Novel Material Designs And Mechanism Investigations Of Electrode And Separator In Lithium-sulfur Battery

The theoretical specific capacity of sulfur(S)in lithium sulfur(Li-S)battery is 1675 mAh g-1,which is much higher than that of commercial-widely used LiFePO4(170 mAh g-1)and LiNixCoyMn1-x-yO2(<300 mAh g-1)cathodes in lithium-ion batteries.On the other hand,the theoretical specific capacity of lithium metal is as high as 3860 mAh g-1,which is more than ten times that of graphite anode(372 mAh g-1)in lithium-ion batteries.Therefore,Li-S battery is expected to meet the demands for energy storage devices with higher energy density.However,the following dilemmas in cathode and anode seriously impair the electrochemical performances of Li-S battery:(1)the conductivities of S and lithium sulfide(Li2S)are poor;(2)The volume change between charging and discharging process is tremendous(ca.80%);(3)The lithium polysulfides(LiPSs)produced in charge and discharge processes are easy to dissolve in the traditional ether-based electrolyte,resulting in "shuttle effect";(4)Lithium dendrite is easy to generate on the surface of lithium metal due to uneven deposition of lithiumion.Besides,there will be a large volume change(100%)in the stripping/deposition process due to the hostless property of lithium metal.To address the above issues,the material modifications of cathode,anode and separator are conduct in this paper.The working mechanism of different modified materials are analyzed and demonstrated by a series of physical measurements,chemical characterizations,battery performance tests and first principle calculations,thereby providing new ideas and technical supports for the material designs in Li-S battery.The main contents and conclusions are as follows:(1)A novel microporous-mesoporous carbon(MC)was prepared by chemical vapor deposition(CVD)and template method.The MC was compounded with magnesium oxide(MgO)nanoparticles and S to prepare the MC/MgO/S(MCMS)composite.The MC can effectively improve the conductivity of the electrode.Meanwhile,MgO can effectively anchor lithium polysulfides(LiPSs)in the process of charge and discharge,therefore inhibiting the shuttle of LiPSs.The results show that when the mass amount of MgO is 10 wt%,the Li-S battery shows the highest discharge specific capacity and the best cycle stability.(2)A silver(Ag)layer with a thickness of 80 nm was coated on the surface of commercial carbon cloth(CC)by thermal evaporation.The experiments and density functional theory(DFT)prove that the lithiophilic Ag layer can effectively induce the homogeneous deposition of Li metal.Li was deposited into the three-dimensional structure of carbon cloth/silver(CC/Ag)by electrochemical deposition method to obtain the three-dimensional lithium metal anode(CC/Ag/Li).Both the symmetrical battery(CC/Ag/Li-CC/Ag/Li),lithium iron phosphate battery(CC/Ag/Li-LiFePO4)and Li-S battery(CC/Ag/Li-S)show that the introducted Ag layer in the current collector can improve the cycle stability of the batteries.(3)The three-dimensional copper foam(3D-Cu)was used as the substrate,then it was coated with Ag layer(100 nm thick)to obtain Ag@3D-Cu composite by thermal evaporation.The Ag@3D-Cu was applied as a three-dimensional current collector while Li2S was used as a cathode to constructe an anode-free full battery.The results show that the lithiophilic and threedimensional Ag@3D-Cu current collector can improve the electrochemical performances of the anode-free full battery.Even under the condition of ultra-high mass loading of Li2S(14.6 mg cm-2),the battery still maintains a stable cycle performance.(4)A novel bismuth ferrite(BiFeO3,BFO)powder with a high purity and a cubic structure was prepared by a hydrothermal method.The BFO powder was applied to modify the the commercial polypropylene(PP)separator with graphene oxide(GO)and conductive acetylene black(AB)to obtain BFO/GO/AB@PP separator.The experimental results show that the BFO with inherent spontaneous polarization enables BFO/GO/AB@PP separator effectively inhibit the shuttle of LiPSs,thereby improving the cycle stability,rate performance,self discharge behavior and lithium anode corrosion in Li-S battery.(5)A novel three-dimensional ordered iron vanadate(FeVO4,FVO)nanocatalyst was prepared by a template method.Then it was combined with commercial carbon nanotube(CNT)to modify PP separator.A series of experiments verify that the FVO/CNT@PP separator can not only largely suppress the shuttle of LiPSs,but also effectively catalyze the redox processes of LiPSs.Owing to these,the Li-S battery assembled with FVO/CNT@PP separator shows enhanced electrochemical performances.