Synthesis And Electrochemical Properties Of Fe₃O₄ Composites As Anode Materials For Lithium Ion Batteries

After entering the 21^stcentury,the developments of lithium-ion batteries?LIBs?are more rapid.With its advantages of portability and environmental protection,LIBs have rapidly occupied the field of portable electronic equipment,and gradually developed into the ideal power source for new energy electric vehicles and hybrid electric vehicles.Currently,graphite has been used as a commercial anode material,but its theoretical specific capacity?372 mAh/g?is too low to meet the increasing demands of LIBs applications in modern life.It is very important for LIBs to explore new anode materials with high capacity and excellent stability.Fe₃O₄ has attracted tremendous attention due to its high theoretical specific capacity,rich earth reserve,low cost,and non-toxicity.But,the Fe₃O₄ material structure suffers from large volume changes and severe powder pulverization during lithium-ion insertion/extraction processes,which results in severe loss of capacity and poor cycling performance.To solve these problems,researchers have proposed many effective methods to improve the properties of Fe₃O₄ materials.An effective method is to synthesize materials with special morphologies and structures.Another method is to prepare Fe₃O₄ and carbon composite materials to maintain structural stability and improve the overall conductivity of the material.In recent years,Transition metal sulfides have been widely used in energy devices due to its high theoretical specific capacity.Sulfides also tend to exhibit better stability and initial Coulomb efficiency than their corresponding oxides.Therefore,we hope to combine the advantages of Fe₃O₄ and FeS_x through a reasonable structural design to prepare composite electrode materials with excellent electrochemical properties for LIBs.The specific work of this paper mainly includes the following two parts:?1?We have successfully prepared Fe₃O₄/FeS composite using hydrothermal synthesis,which iron nitrate as the sole source of iron,urotropine to provide hydroxyl and alkaline conditions,ultrapure water as the reaction solvent,and sublimed sulfur as the sulfur source.The Fe₃O₄/FeS composite has a regular octahedral structure,whcih can effectively inhibit agglomeration and alleviate volume expansion during Li⁺insertion/extraction processes.In addition,the characterization results show that the presence of FeS reduced the decomposition of electrolytes,increased electronic conductivity and stabilized the solid electrolyte interface films.As a result,as anode materials for lithium-ion batteries,the Fe₃O₄/FeS composite exhibits excellent cycle performance with a high specific capacity of 530.7 mAh/g after 100 cycles at 0.1 C,and superior rate performance,compared with prue Fe₃O₄?217 mAh/g after 100 cycle at 0.1 C?.?2?Inspired by the above research,we try to design and develop a novel composite of Fe₃O₄/FeS and carbon material by hydrothermal method to further improve conductivity,so as to further improve the performance of lithium storage performance.Compared with other conductive materials,graphene and its derivatives have higher conductivity,excellent chemical stability and structural flexibility.But,during the hydrothermal reaction,the graphene sheet layer is easy to agglomerate due to its poor dispersibility,which greatly reduces the specific surface area of graphene,and will hinder the infiltration of the electrolyte and electrons transmission.Therefore,we consider introducing graphene derivatives into Fe₃O₄/FeS materials.It is worth noting that graphene oxide is easy to disperse,and the sp² structure is reconstructed in the hydrothermal reaction to obtain excellent conductivity.In this work,we developed a simple one-step hydrothermal method to successfully prepare Fe₃O₄/FeS-reduced graphene oxide?Fe₃O₄/FeS/rGO?composite directly,which is a novel LIBs anode material.The characterization of Fe₃O₄/FeS/rGO composite demonstrates rGO are uniformly deposited on octahedral Fe₃O₄/FeS particles,leading to a strong synergy between them.The excellent structural design can make Fe₃O₄/FeS/rGO composite have higher reversible capacity?744.7 mAh/g at 0.1 C after 50 cycles?and superior rate capability.This outstanding electrochemical behavior can be attributed to the conductivity network of rGO improves the composite electrode conductivity and prevents the aggregation and pulverization of Fe₃O₄/FeS particles during charging and discharging processes.Moreover,the Fe₃O₄/FeS/rGO electrode surface is covered with a thin SEI film and the octahedral structure of Fe₃O₄/FeS particles is still clearly visible,which indicate composite electrode has excellent interface stability.We believe the desigin of this composite structure will provide a new perspective for the further study of other transition metal oxides for LIBs.