| Lithium ion batteries have advantages of high output voltage,good safety performance,and long cycle life.They are used in portable electric vehicles and electronic products.At present,commercial lithium ion batteries use graphite as a negative electrode material,but their low specific capacity(372 mA·h·g-1)limits the improvement of the energy density of lithium ion batteries.The transition metal oxide Fe3O4 has the advantages of high theoretical capacity(926 mA·h·g-1),cheap and easy to obtain,non-toxic.It is expected to become an ideal substitute for graphite anodes,but its volume change during charging and discharging is large,the problem of poor conductivity makes the electrode material prone to agglomeration or swelling,thereby reducing rate performance and cycle stability.In order to solve the above problems,this paper uses heterogeneous doping,morphology control and phosphating methods to control the structure of Fe3O4 to improve the performance of its lithium ion batteries.(1)Ag is doped into Fe3O4 nanospheres by a one-step solvothermal method to improve the performance of its lithium ion battery.In the experiment,FeCl3·6H2O,CH3COONa and AgNO3 were used as raw materials,and ethylene glycol was used as the reducing agent to prepare Ag-doped pomegranate seed-like Fe3O4 composite materials.This method does not use any inert gas or surfactant.Ag nanoparticles significantly improve the conductivity and cyclic stability of Fe3O4 nanospheres.It is used as a anode electrode materials for lithium ion batteries,the specific capacity of the composite is stable after 150 cycles at a current density of 0.5 C(463 mA·g-1)at 550 mA·h·g-1,the calculated specific capacity based on Fe3O4 is 696 mA·h g-1,which shows its excellent cycle stability.In addition,at a higher current density of 2.0 C(1852 mA·g-1),the specific discharge capacity of the composite can still reach 390 mA·h g-1.(2)Preparation of Fe3O4 nanosheets by a two-step method of solvothermal and carbothermal reduction.In the experiment,graphene oxide(GO)was used as a structure directing agent,FeCl3·6H2O and CH3COONa were used as raw materials,and ethylene glycol was used as a reducing agent to synthesize Fe304 nanosheet precursors.Subsequently,heat treatment was performed at 500℃,and Fe3O4 nanosheets were prepared by carbothermal reduction.The thickness of Fe3O4 nanosheets is 90～120 nm,and the particle size is about 10～15 nm.When applied to lithium ion battery anode materials,the current density is 0.2 C(185.2 mA·g-1).The capacity was maintained at 910 mA·h·g-1 after two charge and discharge cycles.Under the condition of large charge and discharge of 0.5 C after 200 cycles,the capacity is still maintained at 500 mA·h·g-1.Graphene enhances the conductivity of Fe3O4 nanosheets,and the layer structure can shorten the lithium ion diffusion path and thereby improve the rate performance and cycle stability of lithium ion battery anode materials.(3)Phosphating the Fe3O4 nanosheet precursor in(2)by solid-phase phosphating to prepare graphene oxide-supported FeP nanocomposites.FeP nanoparticles have a size of 90～150 nm,FeP/GO nanoparticles have a size of 30～65 nm,and the introduction of graphene oxide reduces the size of the nanoparticles,which can suppress the aggregation of the nanoparticles during the phosphating process and can also increase the particle size conductivity of composite materials.FeP/GO is used as a lithium ion battery anode material,the first discharge capacity can reach 1050 mA·h·g-1,and the capacity is maintained 600 mA·h·g-1 after 150 charge and discharge cycles at a current density of 0.2 C(185.2 mA·g-1). |
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