Application Of MOF-derived Carbon-based Composite Material Modified Separator In Lithium-sulfur Batteries

With the increasing demand for new energy electric vehicles and portable electronic equipment,energy storage systems with high energy density and low cost have been extensively studied due to the limited energy density of commercial lithium ion batteries(LIB).Lithium-sulfur(Li-S)batteries with high theoretical specific capacity(1675mAh g-1)and high energy density(2600 Wh kg-1)several times that of traditional LIB have aroused great research interest in recent years,and sulfur itself has the characteristics of low cost,abundant reserves and environmental friendliness.However,it should be emphasized that lithium-sulfur battery,as one of the most promising energy storage systems,still has the many disadvantages that affect practical application.It has poor sulfur conductivity and larger volume expansion,in addition,lithium polysulfide(LiPS)(Li2Sn,4?n?8),its intermediate during discharge,could cause "shuttle effect".In the paper,NiCo2S4@C and CoSe2@N-C/CNT are derived from metal-organic frameworks(MOF).The two composites not only have porous structure and high specific surface area,but also have high conductivity and excellent chemisorption to LiPS.According to these characteristics,the two materials are respectively coated on the traditional commercial separators to form functional separators.The functional separators are then applied to the Li-S batteries to improve the electrochemical performance of them.In the paper,NiCo2S4@C was prepared based on Ni-Co bimetallic organic frameworks.Then,the adsorption characteristics of NiCo2S4@C on LiPS were studied by adsorption experiments on LiPS and a series of test methods such as XPS.The results show that NiCo2S4@C has a strong chemical bonding effect on LiPS.Therefore,it is coated on the traditional separator as a functional layer to effectively adsorb LiPS dissolved in the electrolyte,and its excellent conductivity can make the absorbed LiPS further use,thereby improving the performance of the battery.When the battery is cycled at a rate of 1C,after 200 cycles,it still has a reversible discharge specific capacity of 700 mAh g-1,and the capacity attenuation per cycle is only 0.102%.By using ZIF-67 as a catalyst for in-situ growth of carbon nanotubes,and further through a simple selenization process,a composite(CoSe2@N-C/CNT)with a three-dimensional conductive network was prepared.As a polar metal compound,CoSe2 in the material can effectively capture LiPS,and CoSe2 nanoparticles are uniformly distributed in the structure of N-C/CNT,which can provide an effective electron transport path between active materials.The experimental results show that the electrochemical performance of Li-S batteries with CoSe2@N-C/CNT modified separators has been significantly improved.The battery has a specific discharge capacity of 975 mAh g-1 after 150 cycles at a rate of 0.5C.