Nanostructured Sulfur Composites for Lithium-Sulfur Batteries
Authors: Cheng Jianli , Wang Bin 
Published: February 2020

Lithium-sulfur (Li-S) batteries are expected to an appealing candidate for the next generation of high-energy density batteries. Sulfur cathode can deliver a high specific capacity of 1672 mAh g-1, which is more than five times that of currently widely used LiCoO2.1 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.
An attractive approach to enhance the performance of Li/S batteries is the construction of novel microstructured/nanostructured S cathode, in which sulfur particles are embedded in conductive matrices. 2-4 Herein, both electroactive polymer, poly (N-vinylcarbazole) (PVK) and graphene oxide are introduced into the Li/S systems as a conductive matrix and reservoir of S to enhance the performance of Li/S batteries. Three new nanostructured S composites were reported to address the problems of Li–S battery.5-6

Firstly, Core–shell SQD/PVK nanocomposites were prepared via a facile two-step dissolution–precipitation treatment in which a large number of SQDs (about 5 nm in size) with plenty of internal void spaces were encapsulated in a PVK shell. 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. Meanwhile, the conducting PVK shell coated on the surface of S-core can restrain the polysulfide dissolution and suppress shuttle effect. A superior electrochemical performance was obtained using this core–shell nanocomposite as the cathode material in Li/S batteries.
Then, micron-sized sulfur and PVK composites with reduced graphene oxide wrapping (denoted as PVK/S@RGO) were synthesized by a facile Poly(N-vinylcarbazole)-assisted vibrating-emulsification method. The as-prepared micron-sized PVK/S@RGO composites containing 71 wt.% sulfur exhibit excellent cycling and rate properties with a high discharge capacity of 517.6 mAh g-1 at 0.75 C after 400 cycles .
Finally, free-standing PVK/S@RGO electrodes were prepared by inducing self-assembly of graphene sheets in the mixture of PVK and nano S particles. The 10um ultra-thin PVK/S@RGO electrodes displayed excellent cycling and rate properties with a high discharge capacity of 500 mAh g-1 at 1C after 500 cycles (Figure 1).
Acknowledgment
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, the “Sichuan young-talent” Foundation (NO. 2016JQ0025) and the R&D Foundation of China Academy of Engineering Physics (2014B0302036).

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Figure 1