Preparation Of Surface Doped Porous Carbon Materials And Its Application In High Performance Lithium Sulfur Batteries

With the development of modern society,energy crisis problem become more seriously day by day along with the traditional fossil energy consumption,the rational utilization of new energy has attacted much attention.At present,the research direction of new energy mainly includessolar energy,tidal energy,wind energy,geothermal energy,etc.Although they have made great achievements,the new energy is intermittent,which limits their practical application.The solution is stored the energy in the efficient energy storage systems,and therefore the research and development on electrochemical energy storage system is imperative.At the same time,the rapid development of electric and electronic equipment has also increased the demand for high-performance energy storage system.Some energy stored system like lithium ion batteries have been developed successfully.Although the laboratorial specific energy density of lithium ion battery has reached 250 Wh kg-1,it subjects to the cathode material that the specific energy cannot be improved greatly,and increasing the charging voltage to obtain higher energy will lead to serious security issues.Thus exploiting new battery system is imminent.Among these battery systems,lithium sulfur batteries have attracted more and more attention because of its high theoretic specific energy density as 2600 Wh kg-1 and theoretical specific capacity of 1675 mAh g-1 which is greater than the present commercialization secondary battery.In addition,the active material elemental sulfur is abundant in nature,cheap and environmental friendly which will be conducive to its commercialization.The lithium sulfur battery has a good prospect in the areas about the new type of energy storage devices,electronics equipment,hybrid power cars and other fields.In this thesis,we successfully prepared a three-dimensional porous composite material which can find application in the lithium sulfur battery,and the research content is as follows:(1)The preparation and studying of the nitrogen boron co-doped porous carbon materials.A boron-doped porous carbonmaterial with a termite nest shape(TNPBC)was obtained from a new carbon source,polyaspartic acid,and borax.Importantly,the doping,activation,and pyrolysis were integrated into one step through a low cost and simple methodology.The borax was essential to formation of a high surface porous architecture and provided boron dopants,which,combined with polyaspartic acid,achieves co-doping(B and N)carbon materials with special porous structures.The simultaneous pore-formation and doping leaves an abundance of hetero-atoms exposed on the surface of pores,which enhances the electrostatic interactions between the hetero-atoms and the charged species in the batteries.As a result,the S/TNPBC cathode maintains a stable capacity of 703 mAh g-1 with an excellent Coulombic efficiency of 101.3% after 120 cycles at 0.1 C.Moreover,it exhibits an excellent rate capability with an initial capacity of 650 mA h g-1 at 0.5 C and sustains a capacity of 500 mAh g-1 after 100 cycles.(2)Introducing a carbon interlayer to optimize the cell structure which will improve the comprehensive performance of lithium sulfur battery.Compared to the composite cathode,carbon layer exists as an independent unit which canalleviate the "shuttleeffect" of lithium sulfur batteries better as the barrierand avoid structure collapse causing by volume effect during the cycling process.We place the boron-doped porous carbonmaterialon the membrane uniformlyvia the spraying method,and therebyit improves the overall stability of the battery.Porous characteristics can facilitate the infiltration of electrolyte in the battery.It can reduce the internal resistance of system while maintaining the own physical obstacles and chemical adsorption function.In this chapter,we place the conductive carbon layer between the membrane and the cathode which significantly improves the cycling and rating performance of the battery.As a result,the layer-systemshows a high initial specific discharge capacity of 1400 mAh g-1 and maintains a stable capacity of 800 mAh g-1 with a stable Coulombic efficiency around 100% after 250 cycles at 0.1 C.When the rate enlarges to 1 C,it exhibits a stable capacity of 750 mAh g-1after 25 cycles and sustains a capacity of700 mAh g-1 after 400 cycles.These studies promoted the comprehensive performance of the battery as expected which can provide some new train of thoughts for further optimization.