Structure transformation of sulfur.
Tremendous progress in carbon–sulfur composites has been achieved in recent years. Substantial attention has been paid to the morphology, chemistry and electrochemistry of carbon–sulfur composites. In the future the adsorption, intercalation, and functionalization of carbon materials still require further investigation in terms of the cathode stabilization through the confinement and physical–chemical attraction of polysulfide. For cyclic stability of sulfur, micropore adsorption or possible intercalation between graphene layers have been shown to lead to the best results (Table 2), while surface functionalization can potentially improve this further. In terms of reaction kinetics, however, mesopores, or loose confinement, e.g. graphene wrapping, are beneficial. The large pore volumes of mesoporous carbon and graphene sheets are also advantageous for high loading of sulfur that gives high capacity of the C–S composite. One also needs to bear in mind that partially filled pores can compensate for the volume change of sulfur–lithium sulfides; thus the optimal loading of sulfur is a balance between the desire for maximum capacity and the need to allow for the volume change to ensure stability. Hollow carbons with rigid shells and large internal voids are desirable for this purpose. Similarly, graphene sheets with good flexibility are excellent for buffering volume change.
Several critical factors are suggested here for the rational design of advanced carbon–sulfur composites:
We emphasize here that the correlations between the carbon structure (porosity, surface chemistry, graphitic degree) and the sulfur structure need to be comprehensively studied and optimized (Fig. 14). Facile and low cost material fabrication techniques are also desired. An optimal carbon–sulfur composite will benefit from a combination of fundamental insights and an advanced synthesis approach. We also acknowledge that the sulfur cathode problem is not the only issue that impedes Li–S technology. Problems with the lithium anode, the electrolyte as well as engineering difficulties in fabricating lithium metal batteries are all great challenges to be faced, but recent dramatic progress in carbon–sulfur composites is likely to form the basis for future commercialization of Li–S batteries.
- Pore size: micropores/small mesopores for strongly confining polysulfides.
- Large pore volume for maximum sulfur loading: carbon is not an active cathode material in Li–S batteries and its weight ratio is required to be as low as possible without harming the overall performance of sulfur–carbon cathodes. However, the very low conductivity of the composite when sulfur content increases is a critical problem to be solved.
- Graphitization level: a preferential content of graphitic carbon to facilitate electron conduction to the insulating sulfur/lithium sulfides.
- Electrolyte impregnation and lithium ion migration: a short pathway to preserve fast migration of lithium ions from the bulk electrolyte to active sulfur and the release of lithium ions from lithium sulfides. Voids are required in the final carbon–sulfur composites.
- Flexible or rigid carbon scaffold: volume change occurs during the discharge or charge of sulfur–carbon cathodes; the carbon host should buffer the stress-induced strain and survive over extended cycles.
- Low cost production and easy scale-up: templated ordered mesoporous carbon and CNTs/nanofibers are highly effective due to their periodic structure but are unlikely to be produced at an industrial scale for Li–S batteries owing to their high cost and unsatisfactory performance–cost ratio; novel continuous synthesis techniques, inclusive of but not limited to atomization carbonization and hydrothermal carbonization, are promising.
- An advanced technique to form carbon–sulfur composites: common methods are impregnation with the sulfur melt or sulfur organic solution, disproportionate reaction and in-situ encapsulation, and vapour diffusion. These are post-carbon-synthesis and complicate the industrial processing. A one-step method for fabrication of sulfur–carbon composites is necessary.