Carbon aerogels - Study of structural and mechanical properties for battery applications

Abstract

Carbon aerogels are highly open-porous nanostructured materials in which a gas occupies up to 99.98% of their volume. They show low density, large specific surface area, high pore volume and high electrical conductivity. These properties make carbon aerogels an exciting material for application as cathodes in metal-sulphur batteries [1, 2]. Sulphur as active ma- terial in the positive electrode achieves theoretical gravimetric energy density of of 2600 Wh kg-1 and capacity of 1675 Ah kg-1, by conversion reaction and formation of polysulfides. The conversion reaction of sulphur and the formation of Li2S cause a large volume expansion of about 80%, resulting in possible failure of the porous carbon network. In order to develop carbon aerogel-based cathode materials, an optimisation of their mechanical properties under cyclic deformation becomes imperative. Furthermore, since the microporous range (pore sizes below 2 nm according to the IUPAC definition) is relevant, it is interesting to look at the molecular picture of these nanostructured materials, as the molecular features may well drive the properties at such a lower length scale. In this study, molecular dynamics (MD) has been employed as a computational method to investigate the above-mentioned features in nanoporous carbon. To this end, a first-of-its-kind model consisting of 800,000 atoms has been developed and simulated in LAMMPS [3]. The resulting model structures having a wide range of densities (0.59 to 0.14 g cm-3) are studied for their structural, fractal, and mechanical properties. To this end, the properties such as pore- sizes, pore-size distributions, specific surface areas, surface-to-volume ratios, fractal dimension, pair-correlation functions, and the mechanical properties are investigated and illustrated. The properties obtained are in good agreement with the experimental data and provide a compre- hensive account at the molecular level. As a next step, the proposed model will be applied to study the effects of the above-mentioned volume expansion after infiltrating sulphur at the molecular level.