Charakterisierung des Ionentransportes in neuen Li-Elektrolyten und Untersuchung der Auswirkungen auf die elektrochemischen Eigenschaften

In the first part of this work we study the influence of sulfonate-based zwitterions (ZI) in binary mixtures containing the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) and the lithium salt lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI). Thermal and electrochemical properties are studied by a combination of DSC, conductivity, viscosity measurements, PFG-NMR as well as cyclic voltammetry. Furthermore, the chemical environment of Li+ and TFSI– ions in the mixtures are investigated by 7Li-NMR spectroscopy and Raman spectroscopy. We find that the addition of ZI leads to a suppression of crystallisation processes. A strong impact on the chemical environment of Li+ ions by the addition of ZI occurs. Stable Li+/ZI complexes arise releasing ‘free’ TFSI– anions. A modified Walden plot shows significant differences in the electrolyte transport characteristics. Comparing experimental data with two newly defined reference lines points to typical liquid behaviour at higher temperatures. A dynamic decoupling of ionic conductivity and viscosity occurs at lower temperatures. This phenomenon can be explained by the existence of fast and slow species, which is confirmed by the results of the PFG-NMR measurements in the ternary mixtures. These results indicate the existence of faster TFSI– anions ruling the conductivity and slower Li+/ZI complexes ruling the viscosity. The cathodic shift of the Li+ reduction potential is caused by a stronger binding between the Li+ ions and the ZI’s sulfonate-group. Apart from the lower Li+ diffusivities, this diminishes their applicability in lithium ion batteries. In future work other types of ZI should be tested with anionic groups bearing less Li+ affinity. In the second part Li+ transference numbers in different liquid electrolytes are quantified electrochemically by means of very-low-frequency impedance spectroscopy (VLF-IS). For each electrolyte the VLF-IS experiments are carried out at variable electrode distances inside a newly constructed symmetrical Li | electrolyte | Li cell. This cell allows a clear distinction of different contributions to the cell impedance, in particular the measurement of the Li+ diffusion resistance. The causality of our impedance spectra is checked by Kramers-Kronig transformation tests in order to rule out instationary states within the measurement setup. The impedance spectra are fitted by an equivalent circuit including a Warburg-short element. The VLF-IS method is applied to three different electrolytes: (i) The carbonate-based standard battery electrolyte LP30 with 1 mol/L Li-PF6 dissolved in EC/DMC (1:1 wt.-%). (ii) An equimolar mixture of tetraglyme (G4) and Li-TFSI. (iii) A binary mixture of BMP-TFSI and Li-TFSI. The results are compared to Li+ transport numbers obtained by PFG-NMR. Significant differences between the specific transference numbers and transport numbers are observed. In order to rationalize these discrepancies, we apply Onsager relations to migration and diffusion fluxes in the cell. We show that strong ionic interactions lead to correlated movements of cations and anions into the same direction. Furthermore, we give guidance to the correct application of the potentiostatic polarisation (PP) method by using a specific optimal time interval, in order to determine the correct initial current value. In the third part organic hydrosulphide and -selenide ionic liquids are studied by means of thermogravimetric measurements. By preliminary tests we found out that imidazolium hydrochalcogenides show a remarkable high volatility, allowing their quantitative sublimation below 100 °C at a pressure of 10–2 mbar. To the state of our knowledge a comparable high volatility below the melting temperature is a novelty for the class of aprotic ILs. At elevated temperatures decomposition into the respective alkylimidazoles and 1-methyl-3-alkylimidazolchalcogenones occurs. A reliable determination of vaporisation enthalpies by isothermal thermogravimetric analysis (TGA) is carried out. Comparing experimentally determined evaporation enthalpies at atmospheric pressure with those determined by theoretical DFT calculations emphasize that decomposition products cross over to the gas phase much easier. Consequently, this is confirmed by the characterisation of condensation residues. The theoretical calculations lead to the stable gas phase decomposition products detected by mass spectrometric measurements.