Carbon aerogel for battery applications

Kurzfassung

Carbon aerogels are highly open-porous solid materials in which a gas occupies more than 90% of their volume. They show low density, large surface area, high pore volume and high electrical conductivity. These properties make carbon aerogels an almost perfect cathode material for metal-sulphur batteries. Sulphur as active material in the positive electrode achieves theoretical gravimetric energy density and capacity of 2600 Wh kg-1 and 1675 Ah kg-1 by conversion reaction and formation of polysulfides. Furthermore, low cost of sulphur and high abundance make the metal-sulphur battery very attractive. Nevertheless, there are still several challenges, such as low sulphur utilization, polysulfide shuttle effect, Lithium - dendrite formation, and enormous volume expansion. The conversion reaction of sulphur and the formation of Li2S cause a large volume expansion of about 80%, resulting in reduced electron transport paths and decrease in kinetics. It is due to different densities of sulphur and the various sulphur compounds at ambient conditions Li2S (1.67 g cm-³). Thus, complete conversion of one mole of S8(s) to Li2S(s) occupies 76-80% extra space. The cathode structure should be tolerant enough to stand the large volume expansion and contraction incurred by the discharging and charging of sulphur active material. Flexible carbon aerogels with designed microstructure can accommodate volume expansion due to their ability for reversible deformation till certain degree during cycling, thereby resulting in high-performance cells. They showed high capacity and high Coulombic efficiency after 200 cycles if infiltrated in the gas phase compared to commercial available Ketjenblack. Molecular dynamics (MD) studies also present a detailed insight into the atomic-scale phenomena that underlie the formation of the porous network of carbon aerogels. The AIREBO potential, which is designed to account for the carbon and hydrogen interactions, is used to simulate the porous carbon network. The resulting pores in the structure are then incorporated with sulphur atoms and the behaviour of the system is studied under expansion of sulphur atoms mimicking the discharging and charging cycles.