Improved electrochemical properties of Li1+x(Ni0.3Co0.4Mn0.3)O2−δ (x = 0, 0.03 and 0.06) with lithium excess composition prepared by a spray drying method

Layered Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ (x = 0, 0.03 and 0.06) materials were synthesized through the different calcination times using the spray-dried precursor with the molar ratio of Li/Me = 1.25 (Me = transition metals). The physical and electrochemical properties of the lithium excess and the stoichiometric materials were examined using XRD, AAS, BET and galvanostatic electrochemical method. As results, the lithium excess Li_1.06(Ni_0.3Co_0.4Mn_0.3)O_2−δ could show better electrochemical properties, such as discharge capacity, capacity retention and C rate ability, than those of the stoichiometric Li_1.00(Ni_0.3Co_0.4Mn_0.3)O_2−δ. In this paper, the effect of excess lithium on the electrochemical properties of Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ materials will be discussed based on the experimental results of ex situ X-ray diffraction, transmission electron microscopy (TEM) and galvanostatic intermittent titration technique (GITT)     

Preformed Boehmite Nanoparticles As Coating Materials for Long-Cycling LiCoO2

A simple mechano-thermal coating procedure for the production of long-cycling LiCoO₂ is described. Preformed nanoparticulate boehmite was used as the coating material. The coating procedure resulted in the cathode particles covered in a 20-30 nm thick kernel of boehmite, as revealed by TEM examination. XRD studies showed that the lattice parameter c diminished upon coating, indicating that a substitutional compound of the composition Li _x Al _y Co_1−y O₂ may have formed upon calcination. SEM images, R-factor values from XRD studies and galvanostatic charge-discharge studies showed that a coating level of 1.0 wt % gave an optimal performance in capacity and cyclability. SEM images showed that above the 1.0 wt % coating level, the excess boehmite adhered to the coated cathode particles as sperules. ESCA depth profile analysis showed partial diffusion of aluminum from the surface to the interior. Galvanostatic cycling studies showed that at a coating level of 1.0 wt %, cyclability improved three- and twelve-fold for the two commercial LiCoO₂ samples tested. Cyclic voltammetry revealed that the hexagonal-monoclinic-hexagonal phase transformations during cycling were suppressed in the coated cathode materials.     

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.     

Nano-crystalline LiNi0.5Mn1.5O4 synthesized by emulsion drying method

LiNi_0.5Mn_1.5O₄ spinel has been prepared by an emulsion drying method which can intermix cations very homogeneously at the atomic scale. When the emulsion-dried precursor was fired at 750 °C for 24 h, the observed particle of the LiNi_0.5Mn_1.5O₄ was nano-crystallite, being about 50 nm in diameter. The Rietveld refinement result clearly exhibited that the cubic spinel phase was successfully formed without any secondary phases, indicating that Li and transition metal cations occupied the 8a and 16d sites of the Fd3m structure, respectively. Li deintercalation from the spinel framework brings about a shift in the XRD peak toward higher angles and a peak splitting in the composition range δ=0-0.2 in Li_δNi_0.5Mn_1.5O₄, implying that the host structure is progressively oxidized from Ni²⁺ to Ni⁴⁺ and accompanied by a two phase reaction. The sample calcined at 750 °C for 24 h showed the best cyclability upon cycling due probably to better crystallinity and a smaller particle size. We suggest that this material can be used as a 4.5 V cathode material for Li-ion battery.     

Pickering emulsion polymerization: Preparation of polystyrene/nano-SiO2 composite microspheres with core-shell structure

Polystyrene/nano-SiO₂ composite microspheres (PS/nano-SiO₂) with core-shell structure were successfully synthesized in a Pickering emulsion route using nano-SiO₂ particles as stabilizers, which were organically modified by methacryloxypropyltrimethoxysilane (MPTMS) which containing a reactive CC bond. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transformation infrared spectrum (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). In addition, bare PS spheres could also be obtained by changing synthetic conditions. It was found that the morphology of the resulting products depends on particle concentration, particle wettability and pH value of particle dispersion. A possible mechanism for the formation of the composites with different morphologies is proposed.     

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.     

Texture effect on the electrochemical properties of LiCoO2 thin films prepared by PLD

LiCoO₂ thin films were prepared by pulsed laser deposition (PLD) on Pt/Ti/SiO₂/Si (Pt) and Au/MgO/Si (Au) substrates, respectively. Crystal structures and surface morphologies of thin films were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The LiCoO₂ thin films deposited on the Pt substrates exhibited a preferred (0 0 3) texture with smooth surfaces while the LiCoO₂ thin films deposited on the Au substrates exhibited a preferred (1 0 4) texture with rough surfaces. The electrochemical properties of the LiCoO₂ films with different textures were compared with charge-discharge, dQ/dV, and Li diffusion measurements (PITT). Compared with the (1 0 4)-textured LiCoO₂ thin films, the (0 0 3)-textured thin films exhibited relatively lower electrochemical activity. However, the advantage of the (1 0 4)-textured film only remained for a small number of cycles due to the relatively faster capacity fade. Li diffusion measurements showed that the Li diffusivity in the (0 0 3)-textured film is one order of magnitude lower than that in the (1 0 4)-textured film. As discussed in this paper, we believe that Li diffusion through grain boundaries is comparable to or even faster than Li diffusion through the grains.     

Preparation and characterization of multicomponent porous materials prepared by the sol-gel process

An experimental strategy was developed to obtain transparent Si-Al-Ti-Ni-Mo and Si-Zr-Ti-Ni-Mo sols via the sol-gel process. The sol was prepared from Si(OEt)₄ (TEOS), Al(OBu^s)₃ (OBu^s: C₂H₅CH(CH₃)O), Ti(OEt)₄ (OEt: OCH₂CH₃), Zr(OPrⁿ)₄ (OPrⁿ: OCH₂CH₂CH₃). In both cases nickel nitrate hexahydrate (Ni(NO₃)₂ · 6H₂O) and ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄ · 4H₂O) were the Ni and Mo sources, respectively. The sols were characterized by Fourier Transform Infrared Spectroscopy (FTIR). Assignments of the simultaneous formation of the Si-O-Al, Si-O-Ti, Si-O-Ni, and Si-O-Zr bonds were done. The sols were polymerized at room temperature (293 K) to obtain gels, and these were dried at 423 K and calcined at 573, 853 and 893 K in air. The characterization techniques used were small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The density of the solids was measured following ASTM method D-4892 and the porosity and surface area were determined by N₂ adsorption/desorption isotherms. The corresponding average pore diameters were evaluated using the BJH, HK, and DA methods.     

Preparation and electrochemical properties of LiMn2O4 by the microwave-assisted rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders as cathode materials for lithium-ion batteries were successfully synthesized by a new simple microwave-assisted rheological phase method, which was a timesaving and efficient method. The physical properties of the as-synthesized samples compared with the pristine LiMn₂O₄ obtained from the rheological phase method were investigated by thermogravimetry analysis (TGA), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The as-prepared powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance were examined in detail. The powders resulting from the microwave-assisted rheological phase method were pure, spinel structure LiMn₂O₄ particles of regular shapes with distribution uniformly, and exhibited promising electrochemical properties for battery. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reactions of Li-ion insertion into and extraction from LiMn₂O₄ electrode.     

纳米SiO2改性纯丙乳液的制备与表征

采用硅烷偶联剂KH570对纳米SiO2进行表面改性,有效改善了纳米SiO2的表面性能。通过预乳化种子乳液法制备了纳米SiO2改性纯丙乳液,并对改性前后的SiO2、纯丙乳液及改性纯丙乳液进行红外和X-射线衍射分析表征。     

Nanostructured CeO2 microspheres synthesized by a novel surfactant-free emulsion

Nanostructured CeO₂ microspheres with an average diameter of 11 μm were prepared by a novel surfactant-free emulsion for the first time. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer-Emmet-Teller (BET) measurements were used to characterize the products. The images of SEM showed that the CeO₂ microspheres consisted of petal-like nanostructures with petal thickness in a range of 60 nm to 100 nm. The BET measurements showed the specific surface area of the CeO₂ microspheres exceeded 43 m²/g. The XRD analysis indicated the nanostructured CeO₂ microspheres were of cubic lattice. A possible mechanism of an interfacial precipitation reaction with the droplets of solid-stabilized emulsion as templates was proposed.     

纳米SiO_2改性纯丙乳液的制备与表征

采用硅烷偶联剂KH570对纳米SiO2进行表面改性,有效改善了纳米SiO2的表面性能。通过预乳化种子乳液法制备了纳米SiO2改性纯丙乳液,并对改性前后的SiO2、纯丙乳液及改性纯丙乳液进行红外和X-射线衍射分析表征。     

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.     

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.     

Preparation and characterization of Bi2Se3 nanowires by electrodeposition

The electrodeposition of Bi₂Se₃ nanowires on an anodic aluminum oxide template was investigated by cyclic voltammetry in a tartaric acid aqueous solution. The electrochemical behavior of the Bi₂Se₃ nanowires in the electrolytic solution was also investigated using cyclic voltammetry, and the underpotential deposition mechanism of the Bi₂Se₃ nanowires was determined. According to the cyclic voltammetric curves, −0.20 V vs. SCE (saturated calomel electrode) was chosen as the deposition potential of the Bi₂Se₃ nanowires. The ratio of Bi to Se is nearly 2:3, verified by energy-dispersive X-ray spectroscopy and with the addition of surfactant. X-ray diffraction, scanning electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy indicate that annealing can improve the crystallinity and chemical composition of Bi₂Se₃ nanowires. Surfactant can also improve the surface morphology and composition of the Bi₂Se₃ nanowires.     

Preparation and electrochemical characterization of TiO2 nanowires as an electrode material for lithium-ion batteries

Anatase TiO₂ nanowires containing minor TiO₂(B) phase were prepared by a hydrothermal chemical reaction followed by the post-heat treatment at 400 °C. The phase structure and morphology were analyzed by X-ray diffraction, Raman scattering, transmission electron microscope, and field-emission scanning electron microscopy. The electrochemical properties were investigated by employing constant current discharge-charge test, cyclic voltammetry, and electrochemical impedance techniques. These nanowires exhibited high rate capacity of 280 mAh g⁻¹ even after 40 cycles, and the coulombic efficiency was approximately 98%, indicating excellent cycling stability and reversibility. The electrochemical impedance spectra showed a stable kinetic process of the electrode reaction. These results indicated that the TiO₂ nanowires have promising application for high energy density lithium-ion batteries.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

Structural modification of Li[Li0.27Co0.20Mn0.53]O2 by lithium extraction and its electrochemical property as the positive electrode for Li-ion batteries

In our previous report, we have synthesized Li₂MnO₃-LiCoO₂ solid solutions and have investigated electrochemical properties [J.-M. Kim, T. Sho, N. Kumagai, Electrochem. Commun. 9 (2007) 103]. These materials have showed a long charge plateau at above 4.5 V during the first charge, which disappears with the subsequent cycles. This phenomenon is usually observed in Li₂MnO₃ and Li₂MnO₃-LiMeO₂ system (Me = Ni_1/2Mn_1/2 [Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid State Lett. 4 (2001) 191], Co [K. Numata, C. Sakaki, S. Yamanaka, Solid State Ionics 117 (1999) 257; Y.J. Park, Y.-S. Hong, X. Wu, M.K. Kim, K.S. Ryu, S.H. Chang, J. Electrochem. Soc. 151 (2004) A720], Fe [M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, T. Nakamura, M. Kohzaki, A. Hirano, R. Kanno, J. Electrochem. Soc. 149 (2002) A509], or Cr [B. Ammundsen, J. Paulsen, Adv. Mater. 13 (2001) 943]). In this study, we investigate the relationship between the first lithium extraction process and the electrochemical property of the synthesized Li[Li_0.27Co_0.2Mn_0.53]O₂ material. The crystal structure and electrochemical performance of the synthesized Li[Li_0.27Co_0.20Mn_0.53]O₂ are modified by the Li⁺ extraction.     

Synthesis of spherical LiMn2O4 microparticles by a combination of spray pyrolysis and drying method

Lithium manganese oxide (LiMn₂O₄) has been synthesized by a spray pyrolysis method from the precursor solution; LiNO₃ and Mn(NO₃)₂·6H₂O were stoichiometrically dissolved into distilled water. The synthesized LiMn₂O₄ particles exhibited a pure cubic spinel structure in the X-ray diffraction (XRD) patterns, however they were spherical hollow spheres for various concentrations of precursor solution. Thus, the as-prepared LiMn₂O₄ particles were then ground in a mortar and dispersed into distilled water. To make a well dispersed slurry solution, a dispersion agent was also added into the slurry solution. The LiMn₂O₄ microparticles with a spherical nanostructure were finally prepared by a spray drying method from the slurry solution. The tap density of the LiMn₂O₄ microparticle prepared by a combination of spray pyrolysis and drying method was larger than that by a conventional spray pyrolysis method.The as-prepared samples were sintered at 750 °C for 1 h in air and used as cathode active materials for lithium batteries. Test experiments in the electrochemical cell Li|1 M LiClO₄ in EC:DEC = 1:1|LiMn₂O₄ demonstrate that the sample prepared by the present method is a promising cathode active material for 4 V lithium-ion batteries at high-charge-discharge and elevated temperature operation. The LiMn₂O₄ compounds by the combination of spray pyrolysis and drying method are superior to that by the conventional spray pyrolysis method in terms of electrochemical characteristics and tap density.     

种子乳液聚合法制备纳米SiO_2/苯丙复合乳液

采用种子乳液聚合工艺,用经硅烷偶联剂改性后的纳米SiO2原位合成了纳米SiO2/苯丙复合乳液。通过正交实验确定了最佳原料配比和聚合工艺条件,并对复合乳液性能进行了分析与表征。结果表明,当功能单体的用量为单体总量的5%、阴离子乳化剂十二烷基硫酸钠(SDS)与非离子乳化剂辛基苯基聚氧乙烯醚(OP-10)质量比为1颐1、复合乳化剂总量3%、纳米SiO2含量为单体总量的1.5%、聚合温度为80℃时,可制备具有良好综合性能的复合乳液,其涂膜耐水性、硬度、附着力等均优于普通苯丙乳液。该复合乳液乳胶粒粒度呈单峰分布,平均粒径为101nm,具有核/壳结构。