The High Value-added Synthesis Of Biomass Based Nitrogen Doped Hierachical Carbons And Their Application As Li-S Battery Electrode

Porous carbon with large surface area,suitable pore and high electrical/thermal conductivity are used widely in the electrochemistry fields.In this thesis,a series of porous carbons have been successfully produced by hard template combined with chemical activation using dopamine and prawn shells as the carbon source.Such carbons were used for the preparation of carbon-sulfur composites by the melt diffusion method.The morphologies and structures of the products were investigated using several techniques such as nitrogen adsorption-desorption,transmission electron microscope and X-ray diffraction.Their application as Li-S battery has been studied in detail.The main contents and results are as follows:Nitrogen-doped hierarchical porous carbon shell?HPCs?has been prepared with surface area of 365 m2 g^-1 and pore volume of 1.445 cm3 g^-1 by using polystyrene microspheres as the template and dopamine as both the nitrogen source and carbon precursor.The pore sizes of the obtained HPCs are centered at 1.4?3.0 and 200 nm.Polyaniline?PANI?coating HPCs effectively improves the conductivity of sulfur composites?HPCs/S?.The HPCs/S sample shows the capacity of 1066 mAh g^-1 at the current density of 100 mA g^-1 with the sulfur content of 60%,due to its nitrogen doping induced chemical adsorption of sulfur on the surface areas of the porous carbons.PANI can help to improve the conductivity of the sample,which reaches 0.325 S m^-1 for HPCs/S-60 and 0.433 S m^-1 for HPCs/S/PANI.The capacity and cycle stability of HPCs/S/PANI are higher than the HPCs/S,indicating polyaniline-coated porous carbon material has improved the stability of the composites.Nitrogen-rich porous "green carbons" derived from abundant shrimp shell shows good performance for Li-S batteries.The strategy in this work is highlighted to selective removal of intrinsic CaCO3 in shrimp shell followed by KOH activation to tune the pore sizes of the obtained carbons.The pore sizes of microporous carbon,micro-mesoporous carbon,mesoporous carbon are centered at 0.53?0.84?3.88?5.12 nm,pore volumes are 0.822?0.853?1.025 cm3 g^-1,and their nitrogen contents are 3.63?2.58?6.67%,respectively.On the basis of the different porous structures,the discharge capacity of the obtained carbons as Li-S cathodes follows the order of micro-mesoporous carbon(920 mAh g^-1)>mesoporous carbon(745 mAh g^-1)>microporous carbon(738 mAh g^-1).The high capacity of the micro-mesoporous carbon is attributed to its positive characters such as the coexistence of micro-mesoporous structure,the large pore volume and the high specific surface area.Furthermore,well-dispersed nitrogen in the porous carbons is naturally doped and inherited from shrimp shell,and can help to enhance cycle stability when used as cathodes.As a result,all carbon cathodes exhibit the good cycle stability?>78%?due to their nitrogen doping induced chemical adsorption of sulfur on the surface areas of the porous carbons.Among them,mesoporous carbon cathode shows the best cycle stability with 90%retention within 100 cycles,which is mainly attributed to the synergistic effects of its both large pore size and high nitrogen content.