Hierarchical Porous Carbon Materials:Preparation And Their Application For Lithium-Sulfur Battery Cathodes

Lithium-ion batteries(LIBs)began commercially in the late 1990s and have been widely used in all aspects of our daily life at present.In terms of new energy vehicles,the demand for LIBs is increasing year by year.However,due to the limitation of its low energy density,LIBs cannot meet the growing demands of the market.Lithium-sulfur battery becomes one of the most potential secondary battery systems according to the higher theoretical specific capacity(1675 mAh g-1)and energy density(2500 wh kg-1).In addition,elemental sulfur has advantages of non-toxicity and eco-friendliness.Despite of these strengths,several challenges remain about the cathodes that inhibit it from practical applications,such as poor electrical conductivity,shuttle effect and volume expansion.To solve the above issues,this thesis carried out research work from the perspective of the design of cathode materials.We prepared a variety of hierarchical porous carbon/sulfur compounds and tested their electrochemical performance.In a word,this dissertation mainly deals with two aspects,shown as below:(1)We prepared in-situ nitrogen and sulfur co-doped few-layer graphene(denoted as N,S-FLG)by a simple calcination method.The graphitic C3N4 as template was prefabricated for the synthesis of porous few-layer graphene,during which N 'and S atoms were simultaneously doped with dithiooxamide as doping reagent.Moreover,the calcination temperature was found to exert vital influence on the specific surface area and pore volume of final graphene.At the calcination temperature of 900 ?under argon atmosphere,the resultant N,S-FLG/S composite acquired the best electrochemical performance.N,S-FLG900/S can maintain a reversible capacity of 602 mAh g-1 after 300 cycles at a current density of 0.8 A g-1 and 506.4 mAh g-1 after 500 cycles at a current density of 1.6 A g-1.As a result,the synergy of heteroatom doping,large specific surface area(460.9 m2 g-1)and the high pore volume(1.31 cm3 g-1)of graphene contributes to the improvement of Li-S battery performance.(2)We extracted polysaccharide from loquat leaves by water extraction and alcohol precipitate method.Followed by the calcination and hydrochloric acid washing,hierarchical porous carbon was prepared as a matrix for sulfur cathode.Subsequently,we tested the electrochemical performance of related cells.HPCM/S can deliver a reversible capacity of 603.5 mAh g-1 at a current density of 0.8 A g-1 after 300 cycles and 485.4 mAh g-1 at a current density of 1.6 A g-1 after 500 cycles.In a word,large specific surface area and hierarchical pores contributed to the excellent electrochemical performance.First of all,large specific surface area helps the uniform dispersion of sulfur in the carrier material.In addition,hierarchical porous structure can effectively alleviate the volume change to ensure the stability of the electrode in the process of charging and discharging.