Jiashen Meng

General synthesis of complex nanotubes by gradient electrospinning and controlled pyrolysis

Nanowires and nanotubes have been the focus of considerable efforts in energy storage and solar energy conversion because of their unique properties. However, owing to the limitations of synthetic methods, most inorganic nanotubes, especially for multi-element oxides and binary-metal oxides, have been rarely fabricated. Here we design a gradient electrospinning and controlled pyrolysis method to synthesize various controllable 1D nanostructures, including mesoporous nanotubes, pea-like nanotubes and continuous nanowires. The key point of this method is the gradient distribution of low-/middle-/high-molecular-weight poly(vinyl alcohol) during the electrospinning process. This simple technique is extended to various inorganic multi-element oxides, binary-metal oxides and single-metal oxides. Among them, Li3V2(PO4)3, Na0.7Fe0.7Mn0.3O2 and Co3O4 mesoporous nanotubes exhibit ultrastable electrochemical performance when used in lithium-ion batteries, sodium-ion batteries and supercapacitors, respectively. We believe that a wide range of new materials available from our composition gradient electrospinning and pyrolysis methodology may lead to further developments in research on 1D systems.

VO2 Nanowires Assembled into Hollow Microspheres for High-Rate and Long-Life Lithium Batteries

Development of three-dimensional nanostructures with high surface area and excellent structural stability is an important approach for realizing high-rate and long-life battery electrodes. Here, we report VO2 hollow microspheres showing empty spherical core with radially protruding nanowires, synthesized through a facile and controllable ion-modulating approach. In addition, by controlling the self-assembly of negatively charged C12H25SO4(-) spherical micelles and positively charged VO(2+) ions, six-armed microspindles and random nanowires are also prepared. Compared with them, VO2 hollow microspheres show better electrochemical performance. At high current density of 2 A/g, VO2 hollow microspheres exhibit 3 times higher capacity than that of random nanowires, and 80% of the original capacity is retained after 1000 cycles. The superior performance of VO2 hollow microspheres is because they exhibit high surface area about twice higher than that of random nanowires and also provide an efficient self-expansion and self-shrinkage buffering during lithiation/delithiation, which effectively inhibits the self-aggregation of nanowires. This research indicates that VO2 hollow microspheres have great potential for high-rate and long-life lithium batteries.

General Oriented Formation of Carbon Nanotubes from Metal–Organic Frameworks

Carbon nanotubes (CNTs) are of great interest for many potential applications because of their extraordinary electronic, mechanical and structural properties. However, issues of chaotic staking, high cost and high energy dissipation in the synthesis of CNTs remain to be resolved. Here we develop a facile, general and high-yield strategy for the oriented formation of CNTs from metal-organic frameworks (MOFs) through a low-temperature (as low as 430 °C) pyrolysis process. The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources. The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process. This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs, which further homogeneously incorporate heteroatoms into the CNTs. Specifically, nitrogen-doped CNT-assembled hollow structures exhibit excellent performances in both energy conversion and storage. On the basis of experimental analyses and density functional theory simulations, these superior performances are attributed to synergistic effects between ideal components and multilevel structures. Additionally, the appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. The viable synthetic strategy and proposed mechanism will stimulate the rapid development of CNTs in frontier fields.

Earth Abundant Fe/Mn-Based Layered Oxide Interconnected Nanowires for Advanced K-Ion Full Batteries

K-ion battery (KIB) is a new-type energy storage device that possesses potential advantages of low-cost and abundant resource of K precursor materials. However, the main challenge lies on the lack of stable materials to accommodate the intercalation of large-size K-ions. Here we designed and constructed a novel earth abundant Fe/Mn-based layered oxide interconnected nanowires as a cathode in KIBs for the first time, which exhibits both high capacity and good cycling stability. On the basis of advanced in situ X-ray diffraction analysis and electrochemical characterization, we confirm that interconnected K0.7Fe0.5Mn0.5O2 nanowires can provide stable framework structure, fast K-ion diffusion channels, and three-dimensional electron transport network during the depotassiation/potassiation processes. As a result, a considerable initial discharge capacity of 178 mAh g-1 is achieved when measured for KIBs. Besides, K-ion full batteries based on interconnected K0.7Fe0.5Mn0.5O2 nanowires/soft carbon are assembled, manifesting over 250 cycles with a capacity retention of ∼76%. This work may open up the investigation of high-performance K-ion intercalated earth abundant layered cathodes and will push the development of energy storage systems.

Mesoporous VO2 nanowires with excellent cycling stability and enhanced rate capability for lithium batteries

To combine the merits of the one-dimensional structure and the porous structure, mesoporous VO2 nanowires have been designed and reported for the first time. Excellent cycling stability and enhanced rate performance are obtained and may be attributed to the mesoporous nanowires, realizing both high surface area for more active sites and facile stress relaxation resulting in excellent structure stability. Our results demonstrate that the mesoporous nanowires are favourable for high-rate and long-life lithium batteries.

Carbon-supported and nanosheet-assembled vanadium oxide microspheres for stable lithium-ion battery anodes

Naturally abundant transition metal oxides with high theoretical capacity have attracted more attention than commercial graphite for use as anodes in lithium-ion batteries. Lithium-ion battery electrodes that exhibit excellent electrochemical performance can be efficiently achieved via three-dimensional (3D) architectures decorated with conductive polymers and carbon. As such, we developed 3D carbon-supported amorphous vanadium oxide microspheres and crystalline V2O3 microspheres via a facile solvothermal method. Both samples were assembled with ultrathin nanosheets, which consisted of uniformly distributed vanadium oxides and carbon. The formation processes were clearly revealed through a series of time-dependent experiments. These microspheres have numerous active reaction sites, high electronic conductivity, and excellent structural stability, which are all far superior to those of other lithium-ion battery anodes. More importantly, 95% of the second-cycle discharge capacity was retained after the amorphous microspheres were subjected to 7,000 cycles at a high rate of 2,000 mA/g. The crystalline microspheres also exhibited a high-rate and long-life performance, as evidenced by a 98% retention of the second-cycle discharge capacity after 9,000 cycles at a rate of 2,000 mA/g. Therefore, this facile solvothermal method as well as unique carbon-supported and nanosheet-assembled microspheres have significant potential for the synthesis of and use in, respectively, lithium-ion batteries.

Carbon‐MEMS‐Based Alternating Stacked MoS2@rGO‐CNT Micro‐Supercapacitor with High Capacitance and Energy Density

A novel process to fabricate a carbon-microelectromechanical-system-based alternating stacked MoS2 @rGO-carbon-nanotube (CNT) micro-supercapacitor (MSC) is reported. The MSC is fabricated by successively repeated spin-coating of MoS2 @rGO/photoresist and CNT/photoresist composites twice, followed by photoetching, developing, and pyrolysis. MoS2 @rGO and CNTs are embedded in the carbon microelectrodes, which cooperatively enhance the performance of the MSC. The fabricated MSC exhibits a high areal capacitance of 13.7 mF cm-2 and an energy density of 1.9 µWh cm-2 (5.6 mWh cm-3 ), which exceed many reported carbon- and MoS2 -based MSCs. The MSC also retains 68% of capacitance at a current density of 2 mA cm-2 (5.9 A cm-3 ) and an outstanding cycling performance (96.6% after 10 000 cycles, at a scan rate of 1 V s-1 ). Compared with other MSCs, the MSC in this study is fabricated by a low-cost and facile process, and it achieves an excellent and stable electrochemical performance. This approach could be highly promising for applications in integration of micro/nanostructures into microdevices/systems.

An electrospun hierarchical LiV3O8 nanowire-in-network for high-rate and long-life lithium batteries

Structural and morphological control of the LiV3O8 material has a significant impact on its electrochemical performance. In order to obtain a favorable structure, a hierarchical LiV3O8 nanowire-in-network is designed and constructed by electrospinning through a polymer crosslinking strategy. The crosslinking effect between poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) not only benefits electrospinning, but also realizes a mild multi-step degradation process during calcination. Based on temperature-dependent experiments and thermogravimetric (TG) analysis, the function of polymer blends and the formation mechanism of the structure are discussed in detail. As a cathode for lithium batteries, LiV3O8 exhibits a high initial capacity of 320.6 mA h g−1 at 100 mA g−1 and a high-rate capacity of 202.8 mA h g−1 at 2000 mA g−1. This remarkable performance is attributed to its unique structure, which provides a large effective contact area, low charge transfer resistance, and improved structural stability. Our work indicates that the hierarchical LiV3O8 nanowire-in-network material is a promising cathode for use in high-rate and long-life rechargeable lithium batteries.

Interface-modulated approach toward multilevel metal oxide nanotubes for lithium-ion batteries and oxygen reduction reaction

Metal oxide hollow structures with multilevel interiors are of great interest for potential applications such as catalysis, chemical sensing, drug delivery, and energy storage. However, the controlled synthesis of multilevel nanotubes remains a great challenge. Here we develop a facile interface-modulated approach toward the synthesis of complex metal oxide multilevel nanotubes with tunable interior structures through electrospinning followed by controlled heat treatment. This versatile strategy can be effectively applied to fabricate wire-in-tube and tube-in-tube nanotubes of various metal oxides. These multilevel nanotubes possess a large specific surface area, fast mass transport, good strain accommodation, and high packing density, which are advantageous for lithium-ion batteries (LIBs) and the oxygen reduction reaction (ORR). Specifically, shrinkable CoMn2O4 tube-in-tube nanotubes as a lithium-ion battery anode deliver a high discharge capacity of ~565 mAh·g−1 at a high rate of 2 A·g−1, maintaining 89% of the latter after 500 cycles. Further, as an oxygen reduction reaction catalyst, these nanotubes also exhibit excellent stability with about 92% current retention after 30,000 s, which is higher than that of commercial Pt/C (81%). Therefore, this feasible method may push the rapid development of one-dimensional (1D) nanomaterials. These multifunctional nanotubes have great potential in many frontier fields.

A synergistic effect between layer surface configurations and K ions of potassium vanadate nanowires for enhanced energy storage performance

Layered metal vanadates, especially alkali metal vanadates, have been extensively studied in energy storage. Generally, vanadates exhibit more stable electrochemical performance than pristine vanadium oxides, and different vanadates also vary in the performance. However, the detailed mechanisms of the variation in the performance of vanadates and vanadium oxides are poorly explored. Here we choose and construct three typical layered vanadium-based nanowires (V2O5, KV3O8 and K0.25V2O5), and investigate the origin of the enhanced electrochemical performance of the potassium vanadates compared to V2O5, based on crystal structure analysis, electrochemical tests, ex situ ICP measurements and in situ XRD detections. We demonstrate a synergistic effect between layer surface configurations and K ions of potassium vanadate nanowires, which leads to the great improvement in electrochemical stability of K0.25V2O5. The layer surface configuration of K0.25V2O5 only consists of single-connected oxygen atoms, which provides strong interaction with the K ions. And the stabilized K ions act as “pillars” between interlayers to protect the layered structures from collapse in the charge/discharge process. This work provides a further insight into alkali metal vanadates, and benefits the design of ideal electrode materials in the energy storage field.