Haimei Liu

Centimeter‐Long V2O5 Nanowires: From Synthesis to Field‐Emission, Electrochemical, Electrical Transport, and Photoconductive Properties

Adv. Mater. 2010, 22, 2547–2552 2010 WILEY-VCH Verlag G One-dimensional nanostructures have attracted considerable attention due to their importance in basic scientific research and potential technologic applications. Among them, vanadium pentoxide (V2O5) nanowires have been extensively studied in recent years because of their prospective applications in chemical sensors, field-emitters, catalysts, lithium-ion batteries, actuators, and electrochromic or other nanodevices. Several different approaches have been explored for the synthesis of V2O5 nanowires, such as thermal evaporation methods, hydrothermal/solvothermal syntheses, sol–gel techniques, and electrodeposition. However, the nanowires synthesized by these methods have typical lengths in the micrometer range (most of them are shorter than 10mm);moreover, if one canmake centimeter-long V2O5 nanowires, which should be much more useful compared to short wires for some specific purposes, such as field-emission (FE), device interconnects, and reinforcing fibers in composites. Herein, we fabricated high-quality single-crystalline centimeter-long V2O5 nanowires ( 80–120 nm in diameter, several centimeters in length; aspect ratio >10–10) using an environmental friendly hydrothermal approach without dangerous reagents, harmful solvents, and surfactants. The FE, electrochemical and electrical transport, and photoconductive properties of the synthesized V2O5 nanowires were then investigated in detail. Our results suggest a high potential of utilizing these novel nanowires in field-emitters, lithium-ion batteries, interconnects, and optoelectronic devices. The representative morphologies of the V2O5 nanowires were investigated by FE scanning electron microscopy (SEM), as shown in Figure 1a. Other SEM images (see the Supporting Information, Fig. S1) also confirm the high-yield fabrication of smooth and straight nanowires of 80–120 nm in diameter. Large portions of the nanowires are usually several millimeters or even up to several centimeters in length (inset of Fig. 1a), resulting in an aspect ratio of 10–10. To the best of our knowledge, this is the first time that such ultra-long V2O5 nanowires have been obtained. An X-ray diffraction (XRD) pattern of the sample is shown in Figure 1b. All the diffraction peaks can be indexed to an orthorhombic V2O5 phase with the lattice parameters of a1⁄4 11.54 A, b1⁄4 3.571 A, and c1⁄4 4.383 A, in good agreement with the literature values (Joint Committee on Powder Diffraction Standards (JCPDS) Card, no. 89-0612). No characteristic peaks of any impurities are detected in this pattern. Figure S2 (Supplementary Information) depicts a room temperature micro-Raman spectrum of the ultralong V2O5 nanowires. The peaks, located at 145, 197, 285, 305, 407, 480, 525, 694, and 990 cm , can be assigned to the Raman signature of V2O5. [18,19] A predominant low-wavelength peak at 145 cm 1 is attributed to the skeleton bent vibration (B3g mode), while the peaks at 197 and 285 cm 1 derive from the bending vibrations of OC V OB bond (Ag and B2g modes). The bending vibration of V OC (Ag mode), the bending vibration of V OB V bond (Ag mode), the stretching vibration of V OB V bond (Ag mode), and the stretching vibration of V OC bond (B2g mode) are regarded at about 305, 407, 525, and 694 cm , respectively. The layered structure of V2O5 is stacked up from distorted trigonal bipyramidal atoms that share edges to form (V2O4)n zigzag double chains along the [001] direction and are cross-linked along the [100] direction through the shared corners. The mode of a skeleton bent, corresponding to the peak at 145 cm , provides an evidence for the layered structure of V2O5. Furthermore, the narrow peak centered at 990 cm , corresponding to the stretching of vanadium atoms connected to oxygen atoms through double bonds (V1⁄4O), is also an additional clue to the layer-type structure of V2O5. [22,23] The detailed microstructures of V2O5 nanowires were further studied by transmission electron microscopy (TEM). Figure 2a shows a TEM image of V2O5 nanowires, which demonstrates that the V2O5 nanowires have uniform diameters throughout their entire lengths. An X-ray energy-dispersive spectrum (EDS) acquired from an individual nanowire exhibits strong V and O peaks. The atomic ratio of V and O is close to the 2:5

Synthesis and electrochemical performance of nano-sized Li4Ti5O12 with double surface modification of Ti(III) and carbon

Spinel Li4Ti5O12 nano-particles with double conductive surface modification of Ti(III) and carbon were synthesized by a facile solid-state reaction, in which the polyaniline (PANI) coated TiO2 particles and a lithium salt were used as precursors. On heat treatment under an argon atmosphere containing 5% H2, the carbonization of PANI effectively restricted the particle-size growth of Li4Ti5O12 and reduced the surface Ti(IV) into Ti(III). The surface modification combined with tailored particle size can improve the surface conductivity and shorten the Li-ion diffusion path. Furthermore, both the Ti(III) surface modification and the tailored particles (50–70nm) have the potential to increase the solid solution (single-phase insertion/extraction) during the electrochemical process. Electrochemical analysis indicated that the presence of the solid solution is beneficial for Li-ion mobility. Thereby, the prepared Li4Ti5O12 displays high power performance.

Facile synthesis of NaV6O15 nanorods and its electrochemical behavior as cathode material in rechargeable lithium batteries

A ternary vanadium bronze compound, NaV6O15 (Na0.33V2O5), constructed by highly ordered nanorod structures, was facilely synthesized via a low temperature hydrothermal route using V2O5, H2O2 and NaCl as the precursors. A reaction mechanism involved in present hydrothermal condition was tentatively proposed. The sample was systemically post-treated at different temperatures and well characterized by various techniques. It was found that the prepared NaV6O15 nanorods had a highly crystallined single phase with a preferred c* orientation growth. When used as the cathode material in rechargeable lithium batteries, the NaV6O15 nanorods exhibited stable lithium-ion insertion/deinsertion reversibility and delivered as high as 328 mAh g−1 lithium cycled at the current density of 0.02 A g−1. In galvanostatic cycling test, a specific discharge capacity of around 300 mAh g−1 could be demonstrated for 70 cycles under 0.05 A g−1 current density. According to its unique crystallographic structure and electrochemical characteristics, it is therefore expected that as-prepared NaV6O15 nanorods may be employed as cathode material in rechargeable lithium, sodium-based batteries.

Design and synthesis of a novel nanothorn VO2(B) hollow microsphere and their application in lithium-ion batteries

3D hollow VO2(B) micro-spheres have been fabricated by using the hydrothermal method. The obtained VO2(B) hollow spheres were built up from well-defined VO2(B) nanothorn single-crystals, of which were perpendicularly parked on the sphere surface. When this hollow spheres was used as the cathode material for a lithium-ion battery, it exhibited good electrochemical properties, and its initial discharge capacity is ca. 450 mA h g−1 when recorded at the current density of 10 mA g−1. Furthermore, when recorded at 50 mA g−1, it still delivered 195 mA h g−1 in the first cycle of discharge capacity, and after 50 cycles, it did not show any capacity fading. The results described in the present work may open up another way for the design of novel nanostructured materials for various applications.

Rechargeable Ni-Li battery integrated aqueous/nonaqueous system.

A rechargeable Ni-Li battery, in which nickel hydroxide serving as a cathode in an aqueous electrolyte and Li metal serving as an anode in an organic electrolyte were integrated by a superionic conductor glass ceramic film (LISICON), was proposed with the expectation to combine the advantages of both a Li-ion battery and Ni-MH battery. It has the potential for an ultrahigh theoretical energy density of 935 Wh/kg, twice that of a Li-ion battery (414 Wh/kg), based on the active material in electrodes. A prototype Ni-Li battery fabricated in the present work demonstrated a cell voltage of 3.47 V and a capacity of 264 mAh/g with good retention during 50 cycles of charge/discharge. This battery system with a hybrid electrolyte provides a new avenue for the best combination of electrode/electrolyte/electrode to fulfill the potential of high energy density as well as high power density.

Carbon nanocages with nanographene shell for high-rate lithium ion batteries

Carbon nanocages with a nanographene shell have been prepared by catalytic decomposition of p-xylene on a MgO supported Co and Mo catalyst in supercritical CO2 at a pressure of 10.34 MPa and temperatures ranging from 650 to 750 °C. The electrochemical performance of these carbon nanocages as anodes for lithium ion batteries has been evaluated by galvanostatic cycling. The carbon nanocages prepared at a temperature of 750 °C exhibited relatively high reversible capacities, superior rate performance and excellent cycling life. The advanced performance of the carbon nanocages prepared at 750 °C is ascribed to their unique structural features: (1) nanographene shells and the good inter-cage contact ensuring fast electron transportation, (2) a porous network formed by fine pores in the carbon shell and the void space among the cages facilitating the penetration of the electrolyte and ions within the electrode, (3) thin carbon shells shortening the diffusion distance of Li ions, and (4) the high specific surface area providing a large number of active sites for charge-transfer reactions. These carbon nanocages are promising candidates for application in lithium ion batteries.

Flowerlike Vanadium Sesquioxide: Solvothermal Preparation and Electrochemical Properties

A novel 3D hierarchical flowerlike vanadium sesquioxide (V(2)O(3)) nano/microarchitecture consisting of numerous nanoflakes is prepared via a solvothermal approach followed by an appropriate heating treatment. The as-obtained nanostructured V(2)O(3) flower is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and transmission electron microscopy (TEM) (or/and high-resolution TEM, HRTEM), and it is found that the V(2)O(3) flower is constructed by single-crystalline nanoflakes. Furthermore, it is demonstrated that the surface of the flowerlike V(2)O(3) material is composed of nanostructured pores, which derive from the adsorption/desorption of nitrogen, and that the pore-size distribution depends on the unique three-dimensional interconnection between nanoflakes and on their intrinsic properties. The electrochemical behavior of the V(2)O(3) flower for lithium-ion insertion/extraction in non-aqueous solution as well as the faradaic capacitance for pesudocapacitors in a neutral aqueous solution are also investigated. A reversible discharge capacity as high as 325 mA h g(-1) is obtained at a current density of 0.02 A g(-1) from a LiClO(4)/EC:DEC electrolyte solution (i.e. LiClO(4) in ethyl carbonate and diethyl carbonate). When used as the cathode material of pesudocapacitors in Li(2)SO(4), the flowerlike oxide displayed a very high initial capacitance of 218 F g(-1) at a current density of 0.05 A g(-1). We believe that the good performance of the flowerlike V(2)O(3) electrode is most probably due to its unique 3D hierarchical nano/microarchitecture, which shows that the electrochemical properties of a cathodic material do not only depend on the oxidation state of that material but also-to a large extent-on its crystalline structure and morphology. The aforementioned properties suggest that the present V(2)O(3) flower materials may have a great potential to be employed as electrode materials in rechargeable lithium batteries and pesudocapacitors.