Morphology Control of Ordered Mesoporous Carbons for High Capacity Lithium Sulfur Batteries

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.