Carbon-Decorated Na3V2(PO4)3 as Ultralong Lifespan Cathodes for High-Energy-Density Symmetric Sodium-Ion Batteries

Ternary NASICON-Type Na3.25VMn0.25Fe0.75(PO4)3/NC@CNTs Cathode with Reversible Multielectron Reaction and Long Life for Na-Ion Batteries

Na superionic conductor (NASICON)-structure Na4MnV(PO4)3 (NVMP) electrode materials reveal highly attractive application prospects due to ultrahigh energy density originating from two-electron reactions. Nevertheless, NVMP also encounters challenges with its poor electronic conductivity, Mn dissolution, and Jahn-Teller distortion. To address this issue, utilizing N-doped carbon layers and carbon nanotubes (CNTs) for dual encapsulation enhances the material's electronic conductivity, creating an effective electron transport network that promotes the rapid diffusion and storage of Na+. On this basis, partially substituting Mn in NVMP with Fe, a new sodium superionic conductor (NASICON) structured cathode material has been designed to alleviate Jahn-Teller distortion and prolong the cycling life. The synergistic effect of N-doped double nanocarbon encapsulation and multielectron reactions is employed to promote the optimized Na3.25VMn0.25Fe0.75(PO4)3/NC@CNTs (NVMn0.25Fe0.75P/NC@CNTs) electrode material to deliver fast Na+ diffusion kinetics, high reversible capacity (110.2 mAh g-1 at 0.1 C), and long-term cyclic stability (80.1% of the capacity at 10 C over 2000 cycles). Besides, the electrochemical properties of NVMn0.25Fe0.75P/NC@CNTs composites were investigated in detail at high loads and high window voltages to evaluate their potential for practical applications. The reduction/oxidation processes involved in Fe2+/Fe3+, Mn2+/Mn3+, and V3+/V4+ redox couples and a solid-solution and biphasic reaction mechanism upon repeated de- and re-intercalation processes are revealed via ex-situ XRD and XPS characterization. Finally, the assembled NVMn0.25Fe0.75P/NC@CNTs ∥ hard carbon full cell manifests high capacity (101.1 mAh g-1 at 0.1 C) and good cycling stability (98.2% capacity retention at 1 C after 100 cycles). The rational design with multimetal ion substitution regulation has the potential to open up new possibilities for high-performance sodium-ion batteries.

Investigation of Solid Electrolyte Interphase Regarding Extra Capacity in Fe-Fe3C as a Superior Sodium-Ion Anode Material

Sodium-ion batteries (SIBs) offer promising advantages over lithium-ion batteries (LIBs) due to sodium's abundance and lower cost. However, challenges like thick solid electrolyte interphase (SEI) layers and the larger radius of sodium (1.02 Å vs. 0.76 Å for lithium) make graphite, the most common LIB anode, unsuitable for SIBs. To realize the maximum potential of carbon anode for SIBs, one of the main strategies is to fabricate carbon materials with tailored microstructures and to enhance redox reactivity by incorporating catalytic metals. In this work, the Fe-Fe3C nanoparticles embedded in worm-like graphitic carbon (Fe-Fe3C@GC) were synthesized by a simple chemical vapor deposition. This hybrid structure promotes the catalytic activity to achieve additional capacities through the reversible SEI layer formation, in detail, by the reversible interconversion of ester and ether derivatives as well as conductivity enhancement. Furthermore, in situ formed Fe2O3 from Fe(0) contributed extra capacity. The Fe-Fe3C@GC showed a large reversible discharge capacity of 376.2 mAh g-1 with a fading rate of 0.013% per cycle after 1000 cycles at a current density of 50 mA g-1. A full cell coupled with an FeOF cathode delivered a high energy density of 602.8 Wh kg-1.

Unraveling the Function Mechanism of N-Doped Carbon-Encapsulated Na3V2(PO4)3 Cathode toward High-Performance Sodium-Ion Battery with Ultrahigh Cycling Stability

The NASICON-type Na3V2(PO4)3 (NVP) is recognized as a potential cathode material for Na-ion batteries (SIBs). Nevertheless, its inherent small electronic conductivity induces limited cycling stability and rate performance. Carbon coating, particularly N-doped carbon, has been identified as an effective strategy to address these challenges. Hence, N-doped carbon-coated NVP was successfully produced by a straightforward high-temperature solid-phase method, and the mechanism of N-doped carbon coating in regulating the electrochemical kinetics of NVP was unraveled. The N-doped carbon layer establishes a robust conductive network that interconnects the active particles, facilitating electron transfer within the electrode. SEM images after cycling show that the uniform carbon coating mitigated NVP agglomeration, thereby reducing undesired side reactions between electrode and electrolyte. The discharge capacities of NVP/N-C2 electrodes at 0.1, 0.2, 0.5, 1, 2, 5, and 10C reach 116.0, 114.6, 112.6, 111, 108.7, 104.2, and 99.4 mAh g-1, respectively. Even at 20C, the discharge capacity remains up to 92.2 mAh g-1, which is approximately 80% of the discharge capacity at 0.1C. When the rate returns to 0.1C, the NVP/N-C2 cathode still exhibits a discharge capacity of 115.9 mAh g-1, showing excellent electrochemical reversibility. This study presents a viable approach for fabricating NVP with a N-doped carbon coating, showcasing enhanced sodium storage properties.

The Development and Prospect of Stable Polyanion Compound Cathodes in LIBs and Promising Complementers

Cathode materials are usually the key to determining battery capacity, suitable cathode materials are an important prerequisite to meet the needs of large-scale energy storage systems in the future. Polyanionic compounds have significant advantages in metal ion storage, such as high operating voltage, excellent structural stability, safety, low cost, and environmental friendliness, and can be excellent cathode options for rechargeable metal-ion batteries. Although some polyanionic compounds have been commercialized, there are still some shortcomings in electronic conductivity, reversible specific capacity, and rate performance, which obviously limits the development of polyanionic compound cathodes in large-scale energy storage systems. Up to now, many strategies including structural design, ion doping, surface coating, and electrolyte optimization have been explored to improve the above defects. Based on the above contents, this paper briefly reviews the research progress and optimization strategies of typical polyanionic compound cathodes in the fields of lithium-ion batteries (LIBs) and other promising metal ion batteries (sodium ion batteries (SIBs), potassium ion batteries (PIBs), magnesium ion batteries (MIBs), calcium ion batteries (CIBs), zinc ion batteries (ZIBs), aluminum ion batteries (AIBs), etc.), aiming to provide a valuable reference for accelerating the commercial application of polyanionic compound cathodes in rechargeable battery systems.

High-Energy-Density All-V2O5 Battery

Symmetrical batteries hold great promise as cost-effective and safe candidates for future battery technology. However, they realistically suffer low energy density due to the challenge in integrating high specific capacity with high voltage plateau from the limited choice of bipolar electrodes. Herein, a high-voltage all-V2O5 symmetrical battery with clear voltage plateau is conceptualized by decoupling the cathodic/anodic redox reactions based upon the episteme of V2O5 intercalation chemistry. As the proof-of-concept, a hierarchical V2O5-carboncomposite (VO-C) bipolar electrode with boosted electron/ion transport kinetics is fabricated, which shows high performance as both cathode and anode in their precisely clamped working potential windows. Accordingly, the symmetrical full-battery exhibits a high capacity of 174 mAh g-1 along with peak voltage output of above 2.9 V at 0.5C, remarkable capacity retention of 81% from 0.5C to 10C, and good cycling stability of 70% capacity retention after 300 cycles at 5C. Notably, its energy density reaches 429 Wh kg-1 at 0.5C estimated by the cathode mass, which outperforms most of the existing Li/Na/K-based symmetrical batteries. This study leaps forward the performance of symmetrical battery and provides guidance to extend the scope of future battery designs.

Anchored VN Quantum Dots Boosting High Capacity and Cycle Durability of Na3V2(PO4)3@NC Cathode for Aqueous Zinc-Ion Battery and Organic Sodium-Ion Battery

Na3V2(PO4)3 is a promising high-voltage cathode for aqueous zinc-ion batteries (ZIBs) and organic sodium-ion batteries (SIBs). However, the poor rate capability, specific capacity, and cycling stability severely hamper it from further development. In this work, Na3V2(PO4)3 (NVP) with vanadium nitride (VN) quantum dots encapsulated by nitrogen-doped carbon (NC) nanoflowers (NVP/VN@NC) are manufactured as cathode using in situ nitridation, carbon coating, and structural adjustment. The outer NC layer increases the higher electronic conductivity of NVP. Furthermore, VN quantum dots with high theoretical capacity not only improve the specific capacity of pristine NVP, but also serve as abundant "pins" between NVP and NC to strengthen the stability of NVP/VN@NC heterostructure. For Zn-ion storage, these essential characteristics allow NVP/VN@NC to attain a high reversible capacity of 135.4 mAh g-1 at 0.1 A g-1, and a capacity retention of 91% after 2000 cycles at 5 A g-1. Meanwhile, NVP/VN@NC also demonstrates to be a stable cathode material for SIBs, which can reach a high reversible capacity of 124.5 mAh g-1 at 0.1 A g-1, and maintain 92% of initial capacity after 11000 cycles at 5 A g-1. This work presents a feasible path to create innovative high-voltage cathodes with excellent reaction kinetics and structural stability.

Manipulating amorphous and crystalline hybridization of Na3V2(PO4)3/C for enhancing sodium-ion diffusion kinetics

Na3V2(PO4)3 (NVP) has attracted considerable attention as a promising cathode material for sodium-ion batteries (SIBs). But its insufficient electronic conductivity, limited capacities, and fragile structure hinder its extended application, particularly in scenarios involving rapid charging and prolonged cycling. A hybrid cathode material has been developed to integrate both amorphous and crystalline phases, with the objective of improving the rate performance and Na storage capacity by leveraging bi-phase coordination. Consequently, the combination of amorphous and crystalline phases enhanced the kinetics of Na-ion diffusion, resulting in a 1-2 orders of magnitude enhancement in diffusion dynamics. Furthermore, the existence of amorphous states has been demonstrated to elevate the active Na2 site content, resulting in an increased reversible capacity. This assertion is substantiated by evidence derived from solid-state nuclear magnetic resonance (ss-NMR) and electrochemical characteristics. The innovative bi-phase collaborative material provides a specific capacity of 114 mAh/g at 0.2 C, exceptional rate performance of 82 mAh/g at 10 C, and remarkable long-term cycle stability, retaining 95 mAh/g at 5 C even after 300 cycles. In conclusion, the homogeneous hybridization of amorphous and crystalline phases presents itself as a promising and effective strategy for improving Na-ion storage capacity of cathodes in SIBs.

Ultra-Stable Sodium-Ion Battery Enabled by All-Solid-State Ferroelectric-Engineered Composite Electrolytes

Symmetric Na-ion cells using the NASICON-structured electrodes could simplify the manufacturing process, reduce the cost, facilitate the recycling post-process, and thus attractive in the field of large-scale stationary energy storage. However, the long-term cycling performance of such batteries is usually poor. This investigation reveals the unavoidable side reactions between the NASICON-type Na3V2(PO4)3 (NVP) anode and the commercial liquid electrolyte, leading to serious capacity fading in the symmetric NVP//NVP cells. To resolve this issue, an all-solid-state composite electrolyte is used to replace the liquid electrolyte so that to overcome the side reaction and achieve high anode/electrolyte interfacial stability. The ferroelectric engineering could further improve the interfacial ion conduction, effectively reducing the electrode/electrolyte interfacial resistances. The NVP//NVP cell using the ferroelectric-engineered composite electrolyte can achieve a capacity retention of 86.4% after 650 cycles. Furthermore, the electrolyte can also be used to match the Prussian-blue cathode NaxFeyFe(CN)6-z·nH2O (NFFCN). Outstanding long-term cycling stability has been obtained in the all-solid-state NVP//NFFCN cell over 9000 cycles at a current density of 500 mA g-1, with a fading rate as low as 0.005% per cycle.

Trace topological doping strategy and deep learning to reveal high-rate sodium storage regulation of barium-doped Na3V2(PO4)3

The urgent development of sodium ion batteries has stimulated the rapid innovation of sodium super ionic conductor-type Na3V2(PO4)3 materials with high energy density and ultra-high charge/discharge rates, where the bottlenecks are the activation of multi-electron reactions and the utilization of the third sodium ion. Herein, we design a trace topological doping strategy to introduce barium ions into crystal domains of Na3V2(PO4)3 to partially replace vanadium sites. Deep learning demonstrates that the violation of the inversion symmetry of vanadium by barium substitution can improve the structural stability and change the charge density distribution of vanadium, resulting in the re-distribution of surface electrons and supplying more possible migration paths for sodium ions. Simultaneously, the slight alteration of the crystal structure helps the positive shift of vanadium valence from +3 to +4, providing more multi-electron redox reactions. Among these candidates, NVBP-2 manifests a specific capacity of 65.1 mA h g-1 at 50C rate with superior charge-discharge capability and cycling performance. Moreover, it possesses decent long-term cycling stability with 81.2% capacity retention after 2000 cycles at 50C. In summary, the results indicate that trace topological doping of alkaline metal ions in combination with deep learning has a novel ability to achieve sodium ion storage regulation for sodium ion batteries, which exquisitely provides a new perspective for screening cathode materials with high electrochemical performance.

A high-energy-density NASICON-type Na3V1.25Ga0.75(PO4)3 cathode with reversible V4+/V5+ redox couple for sodium ion batteries

The stable three-dimensional framework and high operating voltage of sodium superionic conductor (NASICON)-type Na3V2(PO4)3 has the potential to work with long cycle life and high-rate performance; however, it suffers from the poor intrinsic electronic conductivity and low energy density. Herein, Ga3+ is introduced into Na3V2(PO4)3 to activate the V4+/V5+ redox couple at a high potential of 4.0 V for enhancing energy density of the materials (Na3V2-xGax(PO4)3). After the partial substitution of Ga3+ for V3+, three redox couples (V2+/V3+, V3+/V4+ and V4+/V5+) of V are reversibly converted in the voltage range of 1.4-4.2 V, suggesting multi-electrons (>2e-) involved in the reversible reaction, and simultaneously the electronic conductivity of the materials is effectively enhanced. As a result, the cathode with x = 0.75 exhibits excellent electrochemical properties: in the voltage range of 2.2-4.2 V, delivering an initial capacity of 105 mAh/g at 1C with a capacity retention rate of 92.3% after 400 cycles, and providing a stable reversible capacity of 88.3 mAh/g at 40C; and in the voltage range of 1.4-4.2 V, presenting the reversible capacity 152.3 mAh/g at 1C (497.6 Wh kg-1), and cycling stably for 1000 cycles at 20C with a capacity decay of 0.02375% per cycle. It is found that the Na3V2-xGax(PO4)3 cathodes possess the sodium storage mechanism of single-phase and bi-phase reactions. This investigation presents a useful strategy to enhance the energy density and cycling life of NASICON-structured polyanionic phosphates by activating high-potential V4+/V5+ redox couple.

Outstanding long cycle stability provide by bismuth doped Na3V2(PO4)3 enwrapped with carbon nanotubes cathode for sodium-ion batteries

Na3V2(PO4)3 (NVP), possessing good ionic conduction properties and high voltage plateau, has been deemed as the most prospective material for sodium ion batteries. However, the weak intrinsic electronic conductivity has hindered its further commercialization. Herein, an ingenious strategy of Bi3+ substitution at V3+ site in NVP system is proposed. The ionic radius of Bi3+ is slightly larger than that of V3+, which can further expand the crystal structure inside the NVP, thus accelerating the migration of Na+. Meanwhile, the appropriate amount of carbon coating and carbon nanotubes (CNTs) enwrapping construct an effective three-dimensional network, which provides a conductive framework for electronic transfer. Furthermore, the introduction of CNTs also inhibit the agglomeration of active grains during the sintering process, reducing the particle size and shortening the diffusion path of Na+. Comprehensively, the conductivity, ionic diffusion ability and structural stability of the modified Na3V2-xBix(PO4)3/C@CNTs (0 ≤ x ≤ 0.05) sample are significantly improved. The Na3V1.97Bi0.03(PO4)3/C@CNTs sample obtains a reversible capacity of 97.8 mAh g-1 at 12C and maintains a value of 80.6 mAh g-1 after 9000 ultra-long cycles. As for the super high rate at 80C, it exhibits a high capacity of 84.34 mAh g-1 and retains a capacity of 73.34 mAh g-1 after 6000 cycles. The superior electrochemical performance is derived from the enhancement of the crystal structure by Bi3+ doping and the highly conductive network consisting of carbon coating layers and enwrapped CNTs.

In-situ constructing porous N-doped carbon skeleton with rich defects from modified polyamide acid to boost the high performance of Na3V2(PO4)3 cathode for full sodium-ion batteries

Generally, the transport of electrons and Na+ is seriously constrained in Na3V2(PO4)3 (NVP) due to intense interactions of V-O and PO bonds. Besides, polyamide acid (PAA) is hardly used in the sol-gel route due to insolubility. This work develops a facile liquid synthesis strategy based on modified PAA, achieving in-situ construction of a porous N-doped carbon framework with rich defects to improve the kinetics of NVP. The addition of triethylamine (TEA) reacts with carboxyls in PAA to achieve acid-base neutralization, turning PAA into polyamide salts with good solubility. The special morphology construction mechanism of this unique system was observed by ex-situ scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). Specifically, PAA undergoes in-situ conversion into chain-like polyimide (PI) through a thermal polymerization mechanism during the pre-sintering process. Meanwhile, NVP precursors are evenly dispersed in the PI fibers, efficiently reducing the particle size. After the final treatment, the favorable porous carbon skeleton could be generated derived from the partial decomposition of PI, on which small active grains are in situ grown. The resulting N-doped carbon substrate contains rich defects, benefiting from the migration of Na+. Furthermore, the porous construction is conducive to alleviating the stress and strain generated by the high current impact, increasing the contact area between electrodes/electrolytes to improve the utilization efficiency of active substances. Comprehensively, the optimized samples exhibit a capacity of 82.1 mAh g-1 at 15C with a retention rate of 95.45 % after 350 cycles. It submits a capacity of 67.6 mAh g-1 at 90C and remains 52.2 mAh g-1 after 1500 cycles. Even in full cells, it reveals a value of 110.6 mAh g-1. This work guides the application of in-situ multiple modifications of polymers in electrode materials.

Recent Advances in Carbon-Based Electrodes for Energy Storage and Conversion

Carbon-based nanomaterials, including graphene, fullerenes, and carbon nanotubes, are attracting significant attention as promising materials for next-generation energy storage and conversion applications. They possess unique physicochemical properties, such as structural stability and flexibility, high porosity, and tunable physicochemical features, which render them well suited in these hot research fields. Technological advances at atomic and electronic levels are crucial for developing more efficient and durable devices. This comprehensive review provides a state-of-the-art overview of these advanced carbon-based nanomaterials for various energy storage and conversion applications, focusing on supercapacitors, lithium as well as sodium-ion batteries, and hydrogen evolution reactions. Particular emphasis is placed on the strategies employed to enhance performance through nonmetallic elemental doping of N, B, S, and P in either individual doping or codoping, as well as structural modifications such as the creation of defect sites, edge functionalization, and inter-layer distance manipulation, aiming to provide the general guidelines for designing these devices by the above approaches to achieve optimal performance. Furthermore, this review delves into the challenges and future prospects for the advancement of carbon-based electrodes in energy storage and conversion.

Boosting Cycling Stability of Polymer Sodium Battery by "Rigid-Flexible" Coupled Interfacial Stress Modulation

The discontinuous interfacial contact of solid-state polymer metal batteries is due to the stress changes in the electrode structure during cycling, resulting in poor ion transport. Herein, a rigid-flexible coupled interface stress modulation strategy is developed to solve the above issues, which is to design a rigid cathode with enhanced solid-solution behavior to guide the uniform distribution of ions and electric field. Meanwhile, the polymer components are optimized to build an organic-inorganic blended flexible interfacial film to relieve the change of interfacial stress and ensure rapid ion transmission. The fabricated battery comprising a Co-modulated P2-type layered cathode (Na_0.67Mn_2/3Co_1/3O₂) and a high ion conductive polymer could deliver good cycling stability without distinct capacity fading (72.8 mAh g^-1 over 350 cycles at 1 C), outperforming those without Co modulation or interfacial film construction. This work demonstrates a promising rigid-flexible coupled interfacial stress modulation strategy for polymer-metal batteries with excellent cycling stability.

How About Vanadium-Based Compounds as Cathode Materials for Aqueous Zinc Ion Batteries?

Aqueous zinc-ion batteries (AZIBs) stand out among many monovalent/multivalent metal-ion batteries as promising new energy storage devices because of their good safety, low cost, and environmental friendliness. Nevertheless, there are still many great challenges to exploring new-type cathode materials that are suitable for Zn2+ intercalation. Vanadium-based compounds with various structures, large layer spacing, and different oxidation states are considered suitable cathode candidates for AZIBs. Herein, the research advances in vanadium-based compounds in recent years are systematically reviewed. The preparation methods, crystal structures, electrochemical performances, and energy storage mechanisms of vanadium-based compounds (e.g., vanadium phosphates, vanadium oxides, vanadates, vanadium sulfides, and vanadium nitrides) are mainly introduced. Finally, the limitations and development prospects of vanadium-based compounds are pointed out. Vanadium-based compounds as cathode materials for AZIBs are hoped to flourish in the coming years and attract more and more researchers' attention.

Pore-forming mechanisms and sodium-ion-storage performances in a porous Na3V2(PO4)3/C composite cathode

Na₃V₂(PO₄)₃ (NVP) is regarded as one of the most promising cathode materials for sodium-ion batteries (SIBs). However, it suffers from a dense bulk structure and low intrinsic electronic conductivity, which lead to limited electrochemical performances. Herein, we propose a surfactant-assisted molding strategy to regulate the pore-forming process in NVP/C composite cathode materials. More precisely, the forming process of the pores in NVP could be easily controlled by utilizing the huge difference in critical micelle concentration of a surfactant (cetyltrimethylammonium bromide, CTAB) in water and ethanol. By reasonably modulating the ratio of water and ethanol in the solution, the as-synthesized NVP/C sample exhibited a three-dimensional interconnected structure with hierarchical micro/meso/macro-pores. Benefiting from these hierarchical porous structures in NVP/C, the structural stability, contact surface with the electrolyte, and electronic/ionic conductivity were improved simultaneously; whereby the optimized porous NVP/C sample exhibited an excellent high-rate performance (61.3 mA h g^-1 at 10 C) and superior cycling stability (90.2% capacity retention after 500 cycles at 10 C).

Activating the Extra Redox Couple of Co2+/Co3+ for a Synergistic K/Co Co-Substituted and Carbon Nanotube-Enwrapped Na3V2(PO4)3 Cathode with a Superior Sodium Storage Property

Na₃V₂(PO₄)₃ (NVP) materials have emerged as a promising cathode for sodium ion batteries (SIBs). Herein, NVP is successfully optimized by dual-doping K/Co and enwrapping carbon nanotubes (CNTs) through a sol-gel method. Naturally, the occupation of K and Co in the Na1 sites and V sites can efficiently stabilize the crystal cell and provide the expanded Na⁺ transport channels. The existence of tubular CNTs could restrict the crystal grain growth and effectively downsize the particle size and provide a shorter pathway for the migration of electrons and ions. Moreover, the amorphous carbon layers combined with the conductive CNTs form a favorable network for the accelerated electronic transportation. Furthermore, the ex situ XPS characterization reveals that an extra redox reaction pair of Co²⁺/Co³⁺ is successfully activated at the high voltage range, resulting in superior capacity and energy density property for KC0.05/CNTs composites. Comprehensively, the optimized KC0.05/CNTs electrode exhibits a distinctive electrochemical property. It delivers an initial reversible capacity of 119.4 mA h g^-1 at 0.1 C, surpassing the theoretic value for the NVP system (117.6 mA h g^-1). Moreover, the KC0.05/CNT electrode exhibits the initial capacity of 113.2 mA h g^-1 at 5 C and 105.8 mA h g^-1 at 10 C, and the maintained capacities at 500 cycles are 105.8 and 100.8 mA h g^-1 with outstanding retention values of 96.6 and 95.3%. Notably, it releases capacities of 99.8 and 84.5 mA h g^-1 at 50 and 100 C, and the capacity retention values at 2500 cycles are 66.2 and 58.8 mA h g^-1, respectively. What is more, the KC0.05/CNTs//Bi₂Se₃ asymmetric full cell exhibits a high capacity of 191.4 mA h g^-1 at 2.65 V, with the energy density being as high as 507 W h kg^-1, demonstrating the eminent practical application potential of KC0.05/CNTs in SIBs.

The Synthesis of Porous Na3 V2 (PO4 )3 for Sodium-Ion Storage

Na₃ V₂ (PO₄ )₃ (NVP) has been regarded as a potential cathode material for sodium-ion batteries (SIBs) due to its excellent structural stability and rapid Na⁺ conductivity. However, its electrochemical performances are restricted by the large bulk structure and poor electronic conductivity. The construction of porous NVP materials is a powerful method to improve the electrochemical properties. This concept aims to provide an overview of recent progress of porous NVP materials for SIBs. Herein, the synthetic strategies and formation mechanisms of porous NVP materials as well as the relationship between the porous structures and electrochemical performances of NVP materials are reviewed. Furthermore, the challenges and prospects for the preparation of porous NVP materials in this field are outlined.

Synthesis and electrochemical properties of Zn2Ti3O8/g-C3N4 composites as anode materials for Li-ion batteries

Zn₂Ti₃O₈/g-C₃N₄ (0, 1, 3 and 8 wt%) composites were prepared via a simple solvothermal method, and their physical and electrochemical properties were systematically analyzed. SEM and HRTEM results show that the Zn₂Ti₃O₈/g-C₃N₄ spherical structures with width sizes of about 500-700 nm are plump and uniform. Moreover, g-C₃N₄ with a large specific surface area can effectively buffer the deformation of Zn₂Ti₃O₈ and reduce the resistance of Zn₂Ti₃O₈ charge transfer and Li⁺ diffusion, thus improving the conductivity of Zn₂Ti₃O₈. The results reveal that Zn₂Ti₃O₈/g-C₃N₄ (3 wt%) had the most outstanding electrochemical performance of all samples. It can deliver discharge (charge) capacities of 444.6 (387.9), 284.5 (280.8), 197.5 (199.9), 149.9 (149.3), 119.2 (118.7) and 81.4 (81) mA h g^-1 cycled at 50, 100, 300, 600, 900, and 1500 mA g^-1, respectively. At the same current densities, pure Zn₂Ti₃O₈ only provides discharge (charge) capacities of 325.4 (283.5), 223.7 (219.5), 142.9 (141.8), 95.4(94.8), 69.4 (69.4) and 38.3 (39.3) mA h g^-1. The results verify that Zn₂Ti₃O₈/g-C₃N₄ materials are expected to be remarkable anode materials for Li-ion batteries.