Core-shell Composite With Hierarchical Porous Morphology For The Cathode Materials Of Lithium Sulfur Battery

Sur^fur(S)is naturally abundant,cheap and environmentally friendly.Lithium sulfur(Li-S)battery has attracted considerable attention in the field of liuthum secondary battery due to its high specifical capacity(1675 mAh·g-1)and energy density(2600 Wh kg'1).However,challenges are still existing in its large-scale cormmercialization,such as the insulation of S and its discharged products,high dissolution of long-chain polysulfides in electrolyte and large volumetric change.Recently,various conducting materials,such as carbon materials(hollow carbon spheres,carbon nano tube,graphene and porous carbon),conductive polymers and metal oxides were used to encapsulate S particles so as to improve the electrochemical conductivity,which greatly contributed to the utilization of active substance.Aiming at improving discharge capacity and cycling performance,this paper synthesized a series of core-shell composites with hierarchical porous morphology for the cathode material of Li-S battery.The main resear-ch contents were listed as follows:(1)Series of biomass-derived carbon materials were synthesized by using KOH actvating method,in which biomass materials such as 1 D tubulose catkins,2 D layered cherry blossoms,3 D blocky bamboo joints were adopted as carbon precursors.The morphologies,speci.fic surface area and pore size ditribution were significantly changed resultin g from different KOH actvatin g numbers.The above-mentioned porous carbon materials were conbined with S and prepared as cathode mater-ials for Li-S battery.The results demonstrated that 1 D tubulose biomass materials with thin cytoderm were appealed to KOH activation for only once,which could protect the carbonaceous skeleton from collapsion,forming a hollow tubular structure with narrow and porous ends.In terms of 2 D layered and 3 D blocky biomass materials were supposed to adopt KOH activation for twice because adequate KOH activation assisted in further pores formation,which greatly improved specific surface area and pore size,forming different hierarchical porous morphologies.Especially,TBC possessed well-developed interconnected micro-mecoporous morphology,which showed high specific surface area(1852.64 m2· g-1)and pore size(1.69 cm3·g-1).TBC/S exhibited good electrochemical performances(initial discharge capacity was of 898.3 mAh·g-1,which remained 448.8 mAh ·g-1 after 100 cycles).(2)A core-shell composite was prepared by adopting BDC/S composite as core and flexible PANI coating as shell.In this part,HCl concentration,ratio of oxidant and aniline,the mass ratio of aniline were respectively studied for the purpose of obtaining an optimal synthesis parameter.The composite materials were further prepared as cathode materials.The results showed that the thin and uniform coating was obtained when HCl concentration,ratio of oxidant and aniline and mass ratio of aniline reached to 1M,1:1 and 20%.As a result,a uniform PANI coating warpped on the surface of BDC with the thickness of 20?30 nm.PANI coating provided both an integrated conducting framework with both physicial confinement and strong chemical adsorption,which restained polysulfides dissolution.PANI@BDC/S-20%displayed high initial discharge capacity(1463 mAh·g-1 at 0.1 C),remarkable long-term cycling performance(366 mAh·g-1 after 500 cycles at 0.5 C)and great rate performance(527 mAh ·g-1 at 2 C).(3)To further improve the cycling and rate performance of this kind of core-shell composite,polar sulfonic acid groups were adopted on PANI coating to synthesize sulfonated polyaniline(SPANI).Used as cathode materials for Li-S battery,SPANI@BDC/S displayed high initial discharge capacity(1484 mAh ·g-1),remarkable cycling performance(853 mAh ·g-1 after 100 cycles at 0.1 C)and excellent rate performance(810.2 mAh ·g-1 at 2 C,682.7 mAh ·g-1 at 2 C and 353.0 mAh ·g-1 at 2 C).The result demonstrated that,as compared with PANI,SPANI possessed the synergistic effects of physicial confinement and stronger chemical adsorption,presenting outstanding electronic and ionic dual conductivity.After that,DFT simulation was further conducted,which confirmed the SPANI possessed higher adsorption to both LiS· and Li2S,and provided fast Li ion transportation.