| The lithium sulfur(Li-S)battery is one of the most promising next generation secondary batteries because of its advantages of high specific capacity and specific energy,and low cost due to the relative abundance of sulfur.However there are several issues to be resolved before wide scale use of Li-S batteries can be realized,e.g.,the low conductivity of sulfur;the volume change of sulfur during charging and discharging;and the shuttle of intermediate polysulfide products during charging and discharging.The shuttle effect is the root cause of poor cycling performance and is currently the problem that Li-S battery research is focused on.Restricting sulfur within porous carbon is an effective strategy to resolve these issues mentioned above.However the preparation of many PCMs is complex and expensive,therefore developing low cost porous carbon becomes the key point in research of Li-S batteries.Templated method is used in this report to prepare porous carbon which composites with sulfur to prepare cathode materials of Li-S battery.The effect of porous carbon parameters on Li-S battery performance is investigated,and further cotton fibers are activated to prepare low cost and high performance cathode material of Li-S battery.Subsquently the promotion mechanisms of polyaniline coating and synergistic effect between carbon membrane and cathode are analyzed and finally high performance Li-S battery cathode system is obtained which can be one reference for development of Li-S batteries.Here a nano size zeolite is used as a template to form a porous carbon material(ZPC)which is further prepared into cathode materials(ZPC-S).And the effects of carbon structure and sulfur content on electrochemical performance of cathode materials are investigated by analysis of morphologies,structure,electrochemical performance,etc.The results show that the surface area and po re volume of ZPC are large which are 708 m2 g-1 and 0.954 cm3 g-1 respectively,also the size of its pore diameter is below 40 nm.Therefore,the electrochemical activity of sulfur is large and polysulfides can be adsorbed by pores effectively.The ZPC-S with 44 wt% S can deliver a first discharge capacity of 1035 m Ah g-1,and maintain 701 m Ah g-1 after 50 cycles at 0.5 C.However,due to the decrease of surface area and pore volume,the ZPC-S with 60 wt% S delivers a first discharge capacity of 973 m Ah g-1,and maintains 672 m Ah g-1 after 50 cycles at 0.5 C.Further the surface of ZPC is modified with polyvinylpyrrolidone(PVP)and amphiphilic porous carbon(A-ZPC)is obtained.The effect of carbon surface polarity on electrochemical performance of cathode is investigated by physical and electrochemical characterizations of carbon and its cathode materials before and after modification.The results show surface polarity of carbon can improve the cycling performance of cathode.And A-ZPC-S with 46 wt% S can maintain a discharge capacity of 691 m Ah g-1 after 300 cycles at 1 C,only decaying 0.142% per cycle.Cotton fiber is used as precursor to prepare low cost porous carbon by activation of KOH,NH3,H3PO4 and Zn Cl2 respectively.The comparison of these carbons as sulfur hosts are conducted by characterizations of morphologies,structure,electrochemical performance,etc.The results show that the PCMs prepared by KOH or NH3 activation can not accomodate high content sulfur due to their small pore volumes both of which are below 0.05 cm3 g-1.However,P-doped porous carbon(PCM-650)is prepared using phosphoric acid as activator.Not only the specific surface area(1420 m2 g-1)and pore volume(1.33 cm3 g-1)are large,also lots of phosphorus groups(2.4 at% P)are introduced.DFT calculations and XPS results show the introduced phosphorus groups adsorb sulfur and sulfides more effectively,and hence PCM-S with 55 wt% S can maintain a discharge capacity of 710 m Ah g-1 after 300 cycles at 1 C,only decaying 0.136% per cycle.In addition,porous carbon materials are also tried to be prepared by zinc chloride activation(PCM-Z)further.PCM-Z has small pore size(<40 nm)with high specific surface area(1056 m2 g-1)and large pore volume(1.75 cm3 g-1)which allow high sulfur loading(PCM-Z-S)and result in a significantly improved specific energy of a Li-S battery.PCM-Z-S is much better than Li-S cathode materials developed above.PCM-Z-S cathode material is coated by polyaniline(PANI)further,and the effect of PANI layer on electroperformance of PCM-Z-S before and after coating is investigated by comparison of morphologies,structure,electrochemical performance,etc The results show that PCM-Z-S has hierarchical structure and large size(1-20 ?m);After modified by polyaniline,not only the shuttle effect of polysulfides is restrained;the filtration of electrolyte is also restrained;Adequate modified 55 wt% sulfur PCM-Z-S,whose sulfur content decrease only 4 wt%,can deliever discharge specific capacity of 1181 m Ah g-1 for the first time at 0.5 C,and maintains 846 m Ah g-1 after 100 cycles,the energy density is also 78 Wh kg-1 larger than that of pristine PCM-Z-S.The best position of carbon membrane is confirmed by investigation of the synergistic effect between carbon membrane and PCM-Z-S cathode: the bottom of carbon membrane should contact with cathode.Meanwhile the porous carbon matrix(UCM)with low cost and a surface structure with some graphene morphology and another porous morphology is developed from cotton fibers by activation of high concentration Zn Cl2.The results show UCM is with high specific surface area and large pore volume;the carbon membrane derived from UCM realizes the effective synergism and dramatically improved the cycling performance of a Li-S battery.Therein,74.5 wt% LCM-S composite is prepared from LCM,with UCM membrane,it can deliver a discharge specific capacity of 691 m Ah g-1 after 300 cycles when area sulfur loading is 1.35 mg cm-2,only decaying 23.2%.When area sulfur loading increasing to 5.55 mg cm-2,an area discharge capacity of 7.18 m Ah cm-2 is obtained.Even the cathode can deliver discharge area capacity of 3.2 m Ah cm-2 at 180 th cycle. |
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