Preparation Of ZIF-8-derived Porous Carbon-based Composites And Their Application In Lithium-sulfur Batteries

Lithium-sulfur battery is an electrochemical system which can store and convert energy by reversible electrochemical reaction between metal lithium and elemental sulfur.It has high theoretical specific energy(2600 Wh·kg^-1)and theoretical specific capacity(1675 mAh·g^-1).At the same time,as a cathode active material,sulfur is cheap,abundant in reserves,non-toxic and environmentally friendly.Lithium-sulfur batteries are therefore considered to be the most commercially promising next-generation secondary batteries.Although lithium-sulfur batteries have many advantages,many technical problems have been found in the process of in-depth study.For example,the poor conductivity of active sulfur and discharge products,volume expansion and contraction,shuttle effect caused by the dissolution and diffusion of intermediate lithium polysulfide lead to rapid capacity decay and poor cycle performance.To solve these problems,finding suitable cathode materials is an effective way to improve the performance of batteries.Metal-organic framework material?MOFs?is a new porous material composed of metal ions?or metal clusters?and multi-functional organic ligands.Because MOFs has the advantages of large specific surface area,high porosity and adjustable pore size,it has been widely used in many fields,and people have also tried to use it as a matrix material for lithium-sulfur battery cathode materials.Porous carbon materials derived from ZIF-8 and porous carbon-metal Sulfide Composites derived from ZIF-8 were prepared using ZIF-8 as precursor.The electrochemical properties of ZIF-8 porous carbon-metal sulfide composites were studied as the matrix of positive sulfur for lithium-sulfur batteries.The specific research contents and conclusions are as follows:?1?ZIF-8,a metal-organic framework material,was synthesized by solvothermal method.Zn-ZDPC?600?,Zn-ZDPC?700?and Zn-ZDPC?800?were synthesized by carbonization at 600?,700?and 800?,respectively.Then zinc was removed by HCl,and ZIF-8-Derived Porous Carbon?ZDPC?were obtained.Then Zn-ZDPC?600?/S,Zn-ZDPC?700?/S Zn-ZDPC?800?/S and ZDPC?800?/S composites were prepared by traditional melt-diffusion method.The sulfur-loaded composites were used as cathode to assemble batteries.XRD,SEM,BET,EDS and XPS were used to characterize the pore distribution,morphology and composition of porous carbon derived from ZIF-8 and ZIF-8containing metallic zinc.It was proved that nitrogen atoms in the ligand were successfully incorporated into porous carbon.Charge and discharge test results show that the first discharge point specific capacity of Zn-ZDPC?800?/S electrode is 1162.4 mAh·g^-1 when the discharge rate is 0.2 C.After 100 cycles,it can still maintain 756.6 mAh·g^-1 and the capacity retention rate is65.1%.Compared with ZDPC?800?/S electrode,the capacity decay is slower,which shows better electrochemical performance.Therefore,MOFs-derived porous carbon materials containing metal Zn have many advantages.Firstly,the porous carbon matrix formed after carbonization can effectively conduct electrons and transport ions.Secondly,nitrogen atoms in ligands are doped in carbon materials,and the polar surface can provide better electrolyte wettability,thus promoting ion diffusion,and its polar surface can also form a strong interaction with polysulfides.Therefore,it can adsorb polysulfides and reduce the diffusion of polysulfides.Finally,metal Zn can promote the transformation from soluble high valence polysulfides to insoluble low valence polysulfides,and improve the reaction kinetics.?2?nitrogen-doped ZIF-8-derived porous carbon-zinc sulfide composites?ZnS-ZDPC?were successfully synthesized by continuous carbonization and vulcanization reaction using ZIF-8 as precursor.It is proved by physical characterization that the synthesized ZnS-ZDPC has regular polyhedron morphology,large specific surface area and porous structure.The large specific surface area can provide more adsorption sites,while the porous structure can limit more sulfur in the pores and alleviate the influence of volume change on the carbon matrix.The nitrogen atoms in the ligand are doped into the carbon matrix by carbonization,forming three different types of nitrogen.Pyridine and pyrrole N provide polysulfide binding sites and form a strong interaction with lithium polysulfide.The polysulfide is adsorbed on the surface of carbon matrix,while graphitized nitrogen improves the conductivity of carbon and facilitates the transport of electrons.The in-situ growth of ZnS,on porous carbon matrix can accelerate the conversion reaction of intermediate product polysulfide,improve the reaction kinetics,reduce the accumulation of polysulfide in organic electrolyte,and effectively inhibit the shuttle effect.Therefore,three-dimensional porous carbon materials with appropriate nitrogen content and zinc sulfide cooperate to promote the electrochemical performance of lithium-sulfur batteries.When the discharge rate is 0.2C,the specific discharge capacity of ZnS-ZDPC/S electrode is 1265.2 mAh·g^-11 for the first time and the capacity decays to 654.1 mAh·g^-1 after 300 circles.It is slower than that of the ZDPC?600?/S electrode,so results in a better cycling performance.