Research On The Lithium Storage Performance And Mechanism Of Carbon Nanofiber Paper/sulfur Composite Electrode

Lithium-ion batteries are widely used in small portable electronic devices today,however,they are unable to meet the high-energy needs of stationary grid energy storage.The limited energy density of traditional lithium-ion batteries also hinders their development in new transportation like electric vehicles,which are more environmentally friendly than gasoline-powered cars.Therefore,it was of great importantance to find more reliable battery technologies that go beyond conventional lithiumion batteries.In this dissertation,on the one hand,addition of conductive functional interlayer in Li-S battery and it's lithium storage mechanism were studied by using carbon fiber paper as raw material.On the other hand,a carbon fiber paper/sulfur composite as the anode material of LIBs was then studied.Besides,the work of preparation and activation mechanism of potato starch based activated carbon and applied in electric double layer supercapacitor was also carried out.The contents include the following aspects.?1?The addition of carbon fiber paper as conductive functional interlayer in Li-S battery and it's lithium storage mechanism.The carbon fiber paper was derived from pyrolysis of electrospun polyacrylonitrile?PAN?paper,and then used as the interlayer between the sulfur cathode and separator.The electrochemical measurements indicated that the discharge capacities of the Li-S batteries increased from 863 mAh g^-1 to 1361 mAh g^-1 after adding the carbon fiber paper.Meanwhile,it has excellent long-term cycling stability,it's capacity retention increased from 37%to 77%.In order to further improve the Li-S batteries'electrical performance,double protection of sulfur cathode?simultaneously employing thermally-treated carbon black/sulfur composite cathode and inserting carbon interlayers?was tested.However,to our surprise,the sulfur cathode resulted in extremely low capacity after the first discharge process.Further study found that there were infinitesimal dissolution of polysulfides in these cells.The poor cycling stability of the cells without carbon interlayers was mainly due to the severe dissolution of polysulfides.Only for the batteries with traditional untreated sulfur cathode and carbon fiber paper interlayers allowing modest dissolution of polysulfides in electrolytes,and then high-efficiency cycling of Li-S batteries can be obtained.Thus this work provides new opportunities for the research about the importance of modest polysulfides dissolution in efficient Li-S batteries.?2?A carbon/sulfur composite as the anode material of LIBs.Carbon/sulfur composites were prepared by powdered sulfur and electrostatic spinning carbon fiber paper,in which the mass ratio of sulfur are 15%,30%,45%and 60%.In the self-supported carbon nanofiber paper/sulfur?with 30%mass ratio,denoted as CNFP/S-30?anode,sulfur can contribute to a reversible capacity of 637 mAh g^-1 within the potential range of 1.5-0 V vs.Li/Li⁺,satisfying the low working potential and high-capacity requirements of commercial anodes.Taking the whole anode'mass into account,the self-supported CNFP/S-30 anode has a reversible capacity of 310 mAh g^-1?without conductive agent,binder,and current collector?,it is much more than a commercial graphite anode with Cu as current collector.More importantly,the CNFP/S-30 anode exhibits outstanding high-rate behavior,maintaining a reversible capacity of 258 mAh g^-1 at a large current density of 8A g^-1,which is much higher than the commercial graphite anode.Besides,the CNFP/S-30 anode has excellent long-term cycling stability,it's capacity retention is more than 95%after 1000 continuous cycles.The full cell composed of CNFP/S-30 anode and Li FePO₄ cathode can be cycled reversibly within the potential range of 3.0-3.2V.These experimental results demonstrate the successful application of CNFP/S-30 anode in high-power LIBs.?3?Preparation and activation mechanism of potato starch based activated carbon and application in electric double layer supercapacitor.Potato-derived activated carbon was prepared through pre-carbonization of mashed potato and subsequent chemical activation with KOH.It is found that the activated carbon obtained through two times of KOH activation?PAC-2?exhibits considerably improved porous properties and supercapacitive performance than that obtained through one time of activation?PAC-1?.The optimized activated carbon?PAC-2?possesses a high surface area of 1930 m² g^-1 and large pore volume of 1.25 cm³ g^-1.At the same time,it has optimized porous structure and exhibits a high specific capacitance of 366 F g^-1 at a current density of 2 A g^-1 in KOH solution and good stability over 10000 cycles.Such a low-cost and high-performance activated carbon is a promising electrode material for practical application in supercapacitors.