Heterogeneous WSx/WO₃ Thorn-Bush Nanofiber Electrodes for Sodium-Ion Batteries

Localized nitride strategy to construct interfacial and electronic modulated WO3/WN nanoparticles for superior lithium-ion storage

The interfacial effect is important for the tungsten trioxide (WO3)-based anode to achieve superior lithium-ion storage performance. Herein, the interfacial effect was constructed by in-situ surface direct nitridation reaction at 600 ℃ for 30 min of the as-synthesis WO3 nanoparticles (WO3/WN). X-ray photoelectron spectroscopy (XPS) analysis confirms evident chemical interaction between WO3 and WN via the interfacial covalent bond (WON). This WO3/WN anode shows a distinct interfacial effect for an efficient interatomic electron migration. Electrochemical kinetic analysis shows enhanced pseudocapacitance contribution. The galvanostatic intermittent titration technique (GITT) result demonstrates improved charge transfer kinetics. Ex-situ X-ray diffraction (XRD) analysis reveals the reversible oxidation and reduction reaction of the WO3/WN anode. The density functional theory (DFT) result shows that the evident interfacial bonding effect can enhance the electrochemical reaction kinetics of the WO3/WN anode. The discharge capacity can reach up to 546.9 mA h g-1 at 0.1 A g-1 after 200 cycles. After 2000 cycles, the capacity retention is approximately 85.97 % at 1.0 A g-1. In addition, the WO3/WN full cell (LiFePO4/C//WO3/WN) demonstrates excellent rate capability and capacity retention ratio. This in-situ surface nitridation strategy is an effective solution for designing an oxide-based anode with good electrochemical performance and beyond.

Design and Realization of Visual and Contact-Type Fast Charging Power Source

Charging power supplies with both fast and visualization functions have a wide range of applications in the information and new energy industries. In this paper, the visualized and contact-type fast charging power supply based on WO3 film and Zn sheet is presented, and the prototype devices are fabricated. Different with the charging method of conventional batteries, charging is achieved by a Zn sheet contacting with a WO3 film moistened with water, resulting in a rapid discoloration of WO3. Theoretical investigation indicates that the interaction between Zn sheet and water molecules is the primary cause of the color change in the WO3 film. The WO3 film completes the colouring state within 10 s in the presence of Zn sheet and water, and the open-circuit voltage of the device is 0.7 V, which can be used to drive various electronics by series-parallel connection. This research introduces a novel method to induce colouring of WO3 films and proposes a fast charging mode different from traditional power sources. It provides valuable insights for the future development of fast charging in the field of electrical energy.

Trifunctional L-Cysteine Assisted Construction of MoO2/MoS2/C Nanoarchitecture Toward High-Rate Sodium Storage

The volume collapse and slow kinetics reaction of anode materials are two key issues for sodium ion batteries (SIBs). Herein, an "embryo" strategy is proposed for synthesis of nanorod-embedded MoO2/MoS2/C network nanoarchitecture as anode for SIBs with high-rate performance. Interestingly, L-cysteine which plays triple roles including sulfur source, reductant, and carbon source can be utilized to produce the sulfur vacancy-enriched heterostructure. Specifically, L-cysteine can combine with metastable monoclinic MoO3 nanorods at room temperature to encapsulate the "nutrient" of MoOx analogues (MoO2.5(OH)0.5 and MoO3·0.5H2O) and hydrogen-deficient L-cysteine in the "embryo" precursor affording for subsequent in situ multistep heating treatment. The resultant MoO2/MoS2/C presents a high-rate capability of 875 and 420 mAh g-1 at 0.5 and 10 A g-1, respectively, which are much better than the MoS2-based anode materials reported by far. Finite element simulation and analysis results verify that the volume expansion can be reduced to 42.8% from 88.8% when building nanorod-embedded porous network structure. Theoretical calculations reveal that the sulfur vacancies and heterointerface engineering can promote the adsorption and migration of Na+ leading to highly enhanced thermodynamic and kinetic reaction. The work provides an efficient approach to develop advanced electrode materials for energy storage.

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

Homologous Heterostructured NiS/NiS2 @C Hollow Ultrathin Microspheres with Interfacial Electron Redistribution for High-Performance Sodium Storage

Nickel sulfides with high theoretical capacity are considered as promising anode materials for sodium-ion batteries (SIBs); however, their intrinsic poor electric conductivity, large volume change during charging/discharging, and easy sulfur dissolution result in inferior electrochemical performance for sodium storage. Herein, a hierarchical hollow microsphere is assembled from heterostructured NiS/NiS2 nanoparticles confined by in situ carbon layer (H-NiS/NiS2 @C) via regulating the sulfidation temperature of the precursor Ni-MOFs. The morphology of ultrathin hollow spherical shells and confinement of in situ carbon layer to active materials provide rich channels for ion/electron transfer and alleviate the effects of volume change and agglomeration of the material. Consequently, the as-prepared H-NiS/NiS2 @C exhibit superb electrochemical properties, satisfactory initial specific capacity of 953.0 mA h g-1 at 0.1 A g-1 , excellent rate capability of 509.9 mA h g-1 at 2 A g-1 , and superior longtime cycling life with 433.4 mA h g-1 after 4500 cycles at 10 A g-1 . Density functional theory calculation shows that heterogenous interfaces with electron redistribution lead to charge transfer from NiS to NiS2 , and thus favor interfacial electron transport and reduce ion-diffusion barrier. This work provides an innovative idea for the synthesis of homologous heterostructures for high-efficiency SIB electrode materials.

Structural tuning of copper sulfide material for sodium-ion batteries

In this work, single-crystal and twin-crystal copper sulfide materials are constructed in a regulatable and controlled manner. Twin boundaries are engineered into the copper sulphide material to significantly improve its electrochemical performance. The results demonstrate that structure tuning with twin crystals is an effective strategy for enhancing electrochemical reactions, and also sheds light on the design of electrode materials for sodium ion storage.

Core-Shell Engineered WO3 Architectures: Recent Advances from Design to Applications

Ongoing efforts to design novel materials with efficient structure-property-performance relations prove challenging. Core-shell structures have emerged as novel materials with controlled production routes and highly tailorable properties that offer extensive advantages in advanced oxidation processing, particularly in photocatalysis and photoelectrochemical applications. WO₃ , which is an optoelectronically active semiconductor material, is a popular material in current studies in the field of photo(electro)catalysis. Considerable progress has been made using core-shell WO₃ architectures, which warrants an evaluation in terms of processing and preparedness for their use in versatile catalytic and energy storage applications. This paper presents an in-depth assessment of core-shell WO₃ architectures by highlighting the design challenges and protocols in powder and thin-film chemical processing. The development of specific core-shell designs for use in targeted applications, such as H₂ production, CO₂ reduction, wastewater treatment, batteries, supercapacitors, and sensing, is analyzed. The fundamental role of WO₃ in core-shell structures to enhance efficiency is also discussed, along with the limitations and improvement strategies. Further, the prospects of core-shell WO₃ architectures in energy conversion and environmental applications are suggested.

MoO3@MoS2 Core-Shell Structured Hybrid Anode Materials for Lithium-Ion Batteries

We explore a phase engineering strategy to improve the electrochemical performance of transition metal sulfides (TMSs) in anode materials for lithium-ion batteries (LIBs). A one-pot hydrothermal approach has been employed to synthesize MoS₂ nanostructures. MoS₂ and MoO₃ phases can be readily controlled by straightforward calcination in the (200–300) °C temperature range. An optimized temperature of 250 °C yields a phase-engineered MoO₃@MoS₂ hybrid, while 200 and 300 °C produce single MoS₂ and MoO₃ phases. When tested in LIBs anode, the optimized MoO₃@MoS₂ hybrid outperforms the pristine MoS₂ and MoO₃ counterparts. With above 99% Coulombic efficiency (CE), the hybrid anode retains its capacity of 564 mAh g⁻¹ after 100 cycles, and maintains a capacity of 278 mAh g⁻¹ at 700 mA g⁻¹ current density. These favorable characteristics are attributed to the formation of MoO₃ passivation surface layer on MoS₂ and reactive interfaces between the two phases, which facilitate the Li-ion insertion/extraction, successively improving MoO₃@MoS₂ anode performance.

Patchwork-Structured Heterointerface of 1T-WS2/a-WO3 with Sustained Hydrogen Spillover as a Highly Efficient Hydrogen Evolution Reaction Electrocatalyst

Using tungsten disulfide (WS2) as a hydrogen evolution reaction (HER) electrocatalyst brought on several ways to surpass its intrinsic catalytic activity. This study introduces a nanodomain tungsten oxide (WO3) interface to 1T-WS2, opening a new route for facilitating the transfer of a proton to active sites, thereby enhancing the HER performance. After H2S plasma sulfurization on the W layer to realize nanocrystalline 1T-WS2, subsequent O2 plasma treatment led to the formation of amorphous WO3 (a-WO3), resulting in a patchwork-structured heterointerface of 1T-WS2/a-WO3 (WSO). Addition of a hydrophilic interface (WO3) facilitates the hydrogen spillover effect, which represents the transfer of absorbed protons from a-WO3 to 1T-WS2. Moreover, the faster response of the cathodic current peak (proton insertion) in cyclic voltammetry is confirmed by the higher degree of oxidation. The rationale behind the faster proton insertion is that the introduced a-WO3 works as a proton channel. As a result, WSO-1.2 (the ratio of 1T-WS2 to a-WO3) exhibits a remarkable HER activity in that 1T-WS2 consumes more protons provided by the channel, showing an overpotential of 212 mV at 10 mA/cm2. Density functional theory calculations also show that the WO3 phase gives higher binding energies for initial proton adsorption, while the 1T-WS2 phase shows reduced HER overpotential. This improved catalytic performance demonstrates a novel strategy for water splitting to actively elicit the related reaction via efficient proton transport.

Core-Shell CoSe2 /WSe2 Heterostructures@Carbon in Porous Carbon Nanosheets as Advanced Anode for Sodium Ion Batteries

Heterojunction, with the advantage of fast charge transfer dynamics, is considered to be an effective strategy to address the low capacity and poor rate capability of anode materials for sodium-ion batteries (SIBs). As well, carbonaceous materials, as a crucial additive, can effectively ameliorate the ion/electron conductivity of integrated composites, realizing the fast ion transport and charge transfer. Here, motivated by the enhancement effect of carbon and heterojunction on conductivity, it is proposed that the CoSe₂ /WSe₂ heterojunction as inner core is coated by carbon outer shell and uniformly embedded in porous carbon nanosheets (denoted as CoSe₂ /WSe₂ @C/CNs), which is used as anode material for SIBs. Combining with density functional theoretical calculations, it is confirmed that the structure of heterojunction can introduce built-in electric-field, which can accelerate the transportation of Na⁺ and improve the conductivity of electrons. Moreover, the introduction of porous carbon nanosheets (CNs) can provide a channel for the transportation of Na⁺ and avoid the volume expansion during Na⁺ insertion and extraction process. As it is expected, CoSe₂ /WSe₂ @C/CNs anode displays ultrastable specific capacity of 501.9 mA h g^-1 at 0.1 A g^-1 over 200 cycles, and ultrahigh rate capacity of 625 mA h g^-1 at 0.1 A g^-1 after 100 cycles.

Black Tungsten Oxide Nanofiber as a Robust Support for Metal Catalysts: High Catalyst Loading for Electrochemical Oxygen Reduction

Black valve metal oxides with low oxygen vacancies are identified to be promising for various industrial applications, such as in gas sensing, photocatalysis, and rechargeable batteries, owing to their high reducibility and stability, as well as considerable fractions of low-valent metal species and oxygen vacancies in their lattices. Herein, the nanofiber (NF) of black oxygen-deficient tungsten trioxide (WO_{3- x} ) is presented as a versatile and robust support for the direct growth of a platinum catalyst for oxygen reduction reaction (ORR). The nonstoichiometric, poorly crystallized black WO_{3- x} NFs are prepared by electrospinning the W precursor into NFs followed by their low-temperature (650 °C) reductive calcination. The black WO_{3- x} NFs have adequate electrical conductivity owing to their decreased bandgap and amorphous structure. Remarkably, the oxygen-deficient surface (surface O/W = 2.44) facilitates the growth of small Pt nanoparticles, which resist aggregation, as confirmed by structural characterization and computational analysis. The Pt-loaded black WO_{3- x} NFs outperform the Pt-loaded crystalline white WO_{3- x} NFs in both the electrochemical ORR activity and the accelerated durability test. This study can inspire the use of oxygen-deficient metal oxides as supports for other electrocatalysts, and can further increase the versatility of oxygen-deficient metal oxides.

Transition metal sulfides meet electrospinning: versatile synthesis, distinct properties and prospective applications

One-dimensional (1D) electrospun nanomaterials have attracted significant attention due to their unique structures and outstanding chemical and physical properties such as large specific surface area, distinct electronic and mass transport, and mechanical flexibility. Over the past years, the integration of metal sulfides with electrospun nanomaterials has emerged as an exciting research topic owing to the synergistic effects between the two components, leading to novel and interesting properties in energy, optics and catalysis research fields for example. In this review, we focus on the recent development of the preparation of electrospun nanomaterials integrated with functional metal sulfides with distinct nanostructures. These functional materials have been prepared via two efficient strategies, namely direct electrospinning and post-synthesis modification of electrospun nanomaterials. In this review, we systematically present the chemical and physical properties of the electrospun nanomaterials integrated with metal sulfides and their application in electronic and optoelectronic devices, sensing, catalysis, energy conversion and storage, thermal shielding, adsorption and separation, and biomedical technology. Additionally, challenges and further research opportunities in the preparation and application of these novel functional materials are also discussed.

Controllable Insertion Mechanism of Expanded Graphite Anodes Employing Conversion Reaction Pillars for Sodium-Ion Batteries

Controlling the structural and reaction characteristics of carbonaceous anode materials is essential to realizing alternative alkali-ion batteries. In this study, we report on expanded graphite material employing MoS_x conversion reaction pillars (EG-MoS_x) inserted into the interlayers and assess them as potential anode candidates for Na-ion batteries. We succeed in a tailored control of the insertion characteristics between one-phase reaction and two-phase reaction by modifying the crystal structure of EG-MoS_x under different thermal treatment conditions. EG-MoS_x-900 anode with an enlarged interlayer of ∼5.38 Å delivers an exceptionally high capacity of 501 mAh g^-1. We successfully solve the irreversible capacity issues of the expanded graphite materials by forming chemical preformation of the solid electrolyte interface (SEI) layer on the electrode surface, thereby significantly increasing coulombic efficiencies of thermally tuned EG-MoS_x (52.20 → 97.25%). We elucidate the electrochemical mechanism and structural properties of the EG-MoS_x anode materials by ex situ characterizations. Inserting active sulfide pillars enables us to overcome the performance limitations of existing Na-ion battery technologies, and we expect that this strategy will be applied to realize another family of alkali-ion batteries.

Enhancing the Capacity and Stability of a Tungsten Disulfide Anode in a Lithium-Ion Battery Using Excess Sulfur

Tungsten disulfide (WS₂) is a transition metal disulfide and a promising anode material due to its layered structure, making it favorable for attaining lithium-ion batteries with rate capability and thermal/mechanical stability. Although WS₂ has a rich redox chemistry and a large density, which can increase the specific capacity and volumetric energy density, it still has an inferior specific capacity and poor long-term stability for practical use due to its insufficient space for the accommodation of lithium ions and large volume change during cycling. Herein, to overcome the chronic limitations of WS₂-based anodes, we propose a micron-sized tungsten disulfide/reduced graphene oxide composite by employing excess sulfur (S_x-WS₂/r-GO). In particular, the excess sulfur modifies the polarity of r-GO by chemically binding on the r-GO sheet during WS₂ formation, leading to an increase in the adsorption strength due to WS₂. Moreover, the excess sulfur increases the lattice parameter of WS₂ and decreases the crystallinity degree, securing additional sites for the accommodation of lithium ions. Therefore, the excess sulfur can increase the specific capacity and impede the separation of the pulverized WS₂ nanoparticles, hence mitigating structural decay during cycling. As a result, the S_x-WS₂/r-GO anode exhibits an enhanced specific capacity of 1426 mAh g^-1 at 0.4 A g^-1 and a high cyclic performance, with 87.9% capacity retention after 4000 cycles at a high current density (2.0 A g^-1).

Highly Efficient Photoelectrochemical Water Splitting Using GaN-Nanowire Photoanode with Tungsten Sulfides

In the present study, we have achieved high-performance photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) coated with tungsten sulfide (W_xS_1-x) (GaN-NW-W_xS_1-x) as a photoanode. The measured current density and applied-bias photon-to-current efficiency were 20.38 mA/cm² and 13.76%, respectively. These values were much higher than those reported previously for photoanodes with any kind of III-nitride nanostructure. The amount of hydrogen gas formed was 1.01 mmol/cm² from 7 h PEC-WS, which was also much higher than the previously reported values. The drastic improvement in the PEC-WS performance using the GaN-NW-W_xS_1-x photoanode was attributed to an increase in the number of photogenerated carriers due to the highly crystalline GaN NWs, and acceleration of separation of photogenerated carriers and consequent suppression of charge recombination because of nitrogen-terminated surfaces of NWs, sulfur vacancies in W_xS_1-x, and type-II band alignment between NW and W_xS_1-x. The degree of impedance matching, evaluated from Nyquist plots, was considered to analyze charge transfer characteristics at the interface between the GaN-NW-W_xS_1-x photoanode and 0.5-M H₂SO₄ electrolyte. Considering the material system and scheme for the PEC-WS, our approach provides an efficient way to improve hydrogen evolution reaction.

Co-Construction of Sulfur Vacancies and Heterojunctions in Tungsten Disulfide to Induce Fast Electronic/Ionic Diffusion Kinetics for Sodium-Ion Batteries

Engineering novel electrode materials with unique architectures has a significant impact on tuning the structural/electrochemical properties for boosting the performance of secondary battery systems. Herein, starting from well-organized WS₂ nanorods, an ingenious design of a one-step method is proposed to prepare a bimetallic sulfide composite with a coaxial carbon coating layer, simply enabled by ZIF-8 introduction. Rich sulfur vacancies and WS₂ /ZnS heterojunctions can be simultaneously developed, that significantly improve ionic and electronic diffusion kinetics. In addition, a homogeneous carbon protective layer around the surface of the composite guarantees an outstanding structural stability, a reversible capacity of 170.8 mAh g^-1 after 5000 cycles at a high rate of 5 A g^-1 . A great potential in practical application is also exhibited, where a full cell based on the WS_{2- x} /ZnS@C anode and the P2-Na_2/3 Ni_1/3 Mn_1/3 O₂ cathode can maintain a reversible capacity of 89.4 mAh g^-1 after 500 cycles at 1 A g^-1 . Moreover, the underlying electrochemical Na storage mechanisms are illustrated in detail by theoretical calculations, electrochemical kinetic analysis, and operando X-ray diffraction characterization.

Super-Expansion of Assembled Reduced Graphene Oxide Interlayers by Segregation of Al Nanoparticle Pillars for High-Capacity Na-Ion Battery Anodes

The applicability of Na-ion batteries is contingent on breakthroughs in alternative electrode materials that have high capacities and which are economically viable. Unfortunately, conventional graphite anodes for Li-ion battery systems do not allow Na-ion accommodation into their interlayer space owing to the large ionic radius and low stabilizing energy of Na in graphite. Here, we suggest a promising strategy for significantly increasing Na capacity by expanding the axial slab space of graphite. We successfully synthesized reconstructed graphite materials via self-assembly of negative graphite oxide (GO) flakes and Al cation (positive) pillars and by subsequent chemical reaction of the obtained Al-GO materials. Al pillars, atomically distributed in graphite interlayers, can extend the slab space by up to ∼7 Å, which is a 2-fold interlayer distance of pristine graphite. An exceptionally high capacity of 780 mAh/g is demonstrated for reconstructed graphite anodes with Al pillars, compared with rGO materials (210 mAh/g). We investigated the electrochemical reaction mechanism and structural changes associated with discharge and charge to emphasize the benefit of using reconstructed graphite as anodes in Na-ion batteries. Our strategy of modifying the interlayer distance by introducing metallic pillars between the layers can help address the low capacity of carbonaceous anodes.

Origin of the Superior Electrochemical Performance of Amorphous-Phase Conversion-Reaction-Based Electrode Materials for Na-Ion Batteries: Formation of a Bicontinuous Metal Network

The realization of conversion type electrode materials in Na-ion batteries (NIBs) has been hindered due to the nucleation property of the active material. During the sodiation, the transition metal (TM) cations reduce to the metallic state, and the respective anions react with the sodium ions. As a result, the metal particles are surrounded by the matrix of the insulating sodium compound, resulting in loss of electrical contact among the TM particles. Here, an amorphous molybdenum sulfide (a-MoS_x) electrode is made highly reversible by suppressing TM particle growth via elongating the cation diffusion pathway. Because of the long distance among Mo atoms in a-MoS_x, the growth of Mo nuclei is limited. This leads to more frequent nucleation and formation of smaller particles (3-5 nm in diameter). Since the smaller particles have a larger surface area than the bigger particles, the electrical contacts among Mo particles are clearly retained. The a-MoS_x anode for NIBs demonstrates a high capacity and excellent cycling retention. This work establishes that the amorphous structure enhances the reversibility and cycling stability of conversion-reaction-based electrodes by elongating the diffusion pathway of the metal ions.

Two-Dimensional Transition Metal Chalcogenides for Alkali Metal Ions Storage

On the heels of exacerbating environmental concerns and ever-growing global energy demand, development of high-performance renewable energy-storage and -conversion devices has aroused great interest. The electrode materials, which are the critical components in electrochemical energy storage (EES) devices, largely determine the energy-storage properties, and the development of suitable active electrode materials is crucial to achieve efficient and environmentally friendly EES technologies albeit the challenges. Two-dimensional transition-metal chalcogenides (2D TMDs) are promising electrode materials in alkali metal ion batteries and supercapacitors because of ample interlayer space, large specific surface areas, fast ion-transfer kinetics, and large theoretical capacities achieved through intercalation and conversion reactions. However, they generally suffer from low electronic conductivities as well as substantial volume change and irreversible side reactions during the charge/discharge process, which result in poor cycling stability, poor rate performance, and low round-trip efficiency. In this Review, recent advances of 2D TMDs-based electrode materials for alkali metal-ion energy-storage devices with the focus on lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium-ion batteries (PIBs), high-energy lithium-sulfur (Li-S), and lithium-air (Li-O₂ ) batteries are described. The challenges and future directions of 2D TMDs-based electrode materials for high-performance LIBs, SIBs, PIBs, Li-S, and Li-O₂ batteries as well as emerging alkali metal-ion capacitors are also discussed.

Review on Recent Progress in the Development of Tungsten Oxide Based Electrodes for Electrochemical Energy Storage

Current progress in the advancement of energy-storage devices is the most important factor that will allow the scientific community to develop resources to meet the global energy demands of the 21st century. Nanostructured materials can be used as effective electrodes for energy-storage devices because they offer various promising features, including high surface-to-volume ratios, exceptional charge-transport features, and good physicochemical properties. Until now, the successful research frontrunners have focused on the preparation of positive electrode materials for energy-storage applications; nevertheless, the electrochemical performance of negative electrodes is less frequently reported. This review mainly focuses on the current progress in the development of tungsten oxide-based electrodes for energy-storage applications, primarily supercapacitors (SCs) and batteries. Tungsten is found in various stoichiometric and nonstoichiometric oxides. Among the different tungsten oxide materials, tungsten trioxide (WO₃ ) has been intensively investigated as an electrode material for different applications because of its excellent charge-transport features, unique physicochemical properties, and good resistance to corrosion. Various WO₃ composites, such as WO₃ /carbon, WO₃ /polymers, WO₃ /metal oxides, and tungsten-based binary metal oxides, have been used for application in SCs and batteries. However, pristine WO₃ suffers from a relatively low specific surface area and low energy density. Therefore, it is crucial to thoroughly summarize recent progress in utilizing WO₃ -based materials from various perspectives to enhance their performance. Herein, the potential- and pH-dependent behavior of tungsten in aqueous media is discussed. Recent progress in the advancement of nanostructured WO₃ and tungsten oxide-based composites, along with related charge-storage mechanisms and their electrochemical performances in SCs and batteries, is systematically summarized. Finally, remarks are made on future research challenges and the prospect of using tungsten oxide-based materials to further upgrade energy-storage devices.

Conductive Electrospun Nanofiber Mats

Conductive nanofiber mats can be used in a broad variety of applications, such as electromagnetic shielding, sensors, multifunctional textile surfaces, organic photovoltaics, or biomedicine. While nanofibers or nanofiber from pure or blended polymers can in many cases unambiguously be prepared by electrospinning, creating conductive nanofibers is often more challenging. Integration of conductive nano-fillers often needs a calcination step to evaporate the non-conductive polymer matrix which is necessary for the electrospinning process, while conductive polymers have often relatively low molecular weights and are hard to dissolve in common solvents, both factors impeding spinning them solely and making a spinning agent necessary. On the other hand, conductive coatings may disturb the desired porous structure and possibly cause problems with biocompatibility or other necessary properties of the original nanofiber mats. Here we give an overview of the most recent developments in the growing field of conductive electrospun nanofiber mats, based on electrospinning blends of spinning agents with conductive polymers or nanoparticles, alternatively applying conductive coatings, and the possible applications of such conductive electrospun nanofiber mats.

Synthesis and gas sensing properties of WS2 nanocrystallites assembled hierarchical WS2 fibers by electrospinning

A gas sensor based on a hierarchical WS2 structure embedded with vertically aligned WS2 nanocrystallites was demonstrated. The three-dimensional (3D) hierarchical structure provides many edge sites of nanocrystallites and an extremely large gas contact volume, resulting in a high gas response. The decreased contact resistance between the 3D hierarchical WS2 fibers and sensor electrode resulted in improved NO2 response. We fabricated a one-dimensional (1D) conductive WS2 fiber using a two-step annealing process under sulfur flow (sulfurization). It delivers a continuous and conductive carrier path and lowers the potential barrier at the interface of the WS2 nanocrystallites (top) and electrospun WS2 fiber (bottom), resulting in an improved gas response. We developed 3D hierarchical WS2 fibers embedded with vertically aligned WS2 nanocrystallites to increase the gas adsorption site in comparison with that of 1D WS2 fibers without WS2 flakes. Vertically aligned WS2 nanocrystallites were formed after a two-step annealing treatment. Sensors based on the 3D hierarchical WS2 fibers embedded with WS2 flakes, showed higher response to NO2 gas in comparison to that of pure WS2 fibers without WS2 flakes.

Hierarchically Porous W-Doped CoP Nanoflake Arrays as Highly Efficient and Stable Electrocatalyst for pH-Universal Hydrogen Evolution

It is still challenging to develop high-efficiency and low-cost non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) in pH-universal electrolytes. Herein, hierarchically porous W-doped CoP nanoflake arrays on carbon cloth (W-CoP NAs/CC) are synthesized via facile liquid-phase reactions and a subsequent phosphorization process. The W-CoP NAs/CC hybrid can be directly employed as a binder-free electrocatalyst and delivers superior HER performance in pH-universal electrolytes. Especially, it delivers very low overpotentials of 89, 94, and 102 mV to reach a current density of 10 mA cm^-2 in acidic, alkaline, and neutral electrolytes, respectively. Furthermore, it shows a nearly 100% Faradaic efficiency as well as superior long-term stability with no decreasing up to 36 h in pH-universal electrolytes. The outstanding electrocatalytic performance of W-CoP NAs/CC can be mainly attributed to the porous W-doped nanoflake arrays, which not only afford rich exposed active sites, but also accelerate the access of electrolytes and the diffusion of H₂ bubbles, thus efficiently promoting the HER performance. This work provides a new horizon to rationally design and synthesize highly effective and stable non-noble metal phosphide-based pH-universal electrocatalysts for HER.

Heterogeneous Metal Oxide-Graphene Thorn-Bush Single Fiber as a Freestanding Chemiresistor

The development of freestanding fiber-type chemiresistors, having high integration ability with various portable electronics including smart clothing systems, is highly demanding for the next-generation wearable sensing platforms. However, critical challenges stemming from the irreversible chemical sensing kinetics and weak reliability of the freestanding fiber-type chemiresistor hinder their practical use. In this work, for the first time, we report on the potential suitability of the freestanding and ultraporous reduced graphene oxide fiber functionalized with WO₃ nanorods (porous WO₃ NRs-RGO composite fiber) as a sensitive nitrogen dioxide (NO₂) detector. By employing a tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose, the numerous mesopores are formed on a wet-spun TCNF-GO composite fiber, unlike a bare GO fiber with dense surface structure. More interestingly, due to the superior wettability of TCNF, the aqueous tungsten precursor is uniformly adsorbed on an ultraporous TCNF-GO fiber, and subsequent heat treatment results in the thermal reduction of a TCNF-GO fiber and hierarchical growth of WO₃ NRs perpendicular to the porous RGO fiber (porous WO₃ NRs-RGO fiber). The freestanding porous WO₃ NRs-RGO fiber shows a notable response to 1 ppm NO₂. Furthermore, we successfully demonstrate reversible NO₂ sensing characteristics of the porous WO₃ NRs-RGO fiber, which is integrated on a wrist-type wearable sensing device.

Transforming Nanofibers into Woven Nanotextiles for Vascular Application

This study investigates the unique properties, fabrication technique, and vascular applications of woven nanotextiles made from low-strength nanoyarns, which are bundles of thousands of nanofibers. An innovative robotic system was developed to meticulously interweave nanoyarns in longitudinal and transverse directions, resulting in a flexible, but strong woven product. This is the only technique for producing seamless nanotextiles in tubular form from nanofibers. The porosity and the mechanical properties of nanotextiles could be substantially tuned by altering the number of nanoyarns per unit area. Investigations of the physical and biological properties of the woven nanotextile revealed remarkable and fundamental differences from its nonwoven nanofibrous form and conventional textiles. This enhancement in the material property was attributed to the multitude of hierarchically arranged nanofibers in the woven nanotextiles. This patterned woven nanotextile architecture leads to a superhydrophilic behavior in an otherwise hydrophobic material, which in turn contributed to enhanced protein adsorption and consequent cell attachment and spreading. Short-term in vivo testing was performed, which proved that the nanotextile conduit was robust, suturable, kink proof, and nonthrombogenic and could act as an efficient embolizer when deployed into an artery.

A densely packed Sb2O3 nanosheet-graphene aerogel toward advanced sodium-ion batteries

As a promising anodic material for rechargeable batteries, Sb2O3 has drawn increasing attention due to its high theoretical capacity and abundant natural deposits. However, poor cyclability and rate performance of Sb2O3 derived from a large volume change during insertion/desertion reactions as well as a sluggish kinetic process restrict its practical application. Herein, we report a facile amorphous-to-crystalline strategy to synthesize a densely packed Sb2O3 nanosheet-graphene aerogel as a novel anode for sodium ion batteries (SIBs). This Sb2O3/graphene composite displays a reversible capacity as high as 657.9 mA h g-1 even after 100 cycles at 0.1 A g-1, along with an excellent rate capacity of 356.8 mA h g-1 at 5.0 A g-1. The superior electrochemical performance is attributed to the synergistic effects of densely packed Sb2O3 nanosheets and graphene aerogel, which serves as both a robust support and stable buffer layer to maintain the structural stability of the nanocomposite, and enhances the electrode kinetics of electrolyte diffusion and electron transfer simultaneously. Hence, this densely-packed two-dimensional Sb2O3 nanosheet-graphene aerogel can be a promising anode material for rechargeable SIBs due to its facile synthesis process and outstanding electrochemical performance.

Nanocomposite Materials for the Sodium-Ion Battery: A Review

Clean energy has become an important topic in recent decades because of the serious global issues related to the development of energy, such as environmental contamination, and the intermittence of the traditional energy sources. Creating new battery-related energy storage facilities is an urgent subject for human beings to address and for solutions for the future. Compared with lithium-based batteries, sodium-ion batteries have become the new focal point in the competition for clean energy solutions and have more potential for commercialization due to the huge natural abundance of sodium. Nevertheless, sodium-ion batteries still exhibit some challenges, like inferior electrochemical performance caused by the bigger ionic size of Na⁺ ions, the detrimental volume expansion, and the low conductivity of the active materials. To solve these issues, nanocomposites have recently been applied as a new class of electrodes to enhance the electrochemical performance in sodium batteries based on advantages that include the size effect, high stability, and excellent conductivity. In this Review, the recent development of nanocomposite materials applied in sodium-ion batteries is summarized, and the existing challenges and the potential solutions are presented.

1D Nanomaterials: Design, Synthesis, and Applications in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have received extensive attention as ideal candidates for large-scale energy storage systems (ESSs) owing to the rich resources and low cost of sodium (Na). However, the larger size of Na⁺ and the less negative redox potential of Na⁺ /Na result in low energy densities, short cycling life, and the sluggish kinetics of SIBs. Therefore, it is necessary to develop appropriate Na storage electrode materials with the capability to host larger Na⁺ and fast ion diffusion kinetics. 1D materials such as nanofibers, nanotubes, nanorods, and nanowires, are generally considered to be high-capacity and stable electrode materials, due to their uniform structure, orientated electronic and ionic transport, and strong tolerance to stress change. Here, the synthesis of 1D nanomaterials and their applications in SIBs are reviewed. In addition, the prospects of 1D nanomaterials on energy conversion and storage as well as the development and application orientation of SIBs are presented.

Formation of multi-shelled nickel-based sulfide hollow spheres for rechargeable alkaline batteries

Metal sulfides with multi-shelled hollow structures are promising electrode materials for rechargeable alkaline batteries due to their abundant redox centers and electrochemically active sites.

Heterogeneous Ti3SiC2@C-Containing Na2Ti7O15 Architecture for High-Performance Sodium Storage at Elevated Temperatures

Rational design of heterogeneous electrode materials with hierarchical architecture is a potential approach to significantly improve their energy densities. Herein, we report a tailored microwave-assisted synthetic strategy to create heterogeneous hierarchical Ti₃SiC₂@C-containing Na₂Ti₇O₁₅ (MAX@C-NTO) composites as potential anode materials for high-performance sodium storage in a wide temperature range from 25 to 80 °C. This composite delivers first reversible capacities of 230 mAh g^-1 at 200 mA g^-1 and 149 mAh g^-1 at 3000 mA g^-1 at 25 °C. A high capacity of ∼93 mAh g^-1 without any apparent decay even after more than 10 000 cycles is obtained at an ultrahigh current density of 10 000 mA g^-1. Moreover, both a high reversible capacity and an ultralong durable stability are achieved below 60 °C for the same composites, wherein a 75.2% capacity retention (∼120 mAh g^-1 at 10 000 mA g^-1) is achieved after 3000 cycles at 60 °C. To the best of our knowledge, both the sodium storage performances and the temperature tolerances outperform those of all the Ti-based sodium storage materials reported so far. The superior sodium storage performances of the as-synthesized composites are attributed to the heterogeneous core-shell architecture, which not only provides fast kinetics by high pseudocapacitance but also prolongs cycling life by preventing particle agglomeration and facilitates the transportation of electrons and sodium ions by large micro/mesopore structure.

Advances and Challenges in Metal Sulfides/Selenides for Next-Generation Rechargeable Sodium-Ion Batteries

Rechargeable sodium-ion batteries (SIBs), as the most promising alternative to commercial lithium-ion batteries, have received tremendous attention during the last decade. Among all the anode materials for SIBs, metal sulfides/selenides (MXs) have shown inspiring results because of their versatile material species and high theoretical capacity. They suffer from large volume expansion, however, which leads to bad cycling performance. Thus, methods such as carbon modification, nanosize design, electrolyte optimization, and cut-off voltage control are used to obtain enhanced performance. Here, recent progress on MXs is summarized in terms of arranging the crystal structure, synthesis methods, electrochemical performance, mechanisms, and kinetics. Challenges are presented and effective ways to solve the problems are proposed, and a perspective for future material design is also given. It is hoped that light is shed on the development of MXs to help finally find applications for next-generation rechargeable batteries.

Facile Solution Synthesis of Tungsten Trioxide Doped with Nanocrystalline Molybdenum Trioxide for Electrochromic Devices

A facile, highly efficient approach to obtain molybdenum trioxide (MoO3)-doped tungsten trioxide (WO3) is reported. An annealing process was used to transform ammonium tetrathiotungstate [(NH4)2WS4] to WO3 in the presence of oxygen. Ammonium tetrathiomolybdate [(NH4)2MoS4] was used as a dopant to improve the film for use in an electrochromic (EC) cell. (NH4)2MoS4 at different concentrations (10, 20, 30, and 40 mM) was added to the (NH4)2WS4 precursor by sonication and the samples were annealed at 500 °C in air. Raman, X-ray diffraction, and X-ray photoelectron spectroscopy measurements confirmed that the (NH4)2WS4 precursor decomposed to WO3 and the (NH4)2MoS4-(NH4)2WS4 precursor was transformed to MoO3-doped WO3 after annealing at 500 °C. It is shown that the MoO3-doped WO3 film is more uniform and porous than pure WO3, confirming the doping quality and the privileges of the proposed method. The optimal MoO3-doped WO3 used as an EC layer exhibited a high coloration efficiency of 128.1 cm2/C, which is larger than that of pure WO3 (74.5 cm2/C). Therefore, MoO3-doped WO3 synthesized by the reported method is a promising candidate for high-efficiency and low-cost smart windows.

Red Phosphorus Nanodots on Reduced Graphene Oxide as a Flexible and Ultra-Fast Anode for Sodium-Ion Batteries

Sodium-ion batteries offer an attractive option for potential low cost and large scale energy storage due to the earth abundance of sodium. Red phosphorus is considered as a high capacity anode for sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However, similar to silicon in lithium-ion batteries, several limitations, such as large volume expansion upon sodiation/desodiation and low electronic conductance, have severely limited the performance of red phosphorus anodes. In order to address the above challenges, we have developed a method to deposit red phosphorus nanodots densely and uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the sodium ion diffusion length and the sodiation/desodiation stresses, and the RGO network also serves as electron pathway and creates free space to accommodate the volume variation of phosphorus particles. The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3 mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g charge/discharge current density in rate capability test, and a 914 mAh/g capacity after 300 deep cycles in cycling stability test at 1593.9 mA/g current density, which marks a significant performance improvement for red phosphorus anodes for sodium-ion chemistry and flexible power sources for wearable electronics.

Flexible Paper-like Free-Standing Electrodes by Anchoring Ultrafine SnS2 Nanocrystals on Graphene Nanoribbons for High-Performance Sodium Ion Batteries

Ultrafine SnS₂ nanocrystals-reduced graphene oxide nanoribbon paper (SnS₂-RGONRP) has been created by a well-designed process including in situ reduction, evaporation-induced self-assembly, and sulfuration. The as-formed SnS₂ nanocrystals possess an average diameter of 2.3 nm and disperse on the surface of RGONRs uniformly. The strong capillary force formed during evaporation leads to a compact assembly of RGONRs to give a flexible paper structure with a high density of 0.94 g cm^-3. The as-prepared SnS₂-RGONRP composite could be directly used as free-standing electrode for sodium ion batteries. Due to the synergistic effects between the ultrafine SnS₂ nanocrystals and the conductive, tightly connected RGONR networks, the composite paper electrode exhibits excellent electrochemical performance. A high volumetric capacity of 508-244 mAh cm^-3 was obtained at current densities in the range of 0.1-10 A g^-1. Discharge capacities of 334 and 255 mAh cm^-3 were still kept, even after 1500 cycles tested at current densities of 1 and 5 A g^-1, respectively. This strategy provides insight into a new pathway for the creation of free-standing composite electrodes used in the energy storage and conversion.

Surfactant-driven direct synthesis of a hierarchical hollow MgO nanofiber–nanoparticle composite by electrospinning

A single step synthesis of hierarchical MgO nanoparticle–nanofiber composite using surfactant driven electrospinning.