The Design And Preparation Of Metal Nitrides And The Research Of Energy Storage Performance In Lithium-Sulfur Batteries

Electrochemical energy storage system is an important part of smart grid,consumer electronics,and the upcoming global energy Internet.The increasing demand for sustainable energy supply requires electrochemical energy storage devices with high performance,high energy density and low cost.The lithium-sulfur battery based on electrochemical redox conversion mechanism is considered as an advanced battery system with great application prospect,and its theoretical energy density is 2600 Wh kg^-1.After more than ten years of in-depth development,the problems faced by lithium-sulfur batteries,such as poor conductivity of sulfur,polysulfide shuttle effect and volume expansion,have been initially solved.Although using light carbon-based materials as sulfur-limiting carriers can effectively increase the area load of sulfur,the volume energy density of sulfur electrode is low due to the low tap density of carbon-based materials,which hinders the practical application of lithium-sulfur batteries.Therefore,the preparation of high-density sulfur electrode is very important.Some high-density non-carbon materials,such as metal oxides or sulfides,can increase the bulk density of sulfur electrodes,but their low conductivity will hinder the electron transmission and ion diffusion rate.Metal nitride has high conductivity and excellent catalytic activity,which can reduce the reaction barrier,speed up the reaction rate and improve the utilization rate of active substance sulfur.At the same time,the enhanced polarity of metal nitride can effectively inhibit the shuttle effect of polysulfide.Therefore,the design and preparation of high-efficiency sulfur-limiting carrier of metal nitride is of great significance for the practical application of lithium-sulfur batteries.Based on the advantages of the above metal nitrides,two kinds of metal nitride-based composite materials,carbon/vanadium nitride（C/VN）and titanium nitride/carbon fiber cloth（Ti N/CC）,were designed as positive limit sulfur carriers for lithium-sulfur batteries.The specific research results are as follows:（1）C/VN nanobelt composites were successfully prepared by hydrothermal-in situ growth-nitridation strategy.The excellent catalytic activity of vanadium nitride effectively promotes the conversion of lithium polysulfide,and the enhanced polarity effectively inhibits the transition dissolution of polysulfide in electrolyte.At the same time,the carbon layer carbonized by ZIF-8provides a large specific surface area and a large number of pores,so that the active substance sulfur can be uniformly confined in these pores.The strong polar chemical bond of VN effectively inhibited the excessive dissolution of lithium polysulfide,and its excellent conductivity and catalytic activity promoted the conversion between lithium sulfide and soluble lithium polysulfide.At a current density of 0.5 C,the discharge specific capacity of S@C/VN anode can reach 413.9m Ah g^-1 after 500 cycles.Under the current density of 1 C,the discharge specific capacity of S@C/VN anode remained at 418.6 m Ah g^-1 after 200 cycles.（2）Using solvothermal-nitridation strategy,flexible self-supporting Ti N/CC composites were successfully prepared on carbon fiber cloth by adjusting solvothermal time and nitridation temperature.Ti O₂/CC was prepared by solvothermal reaction at 180℃for 45 min,and then nitrided at 900℃for 5 hours.Ti N/CC loaded with sulfur showed the best performance.The conductive network provided by Ti N array and carbon fiber cloth greatly improves the conductivity of cathode materials.At the same time,thanks to the three-dimensional structure of Ti N/CC and the enhanced polarity of Ti N,the physical and chemical double adsorption of polysulfide is realized,and the shuttle effect of polysulfide is effectively inhibited.At a high current density of 3 C,the S@Ti N/CC cathode material showed an initial discharge specific capacity of 1087 m Ah g^-1,and the discharge specific capacity remained at 604.1 m Ah g^-1 after 250 cycles.At the same time,the battery also showed excellent rate performance.Under the current density of 5 C,the discharge specific capacity was 666.6 m Ah g^-1,and the coulombic efficiency was close to 100%.