In this work, a novel heterofunctional, bimodally-porous carbon morphology, termed the carbon compartment (CC), is utilized as a sulfur host within a lithium-sulfur battery cathode. A multi-scale model explores the physics and chemistry of the lithium-sulfur battery cathode. The CCs are synthesized through a rapid, low cost process to improve electrode-electrolyte interfacial contact and accommodate volumetric expansion associated with sulfide formation. The CCs demonstrate controllable sulfur loading and ca. 700 mAh g −1 (at 47%-wt S) reversible capacity with high coulombic efficiency due to their unique structures. Density functional theory and ab initio molecular dynamics characterize the interface between the C/S composite and electrolyte during the sulfur reduction mechanism. Stochastic realizations of 3D electrode microstructures are reconstructed based on representative SEM micrographs to study the influence of solid sulfur loading and lithium sulfide precipitation on microstructural and electrochemical properties. A macroscale electrochemical performance model is developed to analyze the performance of lithium-sulfur batteries. The combined multi-scale simulation studies explain key fundamentals of sulfur reduction and its relation to the polysulfide shuttle mechanism: how the process is affected due to the presence of carbon substrate, thermodynamics of lithium sulfide formation and deposition on carbon, and microstructural effects on the overall cell performance. The goal of developing new and efficient renewable energy technologies from intermittent energy sources, such as solar and wind, necessitates the need for effective, economical, and safe energy storage.1-3 While batteries and supercapacitors have been considered the best options to address this issue, their progress is staggered by both challenging synthesis problems and a continually-developing understanding of their fundamental electrochemistry. 4 Currently, lithiumion batteries dominate the market for portable electronic devices; however, their cost and relatively low energy density prevents them from being used in electrical vehicle applications at this juncture. [4][5][6] Going beyond lithium-ion chemistry, lithium-sulfur and lithium-air are among the most promising battery technologies that can potentially meet the required specific energy target of about 1,000 Wh kg −1 needed to improve the viability of electrical vehicles. 5,7The appeal of a sulfur-based cathode lies in its high theoretical capacity that is about one order of magnitude higher than current metal oxide-based cathodes. Sulfur is also cheaper and more environmentally-friendly than today's commercial cathode materials. 7Low density and natural abundancy in the earth's crust imply that the use of elemental sulfur in the manufacture of lithium-sulfur (Li-S) batteries will be cost effective and demonstrate low environmental impact.8 Thus, Li-S batteries hold significant promise due to their high theoretical specific energy of 2,567 Wh kg −1 , 9 assuming the complete electroche...

Single-walled carbon nanotubes grow by decomposition of a carbon-containing precursor gas over metal nanocatalysts. It is known that the shape, size, and chemical nature of the catalysts play significant roles in the nucleation and growth processes. Here, we use reactive molecular dynamics simulations to analyze how the catalyst particle size and the strength of adhesion between the surface and nascent carbon structures may affect the growth process. As a result, we determine if the process leads to cap lift-off or if it causes graphitic encapsulation and, therefore, poisoning of the catalyst. In agreement with the Hafner-Smalley model, our MD simulation results illustrate that the work of adhesion must be weak enough so the curvature energy of a spherical fullerene is less favorable than that of a single-walled carbon nanotube with the same diameter, thus allowing the cap-lifting process to take place. Moreover, we propose that a simple model combining curvature energy and kinetic effects may help to identify regions of single-walled carbon nanotube growth in the phase space defined by work of adhesion, temperature, and catalyst size.

Rational catalyst design requires an atomic scale mechanistic understanding of the chemical pathways involved in the catalytic process. A heterogeneous catalyst typically works by adsorbing reactants onto its surface, where the energies for specific bonds to dissociate and/or combine with other species (to form desired intermediate or final products) are lower. Here, using the catalytic growth of single-walled carbon nanotubes (SWCNTs) as a prototype reaction, we show that the chemical pathway may in-fact involve the entire catalyst particle, and can proceed via the fluctuations in the formation and decomposition of metastable phases in the particle interior. We record in situ and at atomic resolution, the dynamic phase transformations occurring in a Cobalt catalyst nanoparticle during SWCNT growth, using a state-of-the-art environmental transmission electron microscope (ETEM). The fluctuations in catalyst carbon content are quantified by the automated, atomic-scale structural analysis of the time-resolved ETEM images and correlated with the SWCNT growth rate. We find the fluctuations in the carbon concentration in the catalyst nanoparticle and the fluctuations in nanotube growth rates to be of complementary character. These findings are successfully explained by reactive molecular dynamics (RMD) simulations that track the spatial and temporal evolution of the distribution of carbon atoms within and on the surface of the catalyst particle. We anticipate that our approach combining real-time, atomic-resolution image analysis and molecular dynamics simulations will facilitate catalyst design, improving reaction efficiencies and selectivity towards the growth of desired structure.

Single-walled carbon nanotubes are usually synthesized by catalytic growth driven by reaction of a precursor gas over metallic nanoparticles supported on a substrate. Here we use molecular dynamics simulations (MD) with the purpose of determining how the catalyst−substrate strength of adhesion influences the structure of the carbon networks synthesized on the catalyst surface. It is found that the strength of the catalyst/substrate interaction energies defines the shape of the catalyst particle. When these energies are attractive, the nanocatalyst height decreases due to enhanced wetting and in turn favors the lifting up of carbon nanotube caps during the synthesis process. In addition, the presence of an appropriate substrate may avoid catalyst poisoning. This effect may result from repulsion forces from the substrate toward catalyzed carbon atoms, which cause carbon atoms to diffuse to upper layers, thus keeping the catalyst−substrate interface exposed to continuous catalytic activity. However, too strong metal−substrate interactions may take the cluster to the limit of complete wetting, thus promoting the formation of graphene or amorphous carbon over carbon nanotube-like structures. A growth diagram is constructed in the space of metal−substrate vs metal−carbon strengths of adhesion. The growth diagram defines regions of nanotube growth and encapsulation; in the first we are able to identify also zones of higher or lower quality of the nanotubes grown. This theoretical characterization is very useful to guide a controlled synthesis.