Sodium-ion batteries (SIBs) have recently garnered considerable attention because of the greater abundance, wider distribution, and lower cost of Na compared to Li. However, the investigation is insufficient, mainly because Na⁺ is larger and heavier than Li⁺, thereby limiting the Na⁺ insertion and extraction ability from the host materials. Anodes with alloying reactions such as Sn, Ge, and Sb have been considered for SIBs owing to their high gravimetric and volumetric specific capacities. In this study, we devised a one-pot reaction strategy for the in-situ fabrication of a spherical porous nano-SnSb/C composite by employing aerosol spray pyrolysis, and subsequently applied it as an anode in SIBs. The products of spray pyrolysis generally feature three-dimensional spherical hierarchical structures, which are considered to be relatively stable and also act as high-packing-density electrode materials. Additionally, they can be easily handled during the fabrication of the electrode. By adjusting the precursor concentration of SnCl₂·2H₂O and SbCl₃, different sizes for SnSb nanoparticles (10 and 20 nm) were obtained. The crystal structures and morphologies of the as-prepared samples were characterized using X-ray diffraction, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. Thermal gravimetric analysis was carried out to analyze the carbon content of SnSb/C composites by using a TG-DSC analyzer with a heating rate of 5 ℃·min^-1 in air from 25 ℃ to 600 ℃. The specific surface areas of the microspheres were determined by Brunauer-Emmett-Teller analysis. X-ray photoelectron spectroscopy and Raman spectroscopy were used to investigate the studied materials. The micro-nanostructured composite is composed of SnSb nanoparticles (10 and 20 nm); moreover, the carbon content and size of SnSb nanograins could be controlled by altering the reaction conditions. Owing to its unique structure, the obtained nano-composite displays stable cycle performance and high rate capability as the anode for SIBs. The specific capacity of 10-SnSb/C was 722.1 mAh·g^-1 at the first cycle, and the coulombic efficiency of the first cycle was 86.3%. The 10-SnSb/C was stable at different current densities of 100, 1000, and 3000 mA·g^-1, and exhibited specific capacities of 607.7, 645.4 and 452.2 mAh·g^-1, respectively. The reversible capacity reached 623 mAh·g^-1 after 200 cycles at a current density of 1000 mA·g^-1, and the capacity retention rate was 95%. The outstanding performance of SnSb/C was due to its distinctive nanostructure, which could effectively improve the utilization rate of active materials, facilitate the transportation of Na⁺ ions, and prevent the nanoparticle pulverization/agglomeration upon prolonged cycling. The facile synthesis technique and good performance would shed light on the practical development of SnSb/C nanocomposites as high rate capability and long cycle life electrodes for SIBs.

With the development of clean and sustainable energy sources, the demand for large-scale electrochemical energy storage systems has rapidly increased over the last few years. Rechargeable Na-ion batteries (NIBs), one of the most promising energy storage technologies, have received a great deal of attention. Titanium-based P2-type layered oxides are attractive candidates for NIB anode materials, owing to their suitable redox potential, low cost, air stability and high safety. The exposed large interlayers of P2 configuration provide facile channels for Na⁺ insertion/extraction when employed as electrode materials for room temperature, non-aqueous NIBs. In this paper, a novel P2-type Na_0.65Li_0.13Mg_0.13Ti_0.74O₂ is synthesized by a solid-state reaction method. An orthorhombic phase of Na_0.9Mg_0.45Ti_1.55O₂ is observed with the increase in calcination time. During the long calcination process, it is speculated that some lattice Na⁺ and Li⁺ of the previously formed P2 phase compound would be volatilized or extracted by O₂, forming a low Na-content orthorhombic phase based on the layered host structure. In particular, when the precursor was calcined at 1273 K for 24 h, a perfect biphasic hybrid composite was synthesized. The Na storage performance of the pure P2 compound and hybrid composite were evaluated respectively in sodium half cells with voltage range of 0.2–2.5 V. The P2-type electrode can deliver a reversible capacity of 85.1 mAh·g^-1 (theoretical capacity of approximately 108.5 mAh·g^-1), whereas, the sample with the orthorhombic phase shows an enhanced initial reversible capacity of 96.3 mAh·g^-1. Both of the curves are smooth with no observed plateau, indicating the good structural stability of the electrode during cycling. Thus, the hybrid composite exhibits better cycling performance (capacity retention of 89.7% vs. 84.4% for pure P2, after 400 cycles at current density of 1C) and better rate capability (56.6 mAh·g^-1 at 5C vs 47.1 mAh·g^-1 at 2C). These results can be attributed to the introduced second phase, which improves the electron and bulk ion conductivity and helps stabilize the structure. Therefore, this novel two-phase intergrowth composite could serve as a promising anode candidate for the large-scale energy storage application of NIBs. Moreover, this structural design strategy could be used for other layered oxides to improve their energy density and cycling stability.

In recent years, aqueous sodium-ion batteries (ASIBs) have experienced rapid development, and a series of cathode materials for ASIBs has been widely reported. Among these, Na_0.44MnO₂ possesses the most promising prospects due to its low cost, non-toxic nature, simple synthesis, and structural stability. However, the reported capacity of Na_0.44MnO₂ in aqueous electrolyte was ~40 mAh·g⁻¹ (less than its theoretical capacity of 121 mAh·g⁻¹), which limits its practical applications. Recently, we developed a novel alkaline Zn-Na_0.44MnO₂ dual-ion battery using Na_0.44MnO₂ as the cathode, a Zn metal sheet as the anode, and a 6 mol L⁻¹ NaOH aqueous solution as the electrolyte. In this system, the Na_0.44MnO₂ electrode presented excellent electrochemical performance with high reversible capacity (80.2 mAh·g⁻¹ at 0.5C) and outstanding cycling stability (73% capacity retention over 1000 cycles at 10C) in alkaline aqueous electrolyte. When the negative potential window was extended to 0.3 V, the Na_0.44MnO₂ electrode delivered an incredibly high capacity of 345.5 mAh·g⁻¹, which far exceeded the theoretical capacity, but the cycling performance was extremely poor. In that study, X-ray diffraction (XRD) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses revealed that de-intercalation of Na⁺ and formation of Mn(OH)₂ occurred during the discharge process, but the detailed electrochemical mechanism and structural evolution of this process remained unclear. In this study, we used ICP-AES to analyze the elemental composition of discharge products at different discharge depths and found that a small amount of Na⁺ ions extracted from Na_0.44MnO₂ electrode since Discharge-120 (corresponding to the discharge capacity of 120 mAh·g⁻¹), and the extraction rate increased gradually with increasing discharge depth. Scanning electron microscope (SEM) and XRD analyses were also carried out to characterize the morphology and phase changes of Na_0.44MnO₂ electrode during discharge. The results show that the discharge of Na_0.44MnO₂ electrode in the voltage range 1.95–0.3 V could be divided into the three following steps: (1) the potential range above 1.0 V: Na⁺ ions de-intercalate reversibly into the tunnel structure of Na_0.44MnO₂; this discharge mechanism is consistent with that in non-aqueous and neutral aqueous sodium ion batteries. (2) The initial platform region at 1.0 V: in this step, protons (H⁺) began to insert into the Na⁺-vacancies in Na_xMnO₂, and the tunnel structure of Na_xMnO₂ was still maintained. (3) Subsequent slope region: when the Na⁺-vacancies in the tunnel structure were fully occupied by protons, further intercalation led to intensification of charge repulsion in the crystal structure. Thus, the tunnel structure collapsed to form a new Mn(OH)₂ phase, accompanied by the release of Na⁺ from the structure. H⁺ has a smaller radius than Na⁺; therefore, it could insert into the smaller vacancies in Na_0.44MnO₂, resulting in higher specific capacity. However, the insertion of H⁺ will also cause structural damage, which seriously worsens the cycling stability of the Na_0.44MnO₂ electrode.

Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles because of their high energy density and long cycle life. Sodium-ion batteries have broad application prospects in the areas of large-scale electrochemical energy storage systems and low-speed electric vehicles because of their abundant raw materials, low resource cost, safety, and environmental friendliness. However, the development of sodium-ion batteries has been hindered by the low reversibility, sluggish ion diffusion, and large volume variations of the host materials. Suitable electrode materials with decent electrochemical performance must be primarily explored for the successful use of sodium-ion batteries. Since the electrochemical potential and specific capacities of cathode materials have a major impact on the energy densities of sodium-ion batteries, the development of cathode materials is critical. To date, various Na-insertable frameworks have been proposed, and some cathode materials have been reported to deliver reversible capacities approaching their theoretical values. Among them, transition metal oxides show a high reversible capacity and high working potential, but most of them still possess problems such as irreversible phase transition, air instability, and insufficient battery performance. Another type is the Prussian blue analogs. These materials exhibit a favorable operating voltage, cycling stability, and rate capability; however, the main obstacles to their practical application are the control of lattice defects, thermal instability, and low tap density. Polyanionic phosphates are the most promising cathode materials for sodium-ion batteries and have great research value and application prospects because of their stable framework structure, suitable operating voltage, and fast ion diffusion channels. However, their inherent defects, such as poor electronic conductivity and low theoretical energy density, considerably limit their practical applications. Researchers have conducted modification studies through bulk structure adjustment and micro-nano structural control with the goal of improving the performance of phosphate cathode materials and promoting the research and development of sodium-ion energy storage systems. This study reviews the recent advances in phosphate cathode materials for sodium-ion batteries, including orthophosphates, pyrophosphates, fluorophosphates, and mixed phosphate compounds. In this study, the intrinsic relationships among material composition, structure, and electrochemical properties are identified through analyses of the crystal structures, sodium storage mechanisms, and modification strategies of phosphate materials, thereby providing a reference for the continuous modification of polyanion phosphate cathode materials and exploration of high-voltage phosphate cathode materials. Some directions for future research and possible strategies for building advanced sodium-ion batteries are also proposed.

All-solid-state sodium ion batteries (ASIBs) are important for future large-scale energy storage applications. ASIBs have come to occupy an important position in research on advanced secondary batteries in recent years owing to their advantages of abundance in resources, low cost, long lifetime, and high safety. As the key to the success of ASIBs, solid-state electrolytes such as polymers, oxide ceramics, and sulfide glass-ceramics have always attracted immense interest. Chalcogenide electrolytes for ASIBs have high room-temperature conductivity, high elastic modulus, and can be easily pressed into a mold at room temperature; hence, they are the research focus in ASIBs. This paper summarized recent studies on the structure and properties of chalcogenide electrolytes for ASIBs. These studies demonstrate the relationship between the phase structure and ionic conductivity of sulfide-based electrolytes and selenide-based electrolytes. Besides, arguments that the sodium vacancy in the crystal structure dominates ionic conduction, and creating a sodium vacancy via cation substitution is the principal strategy to increasing ionic conduction, are discussed. Further, the intrinsic chemical stability and interface stability between the electrode and electrolyte are highlighted. Based on the soft and hard acid and base theory, some studies adopted various anion/cation ion substitution strategies to improve the chemical stability of chalcogenide electrolytes in humid air. Particularly, the inconsistency in the electrochemical stability window of a representative chalcogenide electrode, Na₃PS₄, as measured by a semi-blocking electrode and calculated by first-principles, is compared. Additionally, to develop all-solid-state Na-S and Na-O₂ batteries with high capacity, the nonnegligible interface instability of the sulfide electrode against the sodium metal anode and feasible solutions are summarized. Next, the research progress on ASIBs using chalcogenide electrolytes is reviewed. Chalcogenide electrolytes are restricted by the electrochemical stability window and chemical compatibility with electrode materials; hence, they are expected to only be applicable to ASIBs using sulfur, sulfide, and organic matter as the cathode and Na-Sn alloy as the anode. However, these ASIBs have long cycling life (> 500 cycles), illustrating their potential applications in large-scale energy storage power stations. Finally, we comprehensively evaluate the ionic conductivity, stability against humid air, stability of the interface, electrochemical stability window, and ease of preparation of typical chalcogenide electrolytes, including Na₃PS₄, Na₃PSe₄, Na₃SbS₄, Na₃SbSe₄, Na₁₀SnPS₁₂, and Na₁₁Sn₂PS₁₂. Moreover, we highlight the challenges and propose possible solutions toward the development of chalcogenide electrolytes in future. Advanced technologies in fine synthesis, in situ characterization, and surface/interface modification are essential to overcome existing challenges and promote the development of chalcogenide-electrolyte-based ASIBs.

Because of their high energy density and long cycle life, lithium-ion batteries (LIBs) have dominated the portable electronics market for over 20 years. However, with the increasing demand for large-scale energy storage systems for grid applications, the price of Li resources has increased owing to the low abundance of Li in Earth's crust and non-uniform distribution on the planet. Because Na has similar physical and chemical properties as Li and is an abundant natural resource, room-temperature sodium-ion batteries (SIBs) are expected to be among the most promising next-generation large grid energy storage devices. It is known that the cathode, anode, separator and electrolyte materials are the main components of batteries. Among these, Na-containing cathode materials are of critical importance. As a cathode material for SIBs, polyanion-type compounds have become a hot research topic owing to their versatile structural frameworks, high thermal stabilities, high ambient stabilities even in the charging state, small volume changes, tunable operating voltage by tuning the chemical environment of the polyanions, and high operating voltages owing to the inductive effects of the polyanionic groups (PO₄³⁻, SO₄²⁻, SiO₄⁴⁻, etc.). In particular, for Earth's abundant resources and inherent stability, polyanion-based compounds are suitable for large-scale stationary energy storage. Taking grid balancing into account, batteries with fast charge rates are in demand, which requires cathodes having high rate capability. However, despite the presence of ion diffusion channels in polyanion compounds, the electronic transport channels are blocked owing to the separation of the metal polyhedral and the strong electronegativity of the anions, leading to poor electron conductivity, which largely limits the rate capability of polyanion compounds. Therefore, it is crucial to understand the inherent limitation of the kinetics in terms of the structural aspects and to determine strategies for improving the rate capability. This review discusses the intrinsic reasons for the factors impacting ion diffusion based on the different structures of polyanion-type cathodes. From the perspectives of surface modification and morphology, strategies for enhancing the transport of sodium ions and electrons at the surface and interface are summarized and discussed. Then, from the standpoint of the hierarchical structures of materials to the design of a structural framework, which have been rarely reported, this review proposes schemes that intrinsically enhance the rate capability of polyanion compounds and provides a perspective on developments that can further improve the rate capability of cathode materials. This review provides suggestions for designing and optimizing high-rate polyanion-type and other kinds of cathodes from both academic and practical viewpoints.

Sodium-ion batteries (SIBs) are promising candidates to replace lithium-ion batteries (LIBs) to meet the emergent requirements of various commercial applications. SIBs and LIBs are similar in many aspects, including their reduction potentials, approximate energy densities, and ionic semidiameters. Analogously, safety issues, including liquid leakage, high flammability, and explosiveness limit the usage of SIBs. All-solid-state batteries have the potential to solve the aforementioned problems. However, polycarbonates as promising solid electrolytes have been rarely exploited in all-solid-state SIBs. In addition, organic electrode materials, including non-conjugated redox polymers, carbonyl compounds, organosulfur compounds, and layered compounds, have been intensively investigated as part of various energy storage systems owing to their low cost, environmental friendliness, high energy density, and structural diversity. Nevertheless, the dissolution of small organic compounds in organic-liquid electrolytes has hindered its further applications. Fortunately, the utilization of solid polymer electrolytes combined with organic electrode materials is a promising method to prevent dissolution into the electrolyte and improve the cycling performance of SIBs. Thus, we proposed the utilization of a poly(propylene carbonate) (PPC)-based solid polymer electrolyte and cellulose nonwoven with a 3, 4, 9, 10-perylene-tetracarboxylicacid-dianhydride (PTCDA) cathode in an all-solid-state sodium battery (ASSS). The solid electrolyte significantly enhanced the safety of the SIB and was successfully synthesized via a facile method. The morphology of the as-prepared solid electrolyte was examined by electron scanning microscopy (SEM). Furthermore, the electrochemical performances of the PTCDA/Na battery with organic-liquid and solid electrolytes at room temperature were compared. The SEM results demonstrated that the solid polymer electrolyte and sodium bis(fluorosulfonyl)imide (NaFSI) were evenly distributed inside the pores of the nonwoven cellulose. The ionic conductivity of the composite solid polymer electrolyte (CSPE) at room temperature was 3.01 × 10^-5 S·cm^-1, suggesting that the CSPE was a promising candidate for commercial applications. In addition, the ASSS showed significantly improved cycling performance at a current density of 50 mAh·g^-1 with a high capacity retention of 99.1%, whereas the discharge capacity of the liquid PTCDA/Na battery was only 24.6mAh·g^-1 after 50 cycles. This indicated that the cycling performance of the PTCDA cathode in the SIB was largely improved by preventing the dissolution of the PTCDA cathode material in the electrolyte. Electrochemical impedance spectroscopy results demonstrated that the CSPE was compatible with the organic cathode electrode.

Lithium-ion batteries (LIBs) are widely used in cellphones, laptops, and electric cars owing to their high energy density and long operational lifetime. However, their further deployment in large-scale energy storage systems is restricted by the uneven distribution of lithium resources (~0.0017% (mass fraction, w) in the Earth's crust). Therefore, alternative energy storage systems composed of abundant elements are of urgent need. Recently, sodium-ion batteries (SIBs) have attracted significant attention and are considered to be a potential alternative for next-generation batteries owing to abundant sodium resources (~2.64% (w) of the Earth's crust), suitable potential (−2.71 V), and low cost. SIBs are similar to LIBs in terms of their physical and electrochemical properties. Previous studies have mainly focused on SIB storage materials, including hard carbon, alloys, and hexacyanoferrate, while the safety of SIBs remains largely unexplored. Similar to LIBs, the current electrolytes used in SIBs are mainly composed of flammable organic carbonate solvents (or ether solvents), sodium salts, and functional additives, which pose possible safety issues. Moreover, the chemical activity of sodium is much higher than that of lithium, leading to a higher risk of fire, thermal runaway, and explosion. To overcome this problem, herein we propose a fluorinated non-flammable electrolyte composed of 0.9 mol∙L⁻¹ NaPF₆ (sodium hexafluorophosphate) in an intermixture of di-(2, 2, 2 trifluoroethyl) carbonate (TFEC) and fluoroethylene carbonate (FEC) in a 7 : 3 ratio by volume. Its physical and electrochemical properties were studied by ionic conductivity, direct ignition, cyclic voltammetry, and charge/discharge measurements, demonstrating excellent flame-retarding ability and outstanding compatibility with sodium electrodes. The electrochemical tests showed that the Prussian blue cathode retained a capacity of 84 mAh∙g⁻¹ over 50 cycles in the prepared electrolyte, in contrast to the rapid capacity degradation in a flammable conventional carbonate electrolyte (74 mAh∙g⁻¹ with 57% capacity retention after 50 cycles). To test the practical application of the proposed electrolyte, a hard carbon anode was used and exhibited exceptional performance in this system. The enhancement mechanism was further verified by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning emission microscopy (SEM) investigations. Polycarbonate on the surface of the cathode played an important role for the studied electrolyte system. The polycarbonate may originate from FEC decomposition, which can enhance the ionic conductivity of the solid electrolyte interface (SEI) layer and reduce impedance. Hence, we believe that this proposed electrolyte may provide new opportunities for the design of robust and safe SIBs for next-generation applications.

Sodium batteries have drawn increasing attention from multiple researchers owing to the abundant reserves and low cost of sodium resources. However, traditional sodium batteries based on organic solvent electrolyte systems have safety risks. Thus, the utilization of solid electrolyte materials instead of organic electrolytes could effectively resolve safety issues and ensure the safe performance of the battery. Solid sodium-ion battery is a promising energy storage device. The sodium ion solid-state electrolytes mainly includes Na-β-Al₂O₃, Na super ionic conductor (NASICON), sulfide, polymer, and borohydride. Inorganic solid electrolytes have the advantage of ionic conductivity compared with polymer solid electrolyte. This paper summarizes the research progress on three common inorganic sodium ion solid electrolytes: Na-β-Al₂O₃, NASICON, and sulfide. Research efforts have mainly focused on increasing ionic conductivity and interface stability. Na-β-Al₂O₃ has been successfully commercialized in high-temperature Na-S and ZEBRA batteries with molten electrodes. Pure β″-Al₂O₃ is difficult to prepare owing to its low thermodynamic stability. The synthesized β″-Al₂O₃ based on traditional solid-state reaction generally contains impurities such as β-Al₂O₃ and NaAlO₂ (around the boundaries). Further improvements are required to develop favorable methods for fabricating pure β″-Al₂O₃ with high production yield, low cost, and well-controlled microstructure. NASICON, one of the most promising ionic conductors for solid sodium-ion batteries, has attracted considerable attention for its high ionic conductivity at room temperature. The general method to enhance ionic conductivity is to increase the bottleneck size by introducing proper substituents. However, the substitution of synthetic elements could result in different optimal calcination temperatures, which would lead to a change in the density of ceramic sintering. β″-Al₂O₃ and NASICON have higher ionic conductivity at room temperature but cannot achieve good performance in the field of high power densities and long-term cycling owing to the poor interface contact with electrode materials. Because the high polarizability and large ionic radius of sulfur atoms weaken the interaction between skeleton and sodium ions, sulfide solid electrolytes often provide higher ionic conductivity at room temperature than analogous oxides. At the same time, sulfide solid electrolytes can be easily pressed into a mold at room temperature. However, sulfide electrolytes have low chemical stability in air because of hydrolysis by water molecules with the generation of H₂S gas, which should be handled in inert gas atmosphere. In conclusion, this review discusses the recent progress in different aspects of ionic conductivity and interface stability.

In the past few decades, new energy industries have developed rapidly due to the threat of the depletion of non-renewable resources. Among them, the lithium-ion battery has attracted significant attention of various researchers. However, lithium-ion batteries are limited by the uneven distribution of lithium resources and high cost. Sodium, which is in the same periodic group as Lithium, can help alleviate the problems related to the limited development of lithium ion batteries owing to the shortage of lithium resources. Sodium ion batteries are cheap, with varying choice of electrolytes, and have relatively stable electrochemical performances. However, the radius of a sodium ion is larger than that of a lithium ion, leading to slow ion transportation as well as changes in the volume of the host material during the charging and discharging processes. Therefore, compared with existing lithium ion batteries, sodium ion battery anode materials are very limited. Moreover, most sodium ion battery electrode materials have low specific capacities and poor cycle retention rates. Among these, ternary metal oxides, which have two different cations and can reversibly react with sodium ions are promising high-capacity anode materials for sodium ion batteries. In this study, Ti₂Nb₂O₉ nanosheets are obtained by ion-exchange and chemical-delaminate methods. The carbon-coated Ti₂Nb₂O₉ nanosheets are obtained after hydrothermal coating with sucrose and calcination. From the thermogravimetric analysis (TG) curve, the carbon content in the composites is calculated to be approximately 8.0%. Owing to the rich reactive sites and a short ion transport pathway, the Ti₂Nb₂O₉/C electrode delivers a high reversible capacity of 265.2 mAh·g^-1 at a current density of 50 mA·g^-1. Even at a high current density of 500 mA·g^-1, the electrode exhibits an excellent electrochemical performance with a reversible capacity of 160.9 mAh·g^-1 after 200 cycles (capacity retention of 75.3%). Additionally, the Ti₂Nb₂O₉/C nanosheets exhibit high reversible capacities of 251.3, 224.6, 197.4, 176.3, and 156.5 mAh·g^-1 at the current densities of 100, 200, 500, 1000, and 2000 mA·g^-1, respectively. It is demonstrated through the use X-ray photoelectron spectroscopy (XPS) that the following process involving the transfer of four electrons occurs: Ti⁴⁺/Ti³⁺, Nb⁵⁺/Nb⁴⁺, during the charging and discharging process of the Ti₂Nb₂O₉ electrode in the voltage range of 0.01–3.0 V. The theoretical specific capacity of Ti₂Nb₂O₉ in this process is calculated to be 252 mAh·g^-1, corresponding to the electrochemical data. Overall, this study demonstrates that the Ti₂Nb₂O₉/C anode nanosheets have an excellent charge-discharge performance, cycle stability, and rate performance in sodium ion batteries, thereby providing a feasible choice for sodium ion battery anode materials.

Na-ion batteries are currently an emerging and low-cost energy storage technology, which have attracted enormous attention and research due to its promising potentiality for large-scale energy storage applications. As the key electrode materials for Na-ion batteries, non-graphite carbonaceous materials have been regarded as the best choice for practical application due to its high sodium storage activity, low-cost and non-toxicity. According to the current research, graphite materials are not suitable to be anode materials of Na-ion batteries for practical application due to its low sodium storage capacity in carbonate electrolytes. Hard carbons have a high capacity of ~300 mAh·g^-1 with low sodium storage potential and thus are suitable for practical applications. Soft carbons have a sodium storage capacity about 200 mAh·g^-1 with sodium storage potential below 1 V vs. Na⁺/Na. Soft carbons usually exhibit excellent rate performances and thus are suitable to be used as anode materials for power Na-ion batteries. Reduced graphene oxide (rGO) has a sodium storage capacity of about 220 mAh·g^-1 and excellent rate performances. A high sodium storage capacity can be obtained by doping heteroatoms and introducing defect sites in rGO. However, the low material density, high sodium storage potential and large irreversible capacity of rGO will restrict its practical application. Porous carbons have high capacities of 300-450 mAh·g^-1 with excellent rate performances because their developed porous structure can provide more defects as the active sites for sodium storage and shorten the diffusion path of Na⁺ to improve rate performances. Carbon nanowires/fibers have good flexibility due to their unique one-dimensional feature and stable sodium storage reversible capacity with good rate performance. These materials have advantages to be flexible electrodes for sodium-based flexible energy storage devices. By introducing N, S and other heteroatoms, heteroatom-doped carbons have more active sites for sodium storage and thus achieve higher sodium storage capacity. In summary, carbon materials with low graphitization degree are important development directions for anode materials of low cost Na-ion batteries. New carbon materials with unique microstructure and morphology have higher sodium storage capacity and rate capability, so they can be used as high power anode materials for sodium storage. Considering many factors, such as cycle life, energy density, power density and manufacturing cost, of practical application, hard carbon anodes is currently the best choice for practical application of Na-ion batteries. In the future, improving SEI stability, increasing Coulombic efficiency and improving electrical conductivity of hard carbon are urgent problems to be solved for practical application. Herein, the recent progress of carbonaceous anode materials is reviewed. The sodium storage mechanism and characteristics of carbon materials are summarized and discussed. Furthermore, the relationship between micro-structures and electrochemical performances, and remained problems of carbon anodes are discussed. This review will promote the development and understanding of carbon anode materials for sodium storage.

Sodium has the advantages of being an abundant resource and having a low cost; thus, sodium ion batteries are considered as one of the best candidates for replacing lithium ion batteries in the future. However, the radius of the sodium ion is larger than that of the lithium ion, and the de-intercalation of the sodium ion will seriously damage the crystal structure of most electrode materials during the charging and discharging process, which considerably limits its charge-discharge specific capacity, cycle performance, and rate performance. However, finding appropriate electrode materials is one of the difficulties in fabricating high-performance sodium ion batteries. Among the many candidate materials, vanadate materials can improve the stability of material structures by introducing cations to increase the coordination numbers of vanadium, thus improving the electrochemical performance of sodium ion batteries. In this paper, an in situ phase separation method to fabricate V₂O₅/Fe₂V₄O₁₃ nanocomposite materials is reported. First, we synthesized hydrated crystalline Fe₅V₁₅O₃₉(OH)₉·9H₂O nanomaterials using a water bath heating method; then, we in situ constructed two-phase nanocomposite V₂O₅ and Fe₂V₄O₁₃ from the single phase by further high-temperature treatment. The morphology, composition, and structure of the electrode materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy(FTIR), as well as other methods. The V₂O₅/Fe₂V₄O₁₃ nanocomposite materials were found to have a more stable structure, higher initial discharge capacity (342 mAh·g^-1 at a current density of 0.1 A·g^-1), longer cycle life, and better rate performance than V₂O₅ nanowires. Therefore, this research on V₂O₅/Fe₂V₄O₁₃ nanocomposite materials has broadened ability to develop new high-performance anode materials for sodium ion batteries.