Preparation Of Nitrogen-doped Porous Carbon Frameworks And Their Application In Lithium-sulfur Batteries

Sulfur（S）is considered to be a promising cathode material because of its high theoretical capacity,low price,abundant reserves and environmental friendliness.However,the development of S cathode materials is limited by its own poor electrical conductivity,severe volume expansion during discharge,serious lithium polysulfide shuttle and lithium dendrite problems.In particular,the generation of lithium dendrites will cause serious safety hazards and hinder the commercial application of S cathode materials.The most effective way to solve the safety hazards caused by lithium dendrites is to replace the non-lithium cathode S with a lithium-rich cathode lithium sulfide（Li₂S）,which can avoid the dependence on the metal lithium anode and realize the assembly of high-safety batteries.However,the improved electrochemical performance of Li₂S is limited by the low electronic conductivity of Li₂S and the severe shuttle of lithium polysulfides.To solve the above problems,the most effective strategy is to use heteroatom-doped carbon materials to combine with Li₂S,thereby improving the electrochemical performance of Li₂S.The introduced heteroatom-doped carbon materials can not only improve the electronic conductivity of Li₂S,but also realize the chemisorption of lithium polysulfides.However,heteroatom-doped carbon materials have the disadvantages of cumbersome preparation method,uncontrollable product structure and morphology,and expensive raw materials,which hinder their large-scale application in Li₂S modification.In view of the above problems,this paper proposes to prepare nitrogen-doped porous carbon frameworks by calcination and etching methods using melamine and yeast as carbon sources.In order to further prove the role of nitrogen-doped porous carbon framework,this paper realizes its recombination with Li₂S by liquid pervaporation method,thereby obtaining Li₂S/nitrogen-doped porous carbon cathode material with excellent electrochemical performance.The main research contents of this paper are as follows:（1）Two-dimensional porous carbon nitride（p-C₃N₄）was prepared via high-temperature calcination and oxidative etching by using melamine as a precursor.Then,Li₂S was composited with highly conductive carbon nanotubes（CNTs）and strongly adsorbed p-C₃N₄ by liquid phase pervaporation to obtain a three-dimensional Li₂S/porous carbon nitride/carbon nanotube composite（3D Li₂S/p-C₃N₄/CNT）.In this paper,the relationship between the microscopic morphology,electrochemical performance and reaction kinetics in 3D Li₂S/p-C₃N₄/CNT composites was systematically investigated through modern characterization techniques.The microscopic results show that the 3D Li₂S/p-C₃N₄/CNT composite exhibits a unique three-dimensional structure.The micro-nano-sized Li₂S（30-200 nm）is successfully filled in the holes and gaps of the 3D p-C₃N₄/CNT framework.The unique three-dimensional structure and micro-nano size have the following two advantages:on the one hand,the 3D p-C₃N₄/CNT framework can not only storage Li₂S,but also improve the conductivity of Li₂S and inhibit the shuttle of lithium polysulfide.on the other hand,the micro-nano size Li₂S is beneficial to reduce the transport distance of lithium ions,thereby improving the reaction kinetics of Li₂S.The unique three-dimensional structure and micro-nano size characteristics of 3D Li₂S/p-C₃N₄/CNT provide the structural basis for its excellent electrochemical performance.Systematic electrochemical tests show that the 3D Li₂S/p-C₃N₄/CNT exhibits ultra-high initial discharge capacity(997.5 m A h g^-1 at 0.2 A g^–1),ultra-long cycle life（≥600 times）and excellent rate capability(500.9 m A h g^-1 at 2.0 A g^–1).（2）A green method for preparing nitrogen-doped porous carbon microspheres（NPCMs）is presented.The NPCMs is prepared by high temperature carbonization and chemical etching of yeast.The obtained NPCMs exhibit ultra-high specific surface area(2005.6 m² g^-1)and hierarchical pore structure.Then,the liquid phase pervaporation method is used to composite of NPCMs and Li₂S,thereby obtaining the NPCMs-Li₂S composite material.Microstructural characterization shows that NPCMs-Li₂S has a typical carbon microsphere structure,and Li₂S is filled in the hierarchical pores of NPCMs.This structure exhibits the following three advantages.First,porous carbon microsphere structure can effectively improve electrode structure stability and cycle life.Second,the hierarchical pores in NPCMs provide fast channels for electron transfer,thereby improving rate performance.Third,rich polar surface of NPCMs is conducive to the strong adsorption of lithium polysulfides,which is beneficial to improve coulomb efficiency.Based on the above advantages,the NPCMs-Li₂S exhibits an ultra-high initial discharge capacity(1077 m A h g^-1),ultra-long cycle life（≥500 times）and excellent rate capability(362 mA h g^-1 at 2.0 A g-1).