Temperature effect on graphite KS44 lithiation in ethylene carbonate + propylene carbonate solution: galvanostatic and impedance study

Graphite Lonza KS44 in a solution 1 M LiClO4 in a propoylene carbonate + ethylene carbonate (1 M:1 M) mixture was lithiated and delithiated galvanostatically at room temperature and at the elevated temperature of 55°C. Voltagetime profiles and complex impedance diagrams were recorded and are discussed for this particular system. It was confirmed that this type of graphite shows a relatively small current loss consumed by exfoliation, if lithiated at room temperature. However, the voltagetime curve of the first charging at 55°C shows a long voltage plateau at 0.7 V vs. Li/Li+, which corresponds to 540 mAh g-1 of irreversible capacity attributed to exfoliation. The solid electrolyte layer formed at elevated temperature, although less protecting in the sense of electrolyte reduction, shows a remarkably higher electrical resistance than that formed at room temperature. A comparison of the impedance diagrams of lithiated and delithiated samples allows the conclusion that mass transfer through the graphite, not that through the solid electrolyte layer, plays a dominant role in the mass transfer limitations

Understanding the influence of conductive carbon additives surface area on the rate performance of LiFePO4 cathodes for lithium ion batteries

Conductive carbon additives with different surface area and particle size, alone or in different combinations, were tested as conductive additives for LiFePO4 cathode materials in lithium ion batteries. Their influence on the conductivity, rate capability as well as the structure of the resulting electrodes was investigated. Mercury porosimetry was carried out to define the porosity and pore size distribution of electrodes, and scanning electron microscopy was used to image their morphology. By comparing the discharge capacity, especially at higher rates, it can be concluded that the electrochemical performance of LiFePO4 cathode material is significantly affected by the surface area, particle size and morphology of the used carbon additives. The best rate performance is achieved with the electrode containing a carbon additive with a specific surface area of 180 m2 g-1. This work reveals that the choice of conductive additive influences discharge capacity of LiFePO4 Li-ion battery cells by as much as 20-30%. This is due to conductive additive's influence on both electronic conductivity and porosity (which determines ionic conductivity) of LiFePO4 electrodes. A system approach to lithium ion battery material research should always consider inactive materials, such as conductive additives and binders, in addition to active materials. © 2013 Elsevier Ltd. All rights reserved

Methanol electrooxidation on supported Pt and PtRu catalysts in acid and alkaline solutions

The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% Pt-Ru (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH at 295 and 333 K using thin-film rotating disk electrode (RDE) method. It was found that the activity of Pt and Pt-Ru for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt2Ru3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt2Ru3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Rurich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol

Investigation of PF6- and TFSI- anion intercalation into graphitized carbon blacks and its influence on high voltage lithium ion batteries

Graphitized carbon blacks have shown a more promising electrochemical performance than the non-treated ones when being applied in small amounts as conductive additives in composite cathode electrodes for lithium ion batteries, due to the absence of surface functional groups which contribute to detrimental side-reactions with the electrolyte. Here, we report that at high potentials of >4.5 V vs. Li/Li+, graphitic structures in carbon black can provide host sites for the partially reversible intercalation of electrolyte salt anions. This process is in analogy to the charge reaction of graphite positive electrodes in dual-ion cells. A standard furnace carbon black with small graphitic structural units, as well as slightly and highly graphitized carbon blacks, were characterized and analyzed with regard to anion intercalation. A LiPF6 containing organic solvent based electrolyte as well as a state-of-the-art ionic liquid based electrolyte composed of LiTFSI in PYR14TFSI were applied. The intercalation of both PF6- and TFSI- could be confirmed by cyclic voltammetry in electrodes made of carbon blacks. When exposed to high potentials, carbon blacks experienced strong activation in the 1st cycle, which promotes the perception for anion intercalation, and thus increases the anion intercalation capacity in the following cycles. The specific capacity from anion intercalation was evaluated by constant current charge-discharge cycling. The obtained capacity was proportional to the graphitization degree. As anion intercalation might be accompanied by decomposition reactions of the electrolyte, e.g., by co-intercalation of solvent molecules, it could induce the decomposition of the electrolyte inside the carbon and thus degradation of the carbon black graphitic structure. In order to avoid side reactions from surface groups and from anion intercalation, the thermal treatment of carbon blacks must be optimized. © the Owner Societies

Influence of thermal treated carbon black conductive additive on the performance of high voltage spinel Cr-doped LiNi0.5Mn1.5O4 composite cathode electrode

In this work, non-graphitized carbon black and graphitized carbon blacks were analyzed with regard to their performance as conductive additives in high voltage spinel Cr-doped LiNi0.5Mn1.5O4 composite cathode electrodes. The degree of graphitization was investigated by X-ray diffraction. X-ray photoelectron spectroscopy was used to investigate the carbon black surface species and their alteration upon graphitization, as well as to the post mortem analysis of the composite cathodes. It was found that the cells with graphitized carbon blacks showed much longer cycling life. The depletion of the functional groups at the carbon black surface by graphitization apparently reduced the side reactions in the composite electrodes. Due to their superior performance, graphitized carbon black should be considered as substitute for the standard carbon black additives in the high voltage cathode electrodes