Tailoring The Pore Structure Of Carbon Nanofibers And Electrochemical Performance Of Lithium Sulfur Batteries

The efficient energy storage system has become one of the key technologies for the low-carbon society in the future.With excellent theoretical specific energy and specific capacity performance,lithium-sulfur battery is one of the most potential practical energy storage devices to meet the current challenges,and it has received extensive attention.Porous carbon nanofibers have great potential in the application of lithium-sulfur battery cathode materials due to their excellent electrical conductivity and rich pore structure.The microporous and mesoporous structure in the cathode material are beneficial to restrain the"shuttle effect" of polysulfides and improve the cycle stability of lithium-sulfur.In this paper,polyacrylonitrile(PAN)and polymethyl methacrylate(PMMA)is used as carbon source and template respectively.The precursor fibers were prepared by electrospinning.The porous carbon nanofibers are obtained by heat treatment of the precursor fibers.The pore structure of carbon nanofibers is optimized by adjusting the ratio of PAN and PMMA precursors.The pre-oxidation temperature is controlled to further increase the volume of micropores and mesopores in porous carbon nanofibers.The electrochemical properties of porous carbon nanofibers were optimized by doping Co to inhibit the "shuttle effect" of polysulfide,the following research was carried out.(1)The influence of PAN/PMMA ratio on the pore structure was studied.Low PMMA content is beneficial to the formation of micropores and mesopores,and the increase of PMMA content easily leads to the merging and growth of micropores.The prepared porous carbon nanofibers(PCNFs-73)have the highest micropore volume(0.2 cm3 g-1)and specific surface area(289.2 m2 g-1),when the ratio of PAN/PMMA is 7:3.The PCNFs-73 sample exhibits excellent rate performance with a capacity of 775 mAh g-1 at a current density of 0.1 C.At a high current density of 5 C,its capacity retention rate can still be maintained at 24.6%(191 mAh g-1).(2)The pore structure of the PCNFs can be effectively regulated by changing the pre-oxidation temperature of the precursor fibers.A low pre-oxidation temperature is more beneficial to increase the rate of micropores and mesopores on the PCNFs.A high pre-oxidation temperature promotes the accumulation of PMMA,and it is easier to form axial through-hole pore structure on the PCNFs.The PCNFs-240 present the highest specific surface area(576.26 m2 g-1)and micropore volume(0.18 cm3 g-1)when the pre-oxidation temperature is 240℃.The PCNFs-240 sample exhibits excellent electrochemical performance with a capacity retention of 1050 mAh g-1 at a current density of 0.1 C,a capacity retention rate of 41%(431 mAh g-1)at a high current density of 5 C,and a capacity retention 470 mAh g-1 after 500 cycles at a current density of 2 C.(3)The effect of Co doping content on the structure and properties of PCNFs was studied.Co-doping can further control the pore structure of carbon nanofibers,promote the conversion of polysulfides and adsorb polysulfides,thereby inhibiting the shuttle effect.The specific surface area of the prepared PCNFs can reach 761 m2 g-1,and it had larger micropores and mesoporous volumes,when the doping amount of Co is 0.3 mmol(PAN=0.7 g,PAN/PMMA=7:3).The reversible capacity of the electrode material is 537 mAh g-1 after 500 cycles at a current density of 2 C,and the capacity decay rate of each cycle is only 0.066%.