The Preparation And Properties Of Metal Doped Porous Carbon/sulfur-based Composite Cathode Materials For Lithium-sulfur Batteries

As a promising energy storage device, the lithium-sulfur battery shows excellent electrochemical performances including high specific capacity and energy density,which have been receiving considerable attention and have been investigated worldwide from the point of environmental issues and energy. Despite these advantages, the further practical applications are severely hindered by several issues:the insulating nature of elemental sulfur and its intermediate polysulfides; severe volumetric expansion during lithiation; the shuttling of polysulfides (Li2Sx,3?x?8)result in the loss of active material and lower the coulombic efficiency of lithium-sulfur battery. In order to solve the above problem, an extensive studies have been conducted, including: 1) the recombination of porous carbon materials and sulfur or lithium sulfide to improve the conductivity of the sulfur cathode and inhibit the sulfide dissolution at the same time; 2) the surface chemical modification and the PANI coating for carbon/sulfur cathode material, the modification of metal oxide and metal nitride to improve the polarity of cathode materials and form strong interaction which can effectively anchor sulfur particles and polysulfides. Base on the electrochemical performance of carbon/sulfur cathode materials, the impacts of different types of carbon/sulfur and different metal compound modified cathode materials on the electrochemical performance of battery further was studied. In this work, the main research contents include the following:(1) The preparation of 3D connected porous carbon/sulfur composite materials(CA-S) and the research of electrochemical performance. The FETEM and FESEM for microstructures of CA and CuS-n/CA-S exhibit a three-dimensional structure with high specific surface area and rich channels which improved the specific capacity of sulfur cathode. The BET surface areas of the CA and CuS-4/CA are 804 and 726 m2 g-1,respectively. In addition, abundant mesoporous and microporous can effectively inhibit the dissolution of polysulfides and improve the cycle stability of the carbon/sulfur cathode materials. Electrochemical research has shown that provides a good electrochemical performance, especially, in the current density of 0.2 C, carbon/sulfur electrode delivers an initial discharge capacity of 1059 mAh g"1 and remains 665 mAh g-1 after 100 cycles.(2) The preparation of a new type of CuS QDs modified carbon/sulfur(CuS-n/CA-S) and the research of electrochemical properties for the sulfur cathode.The well-distributed CuS quantum dots were prepared by a facile chemical method and were dispersed in the carbon channels. On the one hand, CuS quantum dots display good absorbability and anchoring function to polysulfide which effectively inhibit the dissolution of polysulfide and improve the cycle stability of the battery, on the other hand, the high specific surface can effectively facilitate electron and ion transfer.Based on the above advantages, CuS QDs modified cathode materials improve the electrochemical properties of electrodes. In the current density of 0.2 C, CuS QDs modified carbon/sulfur electrode delivers an initial discharge capacity of 1318 mAh g-1 and remains a reversible capacity of 1073 mAh g-1 after 100 cycles. Specially, the CuS QD modified composites remains 840 mAh g-1 after 500 cycles with only 0.05%decay per cycle at 0.5 C.