Preparation Of Biomass-derived Graded Porous Carbon And Its Application In Lithium-sulfur Batteries

At present,with the more and more prominent problems of the energy crisis,the exploration on clean and sustainable energy has gradually become a research hotspot.As a kind of promising energy source,new energy storage system has been applied to all aspects of human life.Due to the high theoretical energy density(2600 Wh/kg),lithium-sulfur batteries have become one of next-generation energy storage systems.In addition,there are merits of elemental sulfur such as abundant resources,economic,and environment-friendly.Considering the above advantages,current researches on lithium-sulfur batteries have attracted extensive attention.However,there are still some challenges hindering the practical application of lithium sulfur batteries,such as the lower conductivity of sulfur and its discharge products,loss of active substances due to the shuttle effect of polysulfides,and volume expansion of electrodes during charging and discharging.In this thesis,above drawbacks are solved by conducting the carbon/sulfur cathode composite materials and carbon-based separator barrier layers.The research work mainly includes following aspects:(1)Orange peel,pomegranate peel,eucommia slag and peanut shell were collected as biomass precursors.Porous carbon materials were prepared by KOH activation method and then porous carbon/sulfur composites were obtained by physical method.Among them,the orange peel-derived carbon of PC-4 with the larger specific surface area(1895.5 m2/g)can increase the accessible area between active substances and electrolytes,and peanut shell-derived carbon of HC-4 with the larger average pore size(2.81 nm)is conducive to the transport of lithium ions.The large specific surface area and pore size of porous carbon can improve the electrochemical performance of lithium-sulfur batteries.The reversible capacity of the PC-4/S and HC-4/S composites as cathode materials was maintained at 306.4 and 364.7 mAh/g after cycling 100 times at current density of 0.1 C,respectively,both showing good electrochemical performance.(2)The porous carbon materials with large specific surface and graded pore structure were synthesized by peanut shell as a precursor,KOH as the main activator:and the KCl,NaCl,MgCl2 and CaCl2 as different auxiliary activators.Among the obtained porous carbon,the addition of 20%KCl assisted activation of HCK-20 possesses a large specific surface area(2199.8 m2/g),and the large number of mesopore,which can alleviate the volume expansion of the materials during the charging and discharging process,and the appropriate amount of micropores can also absorb polysulfides and inhibit the shuttle effect.The initial discharge specific capacity of HCK-20/S composite material reached 856.8 mAh/g,the specific discharge capacity was 525.9 mAh/g after 100 cycles at 0.1 C,and the capacity attenuation per cycle was only 0.30%,showing good performance cycle performance.(3)The graded porous carbon and binder were mixed in NMP solution,and the barrier layer was prepared by roll coating.In addition,the good conductivity of the barrier layer reduced the interface resistance of the positive side of the separator,which maked the transmission speed of lithium ion and electron faster.Among the samples,as HCK-20 the barrier material prepared with HCK-20GM has good conductivity(0.085 S/cm).The initial discharged specific capacity of HCK-20GM as the barrier layer reaches 1190.0 mAh/g,and maintained 277.2 mAh/g after 500 long cycles at 0.1 C,and the decayed rate of the capacity per cycle is only 0.15%,showed good long-cycle stability.