Preparation Of Iron-based Composite Electrode Materials And Their Application In Lithium/sodium-ion Batteries

The development and use of clean energy storage and conversion devices are critical to the sustainable development of society.As a typical energy storage and conversion device,lithium/sodium-ion batteries need to further improve their energy density and power density,while reducing their production cost to meet the needs of large-scale energy storage applications.In lithium/sodium-ion batteries,anode materials play a very important role in their performance.At present,the graphite anode materials for commercial lithium-ion batteries cannot meet the needs of high power and high energy density due to its lower theoretical specific capacity and poor rate performance;meanwhile,sodium-ion batteries exhibit poor capacity and rate performance when graphite is used as the anode materials.Therefore,there is an urgent need to develop new anode materials for lithium/sodium-ion batteries.Recent studies have shown that the multi-electron reaction energy storage mechanism of iron-based electrode materials gives them a high reversible specific capacity,and coupled with their many advantages such as abundant resources,cheap price and environmental friendliness,they are one of the candidates for the new generation of lithium/sodium-ion batteries.However,iron-based electrode materials suffer from large volume changes when inserted with lithium/sodium,low self-conductivity,and tendency to agglomeration.To solve the above problems,this paper obtained a series of high-efficiency iron-based composite electrode materials by compounding iron-based active materials with amorphous carbon(polydopamine,polyacrylonitrile-derived carbon)and reduced graphene oxide(r GO),and deeply analyzed the effects of their microscopic morphology and elemental composition on the lithium/sodium storage performance,and systematically investigated the energy storage mechanism and electrochemical kinetic improvement mechanism of iron-based composites.The main research contents are summarized as follows.(1)A“double carbon”(polydopamine-derived carbon and r GO)synthesis strategy was proposed to successfully prepare Fe₃O₄@C@r GO composite film electrode with“double carbon”effect using inexpensive Fe₂O₃ nanoparticles as precursors and dopamine hydrochloride as the carbon source.The presence of appropriate voids between the active materials Fe₃O₄ and the encapsulated carbon layer effectively alleviates the volume expansion during the insertion of lithium-ions.The r GO in the surface layer can not only play the role of supporting the whole electrode and keeping the structure of the whole electrode stable,but also play a secondary buffering role for the volume expansion of Fe₃O₄.At the same time,the agglomeration of Fe₃O₄ active substances can be avoided,which is conducive to maintaining the microscopic morphology of the composites.The above advantages give Fe₃O₄@C@r GO electrodes outstanding cycling stability(the capacity is 408.2 m Ah g^-1 after 250 cycles at 0.5 A g^-1).In addition,the close contact between the carbon layer and r GO accelerates the charge transfer efficiency,so that the Fe₃O₄@C@r GO composites obtains excellent rate performance(reversible capacity of 371.3 m Ah g^-1 at a large current density of 1 A g^-1).More importantly,Fe₃O₄@C@r GO composites can still exhibit excellent lithium storage performance in full cells(capacity maintained at 85.8 m Ah g^-1 after 100 cycles at a current density of 0.2 C).(2)Fe₂O₃ nanoparticles were uniformly encapsulated in polyacrylonitrile(PAN)fibres by electrospinning technology,and Fe₃O₄@carbon nanofibres-40(Fe₃O₄@CNFs-40)with yolk-shell structure was formed after heat treatment and etching,and the three-dimensional interconnected Fe S₂@CNFs flexible thin film self-supporting electrode was successfully synthesized by further sulfuration strategy.The synthesis method eliminates the need for collectors,binders and conductive additives,thus effectively increasing the percentage of active materials.Meanwhile,the carbonization of PAN forms a three-dimensional continuous conductive carbon network,which facilitates the enhancement of charge transfer kinetics and thus significantly improves the rate performance of the composites,which maintain a capacity of 415.4 m Ah g^-1 and 210m Ah g^-1 at 5 A g^-1 when used as anode materials for lithium-ion and sodium-ion batteries,respectively.In addition,the voids between the carbon layer and Fe S₂ nanoparticles provide additional buffer space for the volume expansion of Fe S₂ and can form a stable solid interface(SEI)in the outer shell layer,thus facilitating the structural stability during cycling,resulting in an electrode materials with outstanding cycling stability performance(the capacity is 590.8 m Ah g^-1 after 500 cycles at 1 A g^-1 in lithium-ion batteries;the capacity is 280 m Ah g^-1 after 400 cycles at 1 A g^-1 in sodium-ion batteries).The above results indicate that the synergistic effect between the unique yolk-shell structural unit in Fe₃O₄@CNFs-40 and the three-dimensional carbon fiber conducting network endows the electrode materials with excellent lithium/sodium storage performance.(3)A general synthetic strategy was proposed to obtain Fe_xQ_y@CNFs-40(Q=Se,P)composites by further selenization and phosphorylation of Fe₃O₄@CNFs-40 obtained by electrospinning as described above.Microstructural characterization shows that both composites have the same yolk-shell structure and three-dimensional conductive network,and both electrode materials exhibit significantly improved rate and long-cycle performance when applied to the anode electrode of sodium-ion batteries.This general synthetic strategy was further confirmed to be practical in the synthesis of iron-based composites for sodium-ion batteries,which can provide a unique idea for the development of new lithium/sodium-ion batteries anode materials for the next generation.