Yunsong Wang

Controlled Synthesis of Core–Shell Carbon@MoS2 Nanotube Sponges as High-Performance Battery Electrodes

Heterogeneous inorganic nanotube structures consisting of multiwalled carbon nanotubes coated by long, continuous MoS2 sheets with tunable sheet number are synthesized using a carbon-nanotube sponge as a template. The resulting 3D porous hybrid sponges have potential applications as high-performance freestanding anodes for Li-ion batteries with excellent specific capacity and cycling stability.

Flexible hybrid carbon nanotube sponges embedded with SnS2 from tubular nanosheaths to nanosheets as free-standing anodes for lithium-ion batteries

Flexible carbon nanotube sponges (CNT sponges) are excellent three-dimensional (3D) porous substrates to fabricate free-standing electrodes applied in lithium-ion batteries, but the low energy density needs to be improved urgently. Hybrid, hierarchical structures designed towards high-performance electrodes have been reported as an efficient route. Here, SnS2 with controllable mass ratios and various morphologies from tubular nanosheaths to nanosheets has been grown in situ on carbon nanotubes in CNT sponges by a facile solvothermal method, taking thiourea as the medium. We propose the formation mechanism for diverse morphologies of SnS2. Also, we demonstrate that the tubular SnS2 nanostructure can be restrictively and directionally grown using CNTs as templates, and has much better reversible capacity and cyclability than its nanoparticle or nanosheet structures on CNTs. Meanwhile, CNT@SnS2 sponges could be used as free-standing and binder-free electrodes with significantly improved areal capacity than bare CNT sponges.

MOF-Derived ZnO Nanoparticles Covered by N-Doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes

Metal-organic frameworks (MOFs) have many promising applications in energy and environmental areas such as gas separation, catalysis, supercapacitors, and batteries; the key toward those applications is controlled pyrolysis which can tailor the porous structure, improve electrical conductivity, and expose metal ions in MOFs. Here, we present a systematic study on the structural evolution of zeolitic imidazolate frameworks hybridized on carbon nanotubes (CNTs) during the carbonization process. We show that a number of typical products can be obtained, depending on the annealing time, including (1) CNTs wrapped by relatively thick carbon layers, (2) CNTs grafted by ZnO nanoparticles which are covered by thin nitrogen-doped carbon layers, and (3) CNTs grafted by aggregated ZnO nanoparticles. We also investigated the electrochemical properties of those hybrid structures as freestanding membrane electrodes for lithium ion batteries, and the second one (CNT-supported ZnO covered by N-doped carbon) shows the best performance with a high specific capacity (850 mA h/g at a current density of 100 mA/g) and excellent cycling stability. Our results indicate that tailoring and optimizing the MOF-CNT hybrid structure is essential for developing high-performance energy storage systems.

Single Carbon Fibers with a Macroscopic‐Thickness, 3D Highly Porous Carbon Nanotube Coating

Carbon fiber (CF) grafted with a layer of carbon nanotubes (CNTs) plays an important role in composite materials and other fields; to date, the applications of CNTs@CF multiscale fibers are severely hindered by the limited amount of CNTs grafted on individual CFs and the weak interfacial binding force. Here, monolithic CNTs@CF fibers consisting of a 3D highly porous CNT sponge layer with macroscopic-thickness (up to several millimeters), which is directly grown on a single CF, are fabricated. Mechanical tests reveal high sponge-CF interfacial strength owing to the presence of a thin transitional layer, which completely inhibits the CF slippage from the matrix upon fracture in CNTs@CF fiber-epoxy composites. The porous conductive CNTs@CF hybrid fibers also act as a template for introducing active materials (pseudopolymers and oxides), and a solid-state fiber-shaped supercapacitor and a fiber-type lithium-ion battery with high performances are demonstrated. These CNTs@CF fibers with macroscopic CNT layer thickness have many potential applications in areas such as hierarchically reinforced composites and flexible energy-storage textiles.

Hyperporous Sponge Interconnected by Hierarchical Carbon Nanotubes as a High-Performance Potassium-Ion Battery Anode

Recently, commercial graphite and other carbon-based materials have shown promising properties as the anode for potassium-ion batteries. A fundamental problem related to those carbon electrodes, significant volume expansion, and structural instability/collapsing caused by cyclic K-ion intercalation, remains unsolved and severely limits further development and applications of K-ion batteries. Here, a multiwalled hierarchical carbon nanotube (HCNT) is reported to address the issue, and a reversible specific capacity of 232 mAh g-1 , excellent rate capability, and cycling stability for 500 cycles are achieved. The key structure of the HCNTs consists of an inner CNT with dense-stacked graphitic walls and a loose-stacked outer CNT with more disordered walls, and individual HCNTs are further interconnected into a hyperporous bulk sponge with huge macropore volume, high conductivity, and tunable modulus. It is discovered that the inner dense-CNT serves as a robust skeleton, and collectively, the outer loose-CNT is beneficial for K-ion accommodation; meanwhile the hyperporous sponge facilitates reaction kinetics and offers stable surface capacitive behavior. The hierarchical carbon nanotube structure has great potential in developing high-performance and stable-structure electrodes for next generation K and other metal-ion batteries.

Tunable Free-Standing Core–Shell CNT@MoSe2 Anode for Lithium Storage

Heterogeneous nanostructuring of MoSe2 over a carbon nanotube (CNT) sponge as a free-standing electrode not only brings higher performance but also eliminates the need for dead elements such as a binder, conductive carbon, and supportive current collectors. Further, the porous CNT sponge can be easily compacted via an intense densification of the active material MoSe2 to produce an electrode with a high mass loading for a significantly improved areal capacity. In this work, we present a tunable coating of MoSe2 on a CNT sponge to fabricate a core-shell MoSe2@CNT anode. The three-dimensional nanotubular sponge is synthesized via a solvothermal process, followed by thermal annealing to improve crystallization. Structural and morphological studies revealed that MoSe2 grew as a layered structure ( d = 0.66 nm), where numbers of layers can be controlled to yield optimized results for Li+ storage. We showed that the 10-layer core-shell CNT@MoSe2 hybrid sponge delivered a discharge capacity of 820.5 mAh g-1 after 100 cycles at 100 mA g-1 with a high cyclic stability and rate capability. Further, an ex situ structural and morphological analysis revealed that ionic storage causes a phase change in MoSe2 from a crystalline to a partial amorphous state for a continuous increase in the capacity with extended cycling. We believe that the strategy developed here will assist users to tune the electrode materials for future energy-storage devices, especially how the materials are changing with the passage of time and their effects on the device performance.

Advancement in Layered Transition Metal Dichalcogenide Composites for Lithium and Sodium Ion Batteries

With an ever increasing energy demand and environmental issues, many state-of-the-art nanostructured electrode materials have been developed for energy storage devices and they include batteries, supercapacitors and fuel cells. Among these electrode materials, L-TMD (layered transition metal dichalcogenide) nanosheets (especially, S (sulfur) and Se (selenium) based dichalcogenides) have received a lot of attention due to their intriguing layered structure for enhanced electrochemical properties. L-TMD composites have recently been investigated not only as a main charge storage specie but also, as a substrate to hold the active specie. This review highlights the recent advancements in L-TMD composites with 0D (0-dimensional), 1D, 2D, 3D and various forms of carbon structures and their potential applications in LIB (lithium ion battery) and SIB (sodium ion battery).