Hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2

The most highlighted point of this work to emphasize is that it is the first trial to use Mn₃O₄ oxide as a precursor to synthesize orthorhombic LiMnO₂ by the hydrothermal method. A well-ordered orthorhombic LiMnO₂ phase was formed by the hydrothermal treatment of Mn₃O₄ with excess LiOH aqueous solution at 170 °C. According to TEM observation, the as-synthesized powder was single crystalline particle oxide. Comparing with other orthorhombic LiMnO₂ prepared by low temperature synthetic route and by high temperature calcination, the orthorhombic LiMnO₂ prepared by the hydrothermal route showed enhanced battery performance as a lithium battery cathode material. We believe that the new hydrothermal synthesis is expected as an excellent alternative of powder preparation method of high capacity cathode material to be used for Li-ion secondary battery.     

One-step hydrothermal routine for pure-phased orthorhombic LiMnO2 for Li ion battery application

This paper reports a simple one-step hydrothermal routine to prepare orthorhombic LiMnO₂ powder for Li ion battery application. Employing Mn₂O₃ and LiOH as the starting materials, hydrothermal reaction operated under 160 °C for 12 h generated pure-phased o-LiMnO₂ powder. The morphological change and reduction in grain size between the reagent and the resultant were revealed by SEM observation, which indicated that LiOH played two important roles in the process, one as the Li ion source to form orthorhombic LiMnO₂ by intercalation and the other as the corrosive medium to control the morphology and reduce the particle size. Detailed investigation showed that the LiOH concentration and the hydrothermal temperature were two key factors influencing phase purity of the final product. Pure-phased o-LiMnO₂ prepared under optimized hydrothermal conditions showed higher capacity and better cyclical performance than the commonly prepared o-LiMnO₂ powder, and therefore promised potential application for lithium ion secondary batteries.     

Efficient microwave hydrothermal synthesis of nanocrystalline orthorhombic LiMnO2 cathodes for lithium batteries

Rod-like orthorhombic LiMnO₂ nanocrystals were successfully synthesized using temperature-controlled microwave hydrothermal route (TCMH) in a short time (30 min) at a temperature as low as 160 °C. o-LiMnO₂ obtained by two different methods was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemistry test. SEM revealed that the product obtained in case of TCMH was rod-like with a diameter of 40 nm, while the nanoparticles with 200 nm diameter were obtained by traditional hydrothermal route (TH). The dramatic formation of o-LiMnO₂ in the microwave hydrothermal field influenced the morphology and crystal structure of the final products. The formation and preferred growth orientation mechanism of o-LiMnO₂ in the microwave irradiation process was discussed. Electrochemistry performance exhibited that the as-synthesized o-LiMnO₂ nanorods reached the maximum discharge capacity of 194.2 mAh g⁻¹ at 0.1 C rate after several cycles between 2.2 and 4.4 V vs. Li⁺/Li at room temperature, which was higher than the electrochemical performance of o-LiMnO₂ obtained by TH. The experimental results showed that the TCMH method provided an effective way for preparing o-LiMnO₂ cathode material in lithium-ion batteries.     

Hydrothermal synthesis of orthorhombic LiMnO2 nano-particles

Orthorhombic LiMnO₂ nano-particles were successfully prepared under mild hydrothermal conditions from KMnO₄ and MnCl₂ sources. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the LiMnO₂ particles. The influence of the molar ratio of Li/Mn and of the holding temperature was studied. The synthesized nano-particles were found to be highly crystallized with a diameter of 50-100 nm.     

Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 0.5, 1.0, and 1.5 wt.% CeO₂ by a polymeric process, followed by calcination at 850 °C for 6 h in air. The surface-coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). XRD patterns of CeO₂-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure or the Fd3m space group of the cathode materials compared to uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using SEM, TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 20 nm. The XPS data illustrated that the CeO₂ completely coated the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and CeO₂-coated LiMn₂O₄ cathode materials were measured in the potential range of 3.0-4.5 V (0.5 C rate) at 30 °C and 60 °C. Among them, the 1.0 wt.% of CeO₂-coated spinel LiMn₂O₄ cathode satisfies the structural stability, high reversible capacity and excellent electrochemical performances of rechargeable lithium batteries.     

IrO2-SiO2 binary oxide films: Preparation, physiochemical characterization and their electrochemical properties

Mixed IrO₂-SiO₂ oxide films were prepared on titanium substrate by the thermo-decomposition of hexachloroiridate (H₂IrCl₆) and tetraethoxysilane (TEOS) mixed precursors in organic solvents. The solution chemistry and thermal decomposition kinetics of the mixed precursors were investigated by ultra violet/visible (UV/vis) spectroscopy and thermogravimetry (TGA) and differential thermal analysis (DTA), respectively. The physiochemical characterization of the resulting materials was conducted by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. It is shown from the UV/vis spectra that the electronic absorption intensity of IrCl₆²⁻ complexes in the precursors decreases in the presence of TEOS, indicating the interaction between these two components. Thermal analysis shows the decomposition reaction of H₂IrCl₆ is inhibited by TEOS in the low temperature range, but the further oxidation reaction at high temperatures of formed intermediates is independent of the presence of silane component. Physical measurements show a restriction effect of silica on the crystallization and crystal growth processes of IrO₂, leading to the formation of finer oxide particles and the porous morphology of the binary oxide films. The porous composite films exhibit high apparent electrocatalytic activity toward the oxygen evolution reaction. In addition, the long-term stability of Ti-supported IrO₂ electrodes is found to apparently improve with appropriate amount of SiO₂ incorporation, as tested under galvanostatic electrolysis.     

Synthesis of SnO2/Mg2SnO4 nanoparticles and their electrochemical performance for use in Li-ion battery electrodes

Uniform crystalline MgSn(OH)₆ nanocubes were synthesized by a hydrothermal method. The influences of reaction conditions were investigated and the results showed that the solvent constituents significantly affected the shape and size of MgSn(OH)₆·SnO₂/Mg₂SnO₄ has been obtained by thermal treatment at 850 °C for 8 h under a nitrogen atmosphere using MgSn(OH)₆ as the precursor. The electrochemical tests of SnO₂/Mg₂SnO₄ revealed that SnO₂/Mg₂SnO₄ has a higher capacity and better cyclability compared to pure SnO₂ or Mg₂SnO₄. The electrochemical performance of SnO₂/Mg₂SnO₄ was sensitive to the size of the nanoparticles. The lithium-driven structural and morphological changes of the electrode after cycling were also studied by the ex-situ XRD pattern and TEM tests. This work indicates that SnO₂/Mg₂SnO₄ is a promising anode material candidate for application in Li-ion batteries.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

Mn influence on the electrochemical behaviour of Li3V2(PO4)3 cathode material

The role played by the substitution of Mn on the electrochemical behaviour of Li₃V₂(PO₄)₃ has been investigated. Independently of the synthesis route, the Mn doping improves the electrochemical features with respect to the undoped samples. Different reasons can be taken into consideration to explain the electrochemical enhancement. In the sol–gel synthesis the capacity slightly enhances due to the Mn substitution on both the V sites, within the solubility limit x = 0.124 in Li₃V_2−xMn_x(PO₄)₃. In the solid state synthesis the significant capacity enhancement is preferentially due to the microstructural features of the crystallites and to the LiMnPO₄ phase formation.     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

Structural colors from TiO2/SiO2 multilayer flakes prepared by sol-gel process

Preparation of TiO₂/SiO₂ multilayer flakes and their application to decorative powders were investigated. In contrast to conventional products prepared through the multicoating of core platelets, the coreless TiO₂/SiO₂ multilayer flakes were prepared by detaching multilayer films from their substrates. These flakes exhibited structural colors, when the optical path length of both the TiO₂ and SiO₂ layers are adjusted to be one fourth of the wavelength of visible light. A multicoating of more than five layers resulted in the propagation of cracks, which prevented the preparation of thick flakes. Paint films fabricated using the multilayer flakes and acrylic resins showed reflectance spectra that were comparable with those obtained for multicoatings on substrates.     

Synthesis and characterization of CeO2 nano-rods

We report a synthesis of two types of CeO₂ nano-rods via the facile and efficient hydrothermal process free from any surfactant and template. The synthesized nano-rods are chemically identified as CeO₂ with the standard fluorite structure but their morphologies are different. The nano-rods prepared with cerium nitrate hexahydrate and sodium phosphate are thicker and shorter with diameter of ∼30 nm and length of ∼100 nm, and those prepared with cerium acetate hydrate and dibasic sodium phosphate are thinner and longer with ∼10 nm in diameter and ∼400 nm in length. Microstructural analyses reveal that the two species of nano-rods have low-energy {111} surfaces and grow along the 〈112〉 direction. As a consequence of their morphologies, the two types of synthesized nano-rods exhibit excellent UV-absorption ability in comparison to the irregular CeO₂ nanoparticles.     

Enhanced electrochemical performance and thermal stability of La2O3-coated LiCoO2

An enhanced electrochemical performance LiCoO₂ cathode was synthesized by coating with various wt.% of La₂O₃ to the LiCoO₂ particle surfaces by a polymeric method, followed by calcination at 923 K for 4 h in air. The surface-coated materials were characterized by XRD, TGA, SEM, TEM, BET and XPS/ESCA techniques. XRD patterns of La₂O₃-coated LiCoO₂ revealed that the coating did not affect the crystal structure, α-NaFeO₂, of the cathode material compared to pristine LiCoO₂. TEM images showed a compact coating layer on the surface of the core material that had an average thickness of about ∼15 nm. XPS data illustrated that the presence of two different environmental O 1s ions corresponds to the surface-coated La₂O₃ and core material. The electrochemical performance of the coated materials by galvanostatic cycling studies suggest that 2.0 wt.% coated La₂O₃ on LiCoO₂ improved cycle stability (284 cycles) by a factor of ∼7 times over the pristine LiCoO₂ cathode material and also demonstrated excellent cell cycle stability when charged at high voltages (4.4, 4.5 and 4.6 V). Impedance spectroscopy demonstrated that the enhanced performance of the coated materials is attributed to slower impedance growth during the charge-discharge processes. The DSC curve revealed that the exothermic peak corresponding to the release of oxygen at ∼464 K was significantly smaller for the La₂O₃-coated cathode material and recognized its high thermal stability.     

Synthesis of LiNi0.9Co0.1O2 from Li2CO3, NiO or NiCO3, and CoCO3 or Co3O4 and their electrochemical properties

Cathode active materials with a composition of LiNi_0.9Co_0.1O₂ were synthesized by a solid-state reaction method at 850 °C using Li₂CO₃, NiO or NiCO₃, and CoCO₃ or Co₃O₄, as the sources of Li, Ni, and Co, respectively. Electrochemical properties, structure, and microstructure of the synthesized LiNi_0.9Co_0.1O₂ samples were analyzed. The curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂ for the first charge–discharge and the intercalated and deintercalated Li quantity Δx were studied. The destruction of unstable 3b sites and phase transitions were discussed from the first and second charge–discharge curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂. The LiNi_0.9Co_0.1O₂ sample synthesized from Li₂CO₃, NiO, and Co₃O₄ had the largest first discharge capacity (151 mA h/g), with a discharge capacity deterioration rate of −0.8 mA h/g/cycle (that is, a discharge capacity increasing 0.8 mA h/g per cycle).     

Antimonides (FeSb2, CrSb2) with orthorhombic structure and their nanocomposites for rechargeable Li-ion batteries

Intermetallic FeSb₂ and CrSb₂ and their nanocomposites (FeSb₂/C and Sb/Cr₃C₂/C) were prepared using solid-state routes, such as heat-treatment and high-energy mechanical milling, in order to enhance the electrochemical properties of Sb. These electrodes were tested as anode materials for rechargeable Li-ion batteries. The reaction mechanism of intermetallic FeSb₂ and CrSb₂ was investigated using ex situ X-ray diffraction and high resolution transmission electron microscopy. The FeSb₂/C and Sb/Cr₃C₂/C nanocomposite electrodes exhibited greatly enhanced electrochemical behaviors compared to the FeSb₂ and CrSb₂ electrodes. Additionally, the Sb/Cr₃C₂/C nanocomposite electrode showed a better electrochemical performance than the FeSb₂/C nanocomposite electrode.     

Orientation-controlled synthesis and characterization of Bi2Te3 nanofilms, and nanowires via electrochemical co-deposition

An electrochemical deposition technique based on co-deposition was used to deposit preferentially oriented Bi₂Te₃ nanostructures (nanofilm, and nanowire). The shared underpotential deposition (UPD) potentials for both Bi and Te co-deposition were determined by cyclic voltammetric measurements. The scanning probe microscopy (scanning tunneling microscopy (STM) and atomic force microscopy (AFM)) and the X-ray diffraction (XRD) data indicated that the electrodeposition of Bi₂Te₃ results in nanofilm-structured deposits with a preferential orientation at (0 1 5) and nanowired-structured deposits with a preferential orientation at (1 1 0) in acidic and basic (in the presence of ethylenediaminetetraacetic acid (EDTA)) medium, respectively. The results show that the nucleation and growth mechanism follows 3D mode in acidic solutions and 2D mode in basic solution containing EDTA additive. The optical characterization performed by reflection absorption Fourier transform infrared (RA-FTIR) spectroscopy showed that the band gap energy of Bi₂Te₃ nanostructures depends on the thickness, size, and shape of the nanostructures and the band gap increases as the deposition time decreases. Moreover, the quantum confinement is strengthened in the wire-like deposits relative to the film-like deposits. Energy dispersive X-ray spectroscopy (EDS) analysis demonstrated that Bi₂Te₃ nanostructures were always in 2:3 stoichiometry, and they were made up of only pure Bi and Te.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Enhanced electrochemical and thermal stability of surface-modified LiCoO2 cathode by CeO2 coating

CeO₂-coated LiCoO₂ particles were successfully synthesized by a sol-gel coating of CeO₂ on the surface of the LiCoO₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiCoO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Cyclic voltammetry curves suggested that the CeO₂ coating suppressed the phase transitions. Unlike pristine LiCoO₂, the CeO₂-coated LiCoO₂ cathode exhibited better capacity retention than the pristine LiCoO₂ electrode in the higher cutoff voltage. Differential scanning calorimetric data revealed the higher thermal stability of the CeO₂-coated LiCoO₂ electrode.     

Synthesis and optical characterization of novel Sr3Ga2O6:Eu phosphor

Alkaline earth metal gallets have been identified as an important ceramic material. The crystal chemistry of many of these gallets is well explored; however, very rare studies regarding optical properties of rare earth (RE) ions doped in such gallets, particularly in Sr₃Ga₂O₆ host, have been carried out. The present study reports on synthesis and characterization of novel Sr₃Ga₂O₆:Eu³⁺ phosphors. The phosphors have been synthesized using a conventional solid state reaction method. Crystal structure, morphology and luminescence properties (excitation, emission and CIE coordinate) of these phosphors have been studied as a function of sintering temperature and Eu³⁺ concentration. X-ray diffraction study reveals that the phosphor sintered at low temperature (900 °C) contains an impurity phase which is removed at higher sintering temperatures and results into cubic crystalline phase of Sr₃Ga₂O₆. Particle size of the phosphor increases with an increase in sintering temperature which results to a red shift in the peak position of excitation band lying in a broad range from 250 to 370 nm. Optimum emission intensity is attained for 0.12 mol% concentration of Eu³⁺ ions; above this concentration, a quenching in emission intensity is observed.     

Effect of CeO2-coating on the electrochemical performances of LiFePO4/C cathode material

The effect of CeO₂ coating on LiFePO₄/C cathode material has been investigated. The crystalline structure and morphology of the synthesized powders have been characterized by XRD, SEM, TEM and their electrochemical performances both at room temperature and low temperature are evaluated by CV, EIS and galvanostatic charge/discharge tests. It is found that, nano-CeO₂ particles distribute on the surface of LiFePO₄ without destroying the crystal structure of the bulk material. The CeO₂-coated LiFePO₄/C cathode material shows improved lithium insertion/extraction capacity and electrode kinetics, especially at high rates and low temperature. At −20 °C, the CeO₂-coated material delivers discharge capacity of 99.7 mAh/g at 0.1C rate and the capacity retention of 98.6% is obtained after 30 cycles at various charge/discharge rates. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.