Hierarchical MoSe₂ yolk-shell microspheres with superior Na-ion storage properties

Metal Selenides Find Plenty of Space in Architecting Advanced Sodium/Potassium Ion Batteries

The rapid evolution of smart grid system urges researchers on exploiting systems with properties of high-energy, low-cost, and eco-friendly beyond lithium-ion batteries. Under the circumstances, sodium- and potassium-ion batteries with the semblable work mechanism to commercial lithium-ion batteries, hold the merits of cost-effective and earth-abundant. As a result, it is deemed a promising candidate for large-scale energy storage devices. Exploiting appropriate active electrode materials is in the center of the spotlight for the development of batteries. Metal selenides with special structures and relatively high theoretical capacity have aroused broad interest and achieved great achievements. To push the smooth development of metal selenides and enhancement of the electrochemical performance of sodium- and potassium-ion batteries, it is vital to grasp the inherent properties and electrochemical mechanisms of these materials. Herein, the state-of-the-art development and challenges of metal selenides are summarized and discussed. Meanwhile, the corresponding electrochemical mechanism and future development directions are also highlighted.

Recent Advancement and Structural Engineering in Transition Metal Dichalcogenides for Alkali Metal Ions Batteries

Currently, transition metal dichalcogenides-based alkaline metal ion batteries have been extensively investigated for renewable energy applications to overcome the energy crisis and environmental pollution. The layered morphologys with a large surface area favors high electrochemical properties. Thermal stability, mechanical structural stability, and high conductivity are the primary features of layered transition metal dichalcogenides (L-TMDs). L-TMDs are used as battery materials and as supporters for other active materials. However, these materials still face aggregation, which reduces their applicability in batteries. In this review, a comprehensive study has been undertaken on recent advancements in L-TMDs-based materials, including 0D, 1D, 2D, 3D, and other carbon materials. Types of structural engineering, such as interlayer spacing, surface defects, phase control, heteroatom doping, and alloying, have been summarized. The synthetic strategy of structural engineering and its effects have been deeply discussed. Lithium- and sodium-ion battery applications have been summarized in this study. This is the first review article to summarize different morphology-based TMDs with their intrinsic properties for alkali metal ion batteries (AMIBs), so it is believed that this review article will improve overall knowledge of TMDs for AMIBS applications.

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

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

Advances and challenges in metal selenides enabled by nanostructures for electrochemical energy storage applications

The development of nanomaterials and their related electrochemical energy storage (EES) devices can provide solutions for improving the performance and development of existing EES systems owing to their high electronic conductivity and ion transport and abundant embeddable sites. Recent progress has demonstrated that metal selenides are attracting increasing attention in the field of EES because of their unique structures, high theoretical capacities, rich element resources, and high conductivity. However, there are still many challenges in their application in EES, and thus the use of nanoscale metal selenide materials in commercial devices is limited. In this review, we summarize recent advances in the nanostructured design of metal selenides (e.g., zero-, one-, two-, and three-dimensional, and self-supported structures) and present their advantages in terms of EES performance. Moreover, some remarks on the potential challenges and research prospects of nanostructured metal selenides in the field of EES are presented.

First-principles calculations of bulk WX2(X = Se, Te) as anode materials for Na ion battery

Two-dimensional transition metal dichalcogenides are promising anode materials for Na ion batteries (NIBs). In this study, we carried out a comprehensive investigation to analyze the structural, electrochemical characteristics, and diffusion kinetics of bulk WX₂(X = Se, Te) by employing first-principles calculation in the framework of density functional theory. We deeply studied the full intercalation of Na⁺in WX₂and diagnosed Na_yX phase through conversion reaction mechanism. The voltage range of 2.05-0.48 V vs Na/Na⁺for Na_yWSe₂and 2.26-0.65 V for Na_yWTe₂(y= 0-3) have been noted. Density of states analysis showed metallic behavior of WX₂(X = Se, Te) during sodiation. The facile pathways for Na⁺mobility through WX₂have shown that tungsten dichalcogenides are inferred as excellent electrode material for NIBs.

Natural ore molybdenite as a high-capacity and cheap anode material for advanced lithium-ion capacitors

Hybrid lithium-ion capacitors (LICs) receive special interests because they work by combining the merits of high-capacity lithium-ion batteries and high-rate capacitors in a Li salt containing electrolyte, so as to bridge the gap between the two devices. One of main challenges for LICs is to develop inexpensive and superior anode materials at high rates. In this work, natural molybdenite was utilized as precursor to achieve the scalable production of cheap MoS₂/carbon composites. This molybdenite-derived MoS₂/carbon electrode can not only exhibit excellent Li⁺-storage performances including ultrahigh specific capacity (1427 mAh g^-1after 1000 cycles at 1 A g^-1) and rate capability (554 mAh g^-1at 10 A g^-1), but also possess four-times higher tap density than that of commercial graphite. By employing MoS₂/carbon as the anode and activated carbon as the cathode, the as-assembled LIC device delivers both high energy//high power density and long cycle lifespan. Furthermore, the price is nearly 200 orders of magnitude lower than the traditional high-purity chemicals, which can be easily scaled up to achieve high-throughput production.

Topological transformation construction of CoSe2/N-doped carbon heterojunction with three-dimensional porous structure for high-performance sodium-ion half/full batteries

Transition metal selenides have been widely used as anode materials for sodium-ion batteries (SIBs) because of their considerable theoretical capacity and good conductivity. Nevertheless, the volume expansion is a serious...

Identifying the Evolution of Selenium-Vacancy-Modulated MoSe2 Precatalyst in Lithium-Sulfur Chemistry

Witnessing compositional evolution and identifying the catalytically active moiety of electrocatalysts is of paramount importance in Li-S chemistry. Nevertheless, this field remains elusive. We report the scalable salt-templated synthesis of Se-vacancy-incorporated MoSe₂ architecture (SeVs-MoSe₂ ) and reveal the phase evolution of the defective precatalyst in working Li-S batteries. The interaction between lithium polysulfides and SeVs-MoSe₂ is probed to induce the transformation from SeVs-MoSe₂ to MoSeS. Furthermore, operando Raman spectroscopy and ex situ X-ray diffraction measurements in combination with theoretical simulations verify that the effectual MoSeS catalyst could help promote conversion of Li₂ S₂ to Li₂ S, thereby boosting the capacity performance. The Li-S battery accordingly exhibits a satisfactory rate and cycling capability even with and elevated sulfur loading and lean electrolyte conditions (7.67 mg cm^-2 ; 4.0 μL mg^-1 _S ). This work elucidates the design strategies and catalytic mechanisms of efficient electrocatalysts bearing defects.

Highly conductive Co3Se4 embedded in N-doped 3D interconnected carbonaceous network for enhanced lithium and sodium storage

Traditional cobalt selenides as active materials in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) would suffer from drastic volume expansions and large stacking effects, leading to a low cycling stability. In this work, we utilized a facile template method for preparing Co₃Se₄@N-CN (CSNC) that encapsulated Co₃Se₄ nanoparticles into 3D interconnected nitrogen-doped carbon network (N-CN). Satisfactorily, it possesses excellent cycling stability with enhanced lithium and sodium energy storage capacity. As an anode material in LIBs, CSNC exhibited a prominent reversible discharge performance of 1313.5 mAh g^-1 after 100 cycles at 0.1 A g^-1 and 835.6 mAh g^-1 after 500 cycles at 1.0 A g^-1. Interestingly, according to the analysis from cyclic voltammetry, the in-situ generated Se might provide extra capacity that leaded to a rising trend of capacity. When utilized as an anode in SIBs, CSNC delivered an outstanding capacity of 448.7 mAh g^-1 after 100 cycles at 0.1 A g^-1 and could retain 328.9 mAh g^-1 (77.2% of that of 0.1 A g^-1) even at a high current density of 5.0 A g^-1. The results demonstrate that CSNC is a superior anode material in LIBs and SIBs with great promise. More importantly, this strategy opens up an effective avenue for the design of transition metal selenide/carbonaceous composites for advanced battery storage systems.

Sheet-to-layer structure of SnSe2/MXene composite materials for advanced sodium ion battery anodes

The synergistic SnSe₂/MXene composite was synthesized through electrostatic assembly and exhibits a good electrochemical performance for use in a sodium-ion battery.

Room temperature ethanol gas-sensing properties based on Ag-doped MoSe2 nanoflowers: experimental and DFT investigation

Nanoflower-like Ag-doped MoSe2 nanocomposites were prepared by a simple hydrothermal method for room-temperature ethanol detection with enhanced sensing performance.

Construction of series-wound architectures composed of metal-organic framework-derived hetero-(CoFe)Se2 hollow nanocubes confined into a flexible carbon skeleton as a durable sodium storage anode

Metal chalcogenides with structural pulverization/degradation and intrinsic low electrical conductivity trigger the challenging issues of serious capacity fading and inferior rate capability upon repeated de-/sodiation cycling. Multiple electroactive heterostructures can integrate the inherent advantages of a strong synergistic coupling effect to improve their electrochemical Na+-storage behavior and structural durability, showing robust mechanical features, fast Na+ immigration and abundant active insertion sites at intriguing heterointerfaces. Hence, a series-wound architecture of metal-organic framework (MOF)-derived heterogeneous (CoFe)Se2 hollow nanocubes confined into a one-dimension carbon nanofiber skeleton ((CoFe)Se2@CNS) was successfully developed via a template-assisted liquid phase anion exchange followed by electrospinning and conventional selenization treatment. When examined as an anode for sodium ion batteries, the (CoFe)Se2@CNS electrode exhibits remarkably enhanced electrochemical Na+-storage performance delivering a high sodiation capacity as high as 213.9 mA h g-1 after 3650 cycles at 5 A g-1 with a capacity degradation rate of only 0.0047% per cycle; specifically, it shows tremendous rate performance and ultrastable cycling durability of 194.7 mA h g-1 at a high rate of 8 A g-1 after 5630 cycles. This work can shed light on a fundamental approach for designing heterostructures of multiple electroactive components toward high-performance alkali metal ion batteries.

Tunable Surface Selenization on MoO2 -Based Carbon Substrate for Notably Enhanced Sodium-Ion Storage Properties

Transition metal chalcogenides with high theoretical capacity are promising conversion-type anode materials for sodium ion batteries (SIBs), but often suffer from unsatisfied cycling stability (hundreds of cycles) caused by structural collapse and agglomerate. Herein, a rational strategy of tunable surface selenization on highly crystalline MoO₂ -based carbon substrate is designed, where the sheet-like MoSe₂ can be coated on the surface of bundle-like N-doped carbon/granular MoO₂ substrate, realizing partial transformation from MoO₂ to MoSe₂ , and creating b-NC/g-MoO₂ @s-MoSe₂ -10 with robust hierarchical MoO₂ @MoSe₂ heterostructures and strong chemical couplings (MoC and MoN). Such well-designed architecture can provide signally improved reaction kinetics and reinforced structural integrity for fast and stable sodium-ion storage, as confirmed by the ex situ results and kinetic analyses as well as the density functional theory calculations. As expected, the b-NC/g-MoO₂ @s-MoSe₂ -10 delivers splendid rate capability and ultralong cycling stability (254.2 mAh g^-1 reversible capacity at 5.0 A g^-1 after 6000 cycles with ≈89.0% capacity retention). Therefore, the tunable surface strategy can provide new insights for designing and constructing heterostructures of transition metal chalcogenides toward high-performance SIBs.

SeC Bonding Promoting Fast and Durable Na+ Storage in Yolk-Shell SnSe2 @SeC

Tin-based compounds have received much attention as anode materials for lithium/sodium ion batteries owing to their high theoretical capacity. However, the huge volume change usually leads to the pulverization of electrode, giving rise to a poor cycle performance, which have severely hampered their practical application. Herein, highly durable yolk-shell SnSe₂ nanospheres (SnSe₂ @SeC) are prepared by a multistep templating method, with an in situ gas-phase selenization of the SnO₂ @C hollow nanospheres. During this process, Se can be doped into the carbon shell with a tunable amount and form SeC bonds. Density functional theory calculation results reveal that the SeC bonding can enhance the charge transfer properties as well as the binding interaction between the SnSe₂ core and the carbon shell, favoring an improved rate performance and a superior cyclability. As expected, the sample delivers reversible capacities of 441 and 406 mAh g^-1 after 2000 cycles at 2 and 5 A g^-1 , respectively, as the anode material for a sodium-ion battery. Such performances are significantly better than the control sample without the SeC bonding and also other metal selenide-based anodes, evidently showing the advantage of Se doping in the carbon shell.

Nanosheets- and nanourchins-like nanostructures of MoSe2 for photocatalytic water purification: kinetics and reusability study

In this paper, we are reporting a simple hydrothermal technique for preparation of MoSe₂ nanostructures (nanourchins and nanosheets) using selenium and sodium molybdate as precursors. Samples are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, UV-Vis spectroscopy, Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). The FESEM revealed that the morphology of materials was varying significantly by changing pH value during synthesis. Photocatalytic degradation of anionic dye (MO), cationic dye (MB), and reduction of Cr(VI) into Cr(III) were performed. Nanosheets and nanourchins showed higher photocatalytic activity, enhanced photocatalytic degradation efficiency is correlated with the higher ^•OH radical concentration, crystallinity of material, and large surface area as evident through XPS, XRD, and BET, respectively. Photocatalysis mechanism along with role of reactive species (^•OH and holes) were explained using trapping experiments. Identification of degraded products was carried out using high-performance liquid chromatography (HPLC). Reaction kinetics and reusability of materials were also studied; wherein, it was observed that the materials have reusable properties.

Simultaneously formed and embedding-type ternary MoSe2/MoO2/nitrogen-doped carbon for fast and stable Na-ion storage

To obtain an electrode material that is capable of manifesting high Na-ion storage capacity during long-term cycling at a rapid discharge/charge rate, ternary heterophases MoSe₂/MoO₂/carbon are rationally designed and synthesized through a supermolecule-assisted strategy. Through using supermolecules that are constructed from MoO₄ ^2- and polydopamine as the precursor and sulfonated polystyrene microspheres as the sacrificial template, the in situ formed ternary phases MoSe₂/MoO₂/carbon are fabricated into a hollow microspherical structure, which is assembled from ultrathin nanosheets with MoSe₂ and MoO₂ nanocrystallites strongly embedded in a nitrogen-doped carbon matrix. In the ternary phases, the MoSe₂ phase contributes to a high Na-ion storage capacity by virtue of its layered crystalline structure with a wide interlayer space, while the surrounding MoO₂ and porous nitrogen-doped carbon phases are conducive to rate behaviour and cycling stability of the ternary hybrids since both the two phases are beneficial for electronic transport and structural stability of MoSe₂ during repeated sodiation/desodiation reaction. The as-prepared MoSe₂/MoO₂/carbon manifests excellent rate behaviour (a Na-ion storage capacity of 461 mA h g^-1 at an extremely high current density of 70 A g^-1) and outstanding cycle performance (610 mA h g^-1 after 1000 cycles).

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.

Yolk-Shell Nanostructures: Syntheses and Applications for Lithium-Ion Battery Anodes

Yolk-shell nanostructures have attracted tremendous research interest due to their physicochemical properties and unique morphological features stemming from a movable core within a hollow shell. The structural potential for tuning inner space is the focal point of the yolk-shell nanostructures in a way that they can solve the long-lasted problem such as volume expansion and deterioration of lithium-ion battery electrodes. This review gives a comprehensive overview of the design, synthesis, and battery anode applications of yolk-shell nanostructures. The synthetic strategies for yolk-shell nanostructures consist of two categories: templating and self-templating methods. While the templating approach is straightforward in a way that the inner void is formed by removing the sacrificial layer, the self-templating methods cover various different strategies including galvanic replacement, Kirkendall effect, Ostwald ripening, partial removal of core, core injection, core contraction, and surface-protected etching. The battery anode applications of yolk-shell nanostructures are discussed by dividing into alloying and conversion types with details on the synthetic strategies. A successful design of yolk-shell nanostructures battery anodes achieved the improved reversible capacity compared to their bare morphologies (e.g., no capacity retention in 300 cycles for Si@C yolk-shell vs. capacity fading in 10 cycles for Si@C core-shell). This review ends with a summary and concluding remark yolk-shell nanostructures.

Construction of 1T-MoSe2 /TiC@C Branch-Core Arrays as Advanced Anodes for Enhanced Sodium Ion Storage

The use of active sites and reaction kinetics of MoSe₂ anodes for sodium ion batteries (SIBs) are highly related to the phase components (1T and 2H phases) and electrode architecture. This study concerns the design and fabrication of wrinkled 1T-MoSe₂ nanoflakes anchored on highly conductive TiC@C nanorods to form 1T-MoSe₂ /TiC@C branch-core arrays by a powerful chemical vapor deposition (CVD)-solvothermal method. The 1T-MoSe₂ branch can be easily transformed into its 2H-MoSe₂ counterpart after a facile annealing process. In comparison to 2H-MoSe₂ , 1T-MoSe₂ has larger interlayer spacing and higher electronic conductivity, which are beneficial for the acceleration of reaction kinetics and capacity improvement. In addition, direct growth of 1T-MoSe₂ nanoflakes on the TiC@C skeleton not only enhance the electrical conductivity, but also contribute to reinforced structural stability. Accordingly, 1T-MoSe₂ /TiC@C branch-core arrays are demonstrated with higher capacity and better rate performance (184 mAh g^-1 at 10 A g^-1 ) and impressive durability over 500 cycles with a capacity retention of approximately 91.8 %. This phase modulation plus branch-core design provides a general method for the synthesis of other high-performance electrode materials for application in electrochemical energy storage.

Hierarchical Tubular-Structured MoSe2 Nanosheets/N-Doped Carbon Nanocomposite with Enhanced Sodium Storage Properties

Intimately coupled carbon/molybdenum-based hierarchical nanostructures are promising anodes for high-performance sodium-ion batteries owing to the combined effects of the two components and their robust structural stability. Mo-polydopamine (PDA) complexes are appealing precursors for the preparation of various Mo-based nanostructures containing N-doped carbon (NC). A facile method for the fabrication of hierarchical tubular nanocomposites with intimately coupled MoSe₂ and NC nanosheets has been developed, which involves the preparation of Mo-PDA hybrid nanotubes through a chemical route followed by two heat treatments. The strong coupling between Mo anions and the catechol groups in dopamine not only restricts the crystallite size but also inhibits agglomeration during selenization, resulting in few-layered MoSe₂ nanosheets embedded in hierarchical NC substrates. The as-synthesized nanotube composites are constructed by assembling primary MoSe₂ /NC nanosheets. This unique structure not only increases the number of active sites but also shortens the diffusion length of ions and enhances the electronic conductivity of electrode materials. The as-synthesized hierarchical MoSe₂ /NC nanotubes deliver a high capacity of 429 mAh g^-1 at 1 A g^-1 after the 150th cycle when used as anodes in sodium-ion batteries. Furthermore, at a high current density of 10 A g^-1 , a high discharge capacity of 236 mAh g^-1 is achieved.

Dual-Functional Template-Directed Synthesis of MoSe2/Carbon Hybrid Nanotubes with Highly Disordered Layer Structures as Efficient Alkali-Ion Storage Anodes beyond Lithium

Sodium/Potassium-ion batteries (SIBs/PIBs) have recently received tremendous attention because of their particular features of cost-effectiveness and promising energy density, which hold great potential for large-scale applications. Nevertheless, it still has a common bottleneck issue that is the sluggish kinetics of Na⁺/K⁺ intercalation, which raises more rigorous requirement on the electrode candidates regarding the morphology, dimension, and architecture. Herein, we have constructed unique MoSe₂-based hybrid nanotubes with wall structures composed of highly disordered MoSe₂ layers embedded in phosphorus and nitrogen co-doped carbon matrix (denoted MoSe₂⊂PNC-HNTs), by a facile two-step strategy using Se nanorods as the dual-functional template, i.e., shape-directed agent and in situ selenization resources. Benefitting from the combined features of the one-dimensional (1D) hollow interior, hybrid wall structure with high disorder, and the phosphorus and nitrogen co-doping-induced abundant defect sites in the carbon matrix, the MoSe₂⊂PNC-HNT anode exhibits high specific capacities of 280 and 262 mA h g^-1 over 200 cycles at the current density of 0.1 A g^-1 for Na⁺ and K⁺ storage, respectively, and achieves remarkable capacity retention rates of 87.0% at 2 A g^-1 over 3500 cycles for Na-ion storage and 80.1% at 1 A g^-1 after 500 cycles for K-ion storage.

SnSe2 Nanoparticles Chemically Embedded in a Carbon Shell for High-Rate Sodium-Ion Storage

The development of advanced anode materials is crucial to enhance the performance of sodium-ion batteries (SIBs). In this study, SnSe₂ nanoparticles chemically embedded in a carbon shell (SnSe₂@C) were fabricated from Sn-organic frameworks and evaluated as an anode material for SIBs. The structural characterization demonstrated that there existed C-Sn chemical bonds between the SnSe₂ nanoparticles and carbon shell, which could strongly anchor SnSe₂ nanoparticles to the carbon shell. Such a structure can not only facilitate charge transfer but also ensure the structural stability of the SnSe₂@C electrode. In addition, the carbon shell also helped in the dispersion of SnSe₂ nanoparticles, thus offering more redox-active sites for Na⁺ storage. The as-prepared SnSe₂@C nanocomposite could deliver good cycling stability and a superior rate capability of 324 mA h g^-1 at 2 A g^-1 for SIBs.

MOF-derived yolk–shell Ni/C architectures assembled with Ni@C core–shell nanoparticles for lightweight microwave absorbents

The yolk–shell Ni/C microspheres assembled by Ni@C core–shell nanoparticles with excellent microwave absorption performance can be simply fabricated by decomposition of a Ni-based metal–organic framework (Ni-MOF).

Advances in the synthesis and design of nanostructured materials by aerosol spray processes for efficient energy storage

The increasing demand for energy storage has motivated the search for highly efficient electrode materials for use in rechargeable batteries with enhanced energy density and longer cycle life. One of the most promising strategies for achieving improved battery performance is altering the architecture of nanostructured materials employed as electrode materials in the energy storage field. Among numerous synthetic methods suggested for the fabrication of nanostructured materials, aerosol spray techniques such as spray pyrolysis, spray drying, and flame spray pyrolysis are reliable, as they are facile, cost-effective, and continuous processes that enable the synthesis of nanostructured electrode materials with desired morphologies and compositions with controlled stoichiometry. The post-treatment of spray-processed powders enables the fabrication of oxide, sulfide, and selenide nanostructures hybridized with carbonaceous materials including amorphous carbon, reduced graphene oxide, carbon nanotubes, etc. In this article, recent progress in the synthesis of nanostructured electrode materials by spray processes and their general formation mechanisms are discussed in detail. A brief introduction to the working principles of each spray process is given first, and synthetic strategies for the design of electrode materials for lithium-ion, sodium-ion, lithium-sulfur, lithium-selenium, and lithium-oxygen batteries are discussed along with some examples. This analysis sheds light on the synthesis of nanostructured materials by spray processes and paves the way toward the design of other novel and advanced nanostructured materials for high performance electrodes in rechargeable batteries of the future.

Synthesis and Electrochemical Energy Storage Applications of Micro/Nanostructured Spherical Materials

Micro/nanostructured spherical materials have been widely explored for electrochemical energy storage due to their exceptional properties, which have also been summarized based on electrode type and material composition. The increased complexity of spherical structures has increased the feasibility of modulating their properties, thereby improving their performance compared with simple spherical structures. This paper comprehensively reviews the synthesis and electrochemical energy storage applications of micro/nanostructured spherical materials. After a brief classification, the concepts and syntheses of micro/nanostructured spherical materials are described in detail, which include hollow, core-shelled, yolk-shelled, double-shelled, and multi-shelled spheres. We then introduce strategies classified into hard-, soft-, and self-templating methods for synthesis of these spherical structures, and also include the concepts of synthetic methodologies. Thereafter, we discuss their applications as electrode materials for lithium-ion batteries and supercapacitors, and sulfur hosts for lithium–sulfur batteries. The superiority of multi-shelled hollow micro/nanospheres for electrochemical energy storage applications is particularly summarized. Subsequently, we conclude this review by presenting the challenges, development, highlights, and future directions of the micro/nanostructured spherical materials for electrochemical energy storage.

Conversion of MoS2 to a Ternary MoS2- xSe x Alloy for High-Performance Sodium-Ion Batteries

MoS₂ has attracted tremendous attention as an anode for Na-ion batteries (NIBs) owing to its high specific capacity and layered graphite-like structure. Herein, MoS₂ is converted to a ternary MoS_{2- x}Se _x alloy through the selenizing process in order to boost the electrochemical performance for Na-ion batteries. Conversion of MoS₂ to MoS_{2- x}Se _x expands interlayer spacing, improves electronic conductivity, and creates more defects. The expanded interlayer spacing decreases Na⁺ diffusion resistance and facilitates Na⁺ fast transfer. The integrated graphene as a conductive network offers effective pathway for electron migration and maintains structural stability of electrodes during cycles. The ternary MoS_1.2Se_0.8/graphene (MoS_1.2Se_0.8/G) electrode demonstrates an extremely high reversible capacity of 509 mA h g^-1 after 200 cycles at 0.1 A g^-1 (capacity retention of 109%) as an anode for sodium-ion batteries. Even at 2 A g^-1 and after 700 cycles, the MoS_1.2Se_0.8/G electrode also displays a relatively high reversible capacity of 178 mA h g^-1. Full cells assembled with Na₃V₂(PO₄)₂F₃ cathodes and MoS_1.2Se_0.8/G anodes reveal high charge/discharge capacities. This work demonstrates that the ternary MoS_{2- x}Se _x alloy could be a potential anode material for Na-ion storage.

Yolk-Shell-Structured Bismuth@N-Doped Carbon Anode for Lithium-Ion Battery with High Volumetric Capacity

As an anode for lithium-ion batteries, metallic bismuth (Bi) can provide a superb volumetric capacity of 3800 mA h cm^-3, showing perspective value for application. It is a pity that the severe volume swelling during the lithiation process leads to the dramatic deterioration of the cycling performances. To overcome this issue, Bi nanorods encapsulated in N-doped carbon nanotubes (yolk-shell Bi@C-N) are elaborately designed through in situ thermal reduction of Bi₂S₃@polypyrrole nanorods. In comparison with the commercial Bi, the lithium storage capacities of Bi@C-N are significantly enhanced, and it presents a stable volumetric capacity of 1700 mA h cm^-3 over 500 cycles at a high current density of 1.0 A g^-1, nearly 2.2 times that of graphite. The N-doped carbon nanotube and the cavity between the carbon wall and Bi jointly contribute to this superior performance. Especially, the failure mechanism of Bi nanorods and the protective effect of the carbon shell are revealed by ex situ TEM, which illuminates the decreasing tendency in the initial 10-20 cycles and the subsequent stable trend of cyclic performance.

TiO2-Coated Interlayer-Expanded MoSe2/Phosphorus-Doped Carbon Nanospheres for Ultrafast and Ultralong Cycling Sodium Storage

Based on multielectron conversion reactions, layered transition metal dichalcogenides are considered promising electrode materials for sodium-ion batteries, but suffer from poor cycling performance and rate capability due to their low intrinsic conductivity and severe volume variations. Here, interlayer-expanded MoSe2/phosphorus-doped carbon hybrid nanospheres coated by anatase TiO2 (denoted as MoSe2/P-C@TiO2) are prepared by a facile hydrolysis reaction, in which TiO2 coating polypyrrole-phosphomolybdic acid is utilized as a novel precursor followed by a selenization process. Benefiting from synergistic effects of MoSe2, phosphorus-doped carbon, and TiO2, the hybrid nanospheres manifest unprecedented cycling stability and ultrafast pseudocapacitive sodium storage capability. The MoSe2/P-C@TiO2 delivers decent reversible capacities of 214 mAh g-1 at 5.0 A g-1 for 8000 cycles, 154 mAh g-1 at 10.0 A g-1 for 10000 cycles, and an exceptional rate capability up to 20.0 A g-1 with a capacity of ≈175 mAh g-1 in a voltage range of 0.5-3.0 V. Coupled with a Na3V2(PO4)3@C cathode, a full cell successfully confirms a reversible capacity of 242.2 mAh g-1 at 0.5 A g-1 for 100 cycles with a coulombic efficiency over 99%.

Facile fabrication, morphology control, and modification of polymeric yolk-shell microspheres

The fabrication and functionalization of polymeric yolk-shell microspheres (YSMs), possessing a hollow shell and a movable core, is interesting but challenging in materials science. Here we report the facile fabrication, morphology control, and fluorescent modification of polymeric YSMs, which have a spherical core of poly(vinylidene fluoride) (PVDF) and a hollow shell of poly(styrene-co-glycidyl methacrylate). First, flower-like microspheres with core-shell structures are synthesized via seeded surface nucleation in an emulsion polymerization of styrene, glycidyl methacrylate, and divinylbenzene by using PVDF microparticles as seeds. Both the feed ratio and the polymerization time are considered to manipulate the core-shell structures of the composite microparticles, which obviously influences the morphology of the YSMs obtained from the subsequent treatment of solvent etching to remove the seed. The hollow volume of the polymeric YSMs is easily adjusted by changing the etching time at different temperatures. Meanwhile, we realized three-dimensionally confined crystallization of PVDF in different morphologies of YSMs. Furthermore, YSMs with the same or different functional groups, inside and outside of the hollow shell, respectively, are chemically modified by the reaction of glycidyl groups on the shell with 2,2'-(ethylenedioxy) bis-ethylamine. Thus, strong fluorescence of the YSMs is observed by subsequent labeling with functional fluorescent groups.

Construction of Hierarchical MoSe2 Hollow Structures and Its Effect on Electrochemical Energy Storage and Conversion

Metal selenides have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including metal-ion batteries and water splitting. However, their practical application is greatly hindered by collapse of the microstructure, thus leading to performance fading. Tuning the structure at nanoscale of these materials is an effective strategy to address the issue. Herein, we craft MoSe₂ with hierarchical hollow structures via a facile bubble-assisted solvothermal method. The temperature-related variations of the hollow interiors are studied, which can be presented as solid, yolk-shell, and hollow spheres, respectively. Under the simultaneous action of the distinctive hollow structures and interconnections among the nanosheets, more intimate contacts between MoSe₂ and electrolyte can be achieved, thereby leading to superior electrochemical properties. Consequently, the MoSe₂ hollow nanospheres prepared under optimum conditions exhibit optimal electrochemical activities, which hold an initial specific capacity of 1287 mA h g^-1 and maintain great capacity even after 100 cycles as anode for Li-ion battery. Moreover, the Tafel slope of 58.9 mV dec^-1 for hydrogen evolution reaction is also attained.

ZnSe Microsphere/Multiwalled Carbon Nanotube Composites as High-Rate and Long-Life Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are considered as one of the most favorable alternative devices for sustainable development of modern society. However, it is still a big challenge to search for proper anode materials which have excellent cycling and rate performance. Here, zinc selenide microsphere and multiwalled carbon nanotube (ZnSe/MWCNT) composites are prepared via hydrothermal reaction and following grinding process. The performance of ZnSe/MWCNT composites as a SIB anode is studied for the first time. As a result, ZnSe/MWCNTs exhibit excellent rate capacity and superior cycling life. The capacity retains as high as 382 mA h g^-1 after 180 cycles even at a current density of 0.5 A g^-1. The initial Coulombic efficiency of ZnSe/MWCNTs can reach 88% and nearby 100% in the following cycles. The superior electrochemical properties are attributed to continuous electron transport pathway, improved electrical conductivity, and excellent stress relaxation.

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

Sodium-ion batteries (SIBs) have huge potential for applications in large-scale energy storage systems due to their low cost and abundant sources. It is essential to develop new electrode materials for SIBs with high performance in terms of energy density, cycle life, and cost. Metal binary compounds that operate through conversion reactions hold promise as advanced anode materials for sodium storage. This Review highlights the storage mechanisms and advantages of conversion-type anode materials and summarizes their recent development. Although conversion-type anode materials have high theoretical capacities and abundant varieties, they suffer from multiple challenging obstacles to realize commercial applications, such as low reversible capacity, large voltage hysteresis, low initial coulombic efficiency, large volume changes, and low cycling stability. These key challenges are analyzed in this Review, together with emerging strategies to overcome them, including nanostructure and surface engineering, electrolyte optimization, and battery configuration designs. This Review provides pertinent insights into the prospects and challenges for conversion-type anode materials, and will inspire their further study.

Phonon thermal transport in a graphene/MoSe2 van der Waals heterobilayer

Combining the best of different monolayers in one ultimate van der Waals (vdW) heterostructure is an appealing approach for practical applications. Recently, a graphene (GR) and molybdenum diselenide (MoSe₂) heterobilayer was successfully fabricated experimentally. The superior electrical conductivity of GR combined with the unique photoelectrical properties and direct bandgap of MoSe₂ can yield many potential applications, such as Li-ion batteries, tunneling field effect transistors and two-dimensional non-volatile memory devices. Efficient heat conduction within the device components is of great importance for nanoelectronic performance. In this work, the cross-plane interfacial thermal resistance (R) and in-plane thermal conductivity (κ) of the GR/MoSe₂ vdW heterobilayer are systematically investigated using classical molecular dynamics (MD) simulations. The predicted R at a temperature of 300 K is equal to 1.91 × 10^-7 K m² W^-1. Effects of several modulators such as temperature, contact pressure and vacancy defects are evaluated, which are all found to have negative correlations with the calculated interfacial thermal resistance. The highest reduction of R amounts to 75% for doubled coupling strength between GR and MoSe₂. Spectral energy density (SED) and phonon density of states (Ph-DOS) analyses are performed to gain further insights into the phonon properties of GR and MoSe₂. Our study provides reasonable guidelines to increase heat dissipation efficiency for future GR/MoSe₂ based applications.