Preparation And Electrochemical Properties Of Iron-based Nanosheets And Their Composite Structures

Lithium ion batteries are widely used because of their light weight and low self-discharge.However,the theoretical specific capacity of commercial lithium-ion battery anode graphite is relatively low,which is difficult to meet people’s demand for high energy density equipment.Iron-based nanosheets are characterized by high specific capacity,large specific surface area and two-dimensional structure can provide a large number of active lithium storage sites.Moreover,iron resource is cheap and environment-friendly,so it is a promising anode material.In this paper,two kinds of iron oxides with different morphologies were prepared by solvothermal method,different kinds of iron nitride were controlled prepared by nitridation and Fe₃O₄@C Core shell structure with different thickness of carbon layer were controlled prepared by carbon coating.In this paper,the electrochemical properties of these materials were systematically studied,the main research contents and results are as follows.（1）Transition metal oxides have high theoretical specific capacity and two-dimensional structure have large active surface area,so they are very promising candidate materials for anode electrode of high energy density energy storage devices.Therefore,different iron oxide nanosheets and their assembling three-dimensional structure can be obtained by one-step solvothermal method.In this paper,we synthesized different thickness hexagonal nanosheets,disk-shaped nanosheets,nanoflowers assembled by nanosheets and rod assembled by nanosheets,and selected hexagonal nanosheets and nanoflowers to assemble lithium-ion batteries.Due to the low conductivity of metal oxides and the huge volume difference before and after charging and discharging,the structure is prone to pulverization or even falling off from the electrode during the long cycle process,resulting in the deterioration of their electrochemical performance.The specific capacity is about 1000 m Ah g^-1at 0.1 A g^-1,At the current density of 8 A g^-1,the specific capacity is only about 10 m Ah g^-1,and the capacity retention rate is less than 1%.（2）In order to improve the low conductivity of iron oxide and the huge volume change before and after charging and discharging,we used the prepared iron oxide as the precursor,selected melamine with high nitrogen content as the nitrogen source,and calcined with melamine to successfully prepare iron nitrides with good conductivity and small volume change.The lithium-ion battery assembled with this anode material shows excellent rate performance.The specific capacity is about 600 m Ah g^-1at 0.1 A g^-1,At the current density of 5 A g^-1,the specific capacity can reach about 375 m Ah g-1,and the capacity retention rate can reach more than 63%.（3）In order to buffer the volume expansion and improve the conductivity of the oxide,making it maintain the high specific capacity of iron oxide and improve the cycle stability.We used the prepared iron oxide as precursor,Fe₃O₄@C nano composite structure were prepared with glucose（GLC）and ethylene glycol（EG）as carbon sources.And then annealed at 350℃,400℃and 450℃respectively,Nanoflowers Fe₃O₄@C annealed at 400℃with ethylene glycol as carbon source,the specific capacity is about920 m Ah g^-1at the current density of 0.1 A g^-1and 650 m Ah g-1 at the current density of 5 A g^-1,and the capacity retention rate is 71%.Nanosheets Fe₃O₄@C annealed at400℃with glucose as carbon source,the specific capacity is about 1150 m Ah g^-1at the current density of 0.1 A g^-1and 900 m Ah g-1 at the current density of 5 A g^-1,and the capacity retention rate is 72%.