Study On Preparation And Modification Of Fe-based Anode Materials For Lithium Ion Batteries

In recent years,lithium-ion batteries are commonly used in portable electronic devices and electric vehicles.With the lack of traditional energy sources and the improvement of environmental awareness,lithium-ion batteries are required to have higher capacity,safety,long life,and good stability.At present,graphite is commonly used as a negative electrode material for lithium ion batteries in commercial batteries.The specific capacity of graphite is only 372 mAh g-1,which is far from the needs of people.Therefore,the research on negative electrode materials has attracted widespread attention.Iron-based oxides have the advantages of non-toxic and pollution-free,low cost,rich resources and high theoretical capacity,but at the same time,there are problems such as large loss of reversible capacity for the first time and poor conductivity.In order to solve these problems,the FeCl3 was employed as the raw material,and the morphology and size of Fe2O3 was controlled by adjusting the temperature.The two-step hydrothermal method was used to prepare Fe2O3/rGO composites with different graphene ratios.Futher,a one-step hydrothermal method(water Pyro-assisted in-situ method,PVP-assisted hydrothermal in-situ method,oleic acid-assisted hydrothermal in-situ method)was applyed to synthesize Fe2O3/rGO composite materials,with the purpose of improving the electrochemical performance of iron-based oxide anode materials.Firstly,the morphology and size of Fe2O3 were controlled by changing the hydrothermal temperature.The SEM results show that as the hydrothermal temperature decreases from 220℃ to 175℃,the morphology changes from spheroid to nanorod shape,and the grain size decreases from 500 nm at 220℃ to about 50 nm at 175℃.Secondly,by adjusting the ratio of Fe2O3 and graphene oxide,a two-step hydrothermal method was used to prepare the Fe2O3/rGO composite.It was found that when the ratio of Fe2O3 and graphene oxide was 1:0.2,graphene did not stack,and Fe2O3 was evenly distributed.And with the increase of graphene content,it is easy to cause the accumulation of graphene,and the phenomenon of micro agglomeration of Fe2O3 occurs.Thirdly,one-step hydrothermal methods(hydrothermal in-situ method,PVP-assisted hydrothermal in-situ method,oleic acid-assisted hydrothermal in-situ method)were used to prepare Fe2O3/rGO composites.The morphology of the prepared alpha-Fe2O3 by different methods was different.Alpha-Fe2O3 prepared by hydrothermal in-situ method and oleic acid-assisted hydrothermal in-situ method is mainly composed of fine spheres,while PVP-assisted hydrothermal in-situ method presents porous ellipsoids.Graphene exhibits typical folds and small blocks.XRD analysis shows that the phase composition of Fe2O3/rGO composites with different graphene ratios consists of alpha-Fe2O3 and graphene phases.Pure Fe2O3 anode materials existed as alpha-Fe2O3 single phase,with high crystallinity and purity.The results of electrochemical cycling performance test show that the electrochemical cycling performance is optimal when the ratio of Fe2O3 and graphene oxide is 1:0.2,compared to pure Fe2O3 anode materials.At a current density of 100 mA g-1,Fe2O3/rGO is cycled for the first time with a discharge capacity of 1372 mAh g-1.The cycle decay rate is slow and after 50 cycles,the cycle reversible capacity is maintained at about 435 mAh g-1.When using FeCl3 and graphene oxide as raw materials,the electrochemical cycling performance of the composites prepared by three different processes shows that the electrochemical performance of Fe2O3/rGO prepared by the oleic acid-assisted hydrothermal method is the best.The Fe2O3/rGO anode material has a first cycle discharge capacity of 1431 mAh g-1 at a current density of 100 mA g-1.After 50 charge and discharge cycles,the cycle reversible capacity remains at 478 mAh g-1.The results of electrochemical impedance spectroscopy show that compared with pure Fe2O3 anode materials,the internal resistance is significantly reduced after the addition of grapheme.The internal resistance order of Fe2O3/rGO composites prepared by three different processes is:R(oleic acid-assisted hydrothermal in-situ method)<R(hydrothermal in-situ method)<R(PVP-assisted hydrothermal in-situ method).The results show that the composite materials prepared by oleic acid-assisted hydrothermal in-situ method have excellent electrochemical performance.