A rationally designed self-standing V2O5 electrode for high voltage non-aqueous all-solid-state symmetric (2.0 V) and asymmetric (2.8 V) supercapacitors

Vanadium pentoxide interfacial layer enables high performance all-solid-state thin film batteries

Lithium cobalt oxide (LiCoO2) is considered as one of the promising building blocks that can be used to fabricate all-solid-state thin film batteries (TFBs) because of its easy accessibility, high working voltage, and high energy density. However, the slow interfacial dynamics between LiCoO2 and LiPON in these TFBs results in undesirable side reactions and severe degradation of cycling and rate performance. Herein, amorphous vanadium pentoxide (V2O5) film was employed as the interfacial layer of a cathode-electrolyte solid-solid interface to fabricate all-solid-state TFBs using a magnetron sputtering method. The V2O5 thin film layer assisted in the construction of an ion transport network at the cathode/electrolyte interface, thus reducing the electrochemical redox polarization potential. The V2O5 interfacial layer also effectively suppressed the side reactions between LiCoO2 and LiPON. In addition, the interfacial resistance of TFBs was significantly decreased by optimizing the thickness of the interfacial modification layer. Compared to TFBs without the V2O5 layer, TFBs based on LiCoO2/V2O5/LiPON/Li with a 5 nm thin V2O5 interface modification layer exhibited a much smaller charge transfer impedance (Rct) value, significantly improved discharge specific capacity, and superior cycling and rate performance. The discharge capacity remained at 75.6% of its initial value after 1000 cycles at a current density of 100 μA cm-2. This was mainly attributed to the enhanced lithium ion transport kinetics and the suppression of severe side reactions at the cathode-electrolyte interface in TFBs based on LiCoO2/V2O5/LiPON/Li with a 5 nm V2O5 thin layer.

Effects of Valence States of Working Cations on the Electrochemical Performance of Sodium Vanadate

Supercapacitors have received much attention as large-scale energy storage devices for high power density and ultralong cycling life. In this work, sodium vanadate Na_0.76V₆O₁₅/poly(3,4-ethylenedioxythiophene) (PEDOT) nanocables with deficient bridge oxygen at the interface (denoted Vo^••-PNVO) have been tailored for supercapacitors through the in situ polymerization of 3,4-ethylenedioxythiophene and studied using three different electrolytes. Experiments and theoretical calculations reveal that all Na⁺, Zn²⁺, and Al³⁺ ions appear as hydrates in aqueous solutions but insert into the crystal structure as Na⁺ ions and Zn²⁺-H₂O and Al³⁺-H₂O hydrates, respectively. In comparison with the Zn²⁺-H₂O and Al³⁺-H₂O hydrates, Na⁺ ions with a smaller radius diffuse more quickly in Vo^••-PNVO. Thus, Vo^••-PNVO delivers better charge storage capability and stability when an electrolyte with Na⁺ ions is used. The results strongly suggest that an electrostatic interaction is significant in determining transport properties and storage capacities, rather than hydrate radii or valence states.

V2O5/Carbon Nanotube/Polypyrrole Based Freestanding Negative Electrodes for High-Performance Supercapacitors

In this study, the vanadium pentoxide (V2O5), functionalized carbon nanotubes (f-CNT), and polypyrrole (PPy) based composites films have been prepared through a facile synthesis method and their electrochemical performance were evaluated as freestanding negative electrodes of supercapacitor. A hydrous V2O5 gel prepared by treating V2O5 powder with H2O2 was mixed with f-CNT to obtain V2O5/f-CNT composite film. V2O5/f-CNT composite was then coated with PPy through vapor phase polymerization method. The PPy deposited on the V2O5/f-CNT prevented the dissolution of V2O5 and thus resulted in an improved the capacitance and cycle life stability for V2O5/f-CNT/PPy composite electrode. V2O5/f-CNT/PPy freestanding negative electrode exhibited a high areal capacitance value (1266 mF cm−2 at a current density of 1 mA cm−2) and good cycling stability (83.0% capacitance retention after 10,000 charge-discharge cycles). The superior performance of the V2O5/f-CNT/PPy composite electrode can be attributed to the synergy between f-CNT with high conductivity and V2O5 and PPy with high-energy densities. Thus, V2O5/f-CNT/PPy composite based electrode can effectively mitigate the drawbacks of the low specific capacitance of CNTs and the poor cycling life of V2O5.

A high-voltage non-aqueous hybrid supercapacitor based on the N2200 polymer supported over multiwalled carbon nanotubes

P(NDI2OD-T2), also known as Polyera ActivInk N2200, is a widely accepted non-fullerene acceptor polymer that is used prominently in the energy harvesting application due to its ease of synthesis, high electron mobility, and other desirable semiconducting properties. With its recent foray into energy storage applications, there is tremendous potential for developing composites of N2200 with carbon nanotubes (CNTs) to improve its electrical properties and extend its applicability. Here we report a facile synthesis of an N2200 composite with multiwalled carbon nanotubes (MWCNTs) following an in situ approach to include MWCNTs into the polymer matrix, improving its electrochemical performance in an organic electrolyte (1 M LiClO₄/propylene carbonate). The composite material with an optimum MWCNT content exhibits prominent redox behavior delivering a specific capacity of 80 mA h g^-1_(polymer) in a standard three-electrode cell. Moreover, the N2200/MWCNT composite material showing a battery-type electrochemical signature could perform as an efficient negative electrode in a high-voltage (2.4 V) hybrid supercapacitor device comprising capacitive activated carbon as the positive electrode.

Ultra-high rate capability of nanoporous carbon network@V2O5 sub-micron brick composite as a novel cathode material for asymmetric supercapacitors

A green biomass-derived nanoporous carbon network (NCN) has been prepared and integrated with V2O5 sub-micron bricks (SMBs). The large surface area and high pore volume of the NCN can not only provide abundant sites for electrochemical reactions but also stabilize the structure of the V2O5 SMBs. The NCN@V2O5 SMB composite, acting as a novel cathode material, delivers a high areal capacitance of 786 mF cm-2 at 0.2 mA cm-2 and superior cycling stability with 89.5% capacitance retention after 5000 cycles. Besides, the electrode achieves an ultra-high rate capability (82% capacitance retention as the current density increases from 0.2 to 5 mA cm-2) since the contribution from the non-diffusion-controlled process is estimated to be as high as 95.5%-98.5% according to the kinetic analysis. Furthermore, the micropores are more favorable than the mesopores at lower current densities (0.2-2 mA cm-2), while the contribution of the external surface area becomes more significant for current densities higher than 2 mA cm-2. Moreover, an asymmetric supercapacitor assembled using this cathode and the NCN anode shows superior electrochemical properties, such as wide operating voltage, long cycle life and large energy density (72.2 μW h cm-2). Their excellent electrochemical features and good eco-friendliness confirm the potential of the NCN@V2O5 SMBs for use as supercapacitors.

Bilayered microelectrodes based on electrochemically deposited MnO2/polypyrrole towards fast charge transport kinetics for micro-supercapacitors

Micro-supercapacitors (MSCs) are promising power solution facilities for miniaturized portable electronic devices. Microfabrication of on-chip MSC with high specific capacitance and high energy density is still a great challenge. Herein, we report a high-performance MnO2/polypyrrole (PPy) microelectrode based MSC (MnO2/PPy-MSC) by modern micromachining technology. Interdigital Au micro current collectors were obtained by photolithography, physical vapor deposition and lift off. A layer of PPy was electrochemically deposited on Au current collectors followed by deposition of urchin-like MnO2 micro/nanostructures. The electrochemical performance of MnO2/PPy-MSC was explored employing LiClO4/PVA gel electrolyte. The assembled MSC demonstrated a high areal capacitance of 13 mF cm-2, an energy density of 1.07 × 10-3 mW h cm-2 and a power density of 0.53 mW cm-2. In addition, the MnO2/PPy-MSC showed an improved cycling stability, retaining 84% of the initial capacitance after 5000 CV cycles at a scan rate of 500 mV s-1. Our proposed strategy provides a versatile and promising method for the fabrication of high-performance MSCs with large-scale applications.

P-Functionalized and O-deficient TiOn/VOm nanoparticles grown on Ni foam as an electrode for supercapacitors: epitaxial grown heterojunction and visible-light-driven photoresponse

P-TiOn-VOm nanowires were grown on nickel foam (NF) via a one-pot hydrothermal method and by further vapor deposition/phosphorization method. It was found that low valence states of titanium oxide and deficient-oxygen coexist in P-TiOn-VOm/NF. Furthermore, (TiO1.25)3.07 (denoted as TiOn) and VO (denoted as VOm) possess similar structures and matched facets, and their epitaxial growth leads to the formation of TiOn/VOm heterostructure with a formation energy of -1.59 eV. P-TiOn-VOm/NF possesses good electron conductivity and electrons can be transferred from Ti to V centers, as evidenced by the DFT calculations and the XPS spectra. As a result, the specific capacity of P-TiOn-VOm/NF can reach 785 C g-1 at 1 A g-1 in the potential range of 0-0.55 V vs. Hg/HgO, which is much larger than those of VOm/NF, P-VOm/NF, and P-TiO2-VOm/NF. On the other hand, the TiOn/VOm heterostructure also favors the separation and transfer of photoinduced electrons and holes, and P-TiOn-VOm/NF exhibits visible-light-driven photoresponse. Under visible light illumination, the specific capacity of P-TiOn-VOm/NF is increased by 6.2% relative to that in the dark. Furthermore, the P-TiOn-VOm/NF//activated carbon (AC) asymmetric supercapacitor (ASC) shows an energy density of 37.2 W h kg-1 at a power density of 1 kW kg-1 and excellent cycling performance with 93.6% capacity retention after 10 000 cycles at 5 A g-1, which is comparable to and even superior to those of titanium oxides and vanadium oxides. A promising achievement has been proposed to improve the energy storage performance of P-TiOn-VOm through P-functionalization and O-deficiency in this work.

Interface Engineering V2 O5 Nanofibers for High-Energy and Durable Supercapacitors

A local electric field is induced to engineer the interface of vanadium pentoxide nanofibers (V₂ O₅ -NF) to manipulate the charge transport behavior and obtain high-energy and durable supercapacitors. The interface of V₂ O₅ -NF is modified with oxygen vacancies (Vö) in a one-step polymerization process of polyaniline (PANI). In the charge storage process, the local electric field deriving from the lopsided charge distribution around Vö will provide Coulombic forces to promote the charge transport in the resultant Vö-V₂ O₅ /PANI nanocable electrode. Furthermore, an ≈7 nm porous PANI coating serves as the external percolated charge transport pathway. As the charge transfer kinetics are synergistically enhanced by the dual modifications, Vö-V₂ O₅ /PANI-based supercapacitors exhibit an excellent specific capacitance (523 F g^-1 ) as well as a long cycling lifespan (110% of capacitance remained after 20 000 cycles). This work paves an effective way to promote the charge transfer kinetics of electrode materials for next-generation energy storage systems.

Asymmetric supercapacitors with high energy densities

The low energy densities of supercapacitors (SCs) are generally limited by the used anodes. To develop SCs with high energy densities, Fe3+ modified V2O5@GQDs (m-V2O5@GQDs) and ZIF-67-derived nanoporous carbon loaded with Mn3O4 (C/N-Mn3O4) were synthesized. After their detailed characterization using electron microscopy, X-ray methods and electrochemical techniques, they were further utilized as the anode and the cathode, respectively, to construct asymmetric supercapacitors (ASCs). The as-synthesized m-V2O5@GQDs improve the poor conductivity of V2O5, contributing greatly to a specific capacitance of 761 F g-1 at a current density of 2 A g-1. With application of a cell voltage of 2 V, an energy density of up to 99.4 W h kg-1 is achieved at a power density of 1000 W kg-1. Such ASCs also exhibit outstanding cycling performance (95% of initial capacitance even after 10 000 charging/discharging cycles). This study thus provides a new way to design and construct ASCs with high energy densities.

Silver-Quantum-Dot-Modified MoO3 and MnO2 Paper-Like Freestanding Films for Flexible Solid-State Asymmetric Supercapacitors

Free-standing paper-like thin-film electrodes have great potential to boost next-generation power sources with highly flexible, ultrathin, and lightweight requirements. In this work, silver-quantum-dot- (2-5 nm) modified transition metal oxide (including MoO₃ and MnO₂ ) paper-like electrodes are developed for energy storage applications. Benefitting from the ohmic contact at the interfaces between silver quantum dots and MoO₃ nanobelts (or MnO₂ nanowires) and the binder-free nature and 0D/1D/2D nanostructured 3D network of the fabricated electrodes, substantial improvements on the electrical conductivity, efficient ionic diffusion, and areal capacitances of the hybrid nanostructure electrodes are observed. With this proposed strategy, the constructed asymmetric supercapacitors, with Ag quantum dots/MoO₃ "paper" as anode, Ag quantum dots/MnO₂ "paper" as cathode, and neutral Na₂ SO₄ /polyvinyl alcohol hydrogel as electrolyte, exhibit significantly enhanced energy and power densities in comparison with those of the supercapacitors without modification of Ag quantum dots on electrodes; present excellent cycling stability at different current densities and good flexibility under various bending states; offer possibilities as high-performance power sources with low cost, high safety, and environmental friendly properties.