The Cathode Choice for Commercialization of Sodium‐Ion Batteries: Layered Transition Metal Oxides versus Prussian Blue Analogs

With the unprecedentedly increasing demand for renewable and clean energy sources, the sodium‐ion battery (SIB) is emerging as an alternative or complementary energy storage candidate to the present commercial lithium‐ion battery due to the abundance and low cost of sodium resources. Layered transition metal oxides and Prussian blue analogs are reviewed in terms of their commercial potential as cathode materials for SIBs. The recent progress in research on their half cells and full cells for the ultimate application in SIBs are summarized. In addition, their electrochemical performance, suitability for scaling up, cost, and environmental concerns are compared in detail with a brief outlook on future prospects. It is anticipated that this review will inspire further development of layered transition metal oxides and Prussian blue analogs for SIBs, especially for their emerging commercialization.     

Advanced Vanadium Oxides for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) represent one of the current research frontiers owing to their low cost, intrinsic safety, environmental friendliness, and other unique features. In the current era, a myriad of investigations are conducted towards the exploration of advanced electrode materials with exceptional energy/power density, superior rate capability, and ultralong cycling life. Notably, vanadium oxides electrode materials have received great attention due to their diversity in chemical compositions and attractive electrochemical properties. In this review, comprehensive and detailed compendium regarding the latest developments and breakthroughs of highly promising vanadium oxides-based electrode materials for advanced-performance SIBs are elucidated. The crystal structures, electrochemical performance, structure-property relationships and sodium storage mechanization of various vanadium oxides are discussed. In addition, further improvement strategies, including lattice engineering, nanostructuring design, surface modification and 3D porous architecting, are summarized. Finally, potential directions of resolving emergent challenges and forward prospects on augmenting the performance of vanadium oxides-based electrode materials to facilitate their commercial application in SIBs are proposed. This review provides pioneering understanding of vanadium oxides-based materials and guiding directions for the development viability of future cutting-edge SIBs.     

Recent Progress in Advanced Organic Electrode Materials for Sodium‐Ion Batteries: Synthesis,Mechanisms, Challenges and Perspectives

Rechargeable sodium‐ion batteries (SIBs) are considered attractive alternatives to lithium‐ion batteries for next‐generation sustainable and large‐scale electrochemical energy storage. Organic sodium‐ion batteries (OSIBs) using environmentally benign organic materials as electrodes, which demonstrate high energy/power density and good structural designability, have recently attracted great attention. Nevertheless, the practical applications and popularization of OSIBs are generally restricted by the intrinsic disadvantages related to organic electrodes, such as their low conductivity, poor stability, and high solubility in electrolytes. Here, the latest research progress with regard to electrode materials of OSIBs, ranging from small molecules to organic polymers, is systematically reviewed, with the main focus on the molecular structure design/modification, the electrochemical behavior, and the corresponding charge‐storage mechanism. Particularly, the challenges faced by OSIBs and the effective design strategies are comprehensively summarized from three aspects: function‐oriented molecular design, micromorphology regulation, and construction of organic–inorganic composites. Finally, the perspectives and opportunities in the research of organic electrode materials are discussed.     

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.     

Overcoming the Unfavorable Kinetics of Na3V2(PO4)2F3//SnPx Full‐Cell Sodium‐Ion Batteries for High Specific Energy and Energy Efficiency

In this work, a full‐cell sodium‐ion battery (SIB) with a high specific energy approaching 300 Wh kg^?1 is realized using a sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃, NVPF) cathode and a tin phosphide (SnP_x) anode, despite both electrode materials having greatly unbalanced specific capacities. The use of a cathode employing an areal loading more than eight times larger than that of the anode can be achieved by designing a nanostructured nanosized NVPF (n‐NVPF) cathode with well‐defined particle size, porosity, and conductivity. Furthermore, the high rate capability and high potential window of the full‐cell can be obtained by tuning the Sn/P ratio (4/3, 1/1, and 1/2) and the nanostructure of an SnP_x/carbon composite anode. As a result, the full‐cell SIBs employing the nanostructured n‐NVPF cathode and the SnP_x/carbon composite anode (Sn/P = 1/1) exhibit outstanding specific energy (≈280 Wh kg^?1_{(cathode+anode)}) and energy efficiency (≈78%); furthermore, the results are comparable to those of state‐of‐the‐art lithium‐ion batteries.     

Recent Tactics and Advances in the Application of Metal Sulfides as High-Performance Anode Materials for Rechargeable Sodium-Ion Batteries

The successful development of post-lithium technologies depends on two key elements: performance and economy. Because sodium-ion batteries (SIBs) can potentially satisfy both requirements, they are widely considered the most promising replacement for lithium-ion batteries (LIBs) due to the similarity between the electrochemical processes and the abundance of sodium-based resources. Among various SIB anode materials, metal sulfides are most extensively studied as materials for high-performance electrodes due to the versatility of their synthesis procedure, utilization potential, and high sodiation capacity. Herein, some of the most effective strategies aimed at effectively alleviating the performance shortcomings of these materials from the materials engineering/design perspective are summarized. In terms of facilitating ion transport in SIBs, which represents one of the most critical aspects of their performance, a specific family of strategies related to a particular operational mechanism is considered rather than categorizing based-on individual sulfide materials. In the foreseeable future, the development of highly functional SIBs electrode materials and utilization of metal sulfides will become highly relevant due to their stability and performance characteristics. Therefore, it is anticipated that this review will guide further research and facilitate the realization of various applications of sulfide-based high-performance rechargeable batteries.     

Rapid Pseudocapacitive Sodium‐Ion Response Induced by 2D Ultrathin Tin Monoxide Nanoarrays

Nanostructured tin‐based anodes are promising for both lithium and sodium ion batteries (LIBs and SIBs), but their performances are limited by the rate capability and long‐term cycling stability. Here, ultrathin SnO nanoflakes arrays are in situ grown on highly conductive graphene foam/carbon nanotubes substrate, forming a unique, flexible, and binder‐free 3D hybrid structure electrode. This electrode exhibits an excellent Na⁺ storage capacity of 580 mAh g^?1 at 0.1 A g^?1, and to the best of our knowledge, has the longest‐reported high‐rate cycling (1000 cycles at 1 A g^?1) among tin‐based SIB anodes. Compared with its LIB performance, the enhanced pseudocapacitive contribution in SIB is proved to be the origin of fast kinetics and long durability of the electrode. Moreover, Raman peaks from the full sodiation product Na₁₅Sn₄ at 75 and 105 cm^?1 are successfully detected and also proved by density functional theory calculations, which could be a promising clue for structure evolution analysis of other tin‐based electrodes.     

A Simple Electrode‐Level Chemical Presodiation Route by Solution Spraying to Improve the Energy Density of Sodium‐Ion Batteries

The formation of a solid electrolyte interface (SEI) on the surface of a carbon anode consumes the active sodium ions from the cathode and reduces the energy density of sodium‐ion batteries (SIBs). Herein, a simple electrode‐level presodiation strategy by spraying a sodium naphthaline (Naph‐Na) solution onto a carbon electrode is reported, which compensates the initial sodium loss and improves the energy density of SIBs. After presodiation, an SEI layer is preformed on the surface of carbon anode before battery cycling. It is shown that a large irreversible capacity of 60 mAh g^?1 is replenished and 20% increase of the first‐cycle Coulombic efficiency is achieved for a hard carbon anode using this presodiation strategy, and the energy density of a Na_0.9[Cu_0.22Fe_0.30Mn_0.48]O₂||carbon full cell is increased from 141 to 240 Wh kg^?1 by using the presodiated carbon anode. This simple and scalable electrode‐level chemical presodiation route also shows generality and value for the presodiation of other anodes in SIBs.     

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.     

MXene‐Bonded Flexible Hard Carbon Film as Anode for Stable Na/K‐Ion Storage

Hard carbon (HC) is a promising anode material for sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), but the volume change during the insertion/extraction of Na⁺ or K⁺ limits the cycle life, especially for PIBs due to the large ion size of K⁺. Moreover, the conventional anodes fabricated through the coating method cannot satisfy the requirement of flexible devices. Here, it is shown that 2D carbide flakes of Ti₃C₂T_x MXene can be used as multifunctional conductive binders for flexible HC electrodes. The use of MXene nanosheets eliminates the need for all the electrochemically inactive components in the conventional polyvinylidene fluoride–bonded HC electrode, including polymer binders, conductive additives, and current collectors. In MXene‐bonded HC electrodes, conductive and hydrophilic MXene 2D nanosheets construct a 3D network, which can effectively stabilize the electrode structure and accommodate the volume expansion of HC during the charge/discharge process, leading to an enhanced electrode capacity and excellent cycle performance as anodes for both SIBs and PIBs. Benefiting from the 3D conductive network, the MXene‐bonded HC film electrodes also present improved rate capability, indicating MXene is a very promising multifunctional binder for next‐generation flexible secondary rechargeable batteries.     

Freestanding Metallic 1T MoS2 with Dual Ion Diffusion Paths as High Rate Anode for Sodium‐Ion Batteries

This work studies for the first time the metallic 1T MoS₂ sandwich grown on graphene tube as a freestanding intercalation anode for promising sodium‐ion batteries (SIBs). Sodium is earth‐abundant and readily accessible. Compared to lithium, the main challenge of sodium‐ion batteries is its sluggish ion diffusion kinetic. The freestanding, porous, hollow structure of the electrode allows maximum electrolyte accessibility to benefit the transportation of Na⁺ ions. Meanwhile, the metallic MoS₂ provides excellent electron conductivity. The obtained 1T MoS₂ electrode exhibits excellent electrochemical performance: a high reversible capacity of 313 mAh g^?1 at a current density of 0.05 A g^?1 after 200 cycles and a high rate capability of 175 mAh g^?1 at 2 A g^?1. The underlying mechanism of high rate performance of 1T MoS₂ for SIBs is the high electrical conductivity and excellent ion accessibility. This study sheds light on using the 1T MoS₂ as a novel anode for SIBs.     

Size Effects in Sodium Ion Batteries

Sodium ion batteries (SIBs) are promising candidates for large-scale energy storage owing to the abundant sodium resources and low cost. The larger Na⁺ radius (compared to Li⁺) usually leads to sluggish reaction kinetics and huge volume expansion. One of the efficient strategies is to reduce the size of electrode materials or the components of electrolytes to a suitable scale where size effect begin to emerge, leading to the improved or varied thermodynamics, kinetics, and mechanisms of sodium storage. However, only a few systematic reviews address size effects in SIBs, which requires further attention urgently. Herein, after a brief discussion of the general size effect, the size-related kinetics, thermodynamics (equilibrium voltage and morphology), and sodium storage mechanisms (phase transition, conversion reaction, interfacial, and nanopore storage) of electrode materials are presented. The size effect on liquid, polymer, and inorganic solid-state electrolytes are discussed as well, including the size of solvent molecules, Na salts, and inorganic fillers. Finally, neutral and adverse size effects are discussed, and some useful strategies are proposed to overcome them. The deep insights into the size effect will provide instructive guidelines for developing SIBs and other new energy storage systems.     

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.     

3D Ordered Porous Hybrid of ZnSe/N-doped Carbon with Anomalously High Na+ Mobility and Ultrathin Solid Electrolyte Interphase for Sodium-Ion Batteries

Transition metal selenides have been widely used in alkali metal ion batteries owing to their high specific capacities and low cost. However, their reaction kinetics and structural stability are usually poor during cycling, along with ambiguous differences in Li/Na/K-storage behaviors. Herein, it is revealed that ZnSe possesses better Na⁺-diffusion kinetics (including lower diffusion barrier, smaller activation energy, and higher diffusion coefficients) in comparison with Li⁺ and K⁺, as evidenced by theoretical calculations and electrochemical studies. The architectural designs of ZnSe-based anode, including nitrogen-doped carbon (N,C) and 3D ordered hierarchical pores (3DOHP) to form a 3DOHP ZnSe@N,C hybrid combined with regulating solid electrolyte interphase (SEI), significantly enhance Na⁺ reaction kinetics and accommodate volume changes. The resulting 3DOHP ZnSe@N,C electrodes exhibit outstanding rate capability and good cycling stability (241.6 mAh g⁻¹ in sodium-ion batteries (SIBs) at 10 A g⁻¹ after 800 cycles), originating from improved electrical conductivity and shortened ion diffusion paths, accompanied by ultrathin and stable SEI with less Na₂CO₃/NaF in organic components and boosted Na₂Se adsorption as sodiation. Moreover, the Na-storage mechanism in 3DOHP ZnSe@N,C hybrid is further revealed by in situ studies. Accordingly, this study provides a new perspective for designing high-performance electrode materials for SIBs.     

Recent Advances and Optimization Strategies on the Electrolytes for Hard Carbon and P-Based Sodium-Ion Batteries

Hard carbons (HCs) and P-based materials (including red phosphorus, black phosphorus, and metal phosphides) represent the most promising anodes for the commercialization of sodium ion batteries. The electrochemical performance of SIBs is determined not only by the structure of electrode materials, but it is also highly dependent on the matched electrolyte. In this review, the optimization, the matching strategies and the advanced characterization techniques of the electrolytes system for HCs and P-based anodes are discussed based on the recent advance. The effective optimization strategies are summarized from three aspects: sodium salts, solvents, and additives. The challenges and perspectives are also discussed in this review.     

CoS Quantum Dot Nanoclusters for High‐Energy Potassium‐Ion Batteries

Potassium‐ion batteries (PIBs) are a promising alternative to lithium‐ion batteries because potassium is an abundant natural resource. To date, PIBs are in the early stages of exploration and only a few anode materials have been investigated. This study reports a cobalt sulfide and graphene (CoS@G) composite as anode electrode for PIBs for the first time. The composite features interconnect quantum dots of CoS nanoclusters uniformly anchored on graphene nanosheets. The coexistence of CoS quantum dot nanoclusters and graphene nanosheets endows the composite with large surface area, highly conductive network, robust structural stability, and excellent electrochemical energy storage performance. An unprecedented capacity of 310.8 mA h g^?1 at 500 mA g^?1 is obtained after 100 cycles, with a rate capability better than an equivalent sodium‐ion batteries (SIBs). This work provides the evidence that PIBs can be a promising alternative to SIBs, especially at high charge–discharge rates. The development of the CoS@G anode material also provides the basis of expanding the library of suitable anode materials for PIBs.     

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.     

Engineering Hierarchical Hollow Nickel Sulfide Spheres for High‐Performance Sodium Storage

Sodium‐ion batteries (SIBs) are considered as promising alternatives to lithium‐ion batteries (LIBs) for energy storage due to the abundance of sodium, especially for grid distribution systems. The practical implementation of SIBs, however, is severely hindered by their low energy density and poor cycling stability due to the poor electrochemical performance of the existing electrodes. Here, to achieve high‐capacity and durable sodium storage with good rate capability, hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters. The formation mechanism of this unique structure is systematically investigated, which is clearly revealed to be Ostwald ripening mechanism on the basis of the time‐dependent morphology evolution. The hierarchical hollow structure provides sufficient electrode/electrolyte contact, shortened Na⁺ diffusion pathways, and high strain‐tolerance capability. The hollow NiS spheres deliver high reversible capacity (683.8 mAh g^?1 at 0.1 A g^?1), excellent rate capability (337.4 mAh g^?1 at 5 A g^?1), and good cycling stability (499.9 mAh g^?1 with 73% retention after 50 cycles at 0.1 A g^?1).     

Enhanced Reversible Sodium‐Ion Intercalation by Synergistic Coupling of Few‐Layered MoS2 and S‐Doped Graphene

Sodium‐ion batteries (SIBs) are regarded as the best alternative to lithium‐ion batteries due to their low cost and similar Na⁺ insertion chemistry. It is still challenging but greatly desired to design and develop novel electrode materials with high reversible capacity, long cycling life, and good rate capability toward high‐performance SIBs. This work demonstrates an innovative design strategy and a development of few‐layered molybdenum disulfide/sulfur‐doped graphene nanosheets (MoS₂/SG) composites as the SIB anode material providing a high specific capacity of 587 mA h g^?1 calculated based on the total composite mass and an extremely long cycling stability over 1000 cycles at a current density of 1.0 A g^?1 with a high capacity retention of ≈85%. Systematic characterizations reveal that the outstanding performance is mainly attributed to the unique and robust composite architecture where few‐layered MoS₂ and S‐doped graphene are intimately bridged at the hetero‐interface through a synergistic coupling effect via the covalently doped S atoms. The design strategy and mechanism understanding at the molecular level outlined here can be readily applied to other layered transition metal oxides for SIBs anode and play a key role in contributing to the development of high‐performance SIBs.     

Unleashing the Potential of Sodium-Ion Batteries: Current State and Future Directions for Sustainable Energy Storage

Rechargeable sodium-ion batteries (SIBs) are emerging as a viable alternative to lithium-ion battery (LIB) technology, as their raw materials are economical, geographically abundant (unlike lithium), and less toxic. The matured LIB technology contributes significantly to digital civilization, from mobile electronic devices to zero electric-vehicle emissions. However, with the increasing reliance on renewable energy sources and the anticipated integration of high-energy-density batteries into the grid, concerns have arisen regarding the sustainability of lithium due to its limited availability and consequent price escalations. In this context, SIBs have gained attention as a potential energy storage alternative, benefiting from the abundance of sodium and sharing electrochemical characteristics similar to LIBs. Furthermore, high-entropy chemistry has emerged as a new paradigm, promising to enhance energy density and accelerate advancements in battery technology to meet the growing energy demands. This review uncovers the fundamentals, current progress, and the views on the future of SIB technologies, with a discussion focused on the design of novel materials. The crucial factors, such as morphology, crystal defects, and doping, that can tune electrochemistry, which should inspire young researchers in battery technology to identify and work on challenging research problems, are also reviewed.