Preparation,Modification And Application Of Polymer-derived Nanostructured Carbon Materials In Lithium Sulfur Batteries

Porous carbon is an important sulfur host material,and its structure and properties play a key role in the performance optimization of lithium-sulfur batteries.Currently,the design and synthesis of porous carbon with high electrical conductivity and rich pore structure are mostly performed by template method and biomass activation.However,the unfavorable factors such as uncontrollable conditions,time consuming and tedious process in its preparation limit its wide application.To address the above problems,this thesis provides a green,simple and template-free method to synthesize carbon nanosheets and porous carbon foam.The nanosheet thickness,pore structure and heteroatom doping were regulated using small molecules to yield hierarchical porous structured carbon materials for lithium-sulfur battery cathode,which exhibits excellent electrochemical properties.This method avoids several defects which induced by the traditional template method or using environmentally unfriendly reagents.The research is as follows:（1）Ultra-thin porous carbon nanosheets（U-PCNS）with adjustable thickness were synthesized from sodium polyacrylate（PAAS）and water.The relationship between the concentration of PAAS solution and the thickness,pore structure of U-PCNS was investigated in detail to elucidate the possible mechanism of structure formation.The electrochemical performance of U-PCNS as a sulfur host material for lithium-sulfur batteries was explored,and the correlations between the thickness,pore structure of carbon nanosheets and lithium ion diffusion,polysulfide ion adsorption,and lithium dendrite formation were investigated in depth to reveal the relationship between structure and performance.The optimized battery based on the U-PCNS/S cathode exhibits a specific capacity of1245.2 mAh g^-1 at 0.2 C and excellent stability（0.042%per cycle decay for 1000 cycles）.（2）Hierarchical porous phosphorus-doped porous carbon foam（PCF）was synthesized by freeze-drying and heat treatment using PAAS as carbon precursor and sodium hexametaphosphate as phosphorus source and pore-forming agent,and used as sulfur host for lithium-sulfur battery cathode.The effects of sodium hexametaphosphate amount and carbonization temperature on carbon structure and heteroatom doping were investigated.The effects of PCF structure and phosphorus doping on the lithium ion diffusion,polysulfide ion adsorption,sulfur reaction kinetics,and lithium dendrite formation were explored to elucidate the relationship between structure and performance.The specific capacity(1452.3 mAh g^-1 at 0.2 C)and cycle stability（0.039%per cycle decay for 1500 cycles）of the lithium-sulfur battery were achieved by pore-making and phosphorus doping modification of the carbon sheet.When the area density was 4.48 mg cm^-2,the area capacity of 5.09 mAh cm^-2 at 0.05 C was achieved.