Aging characteristics of high-power lithium-ion cells with LiNi0.8Co0.15Al0.05O2 and Li4/3Ti5/3O4 electrodes

The impedance rise that results from the accelerated aging of high-power lithium-ion cells containing LiNi_0.8Co_0.15Al_0.05O₂-based positive and graphite-based negative electrodes is dominated by contributions from the positive electrode. Data from various diagnostic experiments have indicated that a general degradation of the ionic pathway, apparently caused by surface film formation on the oxide particles, produces the positive electrode interface rise. One mechanistic hypothesis postulates that these surface films are components of the negative electrode solid electrolyte interphase (SEI) layer that migrate through the electrolyte and separator and subsequently coat the positive electrode. This hypothesis is examined in this article by subjecting cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and Li_4/3Ti_5/3O₄-based negative electrodes to accelerated aging. The impedance rise in these cells was observed to be almost entirely from the positive electrode. Because reduction products are not expected on the 1.55 V Li_4/3Ti_5/3O₄ electrode, the positive electrode impedance cannot be attributed to the migration of SEI-type fragments from the negative electrode. It follows then that the impedance rise results from mechanisms that are “intrinsic” to the positive electrode.     

Lithium insertion/deinsertion properties of new layered vanadium oxides obtained by oxidation of the precursor H2V3O8

The electrochemical behavior of the three new vanadium oxides M_yH_1−yV₃^VO₈ (M=Li, Na, K; y=0.6-0.9) towards lithium insertion was studied. For each compound, the main insertion phenomena, and the lithium amount which can be inserted, were determined from galvanostatic studies performed at different rates. In order to better explain the differences between the compounds, the galvanostatic intermittent titration technique was used and chronoamperometry experiments were performed. Cyclability studies have shown that the performances of M_0.6H_0.4V₃O₈ (M=Li, K) are not better than those of H₂V₃O₈. On the contrary, Na_0.9H_0.1V₃O₈ presents very interesting lithium insertion properties, with a specific capacity of 215 mAh g⁻¹ in the 1.5-4 V voltage range at C/10, and a capacity loss at the 40th cycle remaining smaller than 2.5%. Na_0.9H_0.1V₃O₈ is therefore a good candidate as positive electrode material for rechargeable lithium batteries.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

Auto-ignition route to thermoelectric oxide NaxCo2O4 powder with high compactibility

The ultra fine powder of thermoelectric oxide Na_xCo₂O₄ has been synthesized via an auto-ignition route following with airflow shatter process, which has the flaky shape in different nominal x compositions. Single γ-phase Na_xCo₂O₄ crystal structure was obtained in varying Na contents of 1.4-1.8 at the calcining temperature of 1153 K. The airflow shatter process was found to be beneficial for obtaining uniform particles with smaller the average size and larger BET surface area. The compaction module and the nonlinear exponent of powder compaction are about 4.037 MPa and 4.368 calculated form Huang Peiyun's compacting equation, respectively, which reveals that the Na_xCo₂O₄ powder has high compactibility.     

Al2O3-coated SnO2/TiO2 composite electrode for the dye-sensitized solar cell

A novel Al₂O₃-coated SnO₂/TiO₂ composite electrode has been applied to the dye-sensitized solar cell. In such an electrode, two kinds of energy barriers (SnO₂/TiO₂ and TiO₂/Al₂O₃) were designed to suppress the recombination processes of the photo-generated electrons and holes. After the SnO₂ was modified by colloid TiO₂, the photoelectric conversion efficiency of the SnO₂/TiO₂ composite cell increased to 2.08% by a factor of 2.8 comparing with that of the SnO₂ cell. The Al₂O₃ layer on the SnO₂/TiO₂ composite electrode further suppressed the generation of the dark current, resulting in 37% improvement in device performance comparing with the SnO₂/TiO₂ cell.     

Molten salt synthesis of α-Al2O3 platelets using NaAlO2 as raw material

α-Al₂O₃ platelets were prepared by a molten salt synthesis method when NaAlO₂ was used as raw material. The effects of the stirring rate during the gel preparation, heating temperature, type and addition amount of molten salts, addition of plate-like α-Al₂O₃ seeds, additives such as TiOSO₄ and Na₃PO₄·12H₂O on the morphology of α-Al₂O₃ were studied. High stirring rate during the gel preparation and high heating temperature not only help to restrain the overlapping of α-Al₂O₃ platelets, but also improve the size distribution. When the heating temperature increases to 1200 °C, most of α-Al₂O₃ platelets are hexagonal in its morphology, and the size of platelets becomes relatively uniform. When Na₂SO₄-K₂SO₄ flux is used instead of NaCl-KCl flux, it is easy to obtain α-Al₂O₃ platelets with a big size. When the molar ratio of salt to final Al₂O₃ powders increases to 4:1, most of α-Al₂O₃ platelets are hexagonal, and the overlapping of powders is inhibited. The addition of a small amount of plate-like seeds has a significant effect on the size of α-Al₂O₃ platelets. With the increase of seed amount, the diameter of α-Al₂O₃ platelets tends to decrease. The addition of 5.45 wt.% TiOSO₄ results in the formation of hexagonal α-Al₂O₃ platelets with an average diameter of 5.1 μm and an average thickness of 1.4 μm. Thin α-Al₂O₃ platelets with a discal shape are obtained owing to the co-addition of 0.51 wt.% Na₃PO₄·12H₂O and 3 wt.% TiOSO₄.     

Dynamic corrosion of Al2O3-ZrO2-SiO2 and Cr2O3-containing refractories by molten frits. Part II: Microstructural study

In this work results on dynamic corrosion studies of fused cast Al₂O₃-SiO₂-ZrO₂ and isostatically pressed and sintered Cr₂O₃-based refractories by two crystalline (transparent) frits are described. Experiments have been performed using the “Merry Go Round” test at ≅1500 °C.Microstructural and mineralogical analyses of selected areas from the corroded regions of the studied refractories were performed by reflected light optical microscopy and scanning electron microscopy with analysis by X-ray dispersive energy.Significant differences between the corrosion mechanisms acting in the two types of materials were found. In the fused cast Al₂O₃-SiO₂-ZrO₂ specimens corrosion took place by the dissolution of alumina and zirconia in the frit and in the glass formed by the reaction between the frit and the refractory. In the Cr₂O₃-based materials the corrosion process was controlled by the capillar penetration of the molten frit through the open pores. The reaction between the ZnO from the frits and Cr₂O₃ led to the formation of spinel (ZnCr₂O₄), a high-melting point bonding phase that retarded the frit penetration. Results are discussed using the relevant phase equilibrium diagrams.     

Mechanism transition of cell-impedance-controlled lithium transport through Li1−δMn2O4 composite electrode caused by surface-modification and temperature variation

The mechanism transition of lithium transport through a Li_1−δMn₂O₄ composite electrode caused by the surface-modification and temperature variation was investigated using the galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and the potentiostatic current transient technique. From the analyses of the ac-impedance spectra, experimentally measured from unmodified Li_1−δMn₂O₄ and surface-modified Li_1−δMn₂O₄ with MgO composite electrodes, the internal cell resistance of the MgO-modified Li_1−δMn₂O₄ electrode was determined to be much smaller in value than that of the unmodified electrode over the whole potential range. Moreover, from the analysis of the anodic current transients measured on the MgO-modified Li_1−δMn₂O₄ electrode, it was found that the cell-impedance-controlled constraint at the electrode surface is changed to a diffusion-controlled constraint, which is characterised by a large potential step and simultaneously by a small amount of lithium transferred during lithium transport. This strongly suggests that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport through the MgO-modified Li_1−δMn₂O₄ electrode. Furthermore, from the temperature dependence of the internal cell resistance and diffusion resistance in the unmodified Li_1−δMn₂O₄ composite electrode measured by GITT and EIS, it was concluded that which mechanism of lithium transport will be operative strongly depends on the diffusion resistance as well as on the internal cell resistance.     

Effect of precipitants during the preparation of Cu-ZnO-Al2O3/Zr-ferrierite catalyst on the DME synthesis from syngas

The synthesis of dimethyl ether (DME) from biomass-derived model synthesis gas has been investigated on Cu-ZnO-Al₂O₃/Zr-ferrierite bifunctional catalysts. The catalysts are prepared by co-precipitation–impregnation method using Na₂CO₃, K₂CO₃ and (NH₄)₂CO₃ as the precipitants. The catalytic activity tests reveal that the best yield of DME can be obtained on the catalyst precipitated by using (NH₄)₂CO₃. Detailed characterization studies conducted on the catalysts to measure their properties such as surface area, acidity by temperature-programmed desorption of ammonia (NH₃-TPD), reducibility of Cu oxide by temperature-programmed reduction (TPR), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and copper surface area and particle size measurements by N₂O titration method. Increasing the number of moderate acidic sites and facilitation of easily reducible copper species with small particle size are found to be the prime reasons for the superior functionality of the (NH₄)₂CO₃ precipitated catalyst. The usage of (NH₄)₂CO₃ also leaves no residual ions, whereas the presence of residual K⁺ and Na⁺ ions in the case of K₂CO₃ and Na₂CO₃ precipitated catalysts leads to lower activity and selectivity.     

Modeling the impedance versus voltage characteristics of LiNi0.8Co0.15Al0.05O2

The power-delivery capability of lithium-ion cells based on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes shows a significant dependence on the cell's state-of-charge. One reason for this behavior is the variation of the positive electrode's impedance with the oxide's lithium content. In this article, an electrochemical model based on concentrated solution porous electrode theory is used to model impedance data obtained on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes charged to potentials ranging from 3.55 to 4.55 V versus Li. The parameters obtained from model fits include the exchange-current density and Li-ion diffusion coefficients in the oxide. The variations in these parameters with oxide potential are correlated with structural changes in the material observed during Li-ion intercalation-deintercalation reactions.     

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.     

Li diffusion in LiNi0.5Mn0.5O2 thin film electrodes prepared by pulsed laser deposition

Kinetic and transport parameters of Li ion during its extraction/insertion into thin film LiNi_0.5Mn_0.5O₂ free of binder and conductive additive were provided in this work. LiNi_0.5Mn_0.5O₂ thin film electrodes were grown on Au substrates by pulsed laser deposition (PLD) and post-annealed. The annealed films exhibit a pure layered phase with a high degree of crystallinity. Surface morphology and thin film thickness were investigated by field emission scanning electron microscopy (FESEM). The charge/discharge behavior and rate capability of the thin film electrodes were investigated on Li/LiNi_0.5Mn_0.5O₂ cells at different current densities. The kinetics of Li diffusion in these thin film electrodes were investigated by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). CV was measured between 2.5 and 4.5 V at different scan rates from 0.1 to 2 mV/s. The apparent chemical diffusion coefficients of Li in the thin film electrode were calculated to be 3.13 × 10⁻¹³ cm²/s for Li intercalation and 7.44 × 10⁻¹⁴ cm²/s for Li deintercalation. The chemical diffusion coefficients of Li in the thin film electrode were determined to be in the range of 10⁻¹²-10⁻¹⁶ cm²/s at different cell potentials by GITT. It is found that the Li diffusivity is highly dependent on the cell potential.     

A novel kojic acid amperometric sensor based on hollow CuO/Fe2O3 hybrid microspheres immobilized in chitosan matrix

Hollow CuO/Fe₂O₃ hybrid microspheres with small uniform holes were synthesized using a convenient hydrothermal method and were applied to fabricated an amperometric sensor for kojic acid. The resulting materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) and then were immobilized into the chitosan (Chi) matrix onto a glassy carbon electrode to obtain CuO/Fe₂O₃–Chi/GCE. The potential utility of the constructed electrodes were demonstrated by applying them to the analytical determination of kojic acid concentration. The electrochemical behavior of kojic acid on CuO/Fe₂O₃–Chi/GCE was explored. The modified electrode displayed excellent amperometric response for kojic acid with a linear range from 0.2 μM to 674 μM with a detection limit of 0.08 μM at a signal-to-noise ratio of 3. In order to validate feasibility, the CuO/Fe₂O₃–Chi/GCE has been used for quantitative detecting kojic acid in real samples.     

Preparation of nanocrystalline Sr3Al2O6 powders via citric acid precursor

Sr₃Al₂O₆ was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr₃Al₂O₆ were investigated. Increasing the CA/M promoted the formation of Sr₃Al₂O₆. Single-phase and well-crystallized Sr₃Al₂O₆ was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr₃Al₂O₆ nanoparticles with a diameter of about 50-70 nm were obtained.     

Mass changes accompanying the pseudocapacitance of hydrous RuO2 under different experimental conditions

Pseudocapacitance reaction of hydrous ruthenium oxide was investigated by cyclic voltammetry combined with electrochemical quartz-crystal nanobalance (EQCN) in sulfuric acid as well as in neutral solutions of Na₂SO₄ and K₂SO₄. The ruthenium oxide electrode was prepared by attaching the ruthenium oxide particles on gold covered quartz electrode. The results show that there are different types of charge taking place simultaneously during the redox reaction of ruthenium oxide electrode. Their contribution to the overall charge depends on the experimental conditions. Depending on the potential and electrolyte used the redox reaction of ruthenium oxide is accompanied either by mass loss or by mass gain. The average molar masses of the species exchanged between the solid phase and the electrolyte solution depend on the potential and scan rate. The effect of Nafion™ top layer was also investigated. It has been found that it does not affect significantly the overall specific capacitance of ruthenium oxide electrode but the apparent molar masses of exchanged species decrease in comparison with the uncovered electrodes.     

Effect of TiO2 doped Ni electrodes on the dielectric properties and microstructures of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 multilayer ceramic capacitors

The effects of TiO₂-doped Ni electrodes on the microstructures and dielectric properties of (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ multilayer ceramic capacitors (MLCCs) have been investigated. Nickel paste with a TiO₂ dopant was used as internal electrodes in MLCCs based on (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) ceramic with copper end-termination. The microstructures and defects were analysed by microstructural techniques (SEM/HRTEM) and energy-dispersive spectroscopy (EDS). The continuity of the electrode of the MLCC was measured using a scanning electron microscope, which showed that the continuity of the electrode for the MLCC with a TiO₂-doped Ni electrode was approximately 90%. However, continuity of the electrode for a conventional MLCC was below 80%. The continuity of the TiO₂-doped Ni electrode showed significant improvement in the MLCC, which was due to no reaction between Ni and BCTZ.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

O2 reduction at the Pt/Nafion interface in 85% concentrated H3PO4

The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion® in concentrated 85% H₃PO₄. The effects of Nafion® coatings of different thickness on O₂ electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion® film thickness while the diffusion limiting current decreases with increasing Nafion® film thickness. A O₂ concentration profile model for the Pt/Nafion®/bulk electrolyte has been established, and this model can be used to explain the O₂ reduction polarization results. The performance of Nafion®-modified, high surface area Pt/carbon air cathodes for use in the H₂–air concentrated phosphoric acid fuel cell was also studied.     

Memory characteristics of multi-stacked thin films using La2O3 and LaAlO3 as charge trap layer

Al₂O₃/La₂O₃/Al₂O₃ (ALA) and Al₂O₃/LaAlO₃/Al₂O₃ (A/LAO/A) multi-stacked films were deposited on Si substrates by MOCVD. No interfacial layers (Al_xSi_yO_z) were observed in TEM images, and the thickness ratio of the tunnel oxide (bottom oxide), trap layer (middle oxide), and blocking oxide (top oxide) was about (1:1.3:3) in both films. Memory windows of the (ALA) and (A/LAO/A) films were 1.31 V and 3.13 V, respectively. Each value in the program/erase cycle test was maintained for up to 10⁴ cycles.