Design Of Metal-nitrogen-carbon And Nitrogen-carbon Materials For Lithium-sulfur Batteries

With the development of electric vehicles and industrial equipment,modern industry has higher requirements for energy storage systems.The traditional energy storage system faces problems such as environmental pollution,decreasing reserves,and my country’s over-reliance on imports of traditional energy.Based on the current development of lithium batteries in new energy storage systems,our main batteries are ternary,lithium iron phosphate,and lithium cobalt oxide batteries.Compared with traditional batteries,lithium-sulfur batteries are considered to be one of the most promising next-generation energy storage systems due to their high theoretical energy density,low cost and abundant sulfur content,and environmental friendliness.However,the commercialization of lithium-sulfur batteries is hindered by several existing challenges,such as the insulating properties of active sulfur,the shuttle effect of intermediates,battery self-discharge,and lithium dendrites.Modifying layers by constructing functionalized membranes has been shown to be a feasible and effective way to alleviate the above problems.This thesis focuses on two aspects:On the one hand,we synthesized a nitrogen-rich porous polymer by Schiff base reaction,and prepared a nanoporous cobalt-nitrogen-carbon（Co-N-C）catalyst with a nitrogen-rich polymer precursor,anchored metal cobalt,and finally calcined at high temperature.It is composited with graphene,and the composite material is prepared as a modified separator for lithium-sulfur battery separator coating by vacuum filtration.The modified separator effectively improves the cycling performance(908 m Ah g^-1after 100 cycles at 0.2 C)and rate performance(703 m Ah g^-1at 2 C)of lithium-sulfur batteries.We investigated the mechanism of action of the modified layer of the metal-nitrogen-carbon separator through material characterization,battery testing and theoretical calculations.This is benefited from the abundant（5.51 wt%）and highly dispersed Co active sites that not only strongly bind lithium polysulfides,limiting the dissolution of lithium polysulfides,but also catalyze the acceleration of sulfur redox kinetics.The physical capture and immobilization of lithium polysulfides is achieved through the nanoporous structure and polar nitrogen-rich carbon surface.At the same time,the excellent electrical conductivity further improves the utilization of sulfur.On the other hand,we synthesized a highly nitrogen-doped carbon/graphene（NC/G）that integrates the three functions of high conductivity,strong adsorption and catalysis by a simple one-step method.Benefiting from the polar surface and porous structure of NC/G,it can chemically and physically adsorb lithium polysulfides,inhibit the shuttle effect of lithium polysulfides,accelerate the redox kinetics of lithium polysulfides,and accelerate lithium ion diffusion.Even under extremely low loading of 0.08 mg cm^-2,the NC/G-modified separator still maintains excellent battery cycling performance(831.6 m Ah g^-1after 100 cycles at 0.5 C rate).In terms of practical applications of lithium-sulfur batteries,this work describes the importance and necessity of low loading in practical applications in the separator modification layer of lithium-sulfur batteries.)on the lithium-sulfur battery separator,has three advantages of low cost,easy synthesis,lightness and thinness,these advantages are indispensable factors in the practical application process of the lithium-sulfur battery separator modification layer.