Synthesis Of Oxide-Modified And Nitrogen-Doped Biochar And The Application In Lithium-Sulfur Batteries

Lithium-sulfur batteries have attracted much attention recently due to their high theoretical capacity,energy density,abundant resources and cost effectiveness.However,sulfur cathodes suffer from low conductivity,shuttle effect,uncontrollable deposition of lithium sulfides species and the volume expansion of sulfur,which result in rapid capacity fading and low Coulombic efficiency.To solve these problems,in this text,we designed the nanostructure for sulfur cathodes,and fabricated AZO@C composites and nitrogen-doped biochar using kapok fibers and egg white as both templates and carbon source.Then we studied their phase composition,microstructure and electrochemical performance.The main contents are as follows:In Chapter Three,we report the synthesis of conductive aluminum-doped zinc oxide(AZO)nanoparticles decorated carbon nanoflakes(AZO@C)through a facile biotemplating method using Kapok fibers(KFs)as both the template and carbon source.The microstructure and morphology of the AZO@C composites are characterized by XRD,XPS,SEM and TEM et al.The results indicate that the composites are successfully fabricated with good crystallinity,and the size of the AZO nanoparticles are ranging from 15 nm to 30 nm.In Chapter Four,we fabricate AZO@C/S cathode by combining AZO@C composites with sublimed sulfur.Sulfur cathode based on AZO@C shows better electrochemical performance than those of cathodes based on ZnO and Al2O3 with poor conductivity,with a stable capacity of 927 mAh g-1 at 0.1C after 100 cycles.Even when the current density increase to 0.5C,a reversible capacity of 544 mAh g-1 after 300 cycles can be obtained with the capacity decay per cycle of 0.039% under the sulfur loading of 3.3 mg cm-2.A capacity of 466 mAh g-1 can still be obtainedafter 100 cycles at 0.5C when the sulfur loading increases to 6.96 mg cm-2.The excellent electrochemical performance of the AZO@C/S composite can be attributed to its high conductivity of polar host AZO,which restrains shuttle effect and improves the redox kinetics in the transition of lithium sulfide species at the same time.In Chapter Five,we fabricate Nitrogen-Doped Biochar(N-C)using egg white as both the nitrogen source and carbon source.The results of SEM and BET show that the carbon materials possess good pore structure.When applied in the Lithium-sulfur batteries,the sulfur cathode based on N-C shows a stable capacity of nearly 700 mAh g-1 at 0.1C after 200 cycles with a capacity retention of 84%,much better than that of cathodes based on ordinary activated carbon.The elevated electrochemical performance can be attributed to the formation of the active cites in carbon materials causing by the doping of nitrogen atom,which can adsorb lithium sulfide species effectively and restrain their dissolution.