Preparation Of ZIFs Derivatives/Fe₂O₃ Composites And Their Application In LIBs

In order to meet the growing demand for high-performance lithium-ion batteries in the development of modern society,there is an urgent need for a new generation of electrode materials with higher energy density,better safety,and longer battery life.Transition metal oxides(TMOs)are considered as promising electrodes for LIBs due to their diverse chemical valence states.Most TMOs(such as cobalt oxides,iron oxides,and manganese oxides)are anode materials for conversion-reactive lithium-ion batteries with relatively high theoretical specific capacities,which can meet the demand for high-energy-density batteries.Fe₂O₃are rich in natural resources,non-toxic and non-polluting.However,there are still major problems during the charging and discharging process,especially the cracking caused by the volume change and the detachment of the active material from the electrode sheet.Lead to poor cycle stability and rate performance of the battery.This seriously hinders its wide application in practical production and life.In response to the above problems,there are currently two effective solutions:one is to composite Fe₂O₃with carbon-based materials.The high conductivity of carbon-based materials can effectively improve the problems of poor conductivity of Fe₂O₃.At the same time,carbon-based materials have good flexibility.The properties can be used to protect Fe₂O₃in order to suppress the cracking of the electrode structure.The other is to construct nanostructures,which can not only effectively alleviate the volume effect during discharge/charge process,but also expose more electroactive sites and shorten the Li-ion diffusion distance.In this paper,combining the above two strategies,ZIF-8-derived N-containing porous carbon(NPC)/Fe₂O₃composite nanoparticles were prepared.In the experimental preparation process,the ZIF-8-derived porous carbon obtained by carbonization improves the conductivity of the electrode,and at the same time,the carbon structure can be coated on the outer layer of the active material to suppress its volume expansion.The main research work of this paper is as follows:(1)Fe₂O₃/ZIF-8 composites were synthesized by PVP.The Fe₂O₃/NPC composite material was obtained by subsequent carbonization treatment,and Fe₂O₃/NPC was made into an electrode for the anode of LIBs.ZIF-derived carbon materials usually retain the regular pore structure and high specific surface area,which can not only improve the active sites for charge reactions,but also form high-speed channels for ion transport inside the electrode.It makes the intercalation and deintercalation of lithium ions more convenient.Treating ZIFs materials at high temperature in an inert environment can carbonize the internal organic components and form a compact carbon matrix around the metal compound in situ,which can effectively buffer the volume change of the metal compound during the process of ion de/insertion and accelerate the electron conduction rate.The effects of different preparation conditions on the electrochemical properties of Fe₂O₃/NPC electrode materials were investigated by adjusting the carbonization temperature and the length of ZIF-8 coating treatment time.The investigation shows that the electrochemical performance of the electrode after carbonization treatment at 600°C is better compared with different carbonization temperatures.At a high current of 2000 m A g^-1,the corresponding capacity of Fe₂O₃/NPC electrode material remains 302 m Ah g^-1.In order to explore the cycling ability of the electrode,the electrochemical capacity of the Fe₂O₃/NPC electrode was 365 m Ah g^-1after 200 cycles.Compared with the pure Fe₂O₃electrode,the Fe₂O₃/NPC composite electrode obtained after coating showed more superior electrochemical performance.(2)Preparation and properties of ZIF-67-derived carbon-coated bimetallic oxide Fe₂O₃/Co₃O₄nanoparticle composite electrodes.The derived carbon prepared in this part of the work has better conductivity under the catalysis of metallic cobalt,and the Co₃O₄and Fe₂O₃nanoparticles with interlaced doping distribution suppress the structure collapse through synergistic action during the charging and discharging process.The prepared material is used for Lithium-ion battery anode exhibiting good rate performance and cycle stability.In the previous work,the Fe₂O₃/NPC composite electrode material was obtained by coating Fe₂O₃with ZIF-8 and carbonization,which greatly improved the rate performance of the Fe₂O₃-base electrode.However,it is difficult to suppress the damage of the electrode structure during the charge-discharge process under high current density and long-term cycle by simply coating with carbon layer.Therefore,in the following work,the Fe₂O₃was coated with ZIF-67,and the Fe₂O₃-Co₃O₄/NPC composite electrode structure was obtained after high temperature carbonization,and the reduced cobalt nanoparticles at high temperature could catalyze the conversion of amorphous carbon into graphite carbon to increase its electrical conductivity.At the same time,the volume expansion and contraction are alleviated by the synergistic effect between Co₃O₄and Fe₂O₃.The final results found that the Fe₂O₃-Co₃O₄/NPC nanoparticle blade-coated electrodes have good electrochemical performance(specific capacity of 441.4 m Ah g^-1at 2000 m A g^-1)and excellent cycling stability(The initial reversible capacity is 621 m Ah g^-1after 1000 long cycles at a current density of 1000m A g^-1).Post-cycling SEM and charge-storage mechanism analysis indicate that the excellent lithium storage performance of the Fe₂O₃-Co₃O₄/NPC electrode is attributed to the good structural integrity and Capacitance dominates charge storage behavior.