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It had been proven that carbon aerogel (CA) can effectively improve the conductivity of sulfur (S) and suppress the shuttle effect of polysulfides in Li-S batteries, due to its outstanding electrical conductivity and micropore-rich structure. However, the microstructure of CA was easy to collapse and shrink during carbonization, resulting in a dramatically drop in specific surface area and porosity. Herein, graphene nanosheets (GNs) were introduced in CA to reinforce the structural stability. As-prepared CA/GNs composites were fabricated from in-situ polymerization of resorcinol (R) and formaldehyde (F) in graphene oxide (GO) solution. It was found that the mass concentration of the GO solution had a significantly positive effect on hierarchical porous structure of the CA/GNs composites, which presented improve the electrochemical performances due to the combination of micro-, meso- and macro-pores. It is worth noting that the CA/GNs0.1 sample, obtained from 10 wt % GO solution, possessed the largest specific surface area of 665.477 m2 g−1 and the largest pore volume of 0.912 cm3 g−1. Especially, after incorporated with moderate amount of S (58.86 wt %), the CA/GNs0.1/S cathode exhibited the best electrochemical performances (initial discharging capacity of 1501 mAh g−1 at 0.1C and reversible capacities of 471 mAh g−1 after 100 cycles and 341 mAh g−1 after 500 cycles at 1C). This was attributed to the tunable hierarchical porous microstructure of the CA/GNs0.1 sample. Unfortunately, when increased the S content to 68.39 wt %, the electrochemical performance deteriorated in all aspects because of their sluggish reaction kinetics. These all results suggested that introducing GNs can feasibly enhance the performance of CA as matrix in Li-S batteries.

Nitrogen and sulfur dual-doped graphene (NSG) synthesized by a simple and low-cost one-pot microwave assisted method is applied to immobilize sulfur by the mean of an in situ sulfur deposition approach. Images of scanning electron microscope (SEM) and transmission electron microscope (TEM) show that nano-sulfur (2–20 nm) is wrapped by the transparent NSG nanosheets with wrinkled and folded features. The application of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed the structure and composition of the obtained nitrogen and sulfur dual-doped graphene/sulfur (NSG/S) composites as well. The heteroatom doping content was controlled by adjusting the amount of thiourea. The XPS analysis results indicate that pyridinic-N and pyrrolic-N are dominant among the product, thereby improving the cycle life of the lithium sulfur batteries. The NSG-4/S composites with sulfur loading of 68 wt% exhibits a remarkably higher reversible specific capacity of 1583 mAh g−1, impressive rate capability, splendid specific discharge capacity of 819 mAh g−1 at 0.1 C after 100 cycles and long cycle stability of 640 mAh g−1 after 200 cycles at 1 C.

In this work, graphene quantum dots/sulfur (GQDs/S) composite was prepared as cathode material for lithium sulfur (Li-S) batteries by a feasible chemical method. Multiple polar functional groups in GQDs can provide more active absorption sites to significantly strengthen the chemical absorption with polysulfides, suppressing the ‘shuttle effect’. In addition, the highly dispersed GQDs particles in the solution served as nucleation sites, the dissolved sulfur are reprecipitated around the GQDs, forming a dendritic interlaced network structure to prevent the aggregation of the sulfur, and further enhance the utilization of active substances. A high initial discharge capacity of 1125 mA h g−1 at 0.1C has been obtained in the GQDs/S cell, which is much higher than the initial discharge capacity of 658 mA h g−1 of GQDs/SR prepared by physical melting method. The superior performance of GQDs/S is attributed to the unique dendritic interlaced framework and multiple polar functional groups. The strong chemical effects between polar functional groups and polysulfides have been confirmed by the absorption experiment and DFT calculation. This work develops a new strategy in cathode material design for Li-S batteries.