Study On The Preparation Of Fe-MOF Based Composites And It's Application For Lithium-sulfur Battery

Li-S batteries have an attractive theoretical specific discharge capacity of 1675mA·h/g with low price and non-pollution,which are drawing great extensive research as a hopeful energy-storage system.However,the issues of shuttle effect of polysulfide,poor conductivity of sulfur,and volume expansion during lithiation/delithiation seriously restrict the development of Li-S batteries.Recently,it has been reported that transition metal oxide and sulfide can form metal sulfur bond with polysulfide,which has an effective limiting effect on the diffusion of polysulfide in solution.However,the current application of metal oxide and sulfide in lithium sulfur batteries still has the problems of poor circulation performance and low initial specific capacity,inspired by the strong chemical absorption between metal oxide and sulfide with polysulfides,the composites derived from iron-based metal-organic framework?Fe-MOF?are applied as efficient sulfur host in Li-S batteries in this work.Transition metal iron ion can form strong chemical coordination bond with sulfur,meanwhile,chemical bonds can form between O and S in the compound with Li in polysulfide,wich can restrict the shuttle effect of polysulfide.In addition,the high content of carbon in Fe-MOF derived materials greatly improve the conductivity of cathode materials.Besides,the uniqe porous and hollow structure of MOF derived materials can effectively increase the sulfur load and adapt to the change of sulfur volume during lithiation.It is worth mentioning that the transformation of polysulfide during charge-discharge in lithium-sulfur battery can be catalyzed by FeS₂.In this work,the jujube like Fe₃O₄/C composite was synthesized by high temperature treatment using Fe-1,4-benzenedicarboxylic acid?Fe-BDC?as the precursor.The content of carbon in Fe₃O₄/C,measured by TGA,is 33.2%.The Fe₃O₄/C composite as sulfur hosts in lithium-sulfur battery and its electrochemical properties were studied.The Fe₃O₄/C cathode material manifested a high initial specific discharge capacity of 1323.7mA·h/g at a current density of 0.05 C as well as revealed a high initial specific discharge capacity of 819.2 mA·h/g at a high rate of 1 C.These can be attributed to the following points:1)As the polar material,Fe₃O₄ could form strong chemisorption with polysulfide,suppressing the shuttle effect of polysulfide.2)The Fe₃O₄/C contains abundant carbon,which greatly improves the conductivity of the material and improves the utilization rate of sulfur.3)The hollow structure of the Fe₃O₄/C composite has a large void space,which could furnish adequate space for sulfur reserve as well as provide ample interspace to relief the volume expansion during lithiation.4)The porous structure of Fe₃O₄/C composite could provide more active sites to adsorb LPS.Hollow spherical Fe₂O₃/C composite and Fe₂O₃/FeS₂/C composite were prepared by using Fe-zeolitic imidazolate frameworks?Fe-ZIF?as the precursor for lithium sulfur batteries.In this work,we have mainly studied the performance effect of ferric oxide combined with Fe₂O₃ and FeS₂ heterojunction in lithium-sulfur batteries.First,the Fe₂O₃/C composite was prepared and its electrochemical properties were studied.The Fe₂O₃/C cathode material manifested an initial specific discharge capacity of 805.7mA·h/g at a current density of 0.2 C as well as revealed an initial specific discharge capacity of 374.7 mA·h/g at a high rate of 1 C.The Fe₂O₃/FeS₂/C cathode material was prepared by high temperature treat Fe₂O₃/C and sulfur powder.The Fe₂O₃/FeS₂/C cathode material manifested an initial specific discharge capacity of 972.8 mA·h/g at a current density of 0.2 C as well as revealed an initial specific discharge capacity of 481.5 mA·h/g at a high rate of 1 C.This can be attributed to the polar material Fe₂O₃ has an effective limitation on polysulfide.Moreover,the high content of carbon in Fe₂O₃/C greatly improves the conductivity of cathode materials and it contributes to improve the utilization rate of active sulfur.After composite with FeS₂,the Fe₂O₃/FeS₂/C battery shows a small electrode polarization by cyclic voltammetry curve,which confirms FeS₂acts as a catalyst in charge-discharge process of battery.Finally,this work verifies the Fe₂O₃ and FeS₂ heterojunction beneficial the performance of lithium-sulfur battery.