Study On Coating Modification Of Separators And Their Electrochemical Performance Of Lithium-sulfur Batteries

Because of its high theoretical energy density(2600 Wh kg^-1),low cost and environmental friendliness,Lithium-sulfur batteries have received extensive attention and research from the scientific and industrial circles.However,lithium-sulfur batteries still have problems,such as low utilization of active materials,poor electrochemical reversibility,and rapid capacity decay.From the perspective of modification of lithium-sulfur battery separator materials,this paper builds a functional coating layer on the surface of the separator to improve the utilization of active materials and inhibit the“shuttle effect”,thereby improving the performance of lithium-sulfur batteries.The main research work of this paper is as follows:（1）Preparation of reduced graphene oxide modified separators for Li-S batteries.Graphene oxide（GO）is prepared by the modified Hummers method,after thermal reduction,it is uniformly scraped on the surface of one side of the commercial PP separators to obtain modified separators coated with reduced graphene oxide（r GO@PP）.The r GO coating layer has good electronic/ion conductivity and rich pore structure,which can effectively adsorb the“free”lithium polysulfide dissolved in the electrolyte,and promote the electrochemical conversion of the“free”lithium polysulfide.Therefore,the r GO composite separators inhibits the“shuttle effect”of lithium polysulfide and improves the sulfur utilization efficiency.The batteries using r GO@PP separators delivers a discharge specific capacity of 999.4 m Ah g^-1 in the first cycle at 0.2C,which is much higher than that of the batteries using PP separators.After100 cycles,the Li-S batteries using r GO@PP separators still has a high reversible capacity(599.6 m Ah g^-1).（2）Preparation of MoS₂/reduced graphene oxide composite modified separators for Li-S batteries.MoS₂ nanosheets were grown in situ on the surface of reduced graphene oxide by a simple hydrothermal method,and uniformly scraped on the surface of one side of the commercial PP separators to obtain a MoS₂/reduced graphene oxide coated modified separators（MoS₂/r GO@PP）.The adsorption experiment results of lithium polysulfide solution show that the porous MoS₂/r GO composite material has a higher adsorption capacity of lithium polysulfide than r GO,The main reason is that MoS₂/r GO have more efficient physical and chemical dual adsorption.Moreover,the MoS₂ nanosheets obtained by low-temperature hydrothermal growth have more chemisorption and catalytically active sites,and have obvious catalytic and promotion effects on the electrochemical conversion of polysulfides,thereby increasing the electrochemical reaction rate.Therefore,the Li-S batteries using MoS₂/r GO@PP separators has excellent electrochemical performance:The discharge capacity in the first cycle at 0.2C is as high as 1252.6 m Ah g^-1,the specific capacity of 916.5 m Ah g^-1can still be achieved after 100 cycles.Even under the condition of high sulfur loading(3.2 mg cm^-2),the Li-S batteries using MoS₂/r GO@PP separators delivers a discharge specific capacity of 985.8 m Ah g^-1 in the first cycle,and the capacity remains 647.6m Ah g^-1 after 100 cycles.