Study On Polymer Binders And Modified Separators For Lithium-sulfur Batteries

As a new kind of secondary battery system,lithium-sulfur batteries own great potential,their high theoretical specific capacity and specific energy,abundant reserves,low cost and environmentally friendly features of active material draw lots of attention these years.However,there are still some problems remain to be solved before the practical application of the lithium-sulfur battery,such as the insulate nature of the active material and the discharge end product of the battery,the “shuttle effect” due to the dissolution of long-chain lithium polysulfide in the electrolyte,which is the intermediate product of the battery,the volume change during charge and discharge process,etc.,all of above will lead to the loss of active material during the cycling and affect the capacity release and stability of the battery.In view of the above problems,this paper applies polymers as binders in cathodes and modified materials on separators of lithium-sulfur batteries,in order to improve the charge-discharge specific capacity and cyclic performance of the battery through multiple ways.The main research contents and results are as follows:1.Thiokol was used as the new binder of lithium-sulfur battery.The sulfhydryl group in Thiokol can cut off the polysulfide bond,thus the long-chain lithium polysulfide dissolved in the electrolyte is cut into short-chain lithium polysulfide which is insoluble in the electrolyte,thereby achieving the purpose of reducing shuttle of lithium polysulfide,improving the capacity and stability of the battery.The initial discharge specific capacity of 819 mAh/g was achieved under the constant current charge-discharge rate of 0.1 C,with a reversible specific capacity of 501 mAh/g after 200 cycles.In the multiple rate test from 0.1 C to 1 C,the cycling reversibility after the higher rate cycling was as high as 90.9%,significant improvement was therefore achieved compared to traditional lithium-sulfur batteries.2.Polyaniline was doped with long-chain alkyl protonic acid to prepare long chain polyaniline fluid(LCPANI)as new binder for lithium-sulfur batteries.The conductivity of the doped polyaniline can provide an ion electron transport channel for the electrode,and the long-chain alkyl group can reduce the brittleness of the polyaniline molecule,relieve volume expansion,and thereby improve battery performance.At the rate of 0.1 C,the initial discharge specific capacity of the battery with LCPANI as binder reached 944 mAh/g.At a higher rate of 1 C,the battery with LCPANI as binder could maintain a high capacity retention rate as high as 84.3%,the average single cycle capacity attenuation was only 0.003%,indicating that LCPANI could maintain the stability of the battery structure while improving the ion electron conductivity.In the cyclic performance test of multiple rates,the battery with LCPANI as binder also showed better capacity recovery performance than the conventional one,with a cycle reversibility of up to 94.5%.3.The silane coupling agent ?-mercaptopropyltrimethoxysilane and p-amino thiophenol were grafted onto the surface of oxidized carbon nanotubes to obtain sulfhydryl group-grafted carbon nanotubes(CNTs-SH),respectively.XRD,Raman test,infrared spectroscopy,XPS full spectrum analysis,TEM and SEM spectra all showed that sulfhydryl group-grafted carbon nanotubes had been successfully prepared.The prepared CNTs-SH was mixed with LCPANI and coated on the surface of a lithiumsulfur battery separator for modification.The sulfhydryl group on the surface of CNTs could cut off the polysulfide bond,thus reducing its shuttle.The CNTs could also function as an upper current collector and increase the area of current collector to provide more sites for the reaction.4.The decrease in the ratio of element carbon to sulfur before and after the cycling of the lithium-sulfur battery with the two ?-mercaptopropyltrimethoxysilane and p-amino thiophenol thiolated carbon nanotubes modified separators indicated that the modified separator could reduce the shuttle of long-chain polysulfide.The electrochemical performance test of the battery showed that the CNTs-SH modified separator owned better electrical properties than the original separator of the previous chapter,the initial specific capacity at 0.1 C was as high as 967.9 mAh/g and 972.4 mAh/g,respectively,and the initial discharge specific capacity at high rate 1 C also reached 588 mAh/g and 728 mAh/g,respectively.In the thermal shrinkage test of the separator,when the temperature was raised to 155°C,the original separator shrinkage rate was as high as 43%,and the shrinkage of the two modified separators were only 22.3% and 26%,indicating that the thermal dimensional stability of the modified separator was enhanced,and the safety of the battery was thus guaranteed.5.By diazonium salt grafting and introducing oxygen-containing groups with oxidization on the surface of the carbon nanotubes,p-aminobenzenesulfonic acid was grafted onto the surface of the carbon nanotubes to obtain sulfonate group-grafted sulfonated carbon nanotubes(SCNTs).XRD,Raman test,infrared spectroscopy,XPS full spectrum analysis,TEM and SEM spectra all indicated that both methods had successfully grafted p-aminobenzenesulfonic acid onto the surface of CNTs.The prepared SCNTs was mixed with LCPANI and coated on the surface of a lithium-sulfur battery separator for modification.The negatively charged sulfonic acid group on the surface of the CNTs could surpress the shuttle of the polysulfide anion by electrostatic repulsion,and could also form an ionic bond with the doped polyaniline,which was beneficial to stabilize the internal structure of the battery.The CNTs could also function as an upper current collector and increase the area of current collector to provide more sites for the reaction.6.The decrease in the ratio of element carbon to sulfur before and after the cycling of the lithium-sulfur battery with the two diazonium salt grafting and acylation method SCNTs modified separators indicated that the modified separator could reduce the shuttle of long-chain polysulfide.In both methods,the SCNTs modified separator indeed improved electrical properties compared to the battery of the original separator of chapter three,the initial specific capacity at 0.1 C was as high as 1087.5 mAh/g and 1032.8 mAh/g,respectively,the initial discharge specific capacity at high rate 1 C was also as high as 972 mAh/g and 969 mAh/g,respectively.The liquid absorption test of the separator showed that both modified separators could improve the absorption of the electrolyte by the separator,compared with the 83.2% of original separator,the liquid absorption rate of the modified separators was as high as 100.5% and 101.2%,respectively,which was more favorable for ion transportation,contributing to improve the capacity and stability of the battery.