•V2O5@MV6O15 (M = Na, K) NPs are successfully constructed through a facile “semi-solid” synthetic method.•The V2O5@MV6O15 NPs show much enhanced electrochemical performance as compared to V2O5.•The enhanced performances are attributed to the synergistic effects from the outer MV6O15 layers.•The synergistic effects are confirmed by the in-situ XRD testing results.Synergistic effects of heterostructures can improve electrochemical properties of electrode materials. Herein, core-shell heterogeneous structures of V2O5@MV6O15 (M = Na, K) nanoparticles were designed and successfully synthesized through a facile “semi-solid” method. V2O5@NaV6O15 nanoparticles showed a high discharge capacity of 140 mAh g−1 at 200 mA g−1 which retained 94.9% after 200 cycles, higher than those of V2O5 nanoparticles (116.1 mAh g−1 and 58%). Moreover, through an advanced in-situ XRD technology, the synergistic effect of buffering and smoothing Li+ diffusion from MV6O15 outer layer was revealed. This facile strategy could be widely applied to improve the electrochemical performances of other electrode materials.Download high-res image (123KB)Download full-size image

Graphene has been widely used to enhance the performance of energy storage devices due to its high conductivity, large surface area, and excellent mechanical flexibility. However, it is still unclear how graphene influences the electrochemical performance and reaction mechanisms of electrode materials. The single-nanowire electrochemical probe is an effective tool to explore the intrinsic mechanisms of the electrochemical reactions in situ. Here, pure MnO2 nanowires, reduced graphene oxide/MnO2 wire-in-scroll nanowires, and porous graphene oxide/MnO2 wire-in-scroll nanowires are employed to investigate the capacitance, ion diffusion coefficient, and charge storage mechanisms in single-nanowire electrochemical devices. The porous graphene oxide/MnO2 wire-in-scroll nanowire delivers an areal capacitance of 104 nF/μm2, which is 4.0 and 2.8 times as high as those of reduced graphene oxide/MnO2 wire-in-scroll nanowire and MnO2 nanowire, respectively, at a scan rate of 20 mV/s. It is demonstrated that the reduced graphene oxide wrapping around the MnO2 nanowire greatly increases the electronic conductivity of the active materials, but decreases the ion diffusion coefficient because of the shielding effect of graphene. By creating pores in the graphene, the ion diffusion coefficient is recovered without degradation of the electron transport rate, which significantly improves the capacitance. Such single-nanowire electrochemical probes, which can detect electrochemical processes and behavior in situ, can also be fabricated with other active materials for energy storage and other applications in related fields.

Development of pseudocapacitor electrode materials with high comprehensive electrochemical performance, such as high capacitance, superior reversibility, excellent stability, and good rate capability at the high mass loading level, still is a tremendous challenge. To our knowledge, few works could successfully achieve the above comprehensive electrochemical performance simultaneously. Here we design and synthesize one interwoven three-dimensional (3D) architecture of cobalt oxide nanobrush-graphene@NixCo2x(OH)6x (CNG@NCH) electrode with high comprehensive electrochemical performance: high specific capacitance (2550 F g–1 and 5.1 F cm–2), good rate capability (82.98% capacitance retention at 20 A g–1 vs 1 A g–1), superior reversibility, and cycling stability (92.70% capacitance retention after 5000 cycles at 20 A g–1), which successfully overcomes the tremendous challenge for pseudocapacitor electrode materials. The asymmetric supercapacitor of CNG@NCH//reduced-graphene-oxide-film exhibits good rate capability (74.85% capacitance retention at 10 A g–1 vs 0.5 A g–1) and high energy density (78.75 Wh kg–1 at a power density of 473 W kg–1). The design of this interwoven 3D frame architecture can offer a new and appropriate idea for obtaining high comprehensive performance electrode materials in the energy storage field.

Developing electrode materials with both high energy and power densities holds the key for satisfying the urgent demand of energy storage worldwide. In order to realize the fast and efficient transport of ions/electrons and the stable structure during the charge/discharge process, hierarchical porous Fe3O4/graphene nanowires supported by amorphous vanadium oxide matrixes have been rationally synthesized through a facile phase separation process. The porous structure is directly in situ constructed from the FeVO4·1.1H2O@graphene nanowires along with the crystallization of Fe3O4 and the amorphization of vanadium oxide without using any hard templates. The hierarchical porous Fe3O4/VOx/graphene nanowires exhibit a high Coulombic efficiency and outstanding reversible specific capacity (1146 mAh g–1). Even at the high current density of 5 A g–1, the porous nanowires maintain a reversible capacity of ∼500 mAh g–1. Moreover, the amorphization and conversion reactions between Fe and Fe3O4 of the hierarchical porous Fe3O4/VOx/graphene nanowires were also investigated by in situ X-ray diffraction and X-ray photoelectron spectroscopy. Our work demonstrates that the amorphous vanadium oxides matrixes supporting hierarchical porous Fe3O4/graphene nanowires are one of the most attractive anodes in energy storage applications.

Ultralong H2V3O8 nanowire bundles with length up to hundreds of micrometers were successfully synthesized by a facile hydrothermal approach. The nanowire bundles exhibit a high specific discharge capacity of 325.7 mA h g−1 at 50 mA g−1. While the current density is up to 2000 mA g−1, the initial specific discharge capacities of a H2V3O8 nanowires cathode can reach 121.1 mA h g−1 with a capacity fading of only 0.0425% per cycle for 300 cycles. Electrical transport of a single nanowire is also recorded in situ to detect the evolution of the nanowire during annealing. The conductivity of H2V3O8 nanowire has an increase of three orders of magnitude compared to that of the dehydrated nanowire. The excellent electrochemical performance of H2V3O8 nanowire bundles results from high conductivity and good structural stability. This work demonstrates that H2V3O8 nanowire bundles are a promising cathode material for lithium batteries.

Graphene scrolls have been widely investigated for applications in electronics, sensors, energy storage, etc. However, graphene scrolls with tens of micrometers in length and with other materials in their cavities have not been obtained. Here nanowire templated semihollow bicontinuous graphene scroll architecture is designed and constructed through “oriented assembly” and “self-scroll” strategy. These obtained nanowire templated graphene scrolls can achieve over 30 μm in length with interior cavities between the nanowire and scroll. It is demonstrated through experiments and molecular dynamic simulations that the semihollow bicontinuous structure construction processes depend on the systemic energy, the curvature of nanowires, and the reaction time. Lithium batteries based on V3O7 nanowire templated graphene scrolls (VGSs) exhibit an optimal performance with specific capacity of 321 mAh/g at 100 mA/g and 87.3% capacity retention after 400 cycles at 2000 mA/g. The VGS also shows a high conductivity of 1056 S/m and high capacity of 162 mAh/g at a large density of 3000 mA/g with only 5 wt % graphene added which are 27 and 4.5 times as high as those of V3O7 nanowires, respectively. A supercapacitor made of MnO2 nanowire templated graphene scrolls (MGSs) also shows a high capacity of 317 F/g at 1A/g, which is over 1.5 times than that of MnO2 nanowires without graphene scrolls. These excellent energy storage capacities and cycling performance are attributed to the unique structure of the nanowire templated graphene scroll, which provides continuous electron and ion transfer channels and space for free volume expansion of nanowires during cycling. This strategy and understanding can be used to synthesize other nanowire templated graphene scroll architectures, which can be extended to other fabrication processes and fields.

Ammonium decavanadate single crystalline nanorods with an average diameter of 100 nm, length up to 5 μm, was synthesized at a large scale in an ammonium metavanadate solution by a direct reaction-crystallization growth route. Investigations were conducted by XRD, DSC/TG, Raman, SEM, EDS, TEM, SAED and electrical transport along single nanorod. The results show that ammonium decavanadate nanorods grow along the direction of [1–21]. The individual nanorod exhibits nonlinear current/voltage (I/V) characteristics, with a conductivity of 0.15 S/cm at room temperature. The dominant conduction mechanism is based on small polaron hopping between V5+ and V4+ impurity centers and the I–V curve consists of a linear, Ohmic regime at lower electrical field and a nonlinear one at higher electrical field.

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 VO2+ 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.