Controllable Preparation Of Iron Sulfides (Oxides)/Carbon Composites And Their Sodium (Lithium) Storage Properties

Iron sulfides(oxides)have great development potential in the application of electrode materials for lithium-ion batteries and sodium-ion batteries because of their high theoretical specific capacity,rich raw material resources,low cost and environmental friendliness.However,as electrode materials,iron sulfides(oxides)have some defects,such as serious capacity attenuation and poor capacity retention in the long-time cycle processes,which largely stem from the poor conductivity,poor ion transport kinetics and excessive volume expansion in the charging-discharging processes.In view of the defects of iron sulfides(oxides)as anode material,Fe₂O₃@rGO composites and FeS₂@N-CNTs@rGO composites were prepared by a simple one-step hydrothermal method for the aim of improving the conductivity and structural stability of iron sulfides(oxides),and applied them to lithium-ion battery and sodium-ion battery respectively.The main research results are as follows:(1)Aiming at the problems of poor conductivity and volume expansion in the charging-discharging process of FeS₂ as an anode material of sodium-ion battery,we conceived and designed a hierarchical structure of FeS₂ wrapped by rGO and N-doped multi-wall carbon nanotubes(N-CNTs),which was produced through a hydrothermal method.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)analysis showed that FeS₂was an urchin-like sphere assembled by nano cones(size range:0.8-1.5?m),and N-CNTs and rGO were evenly wrapped on the surface of FeS₂ sphere.In contrast,the size of pure FeS₂sample is large,about 4-17?m.It can be seen that the introduction of N-CNTs and rGO limits the increase of FeS₂ particle size in the preparation process.In addition,the micro-nano hierarchitectures can heighten stability of electrode structure and electrochemical reaction kinetics upon repeated sodiated/desodiated process.Therefore,FeS₂@N-CNTs@rGO exhibits splendid electrochemical performance,including a high capacity of 558 m Ah g^-1 at 0.1 A g^-1and superior rate capacity of 419 m Ah g^-1 at 5 A g^-1.Furthermore,a capacity of 513 m Ah g^-1at 2 A g^-1 after 750 cycles can be noted.X-ray diffraction(XRD)and transmission electron microscopy(TEM)analyses reveal that the reaction mechanism in FeS₂ is ascribed as the reversible formation of Fe and Na₂S.The reaction kinetics of capacitive storage behaviour is studied by cyclic voltammetry(CV)curves.(2)In view of the problems of poor conductivity,poor ion transport dynamics and large volume expansion in the cycle process of Fe₂O₃ as the anode material of lithium-ion battery,we adopted the strategies of optimizing the morphology and structure of materials and combining with carbon materials,and synthesized the target products by a hydrothermal method.On the one hand,Fe₂O₃ was synthesized by a simple hydrothermal method with Fe OOH nanorods as precursors,and the morphology and structure of Fe₂O₃ were optimized by adjusting the hydrothermal reaction time.Finally,complete hexagonal bipyramid Fe₂O₃micro-particles with an average size of about 5.7?m were obtained.On the other hand,in order to further improve the lithium storage performance of Fe₂O₃ micro-particles,we prepared the composites of hexagonal bipyramid Fe₂O₃ and reduced graphene oxide(rGO).The introduction of rGO improves the conductivity of raw materials and buffers the volume expansion of Fe₂O₃ during the cyclic charging-discharging process.As a result,Fe₂O₃@rGO electrode exhibits high initial discharge/charge capacity(1714.6 m Ah g^-1 and 947.8 m Ah g^-1),good reversible capacity(a reversible capacity of 653.6 m Ah g^-1 at 0.1 A g^-1)and long-term cycling performance(a stable capacity of 292 m A h g^-1 can be retained at 1 A g^-1 even after700 cycles).