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Schuster, Jörg (2011): Morphology Control of Ordered Mesoporous Carbons for High Capacity Lithium Sulfur Batteries. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

The focus of this thesis concerns the morphology control of ordered mesoporous carbon (OMC) materials. Ordered mesoporous carbons with diverse morphologies, that are thin films, fibers – embedded in anodic alumina membranes and free-standing – or spherical nanoparticles, have been successfully prepared by soft-templating procedures. The mechanisms of structure formation and processing were investigated with in-situ SAXS measurements and their application in high capacity lithium-sulfur batteries was successfully tested in cooperation with Guang He and Linda Nazar from the University of Waterloo in Canada. The Li-S batteries receive increasing attention due to their high theoretical energy density which is 3 to 5 times higher than from lithium-ion batteries. For this type of battery the specific pore volume is crucial for the content of the active component (sulfur) in the cathode and therefore correlates with the capacity and gravimetric energy density of the battery. At first, mesoporous thin films with 2D-hexagonal structure were obtained through organic-organic self-assembly of a preformed oligomeric resol precursor and the triblock copolymer template Pluronic P123. The formation of a condensed-wall material through thermopolymerization of the precursor oligomers resulted in mesostructured phenolic resin films. Subsequent decomposition of the surfactant and partial carbonization were achieved through thermal treatment in inert atmosphere. The films were crack-free with tunable homogenous thicknesses, and showed either 2D-hexagonal or lamellar mesostructure. An additional, yet unknown 3D-mesostructure was also found. In the second part, cubic and circular hexagonal mesoporous carbon phases in the confined environment of tubular anodic alumina membrane (AAM) pores were obtained by self-assembly of the mentioned resol precursor and the triblock copolymer templates Pluronic F127 or P123, respectively. Casting and solvent-evaporation were also followed by thermopolymerization, thermal decomposition of the surfactant and carbonization through thermal treatment at temperatures up to 1000 °C in an inert atmosphere. For both structures the AAM pores were completely filled and no shrinkage was observed, due to strong adhesion of the carbon wall material to the AAM pore walls. As a consequence of this restricted shrinkage effect, the mesophase system stayed almost constant even after thermal treatment at 1000 °C, and pore sizes of up to 20 nm were obtained. In the third part, the aforementioned mesoporous films and embedded fibers in AAMs were further investigated concerning structure formation and carbonization in an in-situ SAXS study. The in-situ measurements revealed that for both systems the structure formation occurs during the thermopolymerization step. Therefore the process of structure formation differs significantly from the known evaporation-induced self-assembly (EISA) and may rather be viewed as thermally-induced self-assembly. As a result, the structural evolution strongly depends on the chosen temperature, which controls both the rate of the mesostructure formation and the spatial dimensions of the resulting mesophase. In the fourth part the syntheses recipes for AAMs were applied on a presynthesized silica template for synthesis of freestanding mesoporous carbon nanofibers. The syntheses start with casting of carbon nanofibers with a silica precursor solution leading to a porous silica template after calcination with tubular pores mimicking the initial carbon nanofibers. A synthesis concept using triconstituent coassembly of resol, tetraethylorthosilicate as additional silica precursor and Pluronic F127 was applied here. The silica from the additional precursor was found to be beneficial, due to reduced shrinkage and created additional porosity after etching it. Those OMC nanofibers therefore exhibited a very large surface area and a high pore volume of 2486 m2/g and 2.06 cm3/g, respectively. Due to their extremely high porosity values, those fibers were successfully applied as sulfur host and electrode material in lithium-sulfur batteries. The fifth and last part focuses on the synthesis of spherical mesoporous carbon nanoparticles. Therefore the triconstituent coassembly was applied on a silica template with spherical pores, which was derived from the opal structure of colloidal crystals made from 400 nm PMMA spheres. The spherical ordered mesoporous carbon nanoparticles feature extremely high inner porosity of 2.32 cm3/g and 2445 m2/g, respectively They were successfully applied as cathode material in Li-S batteries, where they showed high reversible capacity up to 1200 mAh/g and good cycle efficiency. The final product consists of spherical mesoporous carbon particles with a diameter of around 300 nm and 2D-hexagonal porosity. The particles could be completely separated by sonification to form stable colloidal suspensions. This could be the base for further applications such drug delivery.