Preparation And Electrochemical Properties Of Nano-sized Iron Oxide

Lithium-ion batteries(LIBs),as an efficient energy storage device,has attracted much attention for its high energy density,power density and long cycle life.At present,commercial graphite anode is limited by its low theoretical capacity(372 m Ah/g).Because of its high theoretical capacity(1007 m Ah/g),abundant resources,innocuity and environmental friendliness,Fe₂O₃ is considered as a potential electrode material.But Similar to other transition metal oxides,Fe₂O₃ electrode has some disadvantages during charging and discharging,such as large volume expansion/contraction,poor electrical conductivity and easy agglomeration and pulverization.To solve the above problems,amorphous Fe₂O₃,amorphous Fe₂O₃@C composite and single crystal Fe₂O₃ were prepared in this paper,and their microstructures and electrochemical properties were studied respectively,and their lithium storage mechanism was discussed.(1)preparing amorphous Fe₂O₃ nanospheres with potassium ferricyanide as iron source and ammonium dihydrogen phosphate a s etchant.The effect of hydrothermal temperature(140?/160?/180?/200?)on the structure and morphology of the samples was studied.When the temperature is 140?,the hydrothermal reaction is incomplete,and it dissolves again in the washing process.SEM shows that with the hydrothermal temperature rising to 180?,a large number of fine pores appear on the surface of amorphous nanospheres.When the temperature increases to 200?,a number of randomly interconnected channels are formed on the surface.The effe ct of hydrothermal time(1 h/3 h/6 h/9 h)on the surface morphology of amorphous Fe ₂O₃nanospheres was investigated under the condition of keeping the hydrothermal temperature at 200?.SEM and TEM photos showed that the H⁺concentration in hydrothermal solution seriously impacted the growth and morphology of amorphous Fe₂O₃ nanospheres.Based on ostwald aging and H⁺etching effect,amorphous Fe₂O₃ nanospheres with smooth surface are formed after hydrothermal treatment for 1 h.:After hydrothermal treatment,tiny holes appeared on the surface of nanospheres,and after 6 h,these holes connect,formed random channels.The hydrothermal samples with too long hydrothermal treatment time(9 h)grow into cubes.The electrochemical properties of different hydrothermal temperature and time to synthesize negative electrode materials were compared.it can be concluded that the electrochemical properties of the sample reacted at 200?for 6 h are the best.At the current density of 100 m A/g,the initial discharge capacity of the electrode was 1187.3 m Ah/g,and the reversible specific capacity remained at688.0 m Ah/g after 100 cycles.In order to explore the lithium storage mechanism of amorphous iron oxide electrode,the electrochemical performance of amorphous Fe ₂O₃ material with multi-channel surface obtained by reaction at 200?for 6h was compared with that of crystalline Fe₂O₃ material.The results showed that amorphous Fe₂O₃ electrode is superior to crystalline Fe₂O₃ electrode in cycle,rate,potential lag and EIS performance.The reason may be that the isotropy and grain boundary loss of amorphous Fe₂O₃ not only improve the ability of amorphous electrode to withstand high strain and lithium-ion intercalation,but also provide more Li⁺diffusion channels.In addition,the multi-channel structure on the surface of nanospheres promotes the good contact between internal active substances and electrolyte,and provides volume expansion space for lithium intercalation/deintercalation.(2)According to the study of(1),the capacity of amorphous Fe₂O₃electrode prepared at 200?-6h decreased obviously after 70 cycles.In this experiment,malic acid was used as carbon source to prepare amorphous Fe₂O₃@C composites.The electrochemical test of amorphous Fe ₂O₃@C composite shows that the cycle stability of amorphous Fe ₂O₃@C electrode is greatly improved compared with that of amorphous Fe ₂O₃ electrode without carbon coating,and the capacity of amorphous Fe₂O₃@C electrode decreased by 13%after 100 cycles,which is less than 20%of that of amorphous Fe ₂O₃electrode without carbon coating.Notably,sample agglomeration during carbon membranes may lead to reversible volume reduction.(3)With the purpose of further exploring the cause of amorphous formation,three-dimensional dendritic single crystal Fe₂O₃ material was obtained by using potassium ferricyanide as iron source without ammonium dihydrogen phosphate under the same other conditions.The influence of hydrothermal time(1h/3h/6h)on the microstructure and electrochemical properties of three-dimensional dendritic single crystal Fe₂O₃ was investigated.From SEM and TEM images,it can be seen that the sample after hydrothermal treatment for 1 h is composed of dense massive ferric oxide,which does not grow into a dendritic structure,while Fe₂O₃-3h partially grows into a dendritic structure.There are a few holes on the edge of the main branch,and no holes on the surface of the leaves.The sample after hydrothermal treatment for 1 h completely grows into a dendritic structure with are great quantity of holes on the surface of the leaves.At the current density of 100 m A/g,the initial discharge capacity of the electrode is 1273.5,1312.5 and 1580.0 m Ah/g,and the reversible specific capacity remains 356.8,770.8 and 860.0 m Ah/g after200 cycles.This shows that the hydrothermal 6 h sample has the best cycle performance,because its large specific surface area and a large number of irregular holes in the hydrothermal 6h sample provide more active sites for lithium-ion intercalation and deintercalation,avoiding the structural collapse caused by excessive lithium intercalation,thus improving the cycle performance of the material.