ABSTRACT
In recent years, various attempts have been made to meet the increasing demand for high energy density of lithium-ion batteries (LIBs). The increase in voltage can improve the capacity and the voltage platform performance of the electrode materials. However, as the charging voltage increases, the stabilization of the interface between the cathode material and the electrolyte will decrease, causing side reactions on both sides during the charge-discharge cycling, which seriously affects the high-temperature storage and the cycle performance of LIBs. In this study, a sulfate additive, dihydro-1,3,2-dioxathiolo[1,3,2]dioxathiole 2,2,5,5-tetraoxide (DDDT), was used as an efficient multifunctional electrolyte additive for high-voltage lithium cobalt oxide (LiCoO2). Nanoscale protective layers were formed on the surfaces of both the cathode and the anode electrodes by the electrochemical redox reactions, which greatly decreased the side reactions and improved the voltage stability of the electrodes. By adding 2% (wt.%) DDDT into the electrolyte, LiCoO2 exhibited improved Li-storage performance at the relatively high temperature of 60 °C, controlled swelling behavior (less than 10% for 7 days), and excellent cycling performance (capacity retention rate of 76.4% at elevated temperature even after 150 cycles).
ABSTRACT
LaF3 : Tba3+, Ce3+ nanocrystals were prepared with hydrothermal method with the help of cetyltrimethyl ammonium bromide (CTAB). The effects of pH values of the solution, Ce3+/Tb+ ratio value and reaction time on the luminescent properties were investigated. XRD analysis shows that the as-prepared samples possess hexagonal phase and their main diffraction peaks of samples are similar to the standard card (JCPDS 32-0483). Compared with pure LaF3, the main diffraction peaks of the doped samples have a slight shift, showing existing isomorphous substitution between La3+ and the doped rare earth ions in parent lattice of LaF3. It is found from TEM results that the as-prepared samples have good crystallinity and their average grain sizes change in the range of 20-50 nm. The excitation spectra indicate that the stronger excitation spectrum peaks exist at 250 nm, which is assigned to the transition of 4f --> 5d from Ce3+. When activated at 250 nm, all LaF3 : Tb3+, Ce3+ nanocrystals possess weak blue emission at 490 nm (electric dipole transition, 5D4 --> 7F6) and good green emission at 543 nm (magnetic dipole transition, 5D4 -->7F5). As the Ce3+/Tb+ ratio increases, the fluorescence intensities increase at first and then weaken, and reach the strongest green emission at n(Ce)3+ /n(Tb)3+ = 4. The pH values have some influence on the colors and intensities of the LaF3 : Tb3+, Ce3+ nanocrystals. The sample prepared at pH 9 presents the best color, while the one at pH 7 exhibits the strongest green emission. Besides, increasing reaction time is helpful to improve color purity of sample and enhance its green emission.
ABSTRACT
Using La2O3, Dy2O3, NH4VO3, HNO3 as materials, solvothermal synthesis method was adopted to prepare LaVO4 : Dy3+ nanorods through adjusting the pH values of ethanol-water mixing solution. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscope, infrared spectrum, UV-Vis absorption spectra and fluorescence spectra. The results show that the phase transition from m- to t -LaVO4 : Dy3+ can be found when the pH value changes from 2 to 4. With the increase of the pH value of the mixing solution, the grain size of the sample becomes smaller and the shape of LaVO4 : Dy3+ crystal changes from irregular nanoparticle to one dimension nanorod. Besides, the band gap of the sample decrease from 3.68 (pH 2) to 3.43 eV (pH 10). It is found from FL that the excitation spectra of LaVO4 : Dy3+ nanorods have a little red shift in comparison with irregular nanoparticle. Furthermore, the LaVO4 : Dy3+ nanorod exhibits the strongest yellow emission (4F9/2-6 H13/2) and blue emission(4F9/2-6H15/2), and it possesses the largest Y/B value (1.039).
