Amorphous V2O5 Research Articles

Phase Separation Derived Core/Shell Structured Cu11 V6 O26 /V2 O5 Microspheres: First Synthesis and Excellent Lithium-Ion Anode Performance with Outstanding Capacity Self-Restoration.

Novel amorphous vanadium oxide coated copper vanadium oxide (Cu11 V6 O26 /V2 O5 ) microspheres with 3D hierarchical architecture have been successfully prepared via a microwave-assisted solution method and subsequent annealing induced phase separation process. Pure Cu11 V6 O26 microspheres without V2 O5 coating are also obtained by an H2 O2 solution dissolving treatment. When evaluated as an anode material for lithium-ion batteries (LIBs), the as-synthesized hybrid exhibits large reversible capacity, excellent rate capability, and outstanding capacity self-recovery. Under the condition of high current density of 1 A g-1 , the 3D hierarchical Cu11 V6 O26 /V2 O5 hybrid maintains a reversible capacity of ≈1110 mA h g-1 . Combined electrochemical analysis and high-resolution transmission electron microscopy observation during cycling reveals that the amorphous V2 O5 coating plays an important role on enhancing the electrochemical performances and capacity self-recovery, which provides an active amorphous protective layer and abundant grain interfaces for efficient inserting and extracting of Li-ion. As a result, this new copper vanadium oxide hybrid is proposed as a promising anode material for LIBs.

Electrochemical Synthesis of Amorphous VO2 Colloids and Their Rapid Thermal Transforming to VO2 (M) Nanoparticles with Good Thermochromic Performance.

Amorphous VO2 (a-VO2 ) colloids were synthesized by electrochemical anodic oxidation of metallic vanadium. It was found that the a-VO2 colloids have a cotton-like morphology composed of very small clusters, and that the crystallization temperature of the a-VO2 colloids can be adjusted either by the electrolyte of the anodic oxidation or/and the dispersion agent of the colloids. VO2 (M) nanoparticles (NPs) (and a NP film) with an average size of about 50 nm can be obtained by a rapid thermal annealing of the a-VO2 colloids at 310 °C under air, which is beneficial for practical applications. The VO2 (M) NP film shows an obvious metal-semiconductor transition with a resistance less than 10 Ω in the metallic state. An integral visible transmittance of 40.7 %, a solar transmittance modulation of 9.4 %, and a resistance modulation in the order of 5×104 were realized in the VO2 (M) NP film.

Controllable-multichannel carbon nanofibers-based amorphous vanadium as binder-free and conductive-free electrode materials for supercapacitor

Multichannel carbon nanofibers-based amorphous vanadium (VMCNFs) was fabricated via electrospinning technology and calcination process. The amorphous V2O5 were homogeneous dispersed in the carbon nanofibers with inner hollow channels. One-dimensional structure nanomaterials with a compound inner fistulous pore construction promote the electrochemical property due to the synergistic effect between the hollow inner channels and carbon nanofibers with amorphous vanadium. With changing the addition amount of PS acting as a porogen, the optimized VMCNFs-0.5, in which the molar ratio of VO(acac)2/styrene monomer (SM) was 0.5, exhibited a high specific capacitance of 739 F g−1 at 0.5 A g−1 and good cycling performance with 80.3% capacitance retention after 1500 cycles. Thus this novel method can provide an acceptable idea for practical applications in energy storage.

Effects of amorphous V2O5 coating on the electrochemical properties of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 as cathode material for Li-ion batteries

Lithium-rich layered oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2 coated with V2O5 layers (labeled as LMNCO@V2O5) has been synthesized and its electrochemical properties as cathode material for lithium ion batteries have been measured and compared with pristine Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (labeled as LMNCO) and LMNCO–V2O5 composite. As a lithium ions insertion host material, both the V2O5 in the LMNCO@V2O5 and the LMNCO–V2O5 can reduce the irreversible capacity losses and improve the Coulombic efficiencies of the cathode in the first charge-discharge cycle. However, for improving the cycling stabilities and the high-rate capabilities of the LMNCO, the effects of the V2O5 coating layers in the LMNCO@V2O5 are far beyond the effects of the V2O5 nanoparticles in the LMNCO–V2O5. When charge-discharged galvanostatically at 25 mA g−1 between 2.0 and 4.8 V (vs. Li+/Li), the LMNCO@V2O5 with 3 wt.% V2O5 exhibits a discharge capacity of 279.5 mAh g−1 in the first cycle and maintains a discharge capacity of 269.1 mAh g−1 after 50 cycles, with capacity retention of 96.3%. In contrast, the discharge capacity of the pristine LMNCO changes from 251.2 mAh g−1 in the initial cycle to 202.2 mAh g−1 in the 50th cycle, with capacity retention of 80.2%; and the LMNCO–V2O5 shows the same capacity fading trend as the pristine LMNCO, with no obvious improvement in the capacity retention. At high rate of 1250 mA g−1, the discharge capacity of the LMNCO@V2O5 can reach 113.6 mAh g−1, which is much higher than the capacities that the pristine LMNCO and the LMNCO–V2O5 can reach. Different effects of V2O5 are due to their different roles in the cathode materials. While the V2O5 coating layer in the LMNCO@V2O5 can reduce the charge transfer resistance at the electrode-electrolyte interfaces and improve the transportation of lithium ions among the LMNCO particles, the V2O5 nanoparticles in the LMNCO–V2O5 can only work as a Li+ ions insertion host material.

Thermodynamics of Metal Cation Intercalation into V2O5(aerogel) Hosts

Sol-gel derived V2O5 (aerogel) is an excellent host for intercalation, showing high capacity for guest species. It was first reported as a host for Li+ ions [1], with a capacity of 4 Li/mole of host [2]. The capacity was shown to be reversible by GITT (galvanostatic intermittent titration technique [3]) for the entire range [1,2,4]. Polyvalent cations were also found to have an intercalation capacity of 4 equivalents/mole of V2O5 aerogel host (i.e. Mg+2, Al+3, and Zn+2) [3,4]. Experimentally, the polyvalent cations were intercalated by chemical insertion from organometallic reagents, in parallel to the chemical insertion technique introduced for Li+ ions [5,6]. Thermodynamics of guest-host systems are of importance to the study of batteries that employ intercalation electrodes. Even though thermodynamics does not specify the capacity of the host materials, the energy state of batteries is dictated for which charge can be extracted and stored. Interaction between the guest species and with the host may diminish the system energy substantially as compared to an ideal guest-host process. The diminished energy leads to a lower usable fraction of the available capacity, as reflected in low voltage or as irreversibility in some materials. Our experience with amorphous V2O5, V2O5 xerogels and crystalline V2O5 as lithium hosts demonstrated that disordered structures are generally preferred for high intercalation capacity hosts. The full usable capacity of four Li (per V2O5 unit) in the V2O5 spin-coated xerogel is not matched with amorphous V2O5 in our studies. This illustrated that with proper processing modified structures can provide a significant enhancement for usable capacity. The structural modification achieved with the V2O5 aerogel revealed that the inherent interaction in the M-V2O5 system is eliminated almost entirely. The discussion here utilizes a model for the interactions as the most significant factor in providing a basis for the high capacity for the multivalent guests in the aerogel material [7,8]. Our analysis of the M-V2O5 systems predicts that the effect of interactions is reduced, to yield model predictions that agree extremely well with the experimental results.

Carbon doping to improve the high temperature tribological properties of VN coating

VN and VCN coatings were deposited by the cathodic arc ion plating technique. Microstructures, mechanical properties and tribological properties of both coatings in ambient air were comparatively investigated. Results showed that the sp2C–C, sp3C–C and VC phases were found in VCN coating. The VCN coating exhibited lower friction coefficients and wear rates than VN coating in different temperatures. The lower friction coefficient and better anti-wear property of the VCN coating were closely related to the self-lubricating amorphous carbon and V2O5 phase, indicating its potential application as a lubricant coating for tribo-components under high temperature conditions.

Stabilizing an amorphous V2O5/carbon nanotube paper electrode with conformal TiO2 coating by atomic layer deposition for lithium ion batteries

Amorphous V2O5 (a-V2O5) thin films were conformally coated onto the surface of hydroxyl (–OH) functionalized multi-walled carbon nanotubes (CNTs) and carbon nanotube (CNT) paper using atomic layer deposition (ALD).

Rational selection of amorphous or crystalline V2O5 cathode for sodium-ion batteries.

Vanadium oxide (V2O5), as a potential positive electrode for sodium ion batteries (SIBs), has attracted considerable attention from researchers. Herein, amorphous and crystalline V2O5 cathodes on a graphite paper without a binder and conductive additives have been synthesized via facile anodic electrochemical deposition following different heat treatments. Both the amorphous V2O5 (a-V2O5) cathode and crystalline V2O5 (c-V2O5) cathode show good rate cycling performance and long cycling life. After five rate cycles, the reversible capacities of both the cathodes were almost unchanged at different current densities from 40 to 5120 mA g-1. Long cycling tests with 10 000 cycles were carried out and the two cathodes exhibit excellent cycling stability. The c-V2O5 cathode retains a high specific capacity of 54 mA h g-1 after 10 000 cycles at 2560 mA g-1 and can be charged within 80 s. Interestingly, the a-V2O5 cathode possesses higher reversible capacities than the c-V2O5 cathode at low current densities, whereas it is inversed at high current densities. The c-V2O5 cathode shows faster capacity recovery from 5120 to 40 mA g-1 than the a-V2O5 cathode. When discharged at 80 mA g-1 (long discharge time of 140 min) and charged at 640 mA g-1 (short charge time of 17 min), the a-V2O5 cathode shows a higher discharge capacity than its c-V2O5 counterpart. The different electrochemical performance of a-V2O5 and c-V2O5 cathodes during various electrochemical processes can provide a rational selection of amorphous or crystalline V2O5 cathode materials for SIBs in their practical applications to meet the variable requirements.

Effect of Meso- and Micro-Porosity in Carbon Electrodes on Atomic Layer Deposition of Pseudocapacitive V2O5 for High Performance Supercapacitors

Electrochemical capacitors can deliver high power density and long cyclability, but it is necessary to increase their energy density to promote their widespread use.1,2 The charge storage capacity of supercapacitors can be increased by adding a pseudocapacitance material to create a Faradaic charge storage layer. High surface area activated carbon electrodes can give good power performance, but adding a conformal pseudocapacitance layer could further improve charge storage. The pseudocapacitive properties of V2O5 have been well studied and V2O5 has been incorporated into V2O5/carbon nanotube composites, but it has not been studied with activated carbon.3,4 In this work, we show that atomic layer deposition (ALD) of V2O5 is a viable means to add pseudocapacitance to supercapacitors constructed from activated carbon powders. Commercially available activated carbon powders with varying microporous and mesoporous surface areas were characterized before and after ALD of V2O5 using nitrogen adsorption, FTIR, mass uptake, SEM and TEM analysis. An acid treatment of the activated carbon before ALD aided in V2O5 deposition. For the ALD process, we used vanadium tri-isopropoxide and water to grow thin films of amorphous V2O5 on the activated carbon electrodes. The V2O5 ALD on the high surface-area microporous carbon with pore diameters less than 11 Å decreased the net microporous surface area, thereby decreasing the available area for charge storage in the electrochemical double layer and the capacitance of the supercapacitor. The mesoporous carbon with slightly less starting surface area, however, showed increased charge storage capacity, consistent with more conformal coatings in the larger mesopores. For the mesoporous electrodes, we used cyclic voltammetry, galvanostatic charging/discharging, and electrochemical impedance spectroscopy (EIS) to evaluate detailed changes in supercapacitor performance as a function of pseudocapacitive V2O5 ALD coating thickness. 1.5 nm thick films of V2O5 deposited on mesoporous carbon increased the capacitance by 46%. The V2O5 exhibited a maximum pseudocapacitance of 540 F/g(V2O5) from a 0.5 nm thick V2O5 film. Electrodes with up to 4.5 nm thick V2O5 films maintained high capacitive current (91-93% of total current), similar to uncoated electrodes (89%), and minimal diffusion limited current. Electrodes coated with 33 nm of V2O5 reported no net change in capacitance compared to uncoated electrodes due to decreased surface area (70% decrease) and reduced capacitive current (67% of total current). EIS analysis reported low equivalent series resistance for all of the electrodes, and similar capacitance and diffusion trends as seen with cyclic voltammetry. Electrodes coated with 4.5 nm of V2O5 maintained 89% capacitance after 10,000 charge/discharge cycles, compared to 88% for the uncoated electrodes. This work demonstrates the basic challenges for ALD on highly porous electrodes, and points to further analysis of the V2O5 ALD process that could further improve supercapacitor performance.

THE INFLUENCE OF INTERCALATED IONS ON CYCLIC STABILITY OF V2O5/GRAPHITE COMPOSITE IN AQUEOUS ELECTROLYTIC SOLUTIONS: EXPERIMENTAL AND THEORETICAL APPROACH

An amorphous V2O5 xerogel/graphite composite was synthesized and subjected to electrochemical characterization by cyclic voltammetry, chronopotentiometry and impedance measurements in aqueous LiNO3, NaNO3, KNO3 and Mg(NO3)2 solutions. The kinetics of cyclic intercalation/deintercalation reactions, and the dependence of coulombic capacity on cycle number were examined. The capacity retention depended on the type of inserted ions and decreased in the following order: Mg2+>Li+>Na+>K+. By ultraviolet/visible (UV/VIS) spectroscopy the concentration of dissolved vanadium oxide was determined and correlated with the capacity fade in various electrolyte solutions. By means of Density Functional Theory calculation (DFT), the bond strength between vanadium and triple coordinated oxygen in V2O5 was found to be responsible for the differences in its solubility in various aqueous solutions.

Fast Supercapacitors Based on Graphene‐Bridged V2O3/VOx Core–Shell Nanostructure Electrodes with a Power Density of 1 MW kg−1

Transition metal oxides (TMOs), with their very large pseudocapacitance effect, hold promise for next generation high‐energy‐density electrochemical supercapacitors (ECs). However, the typical high resistivity of TMOs restricts the reported ECs to work at a low charge–discharge (C–D) rate of 0.1–1 V s−1. Here, a novel vanadium oxides core/shell nanostructure‐based electrode to overcome the resistivity challenge of TMOs for rapid pseudocapacitive EC design is reported. Quasi‐metallic V2O3 nanocores are dispersed on graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO2 and V2O5 (called as VOx here) thin shell around V2O3 nanocore acts as the active pseudocapacitive material. With such a graphene‐bridged V2O3/VOx core–shell composite as electrode material, ECs with a C–D rate as high as 50 V s−1 is demonstrated. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg−1 power density at an energy density of 10 Wh kg−1. At the critical 45° phase angle, these ECs have a measured frequency of 114 Hz. All these indicate the graphene‐bridged V2O3/VOx core–shell structure is promising for fast EC development.

Better than crystalline: amorphous vanadium oxide for sodium-ion batteries

Amorphous and nanocrystalline vanadium pentoxide (V2O5) were prepared through a combination of sol–gel processing paired with electrochemical deposition and investigated as cathodes for sodium-ion batteries. Amorphous V2O5 demonstrated superior electrochemical properties upon sodiation as compared to its crystalline counterpart. More specifically, amorphous vanadium pentoxide had a measured capacity of 241 mA h g−1, twice the capacity of its crystalline contemporary at 120 mA h g−1. In addition, the amorphous vanadium pentoxide demonstrated a much higher discharge potential, energy density, and cycle stability. The development of amorphous materials could enable the usage and design of previously unexplored electrode materials; herein, the possible relationship between the improved sodiation properties and the amorphous structure is discussed.

Vanadium oxide electrode synthesized by electroless deposition for electrochemical capacitors

A thin film vanadium oxide electrode was synthesized by a simple electroless deposition method. Surface and structural analyses revealed that the deposited oxide is a mixture of amorphous V2O5 and VO2. Electrochemical characterizations of the synthesized vanadium oxide showed capacitive behavior with good cycle life. The electroless deposition of vanadium oxide is inexpensive, easy to process, and environmentally benign, offering a promising route for electrode development for electrochemical capacitors.

Excited state dynamics of nanocrystalline VO2 with white light continuum time resolved spectroscopy

In an attempt to use ultrafast pump probe spectroscopy technique with white light continuum to reveal wavelength dependent dynamics of VO2, bandgap needs to be opened. Therefore, nanostructured amorphous and crystalline VO2 thin films were prepared with pulsed DC magnetron reactive sputtering. The mean diameters of grains were measured as 22±0.1nm and 44±0.1nm for amorphous and crystalline VO2 thin films, respectively. Temperature dependent resistance measurements show that nanocrystalline VO2 thin film exhibit metal insulator phase transition. The films exhibited dual band gaps (2.3eV, <0.6eV for amorphous films and 1.3eV, 1.8eV for crystalline film). Increased band gaps made it possible to perform time resolved transmission and reflection experiments with white light continuum at fluences above and below photo induced phase transition threshold. Although transmission chance due to photo induced phase transition of VO2 in the literature usually takes places at infrared region of the spectrum, transmission chance was observed in visible as low as 630nm in broadband probe spectra. It was observed that measured time scales depend on not only pump fluence but also probe wavelength. Experiments gave the evidence of the long-lived lower energy non-equilibrium state related to the photo induced phase.

Thermochromic amorphous VO2 coatings grown by APCVD using a single-precursor

This work reports the growth of thermochromic amorphous vanadium dioxide coatings on SnO2-precoated glass substrates by atmospheric pressure chemical vapor deposition using vanadyl (V) triisopropoxide as single-precursor at 450°C. The best reversible transition behavior was at 65.5°C as derived from the transmittance–temperature measurements. This finding is discussed in terms of variations in microstructure occurring during the different nitrogen flow rate studies. We show that the thermochromic behavior of the oxides is determined by the presence of well-defined monoclinic vanadium dioxide.

Electronic density-of-states of amorphous vanadium pentoxide films: Electrochemical data and density functional theory calculations

Thin films of V2O5 were prepared by sputter deposition onto transparent and electrically conducting substrates and were found to be X-ray amorphous. Their electrochemical density of states was determined by chronopotentiometry and displayed a pronounced low-energy peak followed by an almost featureless contribution at higher energies. These results were compared with density functional theory calculations for amorphous V2O5. Significant similarities were found between measured data and computations; specifically, the experimental low-energy peak corresponds to a split-off part of the conduction band apparent in the computations. Furthermore, the calculations approximately reproduce the experimental band gap observed in optical measurements.

Improvement of temperature coefficient of resistance of a VO2 film on an SiN/polyimide/Si substrate by excimer laser irradiation for IR sensors

Using a laser-assisted sputtering process, we have successfully prepared a VO2 film on a SiN/polyimide/Si substrate. We investigated the effects of laser fluence and shot number on the crystallization and temperature coefficient of resistance (TCR) of the deposited VO2 films. At a constant shot number (2500 pulses), the resistivity of the film decreased from 50 to 60 mJ/cm2, and that of the film increased with an increase from 70 to 80 mJ/cm2. At 50 mJ/cm2, the resistivity decreased with increasing shot number. TCR at 30 °C of the amorphous VO2 film on a SiN/polyimide/Si substrate prepared by KrF laser irradiation at 50 mJ/cm2 for 4150 pulses at room temperature is 1.73%/K. In addition, TCR of the film prepared at 70 mJ/cm2 was 3.36%/K at 40 °C. This TCR of the film is much higher than that of the film (1.70%/K) prepared by thermal annealing, which is used in the present IR sensor. The VO2 films prepared by laser sputtering are promising materials for IR sensors, which are increasingly used in industry.

Investigations into Structure and Chemistry of 1D, 2D and 3D Structured Vanadium Oxide Nanomaterials for Li-Ion Batteries

Routes towards the formation of inverted opal vanadium oxide electrodes are presented. Different methods of template infiltration using an IPA diluted solution of vanadium triisopropoxide are discussed and the resulting morphologies investigated using scanning electron microscopy. The effect of different heat treatments and method of sphere removal on morphology and structure is also considered. Solvent template removal retains thehydrolysed amorphous V2O5 structure. Raman scattering spectroscopy identifies the degree of V2O5 crystallinity that results from the different heat treatments. For a thicker inverted opal formed using a polystyrene template as opposed to a monolayer PMMA template, under similar conditions a different phase of vanadium oxide is observed evident by variations inRaman scattering response.

Optimizing Vanadium Pentoxide Thin Films and Multilayers from Dip-Coated Nanofluid Precursors

Using an alkoxide-based precursor, a strategy for producing highly uniform thin films and multilayers of V2O5 is demonstrated using dip coating. Defect-free and smooth films of V2O5 on different surfaces can be deposited from liquid precursors. We show how pinholes are formed due to heterogeneous nucleation during hydrolysis as the precursor forms a nanofluid. Using knowledge of instability formation often found in composite nanofluid films and the influence of cluster formation on the stability of these films, we show how polymer-precursor mixtures provide optimum uniformity and very low surface roughness in amorphous V2O5 and also orthorhombic V2O5 after crystallization by heating. Pinhole and roughness instability formation during the liquid stage of the nanofluid on gold and ITO substrates is suppressed giving a uniform coating. Practically, understanding evolution pathways that involve dewetting processes, nucleation, decomposition, or hydrolysis in complex nanofluids provides a route for improved uniformity of thin films. The method could be extended to improve the consistency in sequential or iterative multilayer deposits of a range of liquid precursors for functional materials and coatings.

Comparison of Structural and Electrochemical Properties of V2O5 Thin Films Prepared by Organic/Inorganic Precursors

Vanadium pentoxide thin films were produced from organic and inorganic precursors by sol gel dip-coating method. Fourier transform infrared spectroscopy and UV Vis spectroscopy were made to figure out structural properties of the films. Electrochemical properties were investigated by cyclic voltammetry. The shape of the curves was in agreement with a typical diffusion controlled cyclic voltammograms of amorphous V2O5 films for a reversible lithium ion intercalation/deintercalation process showing yellow–green–blue multi-electrochromism. V2O5 films, synthesized from organic precursor, indicated lower band gap energy, higher charge capacity as well as homogeneous and low granule size compared to inorganic route.