Charakterisierung von Polyelektrolyten und Lithiumsalzen für elektrochemische Energiespeicher unter Verwendung neu entwickelter Messsysteme

In the first part of this work, three imidazolium-based ionic liquid monomers with polymerizable vinyl groups and the resulting polyelectrolytes have been synthesized and characterized. Particular attention was paid to the purity of the materials. Besides comprehensive monomer and polymer analytics, electrical impedance spectroscopy was carried out to obtain information about the ion conducting properties of the three systems under investigation: poly(3-ethyl-1-vinylimidazolium)-bis(trifluoromethanesulfonyl)imide (P1), poly(3-methyl-1-(4-vinylbenzyl)imidazolium)-bis(trifluoromethanesulfonyl)imide (P2) and poly(1-butyl-3-methyl-2-(4-vinylphenethyl)imidazolium)-bis(trifluoromethanesulfonyl)imide (P3). The pure polymers, which are bis(trifluoromethanesulfonyl)imide (N(Tf)2) anion conductors, exhibit room-temperature conductivities of the order of 10-8 S/cm in the best case. The anion conduction mechanism is strongly influenced by the length of the spacer group between the polymer backbone and the imidazolium cations attached to the side chain. In polymers P1 and P2 with short spacer groups, intra- and inter-cation hopping of the N(Tf)2 anions can be distinguished below the glass transition temperature, while this is not possible in the case of polymer P3 with longer spacer groups. Furthermore, we have studied several mixtures of the best conducting polymer P2 with LiN(Tf)2, zwitterions and monomeric ionic liquid. While the zwitterions were capable of compensating for the conductivity drop due to Li salt addition, the addition of monomeric IL as plasticizer leads to a considerable conductivity enhancement without a significant loss of mechanical stability. In the second part of this work, three lithium salts, lithium bis(pentafluorophenyl)amide LiN(Pfp)2, lithium pentafluorophenyl-trifluoromethyl-sulfonylimide LiN(Pfp)(Tf) and lithium pentafluorophenyl-nonafluorobutyl-sulfonylimide LiN(Pfp)(Nf) were characterized with respect to their thermal and electrochemical properties. LiN(Pfp)2 decomposes around 100 °C, whereas LiN(Pfp)(Tf) and LiN(Pfp)(Nf) show a much higher thermal stability up to temperatures above 300 °C. The ionic conductivity at 100 °C, measured by means of impedance spectroscopy, decreases in the order LiN(Pfp)(Tf) > LiN(Tf)2 > LiN(Pfp)(Nf). Both, the activation energy and entropy for ion conduction in the new salts are lower than in LiN(Tf)2, most likely due to the lower symmetry of the new anions. The electrochemical stability and ionic conductivity of LiN(Pfp)(Tf) and LiN(Pfp)(Nf) solutions in alkyl carbonates are slightly lower than that of the LiN(Tf)2 solution, but still sufficient for application in lithium ion batteries. The high thermal stability of the novel salts and their stability towards hydrolysis makes them attractive candidates for overcoming the drawbacks of LiPF6 - based electrolytes at elevated temperatures. In the third part of this work, a report is given on an electrochemical measurement system. It consists of a temperature-controlled platform and exchangeable measurement cells. The platform contains a Peltier element connected to an external temperature regulator. This construction allows for heating and cooling rates up to 60 °C/min and a temperature stability of ±0,1 °C. The different sample compartments themselves are designed for measurements with different types of samples, like volatile and non-volatile liquids, gels, polymers and other solid electrolytes such as glass ceramics. The required amount of sample volume was significantly reduced as compared to standard measurement cells the liquid sample cell requires between 20 µL and 800 µL. In case of solid electrolytes, sample masses in the range of milligrams are sufficient. In order to perform potential-controlled measurements within these micro-cells, a novel miniaturized Ag/Ag+ reference electrode (RE) suitable for electrochemical measurements in room temperature ionic liquids (RTIL) was developed. The electrode is based on a capillary with an outer diameter of 365 µm and contains a 10 mmol/L solution of a silver salt in a RTIL. The silver salt bears the same type of anion as the RTIL. While potential shifts of several hundred millivolts have been observed for common platinum pseudo-reference electrodes, the newly developed Ag/Ag+ micro-electrode provides a stable and reliable reference potential over a period of more than four weeks, if protected from light and stored in a nitrogen atmosphere. Due to the small dimensions of the RE, it can be placed close to the working electrode. Besides for application in electrochemical micro-cells it is well-suited for potential-controlled in-situ AFM, STM or electrochemical impedance measurements. The electrode characteristics were determined by voltammetric measurements on ferrocene and on cobaltocenium hexafluorophosphate dissolved in an RTIL. The highest expected contamination of the sample with Ag+ ions was calculated and found to be below 4 ppm.