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[en] Silicon and related materials have recently received considerable attention as potential anodes in Li-ion batteries for their high theoretical specific capacities. To overcome the problem of volume variations during the Li insertion/extraction process, in this work, Si/C composites with low carbon content were synthesized from cheap coarse silicon and citric acid by simple ball milling and subsequent thermal treatment. The effects of ball milling time and calcination temperature on the structure, composition and morphology of the composites were systematically investigated by the determination of specific surface area (BET) and particle-size distribution, X-ray diffraction (XRD), O2-TPO, and scanning electron microscopy (SEM). The capacity and cycling stability of the composites were systematically evaluated by electrochemical charge/discharge tests. It was found that both the initial capacity and the cycling stability of the composites were dependent on the milling and calcination conditions, and attractive overall electrochemical performance could be obtained by optimizing the synthesis process.
[en] There is increasing interest in flexible, safe, high-power thin-film lithium-ion batteries which can be applied to various modern devices. Although TiO2 in rutile phase is highly attractive as an anode material of lithium-ion batteries for its high thermal stability and theoretical capacity of 336 mA h g−1 and low price, its inflexibility and sluggish lithium intercalation kinetics of bulk phase strongly limit its practical application for particular in thin-film electrode. Here we show a simple way to prepare highly flexible self-standing thin-film electrodes composed of mesoporous rutile TiO2/C nanofibers with low carbon content (<15 wt.%) by electrospinning technique with outstanding electrochemical performance, which can be applied directly as electrodes of lithium-ion batteries without the further use of any additive and binder. The atmosphere during calcination plays a critical role in determining the flexible nature of thin film and particle size of TiO2 in as-fabricated nanofibers. Big size (10 cm × 4 cm), flexible thin film is obtained after heat treatment under 10%H2–Ar at 900 °C for 3 h. After optimization, the diameter of fibers can reach as small as ∼110 nm, and the as-prepared rutile TiO2 films show high initial electrochemical activity with the first discharge capacity as high as 388 mA h g−1. What is more, very stable reversible capacities of ∼122, 92, and 70 mA h g−1 are achieved respectively at 1, 5 and 10 C rates with negligible decay rate within 100 cycling times.
[en] Pure-phase spinel-type lithium titanate, Li4Ti5O12, was successfully fabricated by cellulose-assisted glycine-nitrate (cellulose-GN) combustion process at reduced temperature using anatase TiO2 solid as raw material of titanium. The influence of cellulose and impregnation sequence on the phase purity, particle size and electrochemical performance of Li4Ti5O12 was investigated. The sequence of preparation was found to have big effect on the phase formation and electrochemical performance of the oxides. High-purity and well-crystallized Li4Ti5O12 oxides were obtained at a calcination temperature of 750 deg. C via the sequence III, for which the cellulose was first adsorbed by the mixed solution of LiNO3 and glycine followed by the impregnation of TiO2 suspension. Compared with solid-state reaction, the cellulose-GN process produced the Li4Ti5O12 oxides with smaller particle size and higher specific capacity due to the lower synthesis temperature. A reversible capacity of 103 mAh/g at 20 C rate and fairly stable cycling performance even at 40 C was achieved.
[en] The formation mechanism of a spinel-type lithium titanate Li4Ti5O12 with TiO2 anatase as raw material, in both a conventional solid-state reaction (SSR) and a cellulose-assisted glycine-nitrate combustion (cellulose-GN) process are comparatively studied. XRD characterization demonstrates high-purity Li4Ti5O12 forms at 750 oC by the cellulose-GN synthesis, which occurs at a temperature at least 100 oC lower than that via SSR. The solid-phase reaction between TiO2 and lithium compounds to form Li-Ti-O spinel and the phase transition of TiO2 from anatase to 'inert' rutile phase occur competitively during both synthesis processes. SEM results suggest that the solid precursor from the cellulose-GN process has a smaller particle size and a more homogenous mixing of the reactants than that in the SSR. Temperature-programmed oxidation experiments demonstrate that cellulose thermal pyrolysis creates a reducing atmosphere, which may facilitate the oxygen-ion diffusion. Both factors facilitate the formation of Li-Ti-O spinel, while the TiO2 anatase transforms to TiO2 rutile during the SSR, which has slow lithium-insertion kinetics. As a result, a high calcination temperature is necessary to obtain a phase-pure Li4Ti5O12. Charge-discharge and EIS tests demonstrate the Li4Ti5O12 obtained by the cellulose-GN process shows much better low-temperature electrochemical performance than that obtained by standard SSR. This improvement attributes to the reduced particle size due to the lower synthesis temperature.
[en] (Ba0.5Sr0.5)1+xCo0.8Fe0.2O3-δ, or BSCF(1 + x), (0 ≤ x ≤ 0.3) oxides were synthesized and investigated as cathodes for intermediate-temperature solid-oxide fuel cells. The A-site cation excess in BSCF(1 + x) resulted in a lattice expansion and the creation of more active sites for oxygen reduction reaction due to the lowered valence states of the B-site ions and the increased oxygen vacancy concentration, which improved the oxygen adsorption process. On the other hand, the A-site excess could also result in higher resistances for oxygen adsorption (due to the formation of BaO and/or SrO impurities), and oxygen-ion transfer (by facilitating the solid-phase reaction between the cathode and the electrolyte). By taking all these factors into account, we found BSCF1.03 to be the optimal composition, which lead to a peak power density of 1026.2 ± 12.7 mW cm-2 at 650 deg. C for a single cell
[en] Co-free oxides with a nominal composition of LnBaFe2O5+δ, where Ln = La, Pr, Nd, Sm, Gd, and Y, were synthesized and phase structure, oxygen content, electronic conductivity, oxygen desorption, thermal expansion, microstructure and electrochemical performance were systematically investigated. Among the series of materials tested, LaBaFe2O5+δ oxide showed the largest electronic conductivity and YBaFe2O5+δ oxide had the smallest thermal expansion coefficient (TEC) of 14.6 × 10−6 K−1 within a temperature range of 200–900 °C. All LnBaFe2O5+δ oxides typically possess the TEC values smaller than 20 × 10−6 K−1. The oxygen content, electronic conductivity and TEC values are highly dependent on the cation size of the Ln3+ dopant. The lowest electrode polarization resistance in air under open circuit voltage condition was obtained for SmBaFe2O5+δ electrode and was approximately 0.043, 0.084, 0.196, 0.506 and 1.348 Ω cm2 at 800, 750, 700, 650 and 600 °C, respectively. The SmBaFe2O5+δ oxide also demonstrated the best performance after a cathodic polarization. A cell with a SmBaFe2O5+δ cathode delivered peak power densities of 1026, 748, 462, 276 and 148 mW cm−2 at 800, 750, 700, 650 and 600 °C, respectively. The results suggest that certain LnBaFe2O5+δ oxides have sufficient electrochemical performance to be promising candidates for cathodes in intermediate-temperature solid oxide fuel cells.
[en] Three kinds of carbon conductive additives, i.e. multi-walled carbon nanotubes (MWNTs), vapour grown carbon fibres (VGCFs) and acetylene carbon blacks (AB), were investigated to improve the electrochemical performance of activated carbon (AC) used as electrode materials for supercapacitors. Galvanostatic charge/discharge and cyclic voltammetric measurements demonstrate that MWNTs are the most effective additive to improve the electrochemical performance of AC under the same conditions. To get the same results for AC in a symmetric supercapacitor, the desired additive amount was 3.0 wt% MWNTs, ∼5.0 wt% VGCFs and ∼9.0 wt% AB, respectively. X-ray diffraction analysis results demonstrate MWNTs have the sharpest (002) peak and highest graphitic degree. Scanning electron microscopy images show MWNTs have a vimineous fibre shape and VGCFs have a stubbed virgate shape. MWNTs run across AC particles and VGCFs distribute discretionarily among AC particles. High-resolution transmission electron microscopy demonstrates the microstructural difference between MWNTs and VGCFs. Some mechanisms were then developed to explain the different performance of the three kinds of carbon conductive additives
[en] New mixed conducting oxides with the composition of Sr1-xYxCo1-yYyO3-δ (x = 0.0-0.8, y = 0.0-0.1) were exploited and synthesized. The resulted materials were investigated by X-ray diffraction, four-probe dc conductivity, temperature-programmed desorption characterization, and oxygen permeability measurement. As compared with the oxides with only one-site (A or B) being Y3+-doped, i.e., Sr1-xYxCoO3-δ and SrCo1-yYyO3-δ, the double-site Y3+-doped ones show improved phase stability, higher electrical conductivity under reduced atmosphere, and higher oxygen permeability and stability. Particularly, Sr0.95Y0.05Co0.95Y0.05O3-δ oxide demonstrates stable cubic perovskite phase in air, oxygen and nitrogen, high electrical conductivity of ∼110 S cm-1 in air and ∼50 S cm-1 in nitrogen, and a maximum permeation flux of 1.35 x 10-6 mol cm-2 s-1 at 900 deg. C under an air/helium gradient. Long-term permeation study at 850 deg. C indicates that Sr0.95Y0.05Co0.95Y0.05O3-δ can operate stably as oxygen semi-permeable membrane
[en] Highlights: • Cobalt oxide nanosheets in situ electrochemical generated from commercial LiCoO_2. • TEM indicates creation of cobalt oxide nanosheets from coarse layered LiCoO_2_. • Coarse-type LiCoO_2 with high tap density shows promising anode performance. • Optimizing weight ratio of LiCoO_2 in electrode, a high capacity was achieved. - Abstract: Cobalt oxides are attractive alternative anode materials for next-generation lithium-ion batteries (LIBs). To improve the performance of conversion-type anode materials such as cobalt oxides, well dispersed and nanosized particulate morphology is typically required. In this study, we describe the in situ electrochemical generation of cobalt oxide nanosheets from commercial micrometer-sized LiCoO_2 oxide as an anode material for LIBs. The electrode material as prepared was analyzed by XRD, FE-SEM and TEM. The electrochemical properties were investigated by cyclic voltammetry and by a constant current galvanostatic discharge–charge test. The material shows a high tap density and promising anode performance in terms of capacity, rate performance and cycling stability. A capacity of 560 mA h g"−"1 is still achieved at a current density of 1000 mA g"−"1 by increasing the amount of additives in the electrode to 40 wt%. This paper provides a new technique for developing a high-performance conversion-type anode for LIBs.
[en] Highlights: ► Sintering temperature of BCZY-Z pellets was reduced by adding ZnO and Na2CO3. ► Chemical stability of BCZY-Z towards CO2 was improved with Na2CO3 addition. ► Good chemical stability against boiling water was observed for BCZY-Z-C2 sample. ► The electrical conductivity is 7.68 × 10−3 S cm−1 for BCZY-Z-C2 sample at 700 °C. ► An anode-supported POFC delivered a peak output 302 mW cm2 at 700 °C. -- Abstract: BaCe0.5Zr0.3Y0.2O3−δ (BCZY) based composite electrolyte materials were fabricated with ZnO sintering aid (BCZY-Z). The effects of Na2CO3 modification on sintering behavior, chemical stability and electrochemical performance were systematically investigated. The X-ray diffraction patterns indicate that the specimens with Na2CO3 addition possessed a single perovskite structure after sintering at 1320 °C for 2 h. The linear shrinkage of 0.5 mol% Na2CO3-modified BCZY-Z sample (BCZY-Z-C2) was about 17.5%, higher than that without Na2CO3 addition (14.9%). Energy dispersive spectrometer shows that Na and C elements still existed and mainly distributed along the grain boundaries. Reactivities with carbon dioxide and boiling water of BCZY-Z and Na2CO3-modified BCZY-Z samples were also evaluated and good chemical stability was observed for Na2CO3-modified BCZY-Z samples. A conductivity of 7.68 × 10−3 S cm−1 for BCZY-Z-C2 was obtained at 700 °C in 3% wet hydrogen atmosphere. An anode-supported fuel cell with thin-film BCZY-Z-C2 as electrolyte was fabricated. The fuel cell delivered a peak power density of 302 mW cm2 and interface resistance value of 0.08 Ω cm2 at 700 °C