| Nowadays,the global petrochemical industry is increasingly exhausted and the environmental pollution is increasingly serious,the development of renewable,recyclable and cleaner new energy is an important means to transform the energy structure of the country.The development of electric vehicles and smart grid is an inevitable trend,which puts forward new requirements on the energy density and cycle life of secondary batteries.Lithium-sulfur batteries are promising candidates for nextgeneration batteries due to their high theoretical capacity(1675 m Ah g-1)and high energy density(2600 Wh kg-1).However,the practical application of lithium-sulfur batteries still faces great challenges.The low conductivity of active sulfur affects the transport of electrons and slows the redox reaction kinetics of the battery;the serious shuttle effect of polysulfides leads to inactivation of active substances on the negative,which greatly limits the actual capacity of lithium-sulfur batteries and leads to unsatisfactory battery cycle performance.And large battery volume expansion and lithium dendrite growth make lithium sulfur battery have great potential safety hazards in practical application.Based on the shuttle effect of polysulfides in lithium-sulfur batteries,researchers optimize electrode materials,structure and separator modification to improve battery performance.In this paper,from the perspective of separator modification,the preparation method of in-situ growth modification material by liquid phase method was developed,and the gradient structure of ZIF-8 modification separator was designed reasonably,which effectively inhibited the shuttle effect of polysulfide and improved the performance of the battery.The specific research contents of this paper are as follows:(1)ZIF-8 particles(ZIF-8/PDA@PP)were in situ grown on polypropylene(PP)separators by a liquid phase method using polydopamine(PDA)as a bridge.The most suitable preparation scheme was selected by changing the reaction conditions,and the composition,structure and microscopic morphology of the modification separator were studied in combination with material characterization.The abundant functional groups of PDA provide strong adsorption,form Zn-O bonds with unsaturated Zn ions in ZIF-8,provide in-situ growth sites for ZIF-8,and enable ZIF-8 to form a firm and dense superstructure on the surface of PP.Thin modification layer and gradient modification of the internal pores of the separator.The polar Zn ions in ZIF-8 have strong adsorption capacity for polysulfides,and the dense modified layer inhibits the shuttle of polysulfides through the dual effects of physical barrier and chemical adsorption,reducing battery capacity loss.At the same time,the ZIF-8 particles and PDA in the pores of the separator can improve the electrolyte distribution in the separator,promote lithium ion transport,achieve uniform lithium deposition and alleviate lithium dendrite growth.(2)The electrochemical properties of the prepared ZIF-8/PDA@PP separator were characterized.The ZIF-8/PDA@PP separator has a higher lithium ion migration number calculated from the time-current curve.The visualized experimental results of polysulfide adsorption demonstrated that the ZIF-8/PDA@PP separator had a significant blocking effect on the shuttle of polysulfides.The lithium-sulfur battery assembled with ZIF-8/PDA@PP separator has better electrochemical performance: at a rate of 0.5 C,the discharge capacity of the first cycle is as high as 1326 m Ah g-1,and the reversible capacity after 100 cycles is still 892 m Ah g-1.Under the condition of high-sulfur-loaded cathode up to 5.8 mg cm-2,the areal capacity is 4.74 m Ah cm-2 after100 cycles at 0.5 C,and the capacity retention rate reaches 82%.The long-term cycling stability at a high rate of 2 C was tested.After 500 cycles,the reversible capacity of the battery was 711 m Ah g-1,and the capacity decay per cycle was only 0.013%. |
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