Preparation And Properties Of Lithium-Sulfur Batteries Cathode By Nucleophilic Ring-Opening Polymerization Of Thiolactone

Lithium-sulfur batteries are considered as one of the most potential secondary battery systems because of its ultra-high theoretical specific capacity(1 675 mAh·g^-1)and energy density(2 600 Wh·kg^-1).Meanwhile,cathode sulfur materials also have many advantages,such as high natural abundance,low cost,non-toxic and environmentally friendly.However,the practical application of lithium-sulfur batteries is hindered and limited by the problems of low conductivity of sulfur,volume expansion during cycling,and the"shuttle effect"of lithium polysulfide.In order to solve the above series of problems,rational design and development of excellent sulfur cathode carriers is regarded as an effective way.In this thesis,a 3D carbon nanotube cross-linked network with sulfhydryl groups was prepared by nucleophilic ring-opening polymerization.Sulfur-containing polymers formed by anchoring elemental sulfur with-SH are used as cathodes for lithium-sulfur batteries and have the following advantages:the stable crosslinking network can play a synergistic effect of physical constraint and chemical adsorption on polysulfides;the sulfhydryl anchored sulfur on the polymer surface forms strong S-S covalent bonds,which improves the utilization of active substances;long-chain polysulfides can be converted into short-chain Li₂S/Li₂S₂through thiol anchoring,thereby inhibiting the shuttle effect caused by soluble polysulfides;conductive carbon nanotubes provide pathway for fast electron transport;the reaction process has the characteristics of high efficiency,rapidity,no catalyst and no by-products.The specific research contents and results of this paper are as follows:（1）N,N’-bis（2-oxothiolan-3-yl）oxamide（BOTO）was generated by the reaction of DL-homocysteine thiolactone hydrochloride and oxalyl chloride.Thiol-containing three-dimensional carbon nanotube cross-linked network（TLP@CNT）was successfully prepared by the in-situ nucleophilic ring-opening polymerization of BOTO and amino groups,which was used as the cathode of lithium-sulfur batteries after melt impregnation with sulfur.By controlling the amount of BOTO,three different cathodes were prepared,and the charge-discharge cycle stability was improved.Among them,the 2-TLP@CNT/S cathode can still retain a specific capacity of 647 mAh·g^-1after 500 cycles at a high rate of1 C.（2）Three different aromatic thiolactone materials containing imide groups（BPDA-T,ODPA-T,BTDA-T）were designed to form a three-dimensional cross-linked network with amino ring-opening polymerization modified carbon nanotubes（BPTP@CNT,ODTP@CNT,BTTP@CNT）,and then melted and impregnated sulfur to obtain multifunctional lithium-sulfur batteries cathode materials.Such cathode materials can not only form S-S bonds with sulfur to anchor sulfur,but also theπelectrons of this conjugated aromatic copolymer can also facilitate rapid electron transfer.Among them,the BTTP@CNT/S cathode can still maintain a specific discharge capacity of 799 m Ah·g^-1after 500 cycles at 1 C.