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The success of high capacity energy storage systems based on lithium (Li) batteries relies on the realization of the promise of Li-metal anodes. Li metal has many advantageous properties, including an extremely high theoretical specific capacity (3860 mAh g–1), the lowest electrochemical potential (–3.040 V versus standard hydrogen electrode), and low density (0.59 g cm–3), which, all together, make it a very desirable electrode for energy storage devices. However, while primary Li batteries are used for numerous commercial applications, rechargeable Li-metal batteries that utilize Li-metal anodes have not been as successful. This article discusses the properties of Li metal in the absence of surface stabilization, as well as three different approaches currently under investigation for stabilizing the surface of Li metal to control its reactivity within the electrochemical environment of a Li-based battery.
The recent development of in-situ liquid stages for (scanning) transmission electron microscopes now makes it possible for us to study the details of electrochemical processes under operando conditions. As electrochemical processes are complex, care must be taken to calibrate the system before any in-situ/operando observations. In addition, as the electron beam can cause effects that look similar to electrochemical processes at the electrolyte/electrode interface, an understanding of the role of the electron beam in modifying the operando observations must also be understood. In this paper we describe the design, assembly, and operation of an in-situ electrochemical cell, paying particular attention to the method for controlling and quantifying the experimental parameters. The use of this system is then demonstrated for the lithiation/delithiation of silicon nanowires.
High-concentration niobium (V)-doped titanium dioxide (TiO2) nanoparticles of the nonequilibrium chemical composition have been synthesized via Ar/O2 radio-frequency thermal plasma oxidation of mist precursor solutions with various Nb5+ concentrations (Nb/(Ti + Nb) = 0–25.0 at.%). The solubility as high as ∼25.0 at.% has not been achieved before by wet-chemical techniques. The preferable anatase formation was attained in the plasma-synthesized powders and was enhanced by the niobium doping. All the powders were heated at high temperatures (600–800 °C) to investigate their phase transformation, band gap variation, inter-particulate binding behavior, and photocatalytic stability. The transformation from anatase to rutile was effectively inhibited by increasing the Nb5+ content. The Nb5+ doping prevented the band gap of TiO2 from narrowing after the heating. At high temperatures, Nb5+ doping could not only preserve particle size but also prevent inter-particulate binding. High concentration (25.0 at.%) Nb5+ doping retained the photocatalytic performance almost invariably irrespective of being thermally treated.
Many recently designated or expanded nature reserves in China were forest farms that ceased operations in the aftermath of the catastrophic Yangtze River floods of 1998. Although the vegetation in many of these areas has been altered significantly during forestry operations, there is now an opportunity to reduce, or even reverse, habitat loss for wildlife species that inhabit these forests. One such species is the globally threatened Reeves's Pheasant Syrmaticus reevesii that is endemic to the forested mountains of central and south-west China. From April 2000 to August 2003, the habitat use by 14 male Reeves's Pheasants was studied by radio-tracking at Dongzhai National Nature Reserve in the Dabie Mountains, central China. Conifer-broadleaf mixed forest was used preferentially in all seasons at the study area scale, as were mature fir plantations and shrubby vegetation. Moreover, young fir plantations were used preferentially during the breeding season at the scale of the home range. Surveys recorded the pheasant in 13 other protected areas in the Dabie Mountains, and indicated that broadly similar habitat types were available in all of them. Furthermore, Reeves's Pheasant were found in habitats similar to those used preferentially at Dongzhai National Nature Reserve. It seems likely that a mosaic of habitats is crucial to meet the various requirements of male Reeves's Pheasants throughout the year and management should therefore concentrate on maintaining this mosaic. It is now important to identify the habitats that produce the most young pheasants so that nesting and brood-rearing habitats can be clearly identified. Further studies on the habitat mosaic would be useful, both at a local scale and also at a larger, landscape scale.
Plasma Enhanced Chemical Vapor Deposition (PECVD) was used to prepare vanadium oxide thin films as cathodes for rechargeable lithium batteries. The reactants consisted of a high vapor pressure liquid source of vanadium (VOCl3) and hydrogen and oxygen gas. Deposition parameters such as the flow rates of H2, O2 and VOCl3, the substrate temperature and the Rf power were optimized, and high deposition rate of 11 Å/s was obtained. Vanadium oxide films with high discharge capacities of up to 408 mAh/g were prepared. The films also showed a superior cycling stability between 4 and 1.5 V at a C/0.2 rate for more than 4400 cycles. The films were amorphous up to a deposition temperature of 300°C, however, deposition on to substrates with textured surfaces facilitated the formation of crystalline films. We demonstrate that both the vanadium oxide material and the PECVD deposition method are very attractive for constructing thin-film rechargeable lithium batteries with high capacity and long-term cyclic stability.
Various factors affecting the coloring and bleaching processes of LixWO3 films have been studied. The rate of the coloring process is limited by the decreasing electromotive force in the LixWO3 film and by the components of the series circuit resistance, including the electrolyte resistance and the diffusion impedance within the film. The bleaching process in a thick film is limited by either the space charge or by the diffusion impedance, depending on the experimental conditions. A more complete and quantitative model of the coloring/bleaching process has been developed and shows good agreement with experimental results. Our analysis also indicates that the lithium concentration value of x near the LixWO3/electrolyte interface can greatly exceed its reversible limit during the coloring process, even though the average x value within the film remains much lower than the reversible limit. This phenomenon may introduce some irreversible structural changes in the film, which in turn may constitute one of the film's degradation mechanisms.
The chemical diffusion coefficients of lithium ions in LixWO3 films were investigated as a function of lithium concentration and film porosity. Thin films were deposited with different porosities by thermal evaporation of WO3 powder in various partial water pressures. Our results indicate that diffusion coefficients increase with film porosity and decrease with increasing lithium concentration. Large diffusion coefficients that were found for small lithium concentrations appear to be due to the contribution of protons generated from ion exchange reactions between lithium and water incorporated in the film. Simultaneous electrical and in situ optical measurements were carried out to study the effect of porosity on the electrochromic properties of LixWO3. The coloring efficiency of porous WO3 films increases by approximately 70% when deposited in partial water pressure of 10−4 Torr, but decreases with further increments in water pressure.