| Sulfur cathode in lithium-sulfur battery has been widespread concern and become a hot research in recent years,because of its high specific capacity of up to mAh g-1,and sulfur resources' abundance,non-toxic,and cheapness and many other advantages in the Earth.However,the practical use of lithium-sulfur battery is still faced with three major challenges:1.active material S is an electronic insulator,but also a bad conductor for ions,reducing the utilization of active substances and is not conducive to the realization of high rate discharge/charge;2.The large volume expansion from S to Li2S,will cause the damage of the cathode structure,affecting the battery cycle performance;3.The most serious problem which application of lithium-sulfur battery facing is that intermediate products polysulfides(PS)during discharging can be dissolved in the electrolyte,and reach the anode reacting with lithium through separator,resulting in the capacity of the irreversible,that is,"shuttle phenomenon".This thesis is based on the modification of the separator,to improve the cycle performance of lithium-sulfur battery:Firstly,we used LiF to modify commercial separator.through the LiF coating and DME interaction,LiF in liquid electrolyte becamed translucent sol state,increasing the viscosity of electrolyte in the LiF coating layer hindering the free migration of polysulfide ions.At the same time,the lower LiF surface has a great effect on the migration of lithium ions during the process of blocking the polysulfide ionization.In the lithium-sulfur battery,the polysulfide ion shuttle effect had a significant inhibition,Through FTIR,TG and DTG,and LSV,we confirmed the interaction between LiF and electrolyte.The LSV test showed that the LiF coated separator had a stable electrochemical window up to 5 V,and the LiF-coated separator was used in lithium-sulfur battery to exhibit good electrochemical performance after 200 cycles at 0.2 C:based on the third cycle discharge specific capacity,capacity retention rate of 69.3%.Without LiNO3 as an additive,almost Coulomb efficiency of above 93%for every cycle was gained on cell after 200 cycles.Subsequently,we prepared the C/3MPMTS coated separator by using the characteristics of 3MPMTS that it could make soluble long chain polysulfide ions dissolved from the cathode electrode turn into insoluble short-chain polysulfide ion in the coating layer.While the conductive carbon in the coating could act as the role of the upper current collector,insoluble polysulfide ions in the coating layer to be used to improve the utilization of active substances.The existence of the upper current collector reduced charge transfer impedance of battery to a large extent,which not only improved the battery cycling stability,but also the rate performance.Finally,we prepared PVDF-HFP/3MPMTS membrane by mixing PVDF-HFP with 3-mercaptopropyltrimethoxysilane(3MPMTS).The PVDF-HFP/3MPMTS membrane was processed by in situ hydrolysis,oxidation and lithiation.(PMPF-HFP/SiO2-SO3Li(PFSS))polymer electrolyte membrane was successfully prepared via hydrolyzing the main chain methoxysilane to SiO2 in the 3MPMTS,and the mercapto-group was oxidized to sulfonic acid(-SO3H)and further was lithiated into-SO3Li.Electron scanning microscopy(SEM)showed that SiO2 were uniformly distributed by in-situ hydrolysis in the polymer membrane.The FT-IR spectra showed that the polymer membrane was rich in sulfonated acid.The results showed that the ionic conductivity of PFSS gel polymer electrolyte was 8.6 × 10-5 S cm-1.LiFePO4/Li cell with PFSS gel polymer electrolyte have a relatively stable cycle performance.All these fully demonstrate the richness of sulfonic acid groups in the films.Severe overcharging phenomena was observed in Li-S cell with PFSS gel polymer electrolyte.by comparing cycling performance of cell with PFSS/pristine separator assembled with that of cell with pristine separator,conclusion was gained that without pore sieving effect,electrostatic repulsion can barely make for blocking PS. |
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