Molybdenum disulfide (MoS2) has received considerable interest for electrochemical energy storage and conversion. In this work, we demonstrate a material on synthesis of a unique hierarchical hollow structure by growing layered MoS2 nanosheets on assembled graphene nanotubes via a template-sacrificed approach (named as graphene@MoS2 nanotubes). As a proof of concept, the graphene@MoS2 nanotubes as the anode materials of lithium-ion batteries exhibit excellent cycling performance at 400 mA g−1 up to 120 cycles without considerable capacity loss (830 mA h g−1, with 96.5% capacity retention) and high rate behavior (502 mA h g−1 at 2000 mA g−1), which is far beyond than that of the pure MoS2 nanosheets and even graphene@MoS2 nanosheets composites. Furthermore, combined with in-situ TEM lithiation experiments, we find a novel conversion reaction mechanism of MoS2 anodes that Li ions induce structural destruction in c-direction following a dynamic layer-by-layer dissociation with Mo/Li2S composites left, rather than well-known multistep reaction in bulk phase. The first-principles computations verify that the surfacial relaxation of Li2S to form an anti-fluorite structure on higher electric conductive LixMoS2 surface is the primarily thermodynamic driving force for activating the above-mentioned reaction. These results are envisaged to be helpful for designing durable conversion-type MoS2 anodes by surface engineering, and the hierarchical tubular feature further points out a new protocol for graphene based hybrid anode for enhanced lithium-ion batteries.Download high-res image (324KB)Download full-size image

Spinel NiCo2O4 is considered a promising supercapacitive material because of its high theoretical capacity (greater than 3000 F g−1), nontoxicity, and safety. Here, we report that electrodes of porous NiCo2O4 nanograss grown in situ and supported on Ni foam achieved remarkable enhancement in electrochemical performance through facile hydrogenation at 300 °C for time periods of 1–4 h. The electrodes synthesized via 3 h of hydrogenation (H-NiCo2O4-3h) exhibited superior comprehensive electrochemical performance compared with the pristine pattern (air-annealed). The peak value of the area capacitance improved from the pristine 0.88 F cm−2 (338.5 F g−1) to 2.1 F cm−2 (807.7 F g−1) of H-3h, an increase of ∼240%. Additionally, the capacity retention from 1 to 30 mA cm−2 improved to a value of 71% (H-NiCo2O4-3h), which was superior to that of non-hydrogenated samples (54%). Furthermore, the long-cycling performance at 10 mA cm−2 exhibited a capacitance activation in H-NiCo2O4-3h within the first 1000 cycles, from 2.4 (923 F g−1) to 3.2 F cm−2 (1230 F g−1), and declined to 1.5 F cm−2 (577 F g−1) after another 2000 cycles; the last value is still greater than that of the pristine pattern (1.3 F cm−2 (500 F g−1)). The prominent electrochemical capacitive properties of hydrogenated NiCo2O4 are attributed to the enhancement in the electrical conductivity observed by an in situ TEM electrical test, resulting from the formation of oxygen vacancies in disordered surface layers (∼5 nm) observed in the hydrogenated samples based on in situ transmission electron microscopy characterization. Our findings provide a scientific explanation for the remarkable hydrogenation-induced electrochemical performance of cobalt oxide or binary nickel cobaltite compounds and offer a new route for the large-scale production of high-performance supercapacitor electrodes.

The growth of a plasma electrolytic oxide (PEO) coating on Al was widely considered as an ejection of molten alumina on the surface, induced by a localized breakdown of the solid coating. However, evidences presented here indicated no breakdown of the insulating coating in the PEO process, based on the X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrometry investigations. There are abundant channels in the coating layer from surface to substrate. However, the SEM observes indicating that the channels in the outer layer with a low-density distribution and a micron-sized diameter, the channels extending into the internal layer with a high-density distribution and a nano-sized diameter. In addition, a continuous thin amorphous alumina layer (AAL) was found between coating and substrate. An intermediate state of the AAL before transition to crystalline, with unusually thick character (∼2.7 μm), is reported for the first time, which presented a typical cross-sectional characteristic of the ionic migration processes. Based on these microstructure observations, a growth mechanism is proposed here to explain the structural features, in which the discharge ignited in the gas-filled channels passing through both the outer layer and the internal layer. The main voltage drop of applying the hole sample loaded on the AAL at a magnitude of ∼1V·nm-1 due to excellent conductivity of discharge plasmas in the channels. Under a high electrical field, the AAL was thickening, ultimately squeezing the coating expanding outward. Abundant moving micro-discharges and localized expansions could result in the growth of the whole coating.

•We find a two-step lithiation mechanism for α-MoO3.•Conversion is the rate-limiting step for lithiation.•Minor deformation is associated with the intercalation process.•MoO3 remains defects free during lithiation cycles.•Electrochemical and mechanical behaviors of MoO3 are different from other oxides.Metal oxides hold the promise of high-capacity anodes for Li-ion batteries. Lithiation of binary metal oxides proceeds with two typical mechanisms: insertion and conversion. We characterize the two-step lithiation behavior of α-MoO3, namely, Li intercalation in the layered α-MoO3 leads to the formation of crystalline Li2MoO3 in the early stage of lithiation, and further Li insertion coverts LixMoO3 to metallic Mo and amorphous Li2O. The intercalation process is thermodynamically more favorable and is accompanied with a minor volumetric change, while the conversion reaction is kinetically slow and induces large deformation. Furthermore, instead of showing significant Li-embrittlement as seen in typical oxides, α-MoO3 remains defects free despite the nearly 100% repetitive volumetric change during lithiation cycles. The reaction mechanism, structural evolution, and mechanical behaviors are unveiled through coordinated in-situ transmission electron microcopy experiments on α-MoO3 nanobelts and first-principles computational studies. The results provide fundamental perspectives in the course of developing reliable high-capacity electrodes for Li-ion batteries.

In this work, a Sn nanoparticle (NP)/carbon nanofiber (CNF) hybrid with unique structure has been designed and fabricated via electrospinning and subsequent heat treatment. The cell assembled by the binder-free Sn NP/CNF hybrid demonstrates an effective capacity (46 mAh g−1 at 200 mA g−1 after 200 cycles) with high coulombic efficiency (up to 99.8%), suggesting a facile strategy for the scalable fabrication of electrochemically stable electrodes for LIBs. For understanding the electrochemical behaviors of the metallic Sn and carbon nanofibers in the lithiation/delithiation process, in situ transmission electron microscopy was applied to study the single hybrid structure. In the first charge/discharge process, real-time size variation of the Sn NP and CNFs was mainly focused, suggesting a two-step lithiation process in the metallic Sn NP. Structural characterization also indicates an irreversible delithiation in a single Sn NP/CNF hybrid structure. The electrochemical performance based on influence of carbonization temperature has also been discussed. The results and fundamental understanding of the lithiation/delithiation in the Sn-based hybrid anodes enables the communities to design flexible high-performance electrodes based on metallic active materials in a rational way.

Achieving economic orientation-controlled growth of monolithic nanowires remains a challenge. We report a simple and low-cost, endotaxial, self-templating, noncatalyzed synthesis of monolithic boron nitride semiconductor nanowires. The method uses orientated control of the nanowires prepared directly on Si substrates through plasma-enhanced chemical vapor deposition without a catalyst. The growth direction of the synthetic monolithic nanowires is controlled as a function of substrate crystal orientation. We measured the vertical electrical properties of the nanowires. Our method provides an alternative strategy to control monolithic nanowire growth in substrates, and may allow for large-scale, low-cost nanowire device manufacture.

•Co3O4@NiCo2O4 nanoforests on Ni foam prepared by one-step solution and annealing process without using nickle source.•Synthesized electrode exhibits capacitive activation during charge-discharge cycling.•Capacitive activation is attributed to enlarged surface area and enhanced electrical conductivity.•Electrochemical induced microstructure change creates new opportunities for developing high performance energy storage materials.We report a simple and cost-effective approach to the synthesis of hierarchical mesporous Co3O4@NiCo2O4 nanoforests on Ni foam for supercapacitor (SC) electrode applications by a coupled one-step solution and annealing process. The synthesized electrode exhibits capacitive activation during charge-discharge cycling (from 0.73 F/cm2 of the pristine state to the peak value of 1.12 F/cm2 after 2000 cycles with only 1.8% loss compared to the peak capacitance after another 2000 cycles). We attribute such dynamic capacitive activation to (1) enlarged electroactive surface area through the formation of Co3O4@NiCo2O4 core-shell structure and (2) enhanced electrical conductivity by forming oxygen vacancies and hydroxyl groups during charge-discharge cycling. Our findings provide a scientific explanation for the capacitive activation in cobalt oxide-binary nickel cobaltite compounds, and a new design guideline for the development of capacitive activation enabled, high performance transitional oxide electrodes.Hierarchical mesporous Co3O4@NiCo2O4 nanoforests supported on Ni foam as supercapacitor electrodes is prepared by a coupled one-step solution and annealing process. The electrochemical performance demonstrates a dynamic capacitive storage behavior, which is attributed to enhanced electroactive surface area via forming Co3O4@NiCo2O4 core-shell structure and enhancement of electrical conductivity by forming oxygen vacancies and hydroxyl groups.

A novel hierarchical porous NiCo2O4 nanograss array directly grown on Ni foam is successfully synthesized through a facile hydrothermal method combined with a thermal treatment. When used as the electrode material for supercapacitors, an areal capacitance of 2.1 F cm−2 was obtained under a current density of 2 mA cm−2. After 4000 charge–discharge cycles, a high areal capacitance retention of 88% could be achieved. The outstanding electrochemical performance was attributed to the novel structure of the hierarchical porous NiCo2O4 nanograss arrays and the Ni foam used as the current collector. The novel hierarchical porous NiCo2O4 nanograss arrays are a promising electrode material for supercapacitors.

•Freestanding NC tungsten films with ultra thin thickness of 38 nm were fabricated.•In situ TEM observation found totally intergranular fracture with no cleavage.•Toughening mechanisms were observed during fracture process.•Nano grain size and freestanding surface may improve the plastic of tungsten.•Theoretically and experimentally explain fracture deflection angle distribution.Crack propagation in sputter-deposited freestanding nanocrystalline tungsten films with a thickness of 38 nm was studied using an in situ transmission electron microscopy (TEM) tensile technique. Distinct from coarse-grained tungsten, a totally intergranular fracture mode and toughening mechanisms were identified in the nanocrystalline tungsten. The transition of the fracture mode indicates that the plasticity is improved, and the improvement may be attributed to the effect of nano-size grains and the freestanding surface, which facilitate a high percentage of grain boundaries (GB) and a weak binding force. A theoretical model of the energy release rate and the deflection angle of cracks was established to quantitatively characterize the preferred deflection angle. The model predictions are in good agreement with the experimental findings.

•Mesporous Ag2O nanotubes were achieved by plasma treatment using corresponding Ag nanowires template.•Kirkendall effect had been involved in the speedy formation of the novel architecture.•The method for synthesis of mesporous Ag2O hollow structures demonstrated the merits of one-step synthesis, speedy fabrication time (<1 min) and low reaction temperature (at room temperature).Mesporous hollow semiconducting materials have received much attention in recent years owing to their unique chemico-physical properties for applications in many fields. Herein, we firstly report a green, rapid, one-step route towards the facile fabrication of mesporous Ag2O nanotube via plasma etching of corresponding silver nanowire templates. Characterization by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) techniques indicates that phase pure cubic Ag2O nanotube with mesporous structure were formed. Time-independent experiments demonstrate Kirkendall effect involved in forming hollow structure at nanoscale in solid–gas reaction. This green and simple strategy is expected to be extended for the rapid fabrication of similar hollow metal–oxide nanostructure and device.

One-dimensional (1D) tubular Ag/MnOx nanocomposites were synthesized by the solvothermal method via the Kirkendall effect between potassium permanganate (KMnO4) and Ag nanowire templates. The morphology and electrochemical performance of Ag/MnOx composites were tuned by varying the pH levels. Tubular MnOx nanosheets with ultrafine Ag nanoparticles were formed in an acidic environment (pH 0.76), whereas the Ag nanoparticles entrapped in amorphous MnO2 nanotubes were prepared in a neutral environment (pH 7.00). Based on a series of volume-dependent experiments, it was confirmed that the Kirkendall effect was involved in the formation of these morphologies. When tested as an electrode for supercapacitors, the hierarchical tubular Ag/MnOx nanosheet composites prepared in an acidic environment exhibited an optimized electrochemical performance, with specific capacitance of 180 F g–1 at current density of 0.1 A g–1, and still maintained 80% of initial capacity after 1000 cycles at current density of 1 A g–1. The proposed synthetic mechanism and the developed synthetic strategy may provide design guidelines for synthesizing other hierarchical transition metal/transition metal oxide nanocomposites.

Spinel NiCo2O4 is considered a promising supercapacitive material because of its high theoretical capacity (greater than 3000 F g−1), nontoxicity, and safety. Here, we report that electrodes of porous NiCo2O4 nanograss grown in situ and supported on Ni foam achieved remarkable enhancement in electrochemical performance through facile hydrogenation at 300 °C for time periods of 1–4 h. The electrodes synthesized via 3 h of hydrogenation (H-NiCo2O4-3h) exhibited superior comprehensive electrochemical performance compared with the pristine pattern (air-annealed). The peak value of the area capacitance improved from the pristine 0.88 F cm−2 (338.5 F g−1) to 2.1 F cm−2 (807.7 F g−1) of H-3h, an increase of ∼240%. Additionally, the capacity retention from 1 to 30 mA cm−2 improved to a value of 71% (H-NiCo2O4-3h), which was superior to that of non-hydrogenated samples (54%). Furthermore, the long-cycling performance at 10 mA cm−2 exhibited a capacitance activation in H-NiCo2O4-3h within the first 1000 cycles, from 2.4 (923 F g−1) to 3.2 F cm−2 (1230 F g−1), and declined to 1.5 F cm−2 (577 F g−1) after another 2000 cycles; the last value is still greater than that of the pristine pattern (1.3 F cm−2 (500 F g−1)). The prominent electrochemical capacitive properties of hydrogenated NiCo2O4 are attributed to the enhancement in the electrical conductivity observed by an in situ TEM electrical test, resulting from the formation of oxygen vacancies in disordered surface layers (∼5 nm) observed in the hydrogenated samples based on in situ transmission electron microscopy characterization. Our findings provide a scientific explanation for the remarkable hydrogenation-induced electrochemical performance of cobalt oxide or binary nickel cobaltite compounds and offer a new route for the large-scale production of high-performance supercapacitor electrodes.