Although 3d transition metal oxides (TMOs) are well-known as promising anodes for Li ion batteries, little is known about the mechanism of electrode process kinetics. In this work, impedance behavior of the flower-like hierarchical CuO electrode is first investigated to understand the kinetics that influences the performances of TMOs toward lithium. The electrochemical impedance spectra are measured at different discharge and charge states during cycling. A modified twoparallel diffusion path model is set up to account for the Nyquist plots. The kinetic parameters in the model that represent the migration of lithium ions through surface-passivating film, charge transfer on active material/electrolyte interfaces, and diffusion of lithium ions in solid material are discussed in detail. On the basis of the analysis of the variation of kinetic parameters, several promising approaches are proposed to improve the electrochemical performances of copper oxides, which can also be applicable to all the 3d transition metal oxides.
' INTRODUCTIONSince Tarascon et al. 1 first reported the excellent electrochemical performance of CoO toward lithium, 3d transition metal oxides (TMOs, where M is Fe, Co, Ni, and Cu) have been widely investigated as promising anodes for lithium ion batteries. [2][3][4][5] The mechanism of Li reactivity in TMOs differs from the classical Li intercalation/deintercalation or Li-alloying process but involves the formation and decomposition of Li 2 O. The electrode reaction of TMOs is shown as followsCompared to the current commercial carbonaceous anodes, TMOs have much higher theoretic capacities and better rate properties, which will hopefully realize the wide application of Li ion batteries in various areas as next-generation electronic devices, electric vehicles, solar energy storage, etc. For instance, the CuO film prepared by spray pyrolysis exhibited high reversible capacity (625 Ah kg -1 ) even over 100 cycles. 6 However, the Coulombic efficiencies and cycling performances of pure TMOs are disappointing. It is commonly attributed to the poor conductivity of active material and incomplete decomposition of Li 2 O during cycling. Thus, surface modification and nanostructure fabrication are developed to improve the electrical conductivity and enhance the electrochemical activity of TMOs. [7][8][9] Although various novel structures and original experiments have been reported, it is still short of theory supports. To effectively overcome the shortcomings of TMOs, it is necessary to understand the kinetics of lithium ion migration through surface film, charge transfer on solid/electrolyte interfaces, and further diffusion in electrode material since these processes indeed govern the polarization and reaction rate of TMOs. It would be desirable to find out what kinetics and relevant properties such as surface resistance, charge transfer resistance, and Li þ diffusion coefficient can improve the electrochemical performances of TMOs.Electrochemical impedance spectroscopy (EIS) is a commonly used technolo...
Nanostructured polyaniline (PANI)-WO 3 hybrid thin films were synthesized via a molecular assembling route in a solution of aniline using peroxotungstic acid (PTA) as the dopant and ammonium persulfate as the oxidant. The films show a porous morphology with nanorod arrays on the surface, and WO 3 is uniformly incorporated into the polymer network. Electrochemical and electrochromic tests including cyclic voltammetry, chronoamperometry and corresponding in situ transmittance of PANI-WO 3 hybrid films compared with neat PANI film and sol-gel WO 3 film were conducted in 0.5 M sulfuric acid solution. The hybrid films, being a dual electrochromic material, varied from royal purple to green, pale yellow and finally dark blue as the applied potential was scanned from 0.8 V to À0.5 V. Compared to sulfate doped PANI film, high colouration efficiency and comparable durability are obtained in the PANI-WO 3 hybrid films. The PANI-WO 3 hybrid films also show faster switching speed and better durability than WO 3 film. The enhanced electrochromic properties such as faster switching speed and better durability are mainly attributed to the combining of advantages of both materials and the formation of the donor-acceptor system.
A monophase nickel phosphide/carbon (Ni5P4/C) composite with a thin carbon shell is controllably synthesized via the two‐step strategy of a wet‐chemistry reaction and a solid‐state reaction. In this fabrication, the further diffusion of phosphorus atoms in the carbon shell during the solid‐state reaction can be responsible for a chemical transformation from a binary phase of Ni5P4‐Ni2P to monophase Ni5P4. Galvanostatic charge‐discharge measurements indicate that the Ni5P4/C composite exhibits a superior, high rate capacibility and good cycling stability. About 76.6% of the second capacity (644.1 mA h g−1) can be retained after 50 cycles at a 0.1 C rate. At a high rate of 3 C, the specific capacity of Ni5P4/C is still as high as 357.1 mA h g−1. Importantly, the amorphous carbon shell can enhance the conductivity of the composite and suppress the aggregation of the active particles, leading to their structure stability and reversibility during cycling. As is confirmed from X‐ray‐diffraction analysis, no evident microstructural changes occur upon cycling. These results reveal that highly crystalline Ni5P4/C is one of the most promising anode materials for lithium‐ion batteries.
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