Uncoated Cathode Research Articles

Observation of radio frequency ring-shaped hollow cathode discharge plasma with MgO and Al electrodes for plasma processing

Various high-density plasma sources have been proposed for plasma processing. Especially, the hollow cathode discharge is one of the powerful ones. In this work, radio-frequency (RF) driven ring-shaped hollow cathode discharges with high secondary-electron emission have been investigated, using an aluminum (Al) cathode, coated or not with magnesium oxide (MgO). The thickness of MgO thin film is approximately 200 nm. The RF discharge voltage for the coated cathode is almost the same as that for the uncoated one, in a wide range of Ar gas pressure, from 5.3 to 53.2 Pa. The results reveal that the plasma density has a peak at an Ar gas pressure of 10.6 Pa for both cathodes. The plasma density for the coated cathode is about 1.5–3 times higher than that for the uncoated one, at various gas pressures. To the contrary, the electron temperature for the coated cathode is lower than temperature obtained with the uncoated cathode, at various gas pressures. Radial profiles of electron saturation current, which is proportional to plasma flux, are also examined for a wide range of gas pressure. Radial profiles of electron temperature at various axial positions are almost uniform for both cathodes so that the diffusion process due to density gradient is dominant for plasma transport. The secondary electrons emitted from the coated cathode contribute to the improvement of the plasma flux radial profile obtained using the uncoated cathode.

Low-temperature electrochemical performances of LiFePO4 cathode materials for lithium ion batteries

The key factors influencing the low-temperature electrochemical performances of LiFePO4 cathode materials are systematically investigated by measuring and comparing the amount of surface carbon, particle size, and conductivities of LiFePO4, LiFePO4/C and electrolyte in the temperatures range of −20 to 40°C. It is found that the low-temperature electrochemical performance of the material is improved obviously by coating the surface of LiFePO4 with carbon. At −20°C, the discharge specific capacity of LiFePO4/C with 4wt% surface carbon is 81.4mAh/g, while the uncoated LiFePO4 only delivers the discharge specific capacity of 27.7mAh/g. The low-temperature electrochemical performance of LiFePO4/C has no significant change as increasing the carbon content, but is further improved by reducing the particle size. In order to understand the difference of low-temperature electrochemical performance between the carbon-coated and uncoated LiFePO4 cathodes, the ionic conductivity of electrolyte, the electronic/ionic conductivities of LiFePO4 and LiFePO4/C at various temperatures were measured. The electronic conductivity of LiFePO4 with 4.9×10−11S/cm at −20°C is found to be responsible for its poor low-temperature electrochemical performance. However, for the LiFePO4/C, the low-temperature electrochemical performance is mainly determined by its ionic conductivity, therefore, the decrease of the particle size can increase the low-temperature electrochemical performance.

Effect of interface modifications on voltage fade in 0.5Li2MnO3·0.5LiNi0.375Mn0.375Co0.25O2 cathode materials

The effects of the coatings Al2O3, LiAlOx, ZrO2, TiO2, AlPO4, and LiPON and of the electrolyte additives 3-hexylthiophene and lithium difluoro (oxalato)borate (LiDFOB) on the voltage fade phenomenon in 0.5Li2MnO3·0.5LiNi0.375Mn0.375Co0.25O2 cathodes were investigated. Cells containing these materials or additives were cycled according to a standard protocol at room temperature between 2.0 and 4.7 V vs. Li+/Li. As expected, the cells containing either an additive or a coated cathode displayed less capacity loss than cells containing an uncoated cathode and no additive. The voltage fade phenomenon was quantified in terms of changes in the average cell voltage (Wh/Ah). The results indicate that, within experimental error, all of the coatings and additives produced little-to-no effect on voltage fade.

Electrochemical Analysis of the Effect of Cr Coating the LiV3O8Cathode in a Lithium Ion Battery with a Lithium Powder Anode

Rechargeable 2032-coin-type cells were produced with Li-powder anodes (i.e., Li-powder electrodes, LPEs) and either Cr-coated lithium trivanadate (Li1+xV3O8, LVO) cathodes or uncoated LVO cathodes. The initial discharge capacity of a cell with an LPE and a Cr-coated LVO cathode (Cellcoated) was 252 mAh g(-1) at a 0.2 C-rate and that of a cell with an LPE and an uncoated LVO cathode (Cellbare) was 223 mAh g(-1). After the 50th cycle, Cellcoated exhibited higher capacity retention (about 89%) than Cellbare (about 78%). Changes in the surface morphology of the Cr-coated LVO cathode were observed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The change in the electrical conductivity of the cell was measured using the impedance analysis. The electrochemical properties of the cells were also evaluated based on the differential capacity curve, voltage profiles, and capacity versus number of cycles.

A Perspective on Coatings to Stabilize High-Voltage Cathodes: LiMn1.5Ni0.5O4 with Sub-Nanometer Lipon Cycled with LiPF6 Electrolyte

High voltage Li-ion cathodes push the limits of stability for both cathode and electrolyte. Here sub-nanometer coatings of an amorphous thin-film electrolyte (Lipon) improved the room temperature and 60°C cycling stability of a LiMn1.5Ni0.5O4 spinel cathode when charged to 4.9 V with a standard LiPF6 carbonate electrolyte. The cathodes delivered superior C-rate performances up to a 5C discharge, when compared to the uncoated cathodes. Enhanced performance extended for at least 100 cycles. Electrochemical impedance spectroscopy indicates that Lipon slows the increase of interface resistance. Thicker 1–3 nm Lipon coatings are sufficiently insulating as to block electronic transport to the cathode particles. Thick coatings also slow Mn dissolution. Results suggest that Lipon may act to scavenge impurities or block active sites that promote electrolyte decomposition. While greatly improved by the Lipon coating, cycling is not yet sufficiently stable for applications requiring thousands of cycles or prolonged operation at higher temperatures. Further work is needed to assess what surface properties of coatings will ultimately stabilize the high voltage cathodes in carbonate liquid electrolyte cells. Comments include insight from other studies of Lipon-coated cathodes and the focus for future research.

Experimental verification of the 3-step model of photoemission for energy spread and emittance measurements of copper and CsBr-coated copper photocathodes suitable for free electron laser applications

This paper presents measurements and analysis of the quantum efficiency (QE) and intrinsic emittance of Cu and CsBr coated Cu photocathodes. The data analysis uses expressions for the quantum efficiency and the intrinsic emittance for metal cathodes previously derived from Spicer's three-step model of photoemission. Data taken with a 257 nm CW laser on (100) Cu crystals indicate an emittance of 0.77 (μm/mm-rms) for CsBr coated and 0.42 (μm/mm-rms) for uncoated cathodes. The high quantum efficiency and low emittance observed for CsBr coated cathodes have applications in free electron laser and other devices requiring high brightness electron beams.

Electrochemical performance of La2O3-coated layered LiNiO2 cathode materials for rechargeable lithium-ion batteries

Uncoated and La2O3-coated LiNiO2 cathode materials were synthesized by polymeric sol gel process using metal nitrate precursors at 600 °C for 10 h. The structure and electrochemical properties of the surface-coated LiNiO2 materials were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry, charge/discharge and electrochemical impedance spectroscopy techniques. X-ray powder diffraction and SEM result show that no significant bulk structural differences were observed between the lanthanum oxide coated and pristine LiMn2O4. The galvanostatic charge/discharge studies on the uncoated and lanthanum oxide-coated LiNiO2-positive material at 0.5-C rate in the potential range between 3 and 4.5 V revealed that lanthanum oxide-coated positive electrode material has enhanced charge/discharge capacities; 2.0 wt.% of lanthanum oxide-coated LiNiO2-positive material has satisfied the structural stability, high reversible capacity and high electrochemical performances.

Electrode performance of a new La0.6Sr0.4Co0.2Fe0.8O3 coated cathode for molten carbonate fuel cells

Abstract To improve the cathode performance in molten carbonate fuel cells (MCFCs), Lanthanum Strontium Cobalt Ferrite (La0.6Sr0.4Co0.2Fe0.8O3, LSCF) of perovskite structure was coated on a porous Ni plate by a vacuum suction method. The electrochemical performance of modified cathode was examined and compared with that of uncoated conventional cathode via single cell operation and electrochemical impedance analysis (EIS). The cell voltage of the single cell using the LSCF coated cathode, measured at 650 °C with current density of 150 mA/cm2 is 0.837 V and it is higher than that of the cell with uncoated conventional cathode, 0.805 V. The higher performance and the lower charge transfer resistance were obtained at 600–700 °C after LSCF coating. The lower activation energy of oxygen reduction reaction was also obtained. The lower activation energy of oxygen reduction reaction after LSCF coating shows that LSCF on lithiated NiO cathode plays a role of catalyst on the oxygen reduction reaction in cathode.

Effect of sputtered lanthanum hexaboride film thickness on field emission from metallic knife edge cathodes

We report experiments and analysis of field emission from metallic knife-edge cathodes, which are sputter-coated with thin films of lanthanum hexaboride (LaB6), a low-work function material. The emission current is found to depend sensitively on the thickness of the LaB6 layer. We find that films thinner than 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode. This result is unexpected due to the higher work function of the bare metal cathode. We show, based on numerical calculation of the electrostatic potential throughout the structure, that the external (LaB6/vacuum) barrier is reduced with respect to uncoated samples for both thin and thick coatings. However, this behavior is not exhibited at the internal (metal/LaB6) barrier. In thinly coated samples, electrons tunnel efficiently through both the internal and external barrier, resulting in current enhancement with respect to the uncoated case. In contrast, the thick internal barrier in thickly coated samples suppresses current below the value for uncoated samples in spite of the lowered external barrier. We argue that this coating thickness variation stems from a relatively low (no higher than 1018 cm−3) free carrier density in the sputtered polycrystalline LaB6.

Enhancement of cell performance using a gadolinium strontium cobaltite coated cathode in molten carbonate fuel cells

To enhance cathode performance, gadolinium strontium cobaltite (Gd 0.6Sr 0.4CoO 3, GSC) is coated onto a porous Ni plate by a vacuum suction method, for use as the cathode in molten carbonate fuel cells (MCFCs). GSC is a mixed ionic and electronic conductor (MIEC) material, and thus has high electronic conductivity and catalytic activity at low temperatures. The electrode performance of the GSC-coated cathode is examined by various methods, such as single cell operation and electrochemical impedance spectroscopy (EIS). At 600 °C, the performance of a single cell using a GSC-coated cathode is 0.813 V. This result is very surprising given that the performance of an uncoated conventional cathode is 0.69 V. Impedance analysis confirms that a dramatic decrease in the charge transfer resistance after GSC coating is primarily responsible for the cell enhancement at low temperature. The reaction orders for O 2 and CO 2 at uncoated and GSC-coated cathodes are also examined via a symmetric cell test, to identify the reaction mechanism of oxygen reduction. The peroxide mechanism, which is known to be a fast reaction, is predominant for the GSC-coated cathode at low temperatures, whereas the superoxide mechanism is predominant for the uncoated cathode.

Enhanced electrochemical and storage properties of La 2/3− XLi 3 XTiO 3-coated Li[Ni 0.4Co 0.3Mn 0.3]O 2

A Li[Ni 0.4Co 0.3Mn 0.3]O 2 cathode was modified by applying a La 2/3− X Li 3 X TiO 3 (LLT) coating. Transmission electron microscope (TEM) images reveal that the coating layer consists of nanoparticles. The coated cathode demonstrated an enhanced rate capability, discharge capacity, and cyclic performance than the uncoated cathode. However, the influence of the coating upon these electrochemical properties is highly dependent upon the composition of the LLT coating layer. Coating layers having high La and low Li contents, such as La 0.67TiO 3, effectively improved the rate capability of the cathode. However, coating layers with a low La and high Li content greatly enhanced the discharge capacity of the cathode under high cut-off voltage (4.8 V) conditions. Overall, the thermal stability of the Li[Ni 0.4Co 0.3Mn 0.3]O 2 electrode was improved by the LLT coating. Storage tests confirmed that the La 2/3− X Li 3 X TiO 3 coating dramatically suppressed the dissolution of transition metals into the electrolyte.

Effect of treated temperature on structure and performance of LiCoO2 coated by Li4Ti5O12

The Li4Ti5O12 (LTO)-coated LiCoO2 powders treated at different temperatures (t=500, 600, 700 and 900°C) and coated contents (x=0, 3%, 5%, 10% and 15%) were prepared. The effects of treated temperatures on structure and performance of LiCoO2 coated by LTO particles have been investigated by X-ray diffraction (XRD) and the galvanostatic charge–discharge test at the high upper voltage limit of 4.5V. 3wt.% and 5wt.% LTO-coated materials expose the absence of secondary phase peaks in the range of the diffraction patterns below 700°C, and distinct additional peaks start appearing when the treated temperature arrives at 900°C. The diffraction patterns of the 15wt.% LTO-coated LiCoO2 shift toward higher angles compared to the other coated and bare LiCoO2. The 3wt.% LTO-coated LiCoO2 cathodes treated at 500, 700 and 900°C exhibit discharge capacities of 157, 162 and 140mAhg−1, respectively, versus capacities of 82mAhg−1 for the uncoated cathodes after 40 cycles. The LTO coatings are shown to suppress the capacity fade by reducing cell polarization.

Polymer coatings as separator layers for microbial fuel cell cathodes

Membrane separators reduce oxygen flux from the cathode into the anolyte in microbial fuel cells (MFCs), but water accumulation and pH gradients between the separator and cathode reduces performance. Air cathodes were spray-coated (water-facing side) with anion exchange, cation exchange, and neutral polymer coatings of different thicknesses to incorporate the separator into the cathode. The anion exchange polymer coating resulted in greater power density (1167±135mWm−2) than a cation exchange coating (439±2mWm−2). This power output was similar to that produced by a Nafion-coated cathode (1114±174mWm−2), and slightly lower than the uncoated cathode (1384±82mWm−2). Thicker coatings reduced oxygen diffusion into the electrolyte and increased coulombic efficiency (CE=56–64%) relative to an uncoated cathode (29±8%), but decreased power production (255–574mWm−2). Electrochemical characterization of the cathodes ex situ to the MFC showed that the cathodes with the lowest charge transfer resistance and the highest oxygen reduction activity produced the most power in MFC tests. The results on hydrophilic cathode separator layers revealed a trade off between power and CE. Cathodes coated with a thin coating of anion exchange polymer show promise for controlling oxygen transfer while minimally affecting power production.

Increased Electrical Output when a Bacterial ABTS Oxidizer is Used in a Microbial Fuel Cell

Microbial fuel cells (MFCs) are a technology that provides electrical energy from the microbial oxidation of organic compounds. Most MFCs use oxygen as the oxidant in the cathode chamber. This study examined the formation in culture of an unidentified bacterial oxidant and investigated the performance of this oxidant in a two-chambered MFC with a proton exchange membrane and an uncoated carbon cathode. DNA, FAME profile and characterization studies identified the microorganism that produced the oxidant as Burkholderia cenocepacia. The oxidant was produced by log phase cells, oxidized the dye 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), had a mass below 1 kD, was heat stable (121°C) and was soluble in ethanol. In a MFC with a 1000 Ω load and ABTS as a mediator, the oxidizer increased cell voltage 11 times higher than atmospheric oxygen and 2.9 times higher than that observed with ferricyanide in the cathode chamber. No increase in cell voltage was observed when no mediator was present. Organisms that produce and release oxidizers into the media may prove useful as bio-cathodes by improving the electrical output of MFCs.

LiNi 0.8Co 0.15Al 0.05O 2 cathode materials prepared by TiO 2 nanoparticle coatings on Ni 0.8Co 0.15Al 0.05(OH) 2 precursors

Synthesis, electrochemical, and structural properties of LiNi 0.8Co 0.15Al 0.05O 2 cathodes prepared by TiO 2 nanoparticles coating on a Ni 0.8Co 0.15Al 0.05(OH) 2 precursor have been investigated by the variation of coating concentration and annealing temperature. TiO 2-coated cathodes showed that Ti elements were distributed throughout the particles. Among the coated cathodes, the 0.6 wt% TiO 2-coated cathode prepared by annealing at 750 °C for 20 h exhibited the highest reversible capacity of 176 mAh g −1 and capacity retention of 92% after 40 cycles at a rate of 1 C (=190 mA g −1). On the other hand, an uncoated cathode showed a reversible first discharge capacity of 186 mAh g −1 and the same capacity retention value to the TiO 2-coated sample at a 1 C rate. However, under a 1 C rate cycling at 60 °C for 30 cycles, the uncoated sample showed a reversible capacity of 40 mAh g −1, while a TiO 2-coated one showed 71 mAh g −1. This significant improvement of the coated sample was due to the formation of a possible solid solution between TiO 2 and LiNi 0.8Co 0.15Al 0.05O 2. This effect was more evident upon annealing the charged sample while increasing the annealing temperature, and at 400 °C, the coated one showed a more suppressed formation of the NiO phase from the spinel LiNi 2O 4 phase than the uncoated sample.

High Performance LiCoO2 Cathode Materials at 60 ° C for Lithium Secondary Batteries Prepared by the Facile Nanoscale Dry-Coating Method

A mass scalable dry-coating method was used on a cathode to improve cycling stability at and, in contrast to the conventional wet-coating method, this one did not require a solvent. Electrochemical results of the , , and coatings on cathodes showed a fast capacity fade, resulting in capacity retention at under 1C rate cycling, whereas an uncoated cathode showed 64% retention. This difference was believed to be because the residual coating layers which did not react with the bulk layer accelerated the side reactions with the electrolytes at , leading to increased formation of the surface films. However, when and were used for dry coating, the coating layers completely disappeared, but a higher concentration of coating elements was found near the surfaces. Overall, the cathodes coated with and exhibited 79 and 88% capacity retention, respectively, at under 1C rate cycling.

Improved Electrochemical Performance of NH[sub 4]AlF[sub 4]-Coated LiCoO[sub 2] Cycled over 4.5 V

Cathode coatings are being developed to improve lithium-ion batteries. In this effort, particles were coated with by a simple dry-coating process to enhance capacity retention on a 4.5 V cutoff cycling. Compared with pristine , this coating greatly improved the electrode cyclability and rate capability. Electrochemical impedance spectroscopy and a cobalt dissolution test showed that the coating protected the particle surface from HF and, thus, suppressed the increase in interfacial impedance between the cathode and electrolyte that occurred with an uncoated cathode material.

Interaction of nitrogen with lithium in lithium ion batteries

The reaction of N 2 with lithium at electrode in lithium ion batteries was reported in this paper. At room temperature, N 2 can react with lithium, mainly at anode, to form Li 3N in an electrochemical system very easily during charge–discharge cycles. Li 3N has been characterized by XPS. Experimental results also revealed that the higher of the current density and higher of the temperature resulted in quicker of the nitrogen-fixation reaction. Moreover, the reaction of nitrogen with Li was faster at the TiO 2-coated cathode than at the uncoated cathode. The reaction can be brought about almost completely in the lithium ion batteries at room temperature. This could be a new method for preparation of Li 3N at room temperature.

Effects of CsI Coating of Carbon Fiber Cathodes on the Microwave Emission From a Triode Virtual Cathode Oscillator

We discuss the effects of cesium iodide (CsI) coating of carbon fiber cathodes on the microwave emission from a triode virtual cathode oscillator. As compared with the uncoated cathode, the CsI-coated carbon fiber cathode significantly improved the diode performance and, most notably, lengthened the microwave pulse, from 150 to 200 ns. The light emission from the diode, the diode perveance, and the diode gap change were introduced to explain the observed extension of microwave pulse. After CsI coating, the carbon fiber cathode exhibited the absence of strong plasma, a slow plasma expansion velocity, and an almost unchanged diode gap during the main voltage pulse. It was found that heavy plasma ions, slow plasma expansion velocities, and long microwave pulses tend to be closely tied. These results show that, given a proper diode design, the carbon fiber cathodes with CsI coating have great promise for generating long-pulse microwave radiation.

M3 ( PO4 ) 2-Nanoparticle-Coated LiCoO2 vs LiCo0.96M0.04O2 ( M = Mg and Zn ) on Electrochemical and Storage Characteristics

The electrochemical cycling and storage characteristics of and nanoparticle-coated cathode materials were compared at room temperature and between 3 and . The doped cathodes showed degraded electrochemical performance at room temperature compared to the uncoated cathode. The first discharge capacities of the uncoated and the doped cathodes were 186 and , respectively. The doped cathodes showed after 30 cycles, while the uncoated cathode showed after 50 cycles at a rate. and -coated showed discharge capacities of 179 and , respectively, and hadsignificantly improved capacity retention, showing 133 and , respectively, after 50 cycles. After storage at , in the electrolytes using charged electrodes, the doped cathodes showed both greatly decreased side reactions with the electrolytes and formation of and phases from Li and Co dissolution. However, the coated cathodes did not show either structural transformation into the and phases or side reactions with the electrolytes.