Nanostructured Sulfur and Composites for Lithium-Sulfur Batteries

Lithium/sulfur (Li/S) batteries have attracted intense attention, because it has a great potential to provide high energy density storage for next generation power storage systems. Li/S batteries have a theoretical energy density of 2600 Wh kg^-1, which is almost three times the energy density of current lithium-ion batteries.¹ Sulfur cathode can deliver a high specific capacity of 1672 mAh g^-1, which is more than five times that of currently widely used LiCoO₂. Meanwhile, sulfur is abundant, low cost and environmentally benign. However, the insulating intrinsic of sulfur, the dissolution of polysulfides, shuttling of polysulfide between the negative and positive electrodes and huge volume expansion are still the main challenges that hinder the Li/S systems practical application.

    To address these issues, much effort on designing and constructing novel microstructured/nanostructured S cathode has been devoted to modifying the cathode materials. Significant advances have been achieved using carbon,² oxides,³ or conducting polymers,⁴as the hosts of S cathode.

   Herein, new nanostructured S cathodes were reported to address the problems of Li–S battery^5-6. We introduce the electroactive polymer, poly (N-vinylcarbazole) (PVK) and graphene oxide, into the Li/S systems as a conductive matrix and reservoir of S. By a facile two-step dissolution-precipitation treatment, novel core-shell S quantum dots/PVK nanocomposites (SQD/PVK) are synthesized, which a large number of sulfur quantum dots (~5 nm) with plenty of internal void spaces are encapsulated in the PVK shell (the nanostructure is showing in Figure 1a). The S-core consisting of uniformly dispersed sulfur quantum dots, and large void spaces, which can form effective transportation pathway of both electrons and ions among these sulfur quantum dots, act as a buffer zone to accommodate the volume expansion during cycling and facilitate the electrolyte wetting. Meanwhile, the conducting PVK shell coated on the surface of S-core can restrain the polysulfide dissolution and suppress shuttle effect. Galvanostatic testing shows that this SQD/PVK nanocomposite could maintain a specific capacity 443.9 mAh g^-1at 0.75 C after 500 cycles (showing in Figure 1b).

    Following, graphene oxide is introduced to the S /PVK system to further improve the electrochemical performance of sulfur. The as-prepared micron-sized PVK/S@RGO composite containing 71 wt.% sulfur exhibits excellent cycling and rate properties. It could maintain a high discharge capacity of 517.6 mAh g^-1at 0.75 C after 400 cycles .

ACKNOWLEDGEMENTS

This work was supported by the Startup Foundation of China Academy of Engineering Physics, Institute of Chemical Materials (KJCX201301 and KJCX201306), National Natural Science Foundation of China (No. 21401177 and 51403193 ), the “1000 plan” from the Chinese Government, and the R&D Foundation of China Academy of Engineering Physics (2014B0302036).

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