Sulfur quantum dots wrapped by conductive polymer shell with internal void spaces for high-performance lithium–sulfur batteries

Abstract

Lithium–sulfur batteries are promising candidates for the next generation of energy storage systems due to the high specific capacity of the sulfur cathode (1675 mA h g⁻¹) and their low cost. However, the intrinsic insulating properties of sulfur, the dissolution of polysulfides, and the huge volume expansion during cycling still hinder their practical application. We introduced the electroactive polymer poly(N-vinylcarbazole) (PVK) into the lithium–sulfur system as a conductive matrix and sulfur reservoir. Using a facile two-step dissolution–precipitation treatment, novel core–shell sulfur quantum dot/PVK (SQD/PVK) nanocomposites were synthesized, in which a large number of SQDs (about 5 nm in size) with plenty of internal void spaces were encapsulated in the PVK shell. The sulfur core consisted of uniformly dispersed SQDs and large void spaces, which formed effective transportation pathways for both electrons and ions among the SQDs. They also acted as a buffer zone to accommodate the volume expansion during cycling and facilitated wetting of the electrolyte. The conducting PVK shell coated on the surface of the sulfur core can restrain dissolution of the polysulfide and suppress the shuttle effect. Galvanostatic testing showed that this SQD/PVK nanocomposite maintained a specific capacity of 687.7 mA h g⁻¹ after 200 cycles at 0.5 C, corresponding to an 89.7% capacity retention with only 0.05% capacity degradation per cycle. Even after long-term cycling, the electrode could still deliver 488.6 mA h g⁻¹ at a rate of 0.5 C after 600 cycles and 443.9 mA h g⁻¹ at 0.75 C after 500 cycles.