Modification And Research Of Fe₃O₄ Anode Materials For Lithium Ion Batteries

In the wake of developments in science and technology,lithium-ion batteries have become the primary choice for energy storage devices.At present,the mainstream anode material for industrially produced lithium-ion batteries is graphite anode.Graphite anode materials have the advantages of abundance,industrialization of preparation processes,excellent chemical stability and so on.However,the lower theoretical specific capacity(372 mAh g^-1)of graphite anode cannot satisfy the needs of lithium-ion batteries for developing high-capacity energy storage devices Therefore,further research on anode materials with high specific capacity is one of the main tasks for developing high-capacity energy storage equipment.Fe₃O₄ possesses a specific capacity of nearly three times that of graphite anodes(924mAh g^-1),and its preparation process is safe and environmentally friendly.It is considered to be one of the promising anode materials for lithium-ion battery which can replace graphite anode.However,there are still some problems to be solved in the practical application of Fe₃O₄ as the negative electrode materials of lithium-ion batteries.For example,during the cycle,the active material Fe₃O₄ would change in volume with the increase of cycle times,the particles would be powdered,and even the active material Fe₃O₄ would be separated from copper foil.At the same time,due to the physical properties of Fe₃O₄,the transmission rate of lithium ion is low,which increases the impedance of the battery system.These problems have seriously affected the practical application value of Fe₃O₄ as a negative electrode material.In this thesis,Fe₃O₄ is combined with highly conductive materials to improve the conductivity of the active material Fe₃O₄,and simultaneously alleviate the gravitational effect of the volume change of the active material Fe₃O₄ during the lithium ion de-intercalation process.Thus,the lithium battery assembled with Fe₃O₄ as a negative electrode material has higher cycling stability.?1?The solid-phase sintering method was used to prepare porous carbon-encapsulated Fe₃O₄ composites?Fe₃O₄@C?,and Fe₃O₄@C with excellent electrochemical properties was obtained by changing the sintering process and the proportion of raw materials.Fe₃O₄@C still had a reversible capacity of 770 mAh g^-1 after130 cycles at a current density of 0.5 A g^-1,and a reversible capacity of 477 mAh g^-1 after500 cycles at a current density of 2 A g^-1.?2?Fe₃O₄@N-C nanoparticles were obtained by simple hydrothermal and low-temperature sintering process with glucose as the carbon source and ferrous sulfate heptahydrate as the iron source.The results showed that the disordered carbon coating structure is helpful to improve the electron,ion transport efficiency and conductivity of Fe₃O₄ nanoparticles as anode materials of lithium-ion batteries.Besides,it can greatly alleviate the volume change of the active material Fe₃O₄ during the charging and discharging process,and improve the electrochemical performance and cycle stability of Fe₃O₄.Fe₃O₄@N-C exhibited a discharge capacity of about 915 mAh g^-1 and a capacity retention rate of about 90.5%after 600 cycles at a current density of 2 A g^-1.?3?In order to exploit the electrode material with high performance under high current,Fe₃O₄-C_?PVP,SDBS,C??PVP?Polyvinyl Pyrrolidone?,SDBS?Sodium Dodecyl Benzene Sulfonate??was prepared by solid-phase sintering surfactant and iron salt.Fe₃O₄-C_?PVP,SDBS,C?showed excellent electrochemical performance.Fe₃O₄-C_?PVP,SDBS,C?composite material had a specific discharge capacity of 418 mAh g^-1 when it is circulated for 300 cycles at a current density of 5 A g^-1.The charge and discharge efficiency was99.3%.However,the morphology of Fe₃O₄-C_?PVP,SDBS,C?was difficult to control.Therefore,we tried to prepare Fe₃O₄ nanomaterials by coprecipitation method,following by using common surfactant PVP to coat Fe₃O₄,but its electrochemical performance is not ideal.Finally,Fe₃O₄-CNTs composite nanomaterials were obtained by CNTs co-precipitation hydrothermal reaction with iron nitrate nonahydrate.The results implied that during the co-precipitation process,the addition of CNTs can provide more Fe₃O₄nucleation sites,inhibit the growth of Fe₃O₄ particles in the process of hydrothermal and calcination,and thus uniform and fine porous nanostructures could form.CNTs is beneficial to improve the ion transmission efficiency,inhibit volume changes,and improve electrochemical performance of Fe₃O₄ materials.The prepared Fe₃O₄-CNTs had a stable discharge capacity of about 230 mAh g^-1 at the 100th lap in a current density of10 A g^-1,and the capacity drops to 179 mAh g^-1 when it was cycled to 9999 laps.Compared to the initial stable capacity?100 laps?,only 50 mAh g^-1 of capacity was lost with charging and discharging efficiency as high as 99.98%.