The State and Challenges of Anode Materials Based on Conversion Reactions for Sodium Storage

Confining WS2 hierarchical structures into carbon core-shells for enhanced sodium storage

The growing demand for clean energy has heightened interest in sodium-ion batteries (SIBs) as promising candidates for large-scale energy storage. However, the sluggish reaction kinetics and significant volumetric changes in anode materials present challenges to the electrochemical performance of SIBs. This work introduces a hierarchical structure where WS2 is confined between an inner hard carbon core and an outer nitrogen-doped carbon shell, forming HC@WS2@NCs core-shell structures as anodes for SIBs. The inner hard carbon core and outer nitrogen-doped carbon shell anchor WS2, enhancing its structural integrity. The highly conductive carbon materials accelerate electron transport during charge/discharge, while the rationally constructed interfaces between carbon and WS2 regulate the interfacial energy barrier and electric field distribution, improving ion transport. This synergistic interaction results in superior electrochemical performance: the HC@WS2@NCs anode delivers a high capacity of 370 mAh g-1 at 0.2 A/g after 200 cycles and retains261 mAh g-1 at 2 A/g after 2000 cycles. In a full battery with a Na3V2(PO4)3 cathode, the Na3V2(PO4)3//HC@WS2@NC full-cell achieves an impressive initial capacity of 220 mAh g-1 at 1 A/g. This work provides a strategic approach for the systematic development of WS2-based anode materials for SIBs.

Sodium-Ion Battery at Low Temperature: Challenges and Strategies

Sodium-ion batteries (SIBs) have garnered significant interest due to their potential as viable alternatives to conventional lithium-ion batteries (LIBs), particularly in environments where low-temperature (LT) performance is crucial. This paper provides a comprehensive review of current research on LT SIBs, focusing on electrode materials, electrolytes, and operational challenges specific to sub-zero conditions. Recent advancements in electrode materials, such as carbon-based materials and titanium-based materials, are discussed for their ability to enhance ion diffusion kinetics and overall battery performance at colder temperatures. The critical role of electrolyte formulation in maintaining battery efficiency and stability under extreme cold is highlighted, alongside strategies to mitigate capacity loss and cycle degradation. Future research directions underscore the need for further improvements in energy density and durability and scalable manufacturing processes to facilitate commercial adoption. Overall, LT SIBs represent a promising frontier in energy storage technology, with ongoing efforts aimed at overcoming technical barriers to enable widespread deployment in cold-climate applications and beyond.

Engineering (FeSn)/S nanocubes heterojunctions for improved sodium ion battery performance

Transition metal sulfides have emerged as compelling anode materials for sodium-ion batteries (SIBs), leveraging their abundant elemental reserves and high theoretical capacities. However, the reaction of sulfur with Na ions is usually accompanied by significant volume dilation, which hinders their further development and application. Hence, constructing bimetallic sulfide (FeSn)/S for SIBs anode material greatly alleviates the circulation attenuation caused by volume expansion. Through constructing bimetallic heterojunction materials from nanocube precursors, the (FeSn)/S anode material retains a high specific capacity of 578 mAh/g at an intense current density of 2 A/g after 1000 cycles, and exhibits an great rate capability, delivering 796 mAh/g at 100 mA/g. The excellent electrochemical performance of the heterojunction material presents a promising solution to the enduring quest for enhanced anode material for SIBs.

A Review of Anode Materials for Dual-Ion Batteries

Distinct from "rocking-chair" lithium-ion batteries (LIBs), the unique anionic intercalation chemistry on the cathode side of dual-ion batteries (DIBs) endows them with intrinsic advantages of low cost, high voltage, and eco-friendly, which is attracting widespread attention, and is expected to achieve the next generation of large-scale energy storage applications. Although the electrochemical reactions on the anode side of DIBs are similar to that of LIBs, in fact, to match the rapid insertion kinetics of anions on the cathode side and consider the compatibility with electrolyte system which also serves as an active material, the anode materials play a very important role, and there is an urgent demand for rational structural design and performance optimization. A review and summarization of previous studies will facilitate the exploration and optimization of DIBs in the future. Here, we summarize the development process and working mechanism of DIBs and exhaustively categorize the latest research of DIBs anode materials and their applications in different battery systems. Moreover, the structural design, reaction mechanism and electrochemical performance of anode materials are briefly discussed. Finally, the fundamental challenges, potential strategies and perspectives are also put forward. It is hoped that this review could shed some light for researchers to explore more superior anode materials and advanced systems to further promote the development of DIBs.

Synthesis of Graphitic Carbon Coated ZnPS3 and its Superior Electrochemical Properties for Lithium and Sodium Ion Storage

Graphitic carbon-coated ZnPS3 is prepared via direct phosphosulfurization and high energy mechanical milling (HEMM) with multiwall carbon nanotubes (MWCNTs) and first introduced as an anode for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). The HEMM process with MWCNTs reduces the particle size of as-synthesized ZnPS3 bulk to 100-500 nm and yields the ≈5 nm thick graphitic carbon coated ZnPS3 nanoparticles, which are the nanocomposites of 5 nm sized nanocrystallites embedded in the amorphous matrix. The ZnPS3 electrode undergoes the combined conversion and alloying reactions with Li and Na ions and exhibits high initial discharge and charge capacities in both LIBs and SIBs. The graphitic carbon-coated ZnPS3 electrode exhibits excellent high-rate capability and long-term cyclability. The superior electrochemical properties can be attributed to high electrical conductivity, high Li ion mobility, and high reversibility and structural stability derived from the graphitic carbon-coated nanoparticles. This study demonstrates that the novel graphitic carbon-coated ZnPS3 is a promising anode material for both LIBs and SIBs and the graphitic carbon coating methodology by HEMM is expected to apply to the various metal oxides, sulfides, and phosphides.

Multifunctional Separator Enables High-Performance Sodium Metal Batteries in Carbonate-Based Electrolytes

Sodium metal has become one of the most promising anodes for next-generation cheap and high-energy-density metal batteries; however, challenges caused by the uncontrollable sodium dendrite growth and fragile solid electrolyte interphase (SEI) restrict their large-scale practical applications in low-cost and wide-voltage-window carbonate electrolytes. Herein, a novel multifunctional separator with lightweight and high thinness is proposed, assembled by the cobalt-based metal-organic framework nanowires (Co-NWS), to replace the widely applied thick and heavy glass fiber separator. Benefitting from its abundant sodiophilic functional groups and densely stacked nanowires, Co-NWS not only exhibits outstanding electrolyte wettability and effectively induces uniform Na+ ion flux as a strong ion redistributor but also favors constructing the robust N,F-rich SEI layer. Satisfactorily, with 10 µL carbonate electrolyte, a Na|Co-NWS|Cu half-cell delivers stable cycling (over 260 cycles) with a high average Coulombic efficiency of 98%, and the symmetric cell shows a long cycle life of more than 500 h. Remarkably, the full cell shows a long-term life span (over 1500 cycles with 92% capacity retention) at high current density in the carbonate electrolyte. This work opens up a strategy for developing dendrite-free, low-cost, and long-life-span sodium metal batteries in carbonate-based electrolytes.

Unveiling the Nature of Superior Sodium Storage in the CoSe2/rGO Nanocomposite

Sodium-ion batteries (SIBs) are considered the most promising alternatives to lithium-ion batteries (LIBs) due to the abundant availability of sodium and their cost-effectiveness. Transition metal selenides (TMSes) are considered promising anodes for SIBs due to their economic efficiency and high theoretical capacity. Nevertheless, overcoming the challenges of sluggish reaction kinetics and severe structural damage is crucial to improving cycle life and rate capability. Herein, a simple microwave hydrothermal process was used to synthesize a nanocomposite of CoSe2 nanoparticles uniformly anchored on reduced graphene oxide nanosheets (CoSe2/rGO). The influences of rGO on the structure and electrochemical performance and Na+ diffusion kinetics are investigated through a series of characterization and electrochemical tests. The resulting CoSe2/rGO nanocomposite exhibits a remarkable initial specific capacity of 544 mAh g-1 at 0.5 A g-1, impressive rate capability (368 mAh g-1 at 20 A g-1), and excellent cycle life and maintains 348 mAh g-1 at 5 A g-1 over 1200 cycles. In addition, the in situ electrochemical impedance spectroscopy (EIS), ex situ X-ray diffraction (XRD), and transmission electron microscopy (TEM) tests are selected to further investigate the sodium storage mechanism.

SnSe Nanosheet Array on Carbon Cloth as a High-Capacity Anode for Sodium-Ion Batteries

Binder-free electrodes offer a great opportunity for developing high-performance sodium-ion batteries (SIBs) aiming at the application in energy storage devices. Tin selenide (SnSe) is considered to be a promising anode material for SIBs owing to its high theoretical capacity (780 mA h g-1). In this work, a SnSe nanosheet array (SnSe NS) on a carbon cloth is prepared using a vacuum thermal evaporation method. The as-prepared SnSe NS electrode does not have metal current collectors, binders, or any conductive additives. In comparison with the electrode of SnSe blocky particles (SnSe BP), the SnSe NS electrode delivers a higher initial charge capacity of 713 mA h g-1 at a current density of 0.1C and maintains a higher charge capacity of 410 mA h g-1 after 50 cycles. Furthermore, the electrochemical behaviors of the SnSe NS electrode are determined via pseudocapacitance and electrochemical impedance spectroscopy measurements, indicating a faster kinetic process of the SnSe NS electrode compared to that of the SnSe BP. Operando X-ray diffraction measurements prove that the SnSe NS exhibits better phase reversibility than the SnSe BP. After the cycles, the SnSe NS electrode still maintains its particular structure. This work provides a feasible method to prepare SnSe nanostructures with high capacity and improved sodium ion diffusion ability.

Double-Walled NiTeSe-NiSe2 Nanotubes Anode for Stable and High-Rate Sodium-Ion Batteries

Electrodes made of composites with heterogeneous structure hold great potential for boosting ionic and charge transfer and accelerating electrochemical reaction kinetics. Herein, hierarchical and porous double-walled NiTeSe-NiSe₂ nanotubes are synthesized by a hydrothermal process assisted in situ selenization. Impressively, the nanotubes have abundant pores and multiple active sites, which shorten the ion diffusion length, decrease Na⁺ diffusion barriers, and increase the capacitance contribution ratio of the material at a high rate. Consequently, the anode shows a satisfactory initial capacity (582.5 mA h g^-1 at 0.5 A g^-1 ), a high-rate capability, and long cycling stability (1400 cycles, 398.6 mAh g^-1 at 10 A g^-1 , 90.5% capacity retention). Moreover, the sodiation process of NiTeSe-NiSe₂ double-walled nanotubes and underlying mechanism of the enhanced performance are revealed by in situ and ex situ transmission electron microscopy and theoretical calculations.

Combustion Activation Induced Solid-State Synthesis for N, B Co-Doped Carbon/Zinc Borate Anode with a Boosting of Sodium Storage Performance

Zinc borates have merits of low voltage polarization and suitable redox potential, but usually suffer from low rate capability and poor cycling life, as an emerging anode candidate for Na+ storage. Here, a new intercalator-guided synthesis strategy is reported to simultaneously improve rate capability and stabilize cycling life of N, B co-doped carbon/zinc borates (CBZG). The strategy relies on a uniform dispersion of precursors and simultaneously stimulated combustion activation and solid-state reactions capable of scalable preparation. The Na+ storage mechanism of CBZG is studied: 1) ex situ XRD and XPS demonstrate two-step reaction sequence of Na+ storage: Zn6 O(OH)(BO3 )3 +Na+ +e- ↔3ZnO+Zn3 B2 O6 +NaBO2 +0.5H2 ①, Zn3 B2 O6 +6Na+ +6e- ↔3Zn+3Na2 O+B2 O3 ②; reaction ① is irreversible in ether-based electrolyte while reversible in ester-based electrolyte. 2) Electrochemical kinetics reveal that ether-based electrolyte possesses faster Na+ storage than ester-based electrolyte. The composite demonstrates an excellent capacity of 437.4 mAh g-1 in a half-cell, together with application potential in full cells (discharge capacity of 440.1 mAh g-1 and stable cycle performance of 2000 cycles at 5 A g-1 ). These studies will undoubtedly provide an avenue for developing novel synthetic methods of carbon-based borates and give new insights into the mechanism of Na+ storage in ether-based electrolyte for the desirable sodium storage.

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

Metal Selenides Anode Materials for Sodium Ion Batteries: Synthesis, Modification, and Application

The powerful and rapid development of lithium-ion batteries (LIBs) in secondary batteries field makes lithium resources in short supply, leading to rising battery costs. Under the circumstances, sodium-ion batteries (SIBs) with low cost, inexhaustible sodium reserves, and analogous work principle to LIBs, have evolved as one of the most anticipated candidates for large-scale energy storage devices. Thereinto, the applicable electrode is a core element for the smooth development of SIBs. Among various anode materials, metal selenides (MSe_x ) with relatively high theoretical capacity and unique structures have aroused extensive interest. Regrettably, MSe_x suffers from large volume expansion and unwished side reactions, which result in poor electrochemistry performance. Thus, strategies such as carbon modification, structural design, voltage control as well as electrolyte and binder optimization are adopted to alleviate these issues. In this review, the synthesis methods and main reaction mechanisms of MSe_x are systematically summarized. Meanwhile, the major challenges of MSe_x and the corresponding available strategies are proposed. Furthermore, the recent research progress on layered and nonlayered MSe_x for application in SIBs is presented and discussed in detail. Finally, the future development focuses of MSe_x in the field of rechargeable ion batteries are highlighted.

Rich Self-Generated Phase Boundaries of Heterostructured VS4 /Bi2 S3 @C Nanorods for Long Lifespan Sodium-Ion Batteries

Rationally designing on sundry multiphase compounds has come into the spotlight for sodium-ion batteries (SIBs) due to enhanced structural stability and improved electrochemical performances. Nevertheless, there is still a lack of thorough understanding of the reaction mechanism of high-active phase boundaries existing between multiphase compounds. Here, a VS₄ /Bi₂ S₃ @C composite anode for SIBs with rich phase boundaries in heterostructure is successfully synthesized. In situ X-ray diffraction analyses demonstrate a multistep redox mechanism in the heterostructures and ex situ transmission electron microscopy results confirm that tremendous self-generated phase boundaries are obtained and well-maintained during cycling, dramatically leading to stable reaction interfaces and better structural integrity. Combining experimental and theoretical results, a self-built-in electric field forming between phase boundaries acts as a dominate driving force for Na⁺ transport kinetics. Benefiting from the fast reaction kinetics of phase boundaries, the heterojunction provides an efficient approach to avoid abnormal voltage failure. As expected, the VS₄ /Bi₂ S₃ @C heterostructure displays superior sodium storage performances, especially an excellent long-term cycling stability (379.0 mAh g^-1 after 1800 cycles at a current density up to 2 A g^-1 ). This work confirms a critical role of phase boundaries on superior reversibility and structural stability, and provides a strategy for analogous conversion/alloying-type anodes.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

A unique two-phase heterostructure with cubic NiSe2 and orthorhombic NiSe2 for enhanced lithium ion storage and electrocatalysis

Two-phase heterostructures have received tremendous attention in energy-related fields as high-performance electrode materials. However, heterogeneous interfaces are usually constructed by introducing foreign elements, which disturbs the investigation of the intrinsic effect of the two-phase heterostructure. Herein, unique heterostructures constructed with orthorhombic NiSe₂ and cubic NiSe₂ phases are developed, which are embedded in in situ formed porous carbon from metal-organic frameworks (MOFs) (O/C-NiSe₂@C). Precisely-controlled selenylation of MOFs is crucial for the formation of the O/C-NiSe₂ heterostructure. The heterogeneous interfaces with lattice dislocations and charge distribution are conducive to the high-speed transfer of electrons and ions during electrochemical processes, so as to improve the electrochemical reaction kinetics for lithium-ion storage and the hydrogen evolution reaction (HER). When used as the anode of lithium-ion batteries (LIBs), O/C-NiSe₂@C shows a superior electrochemical performance to the counterparts with only the cubic phase (C-NiSe₂@C), in view of the cycling performance (719.3 mA h g^-1 at 0.1 A g^-1 for 100 cycles; 456.3 mA h g^-1 at 1 A g^-1 for 1000 cycles) and rate capabilities (344.8 mA h g^-1 at 4 A g^-1). Furthermore, O/C-NiSe₂@C also exhibits better HER properties than C-NiSe₂@C, that is, much lower overpotentials of 154 mV and 205 mV in 0.5 M H₂SO₄ and 1 M KOH, respectively, at 10 mA cm^-2, a smaller Tafel slope as well as stable electrocatalytic activities for 2000 cycles/10 h. Preliminary observations indicate that the unique orthorhombic/cubic two-phase heterostructure could significantly improve the electrochemical performance of NiSe₂ without additional modifications such as doping, suggesting the O/C-NiSe₂ heterostructure as a promising bifunctional electrode for energy conversion and storage applications.

Insights into Electrochemical Processes of Hollow Octahedral Co3Se4@rGO for High-Rate Sodium Ion Storage

Sodium ion batteries (SIBs), as an alternative and promising energy storage system, have attracted considerable attention due to the abundant reserves and low cost of sodium. However, it remains a great challenge to achieve high capacity and rate capability required for practical applications. Herein, hollow octahedral Co₃Se₄ particles encapsulated in reduced graphene oxide (Co₃Se₄@rGO) were designed and synthesized and exhibited excellent electrochemical performances as anodes of SIBs, especially rate capability. Sodiation/desodiation processes and involved mechanisms were investigated by using in situ TEM and in situ XRD. During sodiation, a crystalline Na₂Se layer with numerous amorphous fine Co nanoparticles dispersed on it was observed to appear on the surface of the original Co₃Se₄@rGO particles, and the movable Co-Na₂Se composites further migrated to the rGO network with high electron/ion dual conductivity, resulting in ultrafast sodium storage kinetics and remarkable rate performance of the Co₃Se₄@rGO anode evidenced by delivering a discharge capacity of 229.3 mAh g^-1 at a large current density of 50 A g^-1. Our findings reveal the fundamental mechanism behind the enhanced performance of the Co₃Se₄@rGO anode and offer valuable insights into the rational design of electrode materials for high-performance SIBs.

Binary self-assembly of ordered Bi4Se3/Bi2O2Se lamellar architecture embedded into CNTs@Graphene as a binder-free electrode for superb Na-Ion storage

With the development of various flexible electronic devices, flexible energy storage devices have attracted more research attention. Binder-free flexible batteries, without a current collector, binder, and conductive agent, have higher energy density and lower manufacturing costs than traditional sodium-ion batteries (SIBs). However, preparing binder-free anodes with high electrochemical performance and flexibility remains a great challenge. In this study, a binary self-assembly composite of an ordered Bi₄Se₃/Bi₂O₂Se lamellar architecture wrapped by carbon nanotubes (CNTs) was embedded in graphene with strong interfacial interaction to form Bi₂O₂Se/Bi₄Se₃@CNTs@rGO (BCG), which was used as a binder-free anode for SIBs. A unique "one-changes-into-two" phenomenon was observed: the layered Bi₂Se₃ was transformed into a unique layered Bi₄Se₃/Bi₂O₂Se heterojunction structure, which not only provides more electrochemical channels but also reduces internal stress to improve the stability of the material structure. BCG-2 showed excellent sodium-ion storage, delivering a reversible capacity of 346 mA h/g at 100 mA/g and maintaining a capacity of 235 mA h/g over 50 cycles. Even at a high current density of 1 A/g, it retains a capacity of 105 mA h/g after 1000 cycles. This unique design concept can also be employed in synthesizing other binder-free electrodes to improve their properties.

Sheet-Like Stacking SnS2/rGO Heterostructures as Ultrastable Anodes for Lithium-Ion Batteries

SnS₂-based materials have attracted considerable attention in energy storage and conversion owing to their high lithium activity and theoretical capacity. However, the practical application is severely limited by the low coulombic efficiency and short cycle life due to irreversible side reactions, low conductivity, and serious pulverization in the discharge/charge process. In this study, sheet-like stacking SnS₂/reduced graphene oxide (rGO) heterostructures were developed using a facile solvothermal method. It was found that the composites between SnS₂ nanoplates and rGO nanosheets are closely coupled through van der Waals interactions, providing efficient electron/ion paths to ensure high electrical conductivity and sufficient buffer space to alleviate volume expansion. Therefore, the SnS₂/rGO heterostructure anode can obtain a high capacity of 840 mA h g^-1 after 120 cycles at a current density of 200 mA g^-1 and maintain a capacity of 450 mA h g^-1 after 1000 cycles at 1000 mA g^-1. In situ X-ray diffraction tests showed that SnS₂/rGO undergoes typical initial intercalation, conversion, and subsequent alloying reactions during the first discharge, and most of the reactions are dealloying/alloying in the subsequent cycles. The galvanostatic intermittent titration technique showed that the diffusion of lithium ions in the SnS₂/rGO heterostructures is faster in the intercalation and conversion reactions than in the alloying reactions. These observations help to clarify the reaction mechanism and ion diffusion behavior in the SnS₂ anode materials, thus providing valuable insights for improving the energy efficiency of lithium-ion batteries.

Unveiling the Electrochemical Mechanism of High-Capacity Negative Electrode Model-System BiFeO3 in Sodium-Ion Batteries: An In Operando XAS Investigation

Careful development and optimization of negative electrode (anode) materials for Na-ion batteries (SIBs) are essential, for their widespread applications requiring a long-term cycling stability. BiFeO₃ (BFO) with a LiNbO₃-type structure (space group R3c) is an ideal negative electrode model system as it delivers a high specific capacity (770 mAh g^-1), which is proposed through a conversion and alloying mechanism. In this work, BFO is synthesized via a sol-gel method and investigated as a conversion-type anode model-system for sodium-ion half-cells. As there is a difference in the first and second cycle profiles in the cyclic voltammogram, the operating mechanism of charge-discharge is elucidated using in operando X-ray absorption spectroscopy. In the first discharge, Bi is found to contribute toward the electrochemical activity through a conversion mechanism (Bi³⁺ → Bi⁰), followed by the formation of Na-Bi intermetallic compounds. Evidence for involvement of Fe in the charge storage mechanism through conversion of the oxide (Fe³⁺) form to metallic Fe and back during discharging/charging is also obtained, which is absent in previous literature reports. Reversible dealloying and subsequent oxidation of Bi and oxidation of Fe are observed in the following charge cycle. In the second discharge cycle, a reduction of Bi and Fe oxides is observed. Changes in the oxidation states of Bi and Fe, and the local coordination changes during electrochemical cycling are discussed in detail. Furthermore, the optimization of cycling stability of BFO is carried out by varying binders and electrolyte compositions. Based on that, electrodes prepared with the Na-carboxymethyl cellulose (CMC) binder are chosen for optimization of the electrolyte composition. BFO-CMC electrodes exhibit the best electrochemical performance in electrolytes containing fluoroethylene carbonate (FEC) as the additive. BFO-CMC electrodes deliver initial capacity values of 635 and 453 mAh g^-1 in the Na-insertion (discharge) and deinsertion (charge) processes, respectively, in the electrolyte composition of 1 M NaPF₆ in EC/DEC (1:1, v/v) with a 2% FEC additive. The capacity values stabilize around 10th cycle and capacity retention of 73% is observed after 60 cycles with respect to the 10th cycle charge capacity.

Sodium-Ion Batteries: Current Understanding of the Sodium Storage Mechanism in Hard Carbons : Optimising properties to speed commercialisation

In recent years, sodium-ion batteries (NIBs) have been explored as an alternative technology to lithium-ion batteries (LIBs) due to their cost-effectiveness and promise in mitigating the energy crisis we currently face. Similarities between both battery systems have enabled fast development of NIBs, however, their full commercialisation has been delayed due to the lack of an appropriate anode material. Hard carbons (HCs) arise as one of the most promising materials and are already used in the first generation of commercial NIBs. Although promising, HCs exhibit lower performance compared to commercial graphite used as an anode in LIBs in terms of reversible specific capacity, operating voltage, initial coulombic efficiency and cycling stability. Nevertheless, these properties vary greatly depending on the HC in question, for example surface area, porosity, degree of graphitisation and defect amount, which in turn are dependent on the synthesis method and precursor used. Optimisation of these properties will bring forward the widespread commercialisation of NIBs at a competitive level with current LIBs. This review aims to provide a brief overview of the current understanding of the underlying reaction mechanisms occurring in the state-of-the-art HC anode material as well as their structure-property interdependence. We expect to bring new insights into the engineering of HC materials to achieve optimal, or at least, comparable electrochemical performance to that of graphite in LIBs.

Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release

Bulk isocubanite (CuFe₂S₃) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFe₂S₃ exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g^-1 for more than 1000 cycles at a current rate of 0.5 A g^-1 (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFe₂S₃ is converted to form crystalline CuFeS₂ and nanocrystalline NaFe_1.5S₂ simultaneously. By increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, followed by the reduction of Fe²⁺ to amorphous Fe⁰ while reflections of nanocrystalline Na₂S appear. During charging up to -5 Na/f.u., the intermediate NaFe_1.5S₂ appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFe_1.5S₂ plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that Cu_xS phases play an important role in the region of constant specific capacity.

Conversion-Alloying Anode Materials for Sodium Ion Batteries

The past decade has witnessed a rapidly growing interest toward sodium ion battery (SIB) for large-scale energy storage in view of the abundance and easy accessibility of sodium resources. Key to addressing the remaining challenges and setbacks and to translate lab science into commercializable products is the development of high-performance anode materials. Anode materials featuring combined conversion and alloying mechanisms are one of the most attractive candidates, due to their high theoretical capacities and relatively low working voltages. The current understanding of sodium-storage mechanisms in conversion-alloying anode materials is presented here. The challenges faced by these materials in SIBs, and the corresponding improvement strategies, are comprehensively discussed in correlation with the resulting electrochemical behavior. Finally, with the guidance and perspectives, a roadmap toward the development of advanced conversion-alloying materials for commercializable SIBs is created.

The Emerging of Aqueous Zinc-Based Dual Electrolytic Batteries

As high performance and safety alternatives to the batteries with organic electrolytes, aqueous zinc-based batteries are still far from satisfactory in practical use because of the limitation of the intercalation reaction mechanism and the strict requirements for the cathodes. Very recently, zinc-based dual electrolytic batteries (DEBs), where the cathode and anode are both based on reversible electrolytic reactions, are emerging. It features with electrode-free configuration, thus avoiding the preliminary active materials or electrode fabrication procedures. Meanwhile, the new battery chemistry typically possesses a high specific capacity, output voltage, faster reaction rates, and long cycling life. Herein, the advances of the development of various zinc-based DEBs, including Zn-MnO₂ , Zn-Br₂ , and Zn-I₂ DEBs, are systematically summarized. This review will focus on the working mechanisms of these batteries and how the decoupling catholyte and anolyte affect their output voltages. The perspectives of the opportunities and challenges are also suggested in the aspects of protecting zinc anode, enhancing volumetric energy density, suppressing fast self-discharge, and developing multifunctional integrated zinc-based DEBs.

Realizing High-Performance Li/Na-Ion Half/Full Batteries via the Synergistic Coupling of Nano-Iron Sulfide and S-doped Graphene

Iron sulfide (FeS) anodes are plagued by severe irreversibility and volume changes that limit cycle performances. Here, a synergistically coupled hybrid composite, nanoengineered iron sulfide/S-doped graphene aerogel, was developed as high-capacity anode material for Li/Na-ion half/full batteries. The rational coupling of in situ generated FeS nanocrystals and the S-doped rGO aerogel matrix boosted the electronic conductivity, Li⁺ /Na⁺ diffusion kinetics, and accommodated the volume changes in FeS. This anode system exhibited excellent long-term cyclability retaining high reversible capacities of 422 (1100 cycles) and 382 mAh g^-1 (1600 cycles), respectively, for Li⁺ and Na⁺ storage at 5 A g^-1 . Full batteries designed with this anode system exhibited 435 (FeS/srGOA||LiCoO₂ ) and 455 mAh g^-1 (FeS/srGOA||Na_0.64 Co_0.1 Mn_0.9 O₂ ). The proposed low-cost anode system is competent with the current Li-ion battery technology and extends its utility for Na⁺ storage.

Expanded MoSe2 Nanosheets Vertically Bonded on Reduced Graphene Oxide for Sodium and Potassium-Ion Storage

The cost-efficient and plentiful Na and K resources motivate the research on ideal electrodes for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Here, MoSe₂ nanosheets perpendicularly anchored on reduced graphene oxide (rGO) are studied as an electrode for SIBs and PIBs. Not only does the graphene network serves as a nucleation substrate for suppressing the agglomeration of MoSe₂ nanosheets to eliminate the electrode fracture but also facilitates the electrochemical kinetics process and provides a buffer zone to tolerate the large strain. An expanded interplanar spacing of 7.9 Å is conducive to fast alkaline ion diffusion, and the formed chemical bondings (C-Mo and C-O-Mo) promote the structure integrity and the charge transfer kinetics. Consequently, MoSe₂@5%rGO exhibits a reversible specific capacity of 458.3 mAh·g^-1 at 100 mA·g^-1, great cyclability with a retention of 383.6 mAh·g^-1 over 50 cycles, and excellent rate capability (251.3 mAh·g^-1 at 5 A·g^-1) for SIBs. For PIBs, a high first specific capacity of 365.5 mAh·g^-1 at 100 mA·g^-1 with a low capacity fading of 51.5 mAh·g^-1 upon 50 cycles and satisfactory rate property are acquired for MoSe₂@10%rGO composite. Ex situ measurements validate that the discharge products are Na₂Se for SIBs and K₅Se₃ for PIBs, and robust chemical bonds boost the structure stability for Na- and K-ion storage. The full batteries are successfully fabricated to verify the practical feasibility of MoSe₂@5%rGO composite.

Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions

It remains a great challenge to explore desirable cathodes for sodium-ion batteries to satisfy the ever-increasing demand for large-scale energy storage systems. In this Letter, we report a NASICON-structured Na₄MnCr(PO₄)₃ cathode with high specific capacity and operation potential. The reversible access of the Mn²⁺/Mn³⁺ (3.75/3.4 V), Mn³⁺/Mn⁴⁺ (4.25/4.1 V), and Cr³⁺/Cr⁴⁺ (4.4/4.3 V vs Na/Na⁺) redox couples in a Na₄MnCr(PO₄)₃ cathode endows a distinct three-electron redox reaction during the insertion/extraction process. The highly stable NASICON structure with a small volume variation upon cycling ensures long-time cycling stability (73.3% capacity retention after 500 cycles within the potential region of 2.5-4.6 V). The impedance analysis and interface characterization indicate that the evolution of a cathode electrolyte interphase at high potential is correlated with the capacity fading, while the robustness of the NASICON framework is redemonstrated.

Monoclinic and Orthorhombic NaMnO2 for Secondary Batteries: A Comparative Study

In this manuscript, we report a detailed physico-chemical comparison between the α- and β-polymorphs of the NaMnO2 compound, a promising material for application in positive electrodes for secondary aprotic sodium batteries. In particular, the structure and vibrational properties, as well as electrochemical performance in sodium batteries, are compared to highlight differences and similarities. We exploit both laboratory techniques (Raman spectroscopy, electrochemical methods) and synchrotron radiation experiments (Fast-Fourier Transform Infrared spectroscopy, and X-ray diffraction). Notably the vibrational spectra of these phases are here reported for the first time in the literature as well as the detailed structural analysis from diffraction data. DFT+U calculations predict both phases to have similar electronic features, with structural parameters consistent with the experimental counterparts. The experimental evidence of antisite defects in the beta-phase between sodium and manganese ions is noticeable. Both polymorphs have been also tested in aprotic batteries by comparing the impact of different liquid electrolytes on the ability to de-intercalated/intercalate sodium ions. Overall, the monoclinic α-NaMnO2 shows larger reversible capacity exceeding 175 mAhg−1 at 10 mAg−1.

Sb2O3 nanoparticles anchored on N-doped graphene nanoribbons as improved anode for sodium-ion batteries

A hybrid nanocomposite of Sb2O3 nanoparticles anchored on N-doped graphene nanoribbons is used as anode in SIBs. These hybrid electrodes demonstrate a high charge transfer and improved microstructure, facilitating the Na+ diffusion in the electrode.

Golden Bristlegrass-Like Hierarchical Graphene Nanofibers Entangled with N-Doped CNTs Containing CoSe2 Nanocrystals at Each Node as Anodes for High-Rate Sodium-Ion Batteries

Golden bristlegrass-like unique nanostructures comprising reduced graphene oxide (rGO) matrixed nanofibers entangled with bamboo-like N-doped carbon nanotubes (CNTs) containing CoSe₂ nanocrystals at each node (denoted as N-CNT/rGO/CoSe₂ NF) are designed as anodes for high-rate sodium-ion batteries (SIBs). Bamboo-like N-doped CNTs (N-CNTs) are successfully generated on the rGO matrixed nanofiber surface, between rGO sheets and mesopores, and interconnected chemically with homogeneously distributed rGO sheets. The defects in the N-CNTs formed by a simple etching process allow the complete phase conversion of Co into CoSe₂ through the efficient penetration of H₂ Se gas inside the CNT walls. The N-CNTs bridge the vertical defects for electron transfer in the rGO sheet layers and increase the distance between the rGO sheets during cycles. The discharge capacity of N-CNT/rGO/CoSe₂ NF after the 10 000th cycle at an extremely high current density of 10 A g^-1 is 264 mA h g^-1 , and the capacity retention measured at the 100th cycle is 89%. N-CNT/rGO/CoSe₂ NF has final discharge capacities of 395, 363, 328, 304, 283, 263, 246, 223, 197, 171, and 151 mA h g^-1 at current densities of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 A g^-1 , respectively.

Core-Shell C@Sb Nanoparticles as a Nucleation Layer for High-Performance Sodium Metal Anodes

Sodium metal anode (SMA) is one of the most favored choices for the next-generation rechargeable battery technologies owing to its low cost and natural abundance. However, the poor reversibility resulted from dendrite growth and formation of unstable solid electrolyte interphase has significantly hindered the practical application of SMAs. Herein, we report that a nucleation buffer layer comprising elaborately designed core-shell C@Sb nanoparticles (NPs) enables the homogeneous electrochemical deposition of sodium metal for long-term cycling. These C@Sb NPs can increase active sites for initial sodium nucleation through Sb-Na alloy cores and keep these cores stable through carbon shells. The assembled cells with this nucleation layer can deliver continuously repeated sodium plating/stripping cycles for nearly 6000 h at a high areal capacity of 4 mA h cm^-2 with an average Coulombic efficiency 99.7%. This ingenious structure design of alloy-based nucleation agent opens up a promising avenue to stabilize sodium metal with targeted properties.

Facile and scalable synthesis of α-Fe2O3/γ-Fe2O3/Fe/C nanocomposite as advanced anode materials for lithium/sodium ion batteries

To develop low-cost advanced anode materials for lithium/sodium ion batteries, the chemical reaction equilibrium of Fe(NO₃)₃ and glucose in hot aqueous solution is creatively used to fabricate a new α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite with the primary particle sizes concentrated at 25-80 nm. As anodes for lithium ion batteries, it exhibits a discharge capacity of ∼878 mAh g^-1 after 200 cycles at a current density of 200 mA g^-1. Moreover, even after 1000 cycles at a current density of 3200 mA g^-1, the discharge capacity is as high as ∼532 mAh g^-1, with a capacity retention of over than 100% against that of the second cycle. As anodes for sodium ion batteries, the nanocomposite displays a stable discharge capacity of ∼400 mAh g^-1 at a current density of 100 mA g^-1 and no obvious capacity degradation happens after 200 cycles. During cycling, the α-Fe₂O₃/γ-Fe₂O₃/Fe/C nanocomposite electrodes shows high structural stability and relatively faster reaction kinetics, which should be responsible for its excellent electrochemical performance. This work provides a facile and scalable route to synthesize high-performance and low-cost Fe₂O₃-based nanocomposite for the secondary batteries.

Bimetal Synergistic Effect Induced High Reversibility of Conversion-Type Ni@NiCo2S4 as a Free-Standing Anode for Sodium Ion Batteries

Conversion-type anode materials for sodium ion batteries have received extensive attention because of their relatively high theoretical capacity. However, multiple challenging obstacles stand in the way of their commercial application, especially their poor cycling stability resulting from the bad reversibility of the conversion reaction. Herein, Ni-Co bimetal sulfide was selected and investigated with the goal of improving the reversibility of the conversion reaction owing to the similarity of Ni and Co. As expected, when three-dimensional hierarchical Ni@NiCo₂S₄ (NiCo₂S₄ nanowires growing on the Ni foam) was applied as the free-standing anode for sodium ion batteries, it demonstrated high capacity and excellent cycling stability (90.65%, 100 cycles) compared with those of monometallic sulfides. Various characterization [in situ X-ray diffraction (XRD), ex situ XRD, ex situ X-ray photoelectron spectroscopy, FESEM mapping, and high-resolution transmission electron microscopy] tests confirmed that the Ni-Co alloy was formed during the discharge process and effectively prevented the crystalline grain growth of conversion reaction products, improving the reaction kinetics and reversibility.

Controllable Design of MoS2 Nanosheets Grown on Nitrogen-Doped Branched TiO2 /C Nanofibers: Toward Enhanced Sodium Storage Performance Induced by Pseudocapacitance Behavior

In this work, expanded MoS₂ nanosheets grown on nitrogen-doped branched TiO₂ /C nanofibers (NBT/C@MoS₂ NFs) are prepared through electrospinning and hydrothermal treatment method as anode materials for sodium-ion batteries (SIBs). The continuous 1D branched TiO₂ /C nanofibers provide a large surface area to grow expanded MoS₂ nanosheets and enhance the electronic conductivity and cycling stability of the electrode. The large surface area and doping of nitrogen can facilitate the transfer of both Na⁺ ions and electrons. With the merits of these unique design and extrinsic pseudocapacitance behavior, the NBT/C@MoS₂ NFs can deliver ultralong cycle stability of 448.2 mA h g^-1 at 200 mA g^-1 after 600 cycles. Even at a high rate of 2000 mA g^-1 , a reversible capacity of 258.3 mA h g^-1 can still be achieved. The kinetic analysis demonstrates that pseudocapacitive contribution is the major factor to achieve excellent rate performance. The rational design and excellent electrochemical performance endow the NBT/C@MoS₂ NFs with potentials as promising anode materials for SIBs.

Phosphorus-Doping-Induced Surface Vacancies of 3D Na2 Ti3 O7 Nanowire Arrays Enabling High-Rate and Long-Life Sodium Storage

Sodium-ion batteries have attracted interest as an alternative to lithium-ion batteries because of the abundance and cost effectiveness of sodium. However, suitable anode materials with high-rate and stable cycling performance are still needed to promote their practical application. Herein, three-dimensional Na₂ Ti₃ O₇ nanowire arrays with enriched surface vacancies endowed by phosphorus doping are reported. As anodes for sodium-ion batteries, they deliver a high specific capacity of 290 mA h g^-1 at 0.2 C, good rate capability (50 mA h g^-1 at 20 C), and stable cycling capability (98 % capacity retention over 3100 cycles at 20 C). The superior electrochemical performance is attributed to the synergistic effects of the nanowire arrays and phosphorus doping. The rational structure can provide convenient channels to facilitate ion/electron transport and improve the capacitive contributions. Moreover, the phosphorus-doping-induced surface vacancies not only provide more active sites but also improve the intrinsic electrical conductivity of Na₂ Ti₃ O₇ , which will enable electrode materials with excellent sodium storage performance. This work may provide an effective strategy for the synthesis of other anode materials with fast electrochemical reaction kinetics and good sodium storage performance.

Suppression of Monoclinic Phase Transitions of O3-Type Cathodes Based on Electronic Delocalization for Na-Ion Batteries

As high capacity cathodes, O3-type Na-based oxides always suffer from a series of monoclinic transitions upon sodiation/desodiation, mainly caused by Na⁺/vacancy ordering and Jahn-Teller (J-T) distortion, leading to rapid structural degradation and serious performance fading. Herein, a simple modulation strategy is proposed to address this issue based on refrainment of electron localization in expectation to alleviate the charge ordering and change of electronic structure, which always lead to Na⁺/vacancy ordering and J-T distortion, respectively. According to density functional theory calculations, Fe³⁺ with slightly larger radius is introduced into NaNi_0.5Mn_0.5O₂ with the intention of enlarging transition metal layers and facilitating electronic delocalization. The obtained NaFe_0.3Ni_0.35Mn_0.35O₂ exhibits a reversible phase transition of O3_hex-P3_hex without any monoclinic transitions in striking contrast with the complicated phase transitions (O3_hex-O'3_mon-P3_hex-P'3_mon-P3'_hex) of NaNi_0.5Mn_0.5O₂, thus excellently improving the capacity retention with a high rate kinetic. In addition, the strategy is also effective to enhance the air stability, proved by direct observation of atomic-scale ABF-STEM for the first time.

FeP/C Composites as an Anode Material for K-Ion Batteries

Owing to their natural abundance, the low potential, and the low cost of potassium, potassium-ion batteries are regarded as one of the alternatives to lithium-ion batteries. In this work, we successfully fabricated a FeP/C composite, a novel electrode material for PIBs, through a simple and productive high-energy ball-milling method. The electrode delivers a reversible capacity of 288.9 mA·h·g^-1 (2nd) at a discharge rate of 50 mA g^-1, which can meet the future energy storage requirements. Density functional theory calculations suggest a lower diffusion barrier energy of K⁺ than Na⁺, which allows faster K⁺ diffusion in FeP.

Graphene-Encapsulated CuP2: A Promising Anode Material with High Reversible Capacity and Superior Rate-Performance for Sodium-Ion Batteries

Metal polyphosphides are regarded as the ideal anode candidates for sodium storage because of their high theoretical capacity, reasonable potential, and abundant resource alternative. However, most of them suffer from irreversibility problems, as reflected by their low reversible capacity, inferior Coulombic efficiency (CE), low rate capability, and poor cycling stability. In this work, we systematically compare the electrochemical behavior of a variety of polyphosphides bulks, discovering that the CuP₂ bulks have higher initial reversible capacity (416 mAh g^-1 at 0.1 A g^-1) and CE (74%) compared to the FeP₂, CoP₃, and NiP₂ bulks, which is related to the unique crystal structure of CuP₂. The CuP₂ electrode is optimized by the rational design of encapsulating CuP₂ nanoparticles into three-dimensional graphene networks (CuP₂@GNs), leading to excellent electrochemical performance. In the carbonate electrolyte, the CuP₂@GNs electrode can deliver the reversible capacities of up to 804, 736, 685, 621, and 508 mAh g^-1 at 0.1, 0.5, 1, 2, and 5 A g^-1, respectively, along with a first CE of 66%. The reversible capacity can be up to 737 mAh g^-1 at 0.1 A g^-1 with a first CE of 83% in the ether electrolyte. These excellent performance demonstrates that CuP₂@GNs could be a promising anode material for sodium-ion batteries.

C@TiO2 /MoO3 Composite Nanofibers with 1T-Phase MoS2 Nanograin Dopant and Stabilized Interfaces as Anodes for Li- and Na-Ion Batteries

Integrating layered nanostructured MoS₂ with a structurally stable TiO₂ backbone to construct reciprocal MoS₂ /TiO₂ -based nanocomposites is an effective strategy. C@TiO₂ /MoO₃ composite nanofibers doped with 1T-phase MoS₂ nanograins were fabricated by partially sulfurizing MoO_x /TiO₂ precursors. By controlling a suitable preoxidation temperature before severe thermolysis of polyvinylpyrrolidone (PVP), the MoO_x /TiO₂ precursors formed a polymer-embedded array through coordination of the Mo source and pyrrolidyl groups of PVP. Sulfidation under water/solvent hydrothermal conditions led to partial formation of metallic 1T-phase MoS₂ from the MoO_x precursor with preoxidation at 200 °C. After carbonization, the TiO₂ /MoO₃ /MoS₂ nanograins were encapsulated in a carbon backbone in a vertical pattern, providing both chemical contact for confined electron transport and sufficient space to adapt to volume changes. The obtained carbon-based platform not only has the advantages of an integral structure, but also exhibited ultrastable specific capacities of 540 and 251 mAh g^-1 for Li-ion batteries and Na-ion batteries, respectively, after 100 cycles.

Fe7S8 Nanoparticles Anchored on Nitrogen-Doped Graphene Nanosheets as Anode Materials for High-Performance Sodium-Ion Batteries

Despite high sodium storage capacity and better reversibility, metal sulfides suffer from relatively low conductivity and severe volume change as anode materials of sodium-ion batteries (SIBs). Introducing a conductive carbon matrix is an efficient method to enhance their sodium storage performance. Herein, we present iron sulfide (Fe₇S₈) nanoparticles anchored on nitrogen-doped graphene nanosheets fabricated through a combined strategy of solvothermal and postheating process. The as-prepared composite exhibits appealing cycling stability (a high discharge capacity of 393.1 mA h g^-1 over 500 cycles at a current density of 400 mA g^-1 and outstanding high-rate performance of 543 mA h g^-1 even at 10 A g^-1). Considering the excellent sodium storage performance, this composite is quite hopeful to become a potential candidate as anode materials for future SIBs.

Amorphous CuSnO3 nanospheres anchored on interconnected carbon networks for use as novel anode materials for high-performance sodium ion batteries

Bimetal oxide CuSnO₃ nanospheres encapsulated in carbon networks were explored as novel anode materials for sodium batteries.

Fresh MoO2 as a better electrode for pseudocapacitive sodium-ion storage

A freshly prepared MoO₂ anode with dominant pseudocapacitive sodium-ion storage has been developed for high-rate sodium ion batteries.