Stane Pejovnik

Impact of the Carbon Coating Thickness on the Electrochemical Performance of LiFePO4 / C Composites

Porous, well crystalline LiFePO 4 /C composites with different amounts of carbon have been prepared by a sol-gel technique. The thickness of carbon coatings (paintings) has been determined by high-resolution transmission electron microscopy. It is shown that carbon coating thickness can be controlled by the amount of carbon and it has an impact on the obtained reversible capacity. Furthermore, it is shown that atomic ratio of nonactive Fe(III) phase (presumably Fe 3 P) in as-synthesized LiFePO 4 /C composites depends on the amount of carbon in the composite. Using Mossbauer spectroscopy, we have confirmed that the nonactive Fe(III) remains nearly unchanged in the composite during cycling. The lowest amount of carbon in LiFePO 4 /C composites obtained from citrate anion as a gelling agent was 3.2 wt % and this particular amount corresponds to the carbon coating thickness of about 1-2 nm. The reversible capacity obtained from the above-mentioned composite delivers close to 80% of the theoretical capacity at room temperature with a current density of 170 mA/g (C/1 rate).

The role of carbon black distribution in cathodes for Li ion batteries

Abstract The influence of carbon black distribution/arrangement in cathode composite on cathode performance is studied using three types of active materials: LiMn2O2-spinel, LiCoO2, and LiFePO4. To the active materials, carbon black is added in two different ways: (a) using a conventional mixing procedure and (b) using a novel coating technology (NCT) invented in our laboratory. Different technologies yield different arrangement (distribution) of carbon black around active particles. It is shown that the uniformity of carbon black distribution affects significantly the cathode kinetics, regardless of the type of active particles used. A simple model explaining the influence of carbon black distribution on cathode kinetics is presented.

On the Interpretation of Measured Impedance Spectra of Insertion Cathodes for Lithium-Ion Batteries

In the literature, the interpretation of electrochemical impedance spectra measured on insertion cathode materials is far from being unique. In most cases, various arbitrarily selected equivalent circuits have been used for analysis of spectra whereby the criterion of merit has mainly been the quality of fit. Herein, we propose a different approach. We try to explain the main features such as the high and medium frequency arcs and the low frequency diffusional tail using convenient (simplified) equivalent circuits derived from a quite general description of impedance due to a particulate (porous) system. The proposed models have a clear physical background. The meaning of selected circuit parameters is experimentally verified using carefully modeled experiments on LiFeP0 4 and LiCoO 2 materials. In particular, we discuss the effects of state of charge, external pressure, electrode mass (thickness), and electrolyte concentration on the measured and simulated equivalent circuits. In the last part, we discuss in certain depth the complications arising from poor electronic or ionic contacting (wiring) between different phases constituting electrodes.

Cellulose as a binding material in graphitic anodes for Li ion batteries: a performance and degradation study

Four types of cellulose, in particular carboxy methyl cellulose (CMC), are tested as potential binding materials in graphitic anodes for lithium ion batteries. It is shown that a minimum content of a cellulose which gives acceptable anode properties (reversible capacity>300 mA h g−1 during the first 10 cycles, irreversible loss<20%) is about 2 wt.%, which is less than in the case of conventional polymeric binders (5–10 wt.%). Kinetics of insertion–deinsertion and passivation processes seem not to be affected by the presence of cellulose. Explanation for the electrode failure at cellulose contents lower than 1 wt.% is given based on X-ray diffraction and microscopy investigations. Finally, the structure (distribution) of cellulose in the composite anode material is discussed and (indirectly) checked with a series of experiments. Most results are compared with the corresponding results obtained either with gelatin or conventional polymeric binders or both.

Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules.

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.

Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.

Air-stable monodispersed Mo6S3I6 nanowires

We report on the properties of a new air-stable nanowire material with the chemical formula Mo6S3I6 .T he distinguishing features of the material are rapid one-step synthesis, easy isolation and controllable dispersion into small-diameter wire bundles. Elemental analysis, x-ray diffraction, thermogravimetry, differential thermal analysis, Raman scattering and electron microscopy were used to characterize the material.

A Study of Metal (Ni, Pt, Au)/Yttria-Stabilized Zirconia Interface in Hydrogen Atmosphere at Elevated Temperature

The properties of selected metal (Pt, Ni, Au)/YSZ single crystal interfaces have been investigated by current-voltage measurements and impedance spectroscopy. A three-electrode cell consisting of a quasi-point-contact metal working electrode and Pt reference and counter electrodes were constructed and studied under various conditions. The impedance response consists of at least two processes, both belonging to interfacial properties: a high frequency process with peak frequency between 10 4 and 10 3 Hz, and a low frequency process with peak frequency between 0.5 and 50 Hz. An increase in overall electrochemical reaction rate and in the interfacial capacitance under prolonged anodic polarization was observed. A mechanism based on the spillover of oxide or hydroxide ions on the metal surface is proposed.

Effect of electrode material on the oxidation of H2 at the metal-Sr0.995Ce0.95Y0.05O2.970 interface

Abstract The effect of electrode material on the electrode kinetics of a metal–proton conductor interface has been investigated in a quasi point-contact configuration for four metals: Ni, Ag, Au and Pt. The current–voltage behaviour depends on the nature of the electrode, the hydrogen partial pressure and on temperature. For all systems studied, the anodic part of the polarisation curves displayed limiting current behaviour. For Pt at low anodic polarisation, an additional process determines the overall reaction rate. The apparent reaction order ( q app ) is found to be strongly dependent on electrode type, suggesting that the reaction mechanism is decisively determined by the electrode, rather than by electrolyte surface. The electrocatalytic activity of the metal tested has been classified according to the limiting current density in a humidified atmosphere of N 2 –1% H 2 . The highest i lim has been detected for Ni, followed by Pt, Au and Ag.

Gelatin-pretreated carbon particles for potential use in lithium ion batteries

Abstract Graphite anodes for use in lithium ion batteries were prepared from graphite particles pretreated in a gelatin solution. The content of gelatin in the final anode material was determined from the difference in mass of graphite particles before and after the treatment with gelatin and by thermogravimetric analysis. Forces between a gelatin-coated glass particle and graphite surface were measured in solution using an atomic force microscope. The effect of gelatin content on the characteristics of first charge–discharge cycle is measured and commented in terms of a simple passivation model.