Preparation,Modification And Electrochemical Performance Of Porous Carbon-based Cathode Materials For Lithium Sulfur Batteries

With the widespread use of portable electronic products?e.g.,mobile phones,camera,notebook computers et al.?,the advent of electric?hybrid?vehicles,as well as the excessive use of fossil fuels,the serious energy shortages and environmental pollution problems have been caused.Therefore,the development of clean and efficient energy storage and conversion devices with high energy density has become an extremely urgent topic.Among numerous new energy storage systems,lithium-sulfur?Li-S?batteries have acquired comprehensive attention and investigation because of their excellent theoretical energy density(2600 Wh kg^-1)and specific capacity(1675 mAh g^-1).In addition,the characteristics of environmentally friendliness,non toxicity and abundance of sulfur make Li-S batteries one of candidates for the most potential secondary batteries for commercialization.However,the practical application of Li-S batteries is impeded by several shortcomings,including the inherently poor conductivity of sulfur(5×10^-30 S cm^-1)and Li₂S/Li₂S₂,the large volumetric change?⁸0%?of the electrode and the so-called“shuttle effect”induced by the dissolution of polysulfides in the electrolyte.Therefore,it is critical to develop new cathode materials with special microstructure for Li-S batteries.Carbon materials with good conductivity and adjustable pore structure have been extensively employed as an effective cathode host for inhibiting the dissolution of polysulfides and improving the utilization of sulfur.In this thesis,the low-cost,easily available and green-friendly biomass was selected as a carbon precursor,and porous carbon materials were prepared by a simple high-temperature pyrolysis method.Moreover,the rGO,MoS₂,CNTs and polar heteroatoms?N,O,P,I?were introduced into porous carbons as modifiers.The structure and microstructure properties of the as-obtained materials and their effects on the electrochemical performance of Li-S batteries were studied in detail.The main results were showed as following:1.A series of biomass-derived porous carbon materials using the goat hair as precursor were prepared by H3PO4 activation and different temperatures carbonization.The effects of carbonization temperature on the microstructure,morphology,and electrochemical performance of carbon materials were investigated.The as-prepared carbon materials have a hierarchical porous structure,but the developed degree of pore structure is closely related to carbonization temperature.The obtained carbon sample carbonizatized at 600°C?NOPC-600?has the abundant pores and N,O,P heteroatoms.The charge/discharge performance of the as-prepared sulfur/carbon composite material?S/NOPC-600?was tested at 0.2 C.S/NOPC-600 electrode shows a high discharge capacity of 921 mAh g^-1 and can be maintained at 489 mAh g^-1 after300 cycles.However,the S/NOPC-550 and S/NOPC-650 cathode only deliver the low first discharge capacity of 833 and 517 mAh g^-1.The experimental results indicate that the microstructure of the material has a great influence on the electrochemical performance of Li-S batteries.2.Based on the previous work,various contents of graphene oxide?GO?were introduced to the goat hiar derived carbon material for modification.A series of N,P co-doped reduction graphene oxide/porous carbon?rGO/GPC-n,n=0,1,2,3?hybrid with excellent conductivity were fabricated by the one-pot pyrolysis method.The effects of the incorporated content of graphene on the microstructure,morphology and electrochemical performance of carbon materials were investigated.The experimental results reveal that the prepared rGO/GPC-2 sample?GO:³%?has a loose porous structure and high conductivity.The initial discharge capacity of the corresponding sulfur-containing composite electrode?rGO/GPC-2-S?can be as high as 1420 mAh g^-1 at 0.1 C,and the capacity decay rate is calculated to be only 0.070%per cycle for500 cycles at 1 C.In addition,the rGO/GPC-2-S cathode shows excellent rate performance with discharge capacities of 604 and 500 mAh g^-1 at 1 and 2 C,respectively.The affluent pore structure and N,P heteroatoms of rGO/GPC-2synergistically inhibits the dissolution of polysulfides that can effectively alleviate the shuttle effect;meanwhile,the introduction of rGO greatly improves the conductivity of host materials and the utilization of sulfur.Therefore,the rGO/GPC-2-S electrode possesses excellent electrochemical performance.3.Nitrogen-doped porous carbon using fish scales?FSC?as carbon source was prepared by KOH activation and high-temperature carbonization at 900°C,and then different contents of molybdenum disulfide?MoS₂?were introduced by hydrothermal method.Finally,in order to construct a three-dimensional porous network structure with polarity and high conductivity,FSC/MoS₂/CNTs cathode material was prepared by physically mixing the acid-treated CNTs with FSC/MoS₂.The experimental results show that the most suitable amount of MoS₂ is¹0%,which can effectively inhibit the shuttle behavior of polysulfides to obtain high discharge capacity.The FSC/MoS₂/CNTs@S electrode delivers the first discharge capacity of 1313 mAh g^-1 at0.1 C,and the capacity can still be maintained at 731.3 mAh g^-1 after 200 cycles.The capacity decay rate is only 0.059%per cycle for 500 cycles at 1.0 C,indicating the excellent cycling stability.Meanwhile,FSC/MoS₂/CNTs@S shows excellent rate performance with discharge capacities of 802 and 672 mAh g^-1 at 1 and 2 C,respectively.N heteroatoms and MoS₂ with strong polarity can effectively adsorb polysulfides and inhibit their“shuttle effect”.And MoS₂ possesses catalytic effect,which can accelerate the conversion speed between polysulfides and sulfur.The highly conductive CNTs and porous carbon derived from fish scales provide rich transfer channels for electrons/ions,which is conductive to improving the electrochemical reaction kinetics.Thus,FSC/MoS₂/CNTs@S cathode demonstrates the electrochemical performance.4.N,I dual-doped porous carbon using wheat flour as carbon precursor?NIWFC?was prepared by KOH activation,high temperature pyrolysis and further hydrothermal reaction processes.The microstructure and morphology of the obtained carbon materials were studied.The effects of single-doped and dual-doped matrix materials on the electrochemical performance of Li-S batteries were also explored.The results show that NIWFC has a 3D hierarchical porous structure,and N,I heteroatoms are successfully incorporated into the carbon material.The NIWFC/S cathode achieves a high initial discharge capacity of 1284 mAh g^-1 at 0.1 C.After 100cycles,the reversible capacity is maintained at 917 mAh g^-1,which possesses the glorious electrochemical performance.The porous structure and N,I polar heteroatoms synergistically restrain the shuttle effect of polysulfides and thus improves the electrochemical performance of NIWFC/S material.