Preparation Of Polypropylene Separator Modified By Polymer And Its Electrochemical Behavior In Lithium-sulfur Battery

With the rapid development of society,the increasing demand for energy,as well as the environmental pollution of fossil fuels and the depletion of their own reserves,the demand for new alternative energy is becoming more and more urgent.Lithium-sulfur battery is considered as a strong competitor for the next generation energy storage system because of its high theoretical capacity(1675 m Ah g^-1),energy density(2600 Wh kg^-1)and environmentally friendly,rich resources and low price of active material sulfur.However,its commercialization process is still perplexed by many factors,such as the weak electron and ion conductivity of sulfur electrode-separator interface,the dissolution and shuttle effect of lithium polysulfide and the slow kinetics of sulfur electrode charge-discharge reaction.In order to solve the above problems,the surface of sulfur electrode side separator was modified and regulated by polymer surface modification in order to achieve high capacity density and long cycle performance of lithium-sulfur battery.The main research work is as follows:1.PMMA microsphere arrays modified PP separatorTaking advantage of the fact that PMMA microspheres are easy to self-assemble to form an array structure,PMMA microspheres are arranged orderly on the surface of PP membrane to obtain PMMA-PP separator.The PMMA modified layer has high wettability to the electrolyte,and the contact angle of PMMA-PP separator decreases from 35.0°to 14.3°of PP separator.The microsphere array with 6-7 layers and average diameter of 230 nm was obtained by PMMA self-assembly,and the load was only 0.4287mg cm^-1,which effectively increased the energy density of Li-S battery.Such a neat and tight stacking prolongs the diffusion path of polysulfides,thereby inhibiting the diffusion of polysulfides.The capacity of the 1^st and 100^th discharges of the sulfur electrode with the PMMA-PP separator are 1100 m Ah g^-1 and 613.4 m Ah g^-1(25℃,current density of0.1 m A cm^-2,potential range 1.7-2.8 V),and the coulombic efficiency is maintained at99.5%after several cycles.The electrochemical performance of the PMMA-PP separator battery is much better than that of PP separator battery.2.RAPOP/AB modified PP separatorAlthough the physical barrier can limit the domain of lithium polysulfide,the shuttle effect can not be completely eliminated due to the lack of chemical action.In this work,RAPOP polymers containing a large number of chemical adsorption sites of lithium polysulfide（-N=）were prepared by high temperature Polycondensation,and then RAPOP and AB were physically mixed on the surface of PP separator on the side of sulfur electrode to obtain RAPOP/AB-PP separator.The high electrolyte affinity of AB with good electronic conductivity improves the electrolyte wettability of the separator,which is beneficial to the rapid diffusion of lithium ions.The physical-chemical sulfur retention mechanism of RAPOP/AB layer was analyzed by EIS tests of batteries with different DOD.Through the SEM of S electrode and lithium foil surface after cycling,it was verified that RAPOP/AB coating could form lithium polysulfide aggregation layer on the electrode surface to prevent the further migration of polysulfides.The capacities of Li-S batteries configured with RAPOP/AB-PP-1.7 separators were 1322 and 897 m Ah g^-1after the first and 800^th discharge/charge cycle(25℃,current density of 0.2 m A cm^-2,potential range 1.7-2.8 V),respectively.These values were much higher than those of the batteries configured with AB-PP(723 m Ah g^-1)and PP(481 m Ah g^-1)separators.3.PDMcT/AB modified PP separatorThe shuttle effect was significantly inhibited by the combined action of physical hindrance and chemical adsorption,but the anchored lithium polysulfide was easily deactivated due to the slow transformation kinetics of polysulfides.In this work,PDMcT organic sulfur polymer was obtained by polymerization of DMcT,and PDMcT and AB were coated on the surface of PP separator to prepare a PDMcT/AB-PP separator which suppresses shuttle effect and accelerates the electrochemical reaction of lithium polysulfides.Raman,XPS tests confirmed that PDMcT with a redox potential window similar to S realized its reversible transformation by breaking S-S bond and combining with Li PS to form DMcT·Li S_x（x=4-8）during the redox process.In this process,the combination of DMcT radical and Li₂S_x（x=4-8）accelerates the transformation of Li₂S_x to Li₂S,which is helpful to anchor lithium polysulfide and catalyze the electrochemical reaction of sulfur.The capacities of Li-S batteries configured with PDMcT3/AB-PP separators were 1645 and 821 m Ah g^-1 after the first and 250^th discharge/charge cycle(25℃,current density of 0.2 m A cm^-2,potential range 1.7-2.8 V),respectively.