐members. The combined analysis of operando X‐ray diffraction, ex situ X‐ray absorption spectroscopy, and density functional theory (DFT) calculations for Na_{1– x} Fe_1/2Co_1/2O₂ reveals that the O3‐type phase transforms into a P3‐type phase coupled with Na⁺/vacancy ordering, which has not been observed in O3‐type NaFeO₂. The substitution of Co for Fe stabilizes the P3‐type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3‐type NaFeO₂, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte‐Carlo simulations, the formation of a superlattice originating from the Na⁺/vacancy ordering is found by synchrotron X‐ray diffraction for Na_0.5Fe_1/2Co_1/2O₂, which may involve a potential step in the charge/discharge profiles.     

Stepwise Hollow Prussian Blue Nanoframes/Carbon Nanotubes Composite Film as Ultrahigh Rate Sodium Ion Cathode

Prussian blue and its analogues (PBAs) have been proposed as promising cathode materials for sodium‐ion batteries (SIBs) due to high theoretical capacity and low cost, but they often suffer from poor electronic conductivity and structural instability. Herein, a stepwise hollow cubic framework structure is first designed and a hybridized hierarchical film synthesized from single‐crystal PBA nanoframes/carbon nanotubes (CNTs) composite is demonstrated as a binder‐free ultrahigh rate sodium ion cathode. This hierarchical configuration offers improved tolerance for lattice expansion, reduced sodium ion diffusion path, enhanced electronic conductivity, and optimized redox reactions, thereby achieving the excellent rate capability, high specific capacity, and long cycle life. As expected, the developed FeHCFe nanoframes/CNTs electrode film exhibits a super high rate capacity of 149.2 mAh g^?1 at 0.1C and 35.0 mAh g^?1 at 100C. Moreover, it displays an excellent cycling stability with about 92% capacity retention at 5C after 500 cycles. This work will pave a new way to engineer advanced electrode materials for ultrahigh rate SIBs.     

Applications of Tin Sulfide‐Based Materials in Lithium‐Ion Batteries and Sodium‐Ion Batteries

SnS_x (x = 1, 2) compounds are composed of earth‐abundant elements and are nontoxic and low‐cost materials that have received increasing attention as energy materials over the past decades, owing to their huge potential in batteries. Generally, SnS_x materials have excellent chemical stability and high theoretical capacity and reversibility due to their unique 2D‐layered structure and semiconductor properties. As a promising matrix material for storing different alkali metal ions through alloying/dealloying reactions, SnS_x compounds have broad electrochemical prospects in batteries. Herein, the structural properties of SnS_x materials and their advantages as electrode materials are discussed. Furthermore, detailed accounts of various synthesis methods and applications of SnS_x materials in lithium‐ion batteries, sodium‐ion batteries, and other new rechargeable batteries are emphasized. Ultimately, the challenges and opportunities for future research on SnS_x compounds are discussed based on the available academic knowledge, including recent scientific advances.     

Layer‐Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries

A critical bottleneck that hinders major performance improvement in lithium‐ion and sodium‐ion batteries is the inferior electrochemical activity of their cathode materials. While significant research progresses have been made, conventional single‐phase cathodes are still limited by intrinsic deficiencies such as low reversible capacity, enormous initial capacity loss, rapid capacity decay, and poor rate capability. In the past decade, layer‐based heterostructured cathodes acquired by combining multiple crystalline phases have emerged as candidates with a huge potential to realize performance breakthrough. Herein, recent studies on the structural properties, electrochemical behaviors, and synthesis route optimizations of these heterostructured cathodes are summarized for in‐depth discussions. Particular attention is paid to the latest mechanism discoveries and performance achievements. This review thus aims to promote a deeper understanding of the correlation between the crystal structure of cathodes and their electrochemical behavior, and offers guidance to design advance cathode materials from the aspect of crystal structure engineering.     

Organic Diamine-Regulated Vanadium Oxides as Cathode Materials for High-Performance Sodium Ion Batteries

Pre-intercalating ions between V O layers is considered to be an effective strategy to modulate the interlayer spacing of 2D vanadium oxides. However, the rigid pre-intercalated ions hardly keep stable during repeated charging/discharging process and their sizes limit the extent of interlayer spacing expansion, which inevitably lead to poor rate capability and cycle stability. In this work, aliphatic diamines are adopted as pre-intercalated guests to elastically modulate the interlayer spacing of V O layers by tuning the chain length of the organic diamine molecules. Benefiting from the strong interaction between the terminal doubly protonated amine and the polar negative oxygen bridge of the V O layers, the aliphatic diamine molecules can act as a structural stabilizer between the layers and boost fast Na ion diffusion (10⁻⁸ to 10⁻¹⁰ cm² s⁻¹). The sodium ion battery based on the first synthesized 1,6-hexanediamine pre-intercalated vanadium oxide supported on nickel foam hybrid cathode achieves a large specific capacity of 597 mAh g⁻¹ at 0.09 A g⁻¹, as well as superior rate performance and cycling stability. This work provides a strategy to elastically modulate 2D layered materials with tunable interlayer spacing for batteries based on large-size-ions.     

Enhanced Performance of P2‐Na0.66(Mn0.54Co0.13Ni0.13)O2 Cathode for Sodium‐Ion Batteries by Ultrathin Metal Oxide Coatings via Atomic Layer Deposition

Sodium‐ion batteries are widely considered as promising energy storage systems for large‐scale applications, but their relatively low energy density hinders further practical applications. Developing high‐voltage cathode materials is an effective approach to increase the overall energy density of sodium‐ion batteries. When cut‐off voltage is elevated over 4.3 V, however, the cathode becomes extremely unstable due to structural transformations as well as metal dissolution into the electrolytes. In this work, the cyclic stability of P2‐Na_0.66(Mn_0.54Co_0.13Ni_0.13)O₂ (MCN) electrode at a cut‐off voltage of 4.5 V is successfully improved by using ultrathin metal oxide surface coatings (Al₂O₃, ZrO₂, and TiO₂) deposited by an atomic layer deposition technique. The MCN electrode coated with the Al₂O₃ layer exhibits higher capacity retention among the MCN electrodes. Moreover, the rate performance of the MCN electrode is greatly improved by the metal oxide coatings in the order of TiO₂ < Al₂O₃ < ZrO₂, due to increased fracture toughness and electrical conductivity of the metal oxide coating layers. A ZrO₂‐coated MCN electrode shows a discharge capacity of 83 mAh g^?1 at 2.4 A g^?1, in comparison to 61 mAh g^?1 for a pristine MCN electrode. Cyclic voltammetry and electrochemical impedance analysis disclose the reduced charge transfer resistance from 1421 to 760.2 Ω after cycles, suggesting that the metal oxide coating layer can effectively minimize the undesirable phase transition, buffer inherent stress and strain between the binder, cathode, and current collector, and avoid volumetric changes, thus increasing the cyclic stability of the MCN electrode.     

Ultrathin‐Nanosheet‐Induced Synthesis of 3D Transition Metal Oxides Networks for Lithium Ion Battery Anodes

A general ultrathin‐nanosheet‐induced strategy for producing a 3D mesoporous network of Co₃O₄ is reported. The fabrication process introduces a 3D N‐doped carbon network to adsorb metal cobalt ions via dipping process. Then, this carbon matrix serves as the sacrificed template, whose N‐doping effect and ultrathin nanosheet features play critical roles for controlling the formation of Co₃O₄ networks. The obtained material exhibits a 3D interconnected architecture with large specific surface area and abundant mesopores, which is constructed by nanoparticles. Merited by the optimized structure in three length scales of nanoparticles–mesopores–networks, this Co₃O₄ nanostructure possesses superior performance as a LIB anode: high capacity (1033 mAh g^?1 at 0.1 A g^?1) and long‐life stability (700 cycles at 5 A g^?1). Moreover, this strategy is verified to be effective for producing other transition metal oxides, including Fe₂O₃, ZnO, Mn₃O₄, NiCo₂O₄, and CoFe₂O₄.     

General Synthesis of N‐Doped Macroporous Graphene‐Encapsulated Mesoporous Metal Oxides and Their Application as New Anode Materials for Sodium‐Ion Hybrid Supercapacitors

A general method to synthesize mesoporous metal oxide@N‐doped macroporous graphene composite by heat‐treatment of electrostatically co‐assembled amine‐functionalized mesoporous silica/metal oxide composite and graphene oxide, and subsequent silica removal to produce mesoporous metal oxide and N‐doped macroporous graphene simultaneously is reported. Four mesoporous metal oxides (WO_{3? x} , Co₃O₄, Mn₂O₃, and Fe₃O₄) are encapsulated in N‐doped macroporous graphene. Used as an anode material for sodium‐ion hybrid supercapacitors (Na‐HSCs), mesoporous reduced tungsten oxide@N‐doped macroporous graphene (m‐WO_{3? x} @NM‐rGO) gives outstanding rate capability and stable cycle life. Ex situ analyses suggest that the electrochemical reaction mechanism of m‐WO_{3? x} @NM‐rGO is based on Na⁺ intercalation/de‐intercalation. To the best of knowledge, this is the first report on Na⁺ intercalation/de‐intercalation properties of WO_{3? x} and its application to Na‐HSCs.     

Manipulation of Disodium Rhodizonate: Factors for Fast‐Charge and Fast‐Discharge Sodium‐Ion Batteries with Long‐Term Cyclability

Organic sodium‐ion batteries (SIBs) are one of the most promising alternatives of current commercial inorganic lithium‐ion batteries (LIBs) especially in the foreseeable large‐scale flexible and wearable electronics. However, only a few reports are involving organic SIBs so far. To achieve fast‐charge and fast‐discharge performance and the long‐term cycling suitable for practical applications, is still challenging. Here, important factors for high performance SIBs especially with high capacity and long‐term cyclability under fast‐charge and fast‐discharge process are investigated. It is found that controlling the solubility through molecular design and determination of the electrochemical window is essential to eliminate dissolution of the electrode material, resulting in improved cyclability. The results show that poly(vinylidenedifluoride) will decompose during the charge/discharge process, indicating the significance of the binder for achieving high cyclability. Beside of these, it is also shown that decent charge transport and ionic diffusion are beneficial to the fast‐charge and fast‐discharge batteries. For instance, the flake morphology facilitates the ionic diffusion and thereby can lead to a capacitive effect that is favorable to fast charge and fast discharge.     

General Synthesis of Dual Carbon‐Confined Metal Sulfides Quantum Dots Toward High‐Performance Anodes for Sodium‐Ion Batteries

Sponge‐like composites assembled by cobalt sulfides quantum dots (Co₉S₈ QD), mesoporous hollow carbon polyhedral (HCP) matrix, and a reduced graphene oxide (rGO) wrapping sheets are synthesized by a simultaneous thermal reduction, carbonization, and sulfidation of zeolitic imidazolate frameworks@GO precursors. Specifically, Co₉S₈ QD with size less than 4 nm are homogenously embedded within HCP matrix, which is encapsulated in macroporous rGO, thereby leading to the double carbon‐confined hierarchical composites with strong coupling effect. Experimental data combined with density functional theory calculations reveal that the presence of coupled rGO not only prevents the aggregation and excessive growth of particles, but also expands the lattice parameters of Co₉S₈ crystals, enhancing the reactivity for sodium storage. Benefiting from the hierarchical porosity, conductive network, structural integrity, and a synergistic effect of the components, the sponge‐like composites used as binder‐free anodes manifest outstanding sodium‐storage performance in terms of excellent stable capacity (628 mAh g^?1 after 500 cycles at 300 mA g^?1) and exceptional rate capability (529, 448, and 330 mAh g^?1 at 1600, 3200, and 6400 mA g^?1). More importantly, the synthetic method is very versatile and can be easily extended to fabricate other transition‐metal‐sulfides‐based sponge‐like composites with excellent electrochemical performances.     

Sodium‐Ion Batteries: Building Effective Layered Cathode Materials with Long‐Term Cycling by Modifying the Surface via Sodium Phosphate

Surface stabilization of cathode materials is urgent for guaranteeing long‐term cyclability, and is important in Na cells where a corrosive Na‐based electrolyte is used. The surface of P2‐type layered Na_2/3[Ni_1/3Mn_2/3]O₂ is modified with ionic, conducting sodium phosphate (NaPO₃) nanolayers, ≈10 nm in thickness, via melt‐impregnation at 300 °C; the nanolayers are autogenously formed from the reaction of NH₄H₂PO₄ with surface sodium residues. Although the material suffers from a large anisotropic change in the c‐axis due to transformation from the P2 to O2 phase above 4 V versus Na⁺/Na, the NaPO₃‐coated Na_2/3[Ni_1/3Mn_2/3]O₂/hard carbon full cell exhibits excellent capacity retention for 300 cycles, with 73% retention. The surface NaPO₃ nanolayers positively impact the cell performance by scavenging HF and H₂O in the electrolyte, leading to less formation of byproducts on the surface of the cathodes, which lowers the cell resistance, as evidenced by X‐ray photoelectron spectroscopy and time‐of‐flight secondary‐ion mass spectroscopy. Time‐resolved in situ high‐temperature X‐ray diffraction study reveals that the NaPO₃ coating layer is delayed for decomposition to Mn₃O₄, thereby suppressing oxygen release in the highly desodiated state, enabling delay of exothermic decomposition. The findings presented herein are applicable to the development of high‐voltage cathode materials for sodium batteries.