| Lithium sulfur(Li-S)batteries have been attracted much attention of the researchers in the field of lithium secondary battery,due to the high theoretical capacity(1675 mA·h/g),rich resources,low cost and no pollution.However,there are still some problems,such as the insulation of sulfur,easy dissolution of long chain polysulfide ions in the electrolyte solution and the shuttle effect during the charge and discharge process,limiting Li-S batteries to a large-scale application.In recent years,porous carbon,carbon nanotubes,graphene and metal oxides are usually combined with sulfur as cathode materials to improve the conductivity and utilization of the active materials,thereby enhancing the electrochemical properties.The porous carbon with a highly developed pore structure and a huge specific surface area has prominent application prospect in storing sulfur and limiting loss of polysulfide ions.In this paper,a series of studies about preparation process and structure optimization of porous carbon materials were carried out for the purpose of improving the specific capacity and modifying the cycle performance.The main research contents are as follows:(1)The hydrothermal pre-carbonized precursor which was used glucose as carbon source and acetamide as nitrogen source was pore-formed by KOH activating agent during sintering process,preparing the N-doped activated carbon(NPC).It is clarified that the effect of the amount of activating agent on the pore structure of the sample,the pore-forming principle and microstructure regulation of activating agent.Finally,the S/NPC composite cathode material was prepared by combining the sulfur with the NPC samples by ball milling and melting infusion.The S/NPC4 composites prepared by the porous carbon NPC4 and sulfur processed the best electrochemical performance when the weight ratio of activator was 4 times.The initial discharge specific capacity is 1178.5 mA·h/g at 0.1 C and it remains 654.67 mA·h/g after 50 cycles.The capacity retention rate is up to 55.6%.(2)Two kinds of biomass active porous carbon materials were prepared by one-step activated carbonization method and two-step carbonization activation method.The scanning electron microscopy(SEM)was used to observe the effect of carbon monoxide.The porous carbon(O-ABC)prepared by one-step carbonization activated method has an internal and connected porous structure with a graphene-like lamellar structure on side wall and with a very high specific surface area(1824.39 m2/g)and pore volume(0.8081 cm3/g).The S/O-ABC composites prepared by porous carbon(O-ABC)and sulfur possess the best electrochemical performance during charging and discharging.The results show that the first discharge capacity of the S/O-ABC composite is 1375 mA·h/g at 0.1 C,and it still maintains 737 mA·h/g after 50 cycles.The first discharge specific capacity of porous carbon(prepared by one-step carbonization activation method)/sulfur composites cathode material increases significantly,thanks to its large specific surface area and graphene-like structure,not only enhancing the conductivity of the active material,but also providing sufficient active sites for the elemental sulfur,in turn increasing the utilization of active substances.(3)A new kind of porous carbon material(TC)is prepared via a "hard template" method using sucrose as carbon source and nanoscale CaCO3 particles as hard template.The effects of template dosage and sintering temperature on porous structure were studied.The results show that the pore structure of the TC material is produced by three steps:the primary step attributing to the shape of CaCO3 nanoparticles,the second step due to the CO2 formed by CaCO3 pyrolysis,and the third step ascribing to the decomposition of nanoscale CaO particles etching by HCl.When the amount of CaCO3 is equal to the amount of carbon,the product shows the highest initial discharge specific capacity.Meantime when the heat treatment time is 60 min,the product delivers the best capacity retention rate. |
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