Electrospinning Construction And Electrochemical Performance Of Nanofibers Separator For Lithium Sulfur Battery

Lithium sulfur(Li-S)battery is a potential secondary battery owing to its high specific energy density of 2600 Wh kg^-1and sulfur cathode of theoretical specific capacity of 1675mAh g^-1.However,Li-S battery will have rapid capacity degradation and poor cycle stability due to the volume expansion of elemental sulfur,the low electronic conductivity of sulfur cathode and lithium sulfide,and the shuttle effect caused by soluble polysulfides.In order to solve the above problems,in this thesis,based on electrospun nanofiber membrane,three new types of separators for lithium sulfur battery are designed and constructed to inhibit the shuttle effect of polysulfide,improve the redox reaction kinetics and enhance the electronic conductivity of the cathode.The effects of the functional separators on inhibiting the shuttle effect of polysulfide,promoting the rapid electrons transport and improving the electrochemical performance of the battery are systematically studied.The main research contents and results are as follows:(1)PVDF&[PVDF@PPy]Janus separator is designed and prepared by electro-spinning combined with vapor phase polymerization.PVDF&[PVDF@PPy]Janus separator has two layers:one is PVDF layer with high porosity and specific surface area,which can hinder polysulfides and inhibit the shuttle effect of polysulfides.The other layer is PVDF@PPy layer with good conductivity,which can reduce the interface resistance and reuse the adsorbed polysulfides.As a result,for the assembled battery by using PVDF&[PVDF@PPy]Janus separator as separator,the initial discharge capacity of the battery is 1053 mAh g^-1 at 0.1 C rate.After 200 cycles,the discharge capacity is 546 mAh g^-1,and the decay rate of each cycle is 2.4%.When the current density increases to 0.2 C,the initial discharge capacity of the battery is 817 mAh g^-1.In contrast,the initial discharge capacity of the battery assembled with PP separator is 853 mAh g^-1.After 200 cycles,the discharge capacity is only 217 mAh g^-1.PVDF&[PVDF@PPy]Janus separator significantly improves the discharge capacity and rate performance of the battery.(2)The traditional polypropylene(PP)separator has been modified.By electrospinning a layer of conductive PAN/PANI nanofibers film on PP surface,PP&[PAN/PANI]composite nanofibers film is obtained.The PAN/PANI nanofiber membrane between the positive electrode and PP membrane can inhibit the shuttle of polysulfide and improve the electron transport rate on the cathode.For the assembled battery by using PP&[PAN/PANI]composite nanofibers film as separator,at 0.1 C rate,the initial discharge capacity of the battery is 1017 mAh g^-1.After 200 cycles,the discharge capacity is 479 mAh g^-1.The discharge capacity and cycle stability of the battery are improved effectively.(3)PAN&[PAN@PPy]nanofibers membrane is designed and fabricated by the combination of electrospinning with vapor-phase polymerization.CoS nanoparticles,prepared via ion exchange method,are electro-sprayed onto the PAN&[PAN@PPy]nanofibers membrane to obtain PAN&[PAN@PPy/CoS]composite nanofibers membrane.One layer of the composite nanofibers membrane is PAN nanofibers layer,and the other is the conductive PAN@PPy/CoS nanofibers layer.CoS nanoparticles can not only anchor polysulfides,but also can catalyze the conversion of polysulfides and accelerate the reaction kinetics.The assembled battery by using PAN&[PAN@PPy/CoS]nanofibers membrane as the separator delivers that the initial capacity is 1092 mAh g^-1 at 0.1 C.After200 cycles,the discharge capacity can still maintain to 639 mAh g^-1,which is 58%of the initial capacity.The electrochemical performance of the lithium sulfur battery is significantly improved.