Elektrochemische Synthese von Lithium-, Silizium- und Germanium-Nanostrukturen in ionischen Flüssigkeiten
The launch of full electric vehicles to the market requires a significant increase of energy densities of lithium ion batteries to enhance the range of these vehicles. Therefore it is necessary to improve batteries particularly in the fields of electrodes and electrolytes. The use of nanostructured anodes thereby turned out as a promising option. This work focused on the electrochemical synthesis of nanostructures made of relevant anode materials using ionic liquids. At first lithium was deposited in an ionic liquid, 1-butyl-1- methylpyrrolidinum bis(trifluoromethylsulfonyl)imide ([Py1,4] TFSI), with LiTFSI within different templates. For the preparation of macroporous structures lithium was deposited inside a matrix of colloidal polystyrene spheres and the deposition inside the pores of a polycarbonate membrane induced the formation of nanotubes. The polystyrene templates for the deposition were prepared by applying several layers of polystyrene spheres on a copper plate. For the directed growth of lithium nanotubes membranes with a porediameter of ~200 nm were used. The dimensions were thus given by the pores of the polycarbonate membrane. After a deposition time of 15 minutes solely nanotubes occurred which were evenly distributed over the whole electrode surface and partly free standing. Furthermore, this work focussed on the synthesis of silicon nanowires. For this purpose silicon was deposited inside the ~90 nm wide pores a polycarbonate membrane. The electrochemical deposition was done from [P1,4] TFSI with SiCl4 as silicon source. By the use of potentiostatic deposition of silicon comparatively short nanowires of only 500 nm were formed, whereas pulsed deposition produced significantly longer nanowires of up to 2.5 μm. Additionally, experiments with SiBr4 as silicon source were carried out, since it has around ambient temperature a significantly lower vapour pressure than SiCl4. The same process as described for the silicon nanowires was used to form germanium nanotubes. Furthermore, it was shown that germanium nanotubes could also be prepared by using a membrane with the smaller pore diameter (90 nm).
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