Showing papers in "Electrochemical and Solid State Letters in 2008"

Silicon/Graphite Composite Electrodes for High-Capacity Anodes: Influence of Binder Chemistry on Cycling Stability

Abstract: The binder's influence on the cycling stability of high-energy anodes for lithium-ion batteries is demonstrated. Varying the binder's nature strongly influences the composite electrode's performance on deep charging/discharging. If sodium-carboxymethylcellulose is used as binding agent, then a chemical bond between binder and silicon particles can be detected (attenuated total reflection-Fourier transform infrared spectroscopy). Consequently, a reaction mechanism that describes a condensation reaction between the binder and the silicon particles is verbalized. It is shown that, not necessarily the binder's physical flexibility, but its chemical interaction with the active masses is the major claim leading to long-lasting reversible lithium uptake/release.

The Importance of Interphase Contacts in Li Ion Electrodes: The Meaning of the High-Frequency Impedance Arc

Abstract: Li insertion electrodes are made by pressing a mixture of active material and additives on a metallic substrate. Here we estimate how various interphase contacts affect the electrode kinetics. We apply variable external mechanical pressure onto different cathodes and measure their impedance response. Similar experiments are performed on dry composites in contact with: Al or Cu foil, or Ag paste. Most surprisingly, we find that the high-frequency impedance arc is due to the contact impedance between the metal and the electrode material. This is in fundamental contradiction with previous interpretations. We propose an equivalent circuit explaining the observed phenomena.

Impact of Loading in RRDE Experiments on Fe-N-C Catalysts: Two- or Four-Electron Oxygen Reduction?

Abstract: We have investigated the impact of electrocatalyst loading on rotating ring-disk electrode (RRDE) experiments for the oxygen reduction reaction on Fe-N-C catalysts (ORR) in acid medium. In particular, the fraction of H 2 O 2 produced as a function of catalyst loading was studied. A dramatic increase in H 2 O 2 release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst (NNMC), the fraction of produced H 2 O 2 varied between less than 5% and greater than 95%, depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via H 2 O 2 , and if the catalyst is sparsely loaded, the produced H 2 O 2 cannot be efficiently reduced to H 2 O before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.

Efficient Reduction of CO2 in a Solid Oxide Electrolyzer

Abstract: When the economy is based on renewable energy resources, such as wind and solar, the major source of H2 for chemical production and energy storage will be from the electrolysis of water. The ability to reduce CO2 efficiently by a similar process could also play a role in reducing greenhouse gas emissions and moving us toward a more sustainable economy. 1 CO produced in this manner could be used in chemical production or reacted with H2 to produce liquid fuels via the Fischer‐Tropsch reaction. 2 Solid oxide electrolyzers SOEs, which are essentially solid oxide fuel cells SOFCs operated in reverse, are capable of higher water electrolysis efficiencies compared to solution-based electrolysis cells because they operate at higher temperatures 925 K. The higher operating temperatures result in a lower Nernst potential, the thermodynamic potential required for water splitting, and in lower electrode overpotentials. 3 The electrode overpotential is the difference between the actual electrode potential and the Nernst potential, and is a measure of the lost efficiency in the cell. SOEs also differ from low-temperature, solution-based electrolyzers in that the electrolyte membrane conducts oxygen anions, rather than protons. The material most often used for the electrolyte is yttria-stabilized zirconia YSZ, a material that is a good oxygen-anion conductor and an electronic insulator. In an SOE, the cathode the fuel-side electrode reaction for water electrolysis is the electrochemical dissociation of steam to produce H2 and O 2 anions, Reaction 1, while recombination of the oxygen ions to O2, Reaction 2, occurs at the anode the air-side electrode H2 O+2 e  → O 2 +H 2

Origin of Subthreshold Swing Improvement in Amorphous Indium Gallium Zinc Oxide Transistors

Abstract: The effect of the channel deposition pressure on the device performance of amorphous indium-gallium-zinc oxide (a-IGZO) transistors was investigated in detail. The performance of the fabricated transistors improved monotonously with decreasing chamber pressure: at a pressure of 1 mTorr, the field-effect mobility (μ FE ) and subthreshold gate swing (S) of the a-IGZO thin-film transistors were dramatically improved to 21.8 cm 2 /Vs and 0.17 V/decade, respectively, compared to those (11.4 cm 2 /Vs and 0.87 V/decade) of the reference transistors prepared at 5 mTorr. This enhancement in the subthreshold characteristics was attributed to the reduction of the bulk defects of the a-IGZO channel, which might result from the greater densification of the a-IGZO films at the lower deposition pressure.

Electrochemical Characterizations of Germanium and Carbon-Coated Germanium Composite Anode for Lithium-Ion Batteries

Abstract: The electrochemical reaction mechanism of germanium with lithium at room temperature was investigated. Two distinct voltage plateaus corresponding to the formation of Li 9 Ge 4 and Li 7 Ge 2 were observed, and the final products were identified as a mixture of Li 15 Ge 4 and Li 22 Ge 5 phases. A three-step reaction mechanism of germanium with lithium was suggested. Carbon-coated germanium composites prepared by mechanical milling and pyrolysis were evaluated as anode materials for lithium-ion batteries. The composite materials, Ge-mesocarbon microbeads and Ge-poly(vinyl alcohol), showed good cyclability and retained fairly large capacities of 473 and 579 mAh/g, respectively, up to 50 cycles when cycled within a limited voltage window.

Solution-Processed Indium-Zinc Oxide Transparent Thin-Film Transistors

Abstract: Transparent thin-film transistors (TTFTs) with an indium-zinc oxide (IZO) active layer by the solution-processed deposition method were fabricated and their TFT characterization was examined. Solution-processed IZO thin films were amorphous and highly transparent with >90% transmittance in the visible region with an optical bandgap of 3.1 eV. Spin-coated IZO TTFTs were operated in depletion mode and showed a field-effect mobility as high as 7.3 cm 2 /V s, a threshold voltage of 2.5 V, an on/off current ratio greater than 10 7 , and a subthreshold slope of 1.47 V/decade.

Degradation Mitigation in Polymer Electrolyte Membranes Using Cerium Oxide as a Regenerative Free-Radical Scavenger

Abstract: The efficacy of CeO 2 nanoparticles in mitigating free-radical-induced polymer electrolyte membrane (PEM) degradation is investigated. Commercially obtained CeO 2 and nanoparticles synthesized in-house were incorporated within a recast Nafion membrane. Membrane electrode assemblies were prepared using Nafion and Nafion-CeO 2 composite membranes (0.5, 1, and 3 wt % CeO 2 ). The composite membranes exhibited very similar proton conductivities (∼35 mS/cm) and hydrogen crossover (∼ 1 mA/cm 2 ) as Nafion. However, the fluoride emission rate (from accelerated tests) was lowered by more than 1 order of magnitude upon addition of CeO 2 into the Nafion membrane, suggesting that CeO 2 nanoparticles have tremendous potential to greatly enhance membrane durability.

Nanostructured Li2FeSiO4 Electrode Material Synthesized through Hydrothermal-Assisted Sol-Gel Process

Abstract: A carbon-coated Li 2 FeSiO 4 material with uniform nanoparticles (approximately 40-80 nm in diameter) is synthesized by a synthesis route, i.e., a hydrothermal-assisted sol-gel process. As an electrode material for rechargeable lithium batteries, the Li 2 FeSiO 4 sample shows high rate capability and excellent capacity retention. The Li 2 FeSiO 4 electrode delivers a discharge capacity of 160 mAh g -1 at C/16 rate, corresponding to 96% of the theoretical value. A discharge capacity of about 91 and 78 mAh g -1 can be obtained at 5 and 10 C rate, respectively. No capacity loss can be observed up to 50 cycles.

TEM Study of Fracturing in Spherical and Plate-like LiFePO4 Particles

Abstract: An investigation of fracturing in LiFePO{sub 4} particles as a function of the particle morphology and history is presented. Two types of samples, one subjected to electrochemical cycling and another to chemical delithiation are compared. We observe the formation of micro fractures parallel to low indexed lattice planes in both samples. The fracture surfaces are predominantly parallel to (100) planes in the chemically delithiated powder and (100) and (010) planes in the electrochemically cycled powder. A consideration of the threshold stresses for dislocation glide shows that particle geometry plays an important role in the observed behavior.

Engineering Composite Oxide SOFC Anodes for Efficient Oxidation of Methane

Abstract: Ceramic anodes for solid oxide fuel cells SOFCs were prepared by aqueous impregnation of nitrate salts to produce composites with 45 wt % La0.8Sr0.2Cr0.5Mn0.5O3 LSCM in a 65% porous yttria-stabilized zirconia YSZ scaffold. Scanning electron micrographs indicate that the LSCM coats the YSZ pores following calcination at 1473 K. Composites produced in this manner exhibit conductivities at 1073 K of approximately 1 S/cm in air and 0.1 S/cm in humidified H2. A SOFC with a composite anode composed of 45 wt % LSCM, 0.5 wt % Pd, and 5 wt % ceria exhibited maximum power densities at 1073 K of 1.1 and 0.71 W cm 2 in humidified 3% H2O H2 and methane, respectively.

Improvement of High-Rate Performance of All-Solid-State Lithium Secondary Batteries Using LiCoO2 Coated with Li2O – SiO2 Glasses

Abstract: Development of rate capability is one of the most important issues to be solved in all-solid-state lithium secondary batteries. The electrochemical performance of these batteries has been highly improved by coating LiCoO 2 particles with Li 2 O-SiO 2 thin film. The interfacial resistance between LiCoO 2 and the glass-ceramic electrolyte was decreased by those coatings. The rate capabilities of the cells using the coated LiCoO 2 particles were superior to that of the cell using noncoated LiCoO 2 . The Li 2 Si 3 coating was more effective than the SiO 2 coating in enhancing rate performance, suggesting that lithium-ion conductivity of the coating materials is important for high-rate performance.

Solid Oxide Fuel Cell Ni–YSZ Anodes: Effect of Composition on Microstructure and Performance

Abstract: Ni-YSZ/YSZ/LSM-YSZ (La 0.8 Sr 0.2 MnO 3 -yttria-stabilized zirconia) solid oxide fuel cells (SOFCs) with varying Ni-YSZ active-layer compositions were compared electrochemically and structurally using a focused ion beam scanning electron microscope. Stereological analyses showed that the highest triple-phase boundary (TPB) density was at a Ni:YSZ volume ratio of ∼0.5. Although the electrode polarization resistance was minimized at the same composition as the TPB density maximum, the TPB density variation was too small to explain the observed variation in polarization resistance. This implies that polarization resistance was determined not only by TPB density, but also by other structural factors, such as phase tortuosity and contiguity.

The Effect of Deposition Conditions and Annealing on the Performance of High-Mobility GIZO TFTs

Abstract: The influence of oxygen content, radio-frequency (rf) sputtering power, and postdeposition annealing on the electrical properties of gallium-indium-zinc oxide (GIZO) thin-film transistors (TFTs) is analyzed. Little to no oxygen content in the sputtering chamber is crucial to obtain high-performance devices, even before annealing. In contrast, a high oxygen content and rf power lead, respectively, to unstable/poor performing and depletion mode TFTs before annealing, and mainly for these "nonideal" conditions, annealing proves to be effective to improve device performance/stability and to decrease the performance discrepancy among TFTs processed under different oxygen and rf power conditions. Best TFTs present a field-effect mobility of 46 cm 2 /V s, subthreshold swing of 0.26 V/dec, threshold voltage of 0.70 V, and an on/off ratio 10 8 -10 9 . These results are a consequence of the optimized processing and of the usage of proper GIZO target composition, 1:2:1 mol.

Transparent ZnO-TFT Arrays Fabricated by Atomic Layer Deposition

Abstract: Transparent ZnO thin film transistor (TFT) array of 176 X 144 (106 dpi) was fabricated on glass substrate. The V th of the TFT with inverted coplanar structure is about 0.8 V and the mobility is 1.13 cm 2 /V s. The active layer (ZnO), gate insulator (Al 2 O 3 ), and source-drain electrode (ZnO:Al) were deposited by atomic layer deposition. We also compared the performance of TFTs fabricated by lift-off and wet-etching process as the patterning processes of ZnO layer. The carrier density of the ZnO layer was carefully adjusted to reduce off-current of TFT. Good contact with small contact resistance was formed between the active layer and the source-drain electrode.

Study of the Interface Between Li4Ti5O12 Electrodes and Standard Electrolyte Solutions in 0.0-5.0 V

Abstract: Quasi 6-day oscillations in E-s occurrence were observed to occur in relation with planetary wave activity at Wuhan, China (30.6 degrees N, 114.5 degrees E). Wavelet analysis of E-s occurrence time series revealed that a strong 5- to 7-day oscillation was present during the period from about day 120 to 137 of 2003. The same quasi 6-day planetary wave oscillation was also found to dominate the spectrum of concurrent wind data measured in the 80- to 100-km region by a meteor radar, in agreement with the E-s occurrence. There is also a great deal of similarity between the 5- to 7-day band-pass filtered waveforms of E-s occurrence and the wind data. By estimating the wave phase changes with attitude, the quasi 6-day PW in the zonal wind was found to be in phase with the sporadic E layer critical frequency, f(o)E(s), at about 115 km, close to the observed E-s height. The quasi 6-day PW modulation was also present in the amplitude of the 12-h and 24-h periodicities which existed in the f(o)E(s) time series. The present results provide new evidence in favor of a planetary wave indirect role on E-s formation through the modulation of tides, which in line with previous studies by Haldoupis and Pancheva (2002), and Pancheva et al. (2003).

Air Exposure Effect on LiFePO4

Abstract: The impact of ambient air exposure on LiFePO 4 /C nanocomposites has been investigated. A pristine sample was prepared without any exposure to ambient air through the whole process of synthesis and characterization and compared to the exposed samples. A small amount of lithium deintercalates from the olivine structure during exposure, a majority of which can be electrochemically reintercalated. This phenomenon changes the initial surface and bulk properties and should be taken into account when diminishing the particle size of LiFePO 4 . Keeping nanocomposites away from oxidative moisture atmosphere could be a solution to minimize these side reactions.

Is LiFePO4 Stable in Water? Toward Greener Li–Ion Batteries

Abstract: The stability of LiFePO 4 in water was investigated. From high-resolution transmission electron microscopy observation, electron energy-loss spectroscopy analyses, Mossbauer experiments, and chemical analyses, we showed that a Li 3 PO 4 layer is present at the LiFePO 4 grains surface after immersion in water. This layer, which is a few nanometers thick, is accompanied by an increase of the Fe III percentage in the grains and a continuous dissolution of the layer. If experimental conditions such as immersion time, pH, and LiFePO 4 concentration are set to optimal values, the transition to an aqueous route for preparing LiFePO 4 -based positive electrodes could, in fact, be achieved.

A Dilatometric Study of Lithium Intercalation into Powder-Type Graphite Electrodes

Abstract: The thickness change of porous multiparticle graphite electrodes upon first electrochemical lithium intercalation and extraction was investigated by means of electrochemical dilatometry. For all investigated graphites, expansion starts well above 0.5 V vs Li/Li + and thus, before any unsolvated Li intercalation compounds are formed. This finding can be explained by solvent co-intercalation during the first charge prior to SEI formation, as described by the Besenhard model. At potentials <0.3 V vs Li/Li + , i.e., in the region of unsolvated Li + intercalation, the dilatation curves reveal the same major staging features that are seen in the corresponding potential profiles.

Enhancement in Three-Phase Boundary of SOFC Electrodes by an Ion Impregnation Method : A Modeling Comparison

Abstract: Solid oxide fuel cells SOFCs are promising to be the nextgeneration energy-conversion devices due to their high efficiency and ultralow pollution emission. 1 Many efforts have been made recently to lower their operating temperatures from conventional 1000°C to 600–800°C, in order to significantly reduce the manufacturing cost and improve the stability of the SOFC system. At a reduced operating temperature, the electrode performance becomes the most important determinant of the overall cell output, especially when a thin-film electrolyte i.e., 5–20 m is applied. The electrode performance is believed to be determined by the sum of various polarizations typically associated with the length of the socalled three-phase boundary TPB where the electronic conductor, ionic conductor, and gases are in contact with each other so that the electrochemical reaction can take place. Therefore, a large TPB length is generally essential for high electrode performance. Several experimental studies demonstrated the inverse proportionality of activation overpotential with respect to the TPB length in standard composite electrodes such as Ni–yttria-stabilized zirconia YSZ anodes 2,3 and La,SrMnO3 LSM–YSZ cathodes. 4,5 A composite electrode is usually composed of an electronic conducting phase, an oxygen-ion conducting phase, and pores for gas transportation. Typical examples of the electronic phases are Ni and LSM, which also serve as the electrocatalyst in the anode and cathode, respectively. The ionic phase is generally an electrolyte material such as YSZ and doped ceria DCO. The TPB length of such a composite electrode is dominantly affected by its microstructure characteristics including particle size, porosity, and distribution state of the electronic and ionic conducting phases. Various electrode models have been established to predict and improve the performance of the composite electrode with regards to its microstructure parameters.

Correlating Sulfur Poisoning of SOFC Nickel Anodes by a Temkin Isotherm

Abstract: Experimental data on sulfur's impact on solid oxide fuel cell (SOFC) nickel anodes have been analyzed using an isotherm for the sulfur coverage on the Ni surface: θ s = 1.45 - 9.53 X 10 -5 T+ 4.17 X 10 -5 T ln Y, where Y is the ratio between the partial pressures of hydrogen sulfide and hydrogen. The loss of performance, P l , of the SOFC can be correlated by P l = k*(θ s - θ min ), where θ min > 0 depends on the current density and anode material. The data range covered is 700-1000°C, concentration of hydrogen sulfide 0.05-50 ppm.

Improving the Performance of Lithium Manganese Phosphate Through Divalent Cation Substitution

Abstract: Highly crystalline samples of LiMnPO{sub 4} and its analogs with partial substitution of Mn by divalent Mg, Cu, Zn, and Ni were prepared by hydrothermal synthesis and characterized by x-ray diffraction and infrared spectroscopy. Chemical oxidation produced two-phase mixtures of the initial phases LiMn{sub (1-y)}M{sub y}PO{sub 4} and the delithiated forms, Li{sub y}Mn{sub (1-y)}M{sub y}PO{sub 4}, all with the olivine structure. The extent of oxidation depended upon the quantity of oxidizing agent used and on the identity of the substituent ions. Mg, Ni and Cu were found to increase the level of delithation relative to that in pure LiMnPO{sub 4}. Mg was also shown to reduce the tendency of the oxidized phase to absorb water.

Thermal Stability of LiCoPO4 Cathodes

Abstract: The thermal stability of LiCoPO 4 cathodes charged to different lithium contents was studied by synchrotron diffraction and differential thermal analysis. Both olivine-like phases Li Z COPO 4 (z = 0.6) and CoPO 4 appearing during the delithiation of LiCoPO 4 are unstable upon heating, and decompose readily in the range 100-200°C. The decomposition of lithium-poor phases leads to gas evolution and the crystallization of CO 2 P 2 O 7 . The role of carbon present in the electrochemically delithiated samples is discussed. The significantly lower stability of charged LiCoPO 4 in comparison with LiFePO 4 is a serious challenge for the application of this material in rechargeable Li-ion batteries.

Conversion of Solid Carbonaceous Fuels in a Fluidized Bed Fuel Cell

Abstract: A fluidized bed direct carbon fuel cell was employed to achieve direct conversion of solid fuels into electricity. Power was generated from pulverized Lower Kittanning (bituminous) coal, synthetic carbon, and biomass in a single process step. Current-voltage characteristics exhibited typical fuel cell behavior. Fluidization in flowing CO{sub 2} overcomes the difficulty of attaining solid fuel-to-anode contact and generates CO in situ via the Boudouard reaction. A mechanistic reaction pathway is proposed for anodic oxidation of the solid fuel. Conversion was verified by gas analysis of oxidation products in the flue stream and by oxygen mass balance.

Evidence of Transition-Metal Accumulation on Aged Graphite Anodes by SIMS

Abstract: In cells containing Li{sub 1.05}(Ni{sub 1/3}Co{sub 1/3}Mn{sub 1/3}){sub 0.95}O{sub 2}-based positive and graphite-based negative electrodes, a significant portion of cell impedance rise on aging is known to be from the negative electrode. One possible reason for this impedance rise is the dissolution of transition-metal elements from the oxide electrode that accumulate and create a high-impedance layer at the negative electrode-electrolyte interface. This article details dynamic secondary ion mass spectrometry (SIMS) measurements, which provide a relative comparison of Mn, Co, and Ni contents on fresh, formed, and aged graphite electrodes. The data clearly indicate that these transition-metal elements accumulate at the electrode surface and diffuse into the electrode during cell aging.

Ni Infiltration as a Possible Solution to the Redox Problem of SOFC Anodes

Abstract: In the present paper the use of Ni infiltration into preformed porous yttria-stabilized zirconia structures is investigated as a possible solution to the redox problem of Ni-based solid oxide fuel cell (SOFC) anodes and anode supports. It is shown that cermets containing 12-16 wt % Ni prepared by infiltration exhibited a conductivity above 300 S cm -1 at room temperature, which dropped by 20% after one redox cycle, but was not accompanied by bulk dimensional changes. The dimensional stability is attributed to the properties of the microstructure that is produced by infiltration.

NiCo2O4 / C Nanocomposite as a Highly Reversible Anode Material for Lithium-Ion Batteries

Abstract: A NiCo 2 O 4 /C nanocomposite has been synthesized by a hydrothermal method followed by a calcination. X-ray powder diffraction and transmission electron microscopy measurements demonstrated the composite was composed of crystalline NiCo 2 O 4 and amorphous carbon, and NiCo 2 O 4 and carbon particles amalgamated together with good affinity. The electrochemical results showed as high as 914.5 mAh/g reversible capacity could be achieved at 40 mA/g current density in the potential range of 0.01-3.0 V. The initial coulombic efficiency of the composite was 79.2%, and the capacity retention was 78.3% up to 50 cycles. The superior electrochemical performance indicated that the NiCo 2 O 4 /C nanocomposite might be a promising alternative to conventional graphite-based anode materials for lithium-ion batteries.

Growth of High-Dielectric-Constant TiO2 Films in Capacitors with RuO2 Electrodes

Abstract: Titanium dioxide thin films were grown on RuO 2 layers by atomic layer deposition. The stabilizing effect of the bottom rutile-type RuO 2 layer resulted in growth of the TiO 2 rutile films at temperatures above 275°C. Stabilization of the TiO 2 rutile phase occurred due to local epitaxial growth of the polycrystalline RUO 2 /TiO 2 /RUO 2 structure, as revealed by transmission electron microscopy. A dielectric constant as high as 155 and equivalent oxide thickness (EOT) as low as 0.5 nm were determined from the capacitance-voltage measurements for the TiO 2 films grown above 275°C. A leakage current density of 10 -3 A/cm 2 at 1 V bias voltage was obtained for the films with EOT equal to 0.5 nm.

Ab Initio Study of the Surface Properties and Nanoscale Effects of LiMnPO4

Abstract: Using first-principles calculations, we investigate the surface energies, equilibrium morphology, and surface redox potentials for LiMnPO 4 in the olivine structure Low-energy surfaces are found in the [100], [010], [011], [101], [201], and [301] directions of the orthorhombic structure With the calculated surface energies, we provide the thermodynamic equilibrium shape for the LiMnPO 4 crystal through a Wulff construction The dominating surfaces in the Wulff shape are (010), (011), and (201) Most of the surfaces in the Wulff shape have lower Li extraction potentials than the bulk, except for the (100) and (011) surfaces