Application Of Iron-Based Composites In High-Power Lithium-Ion Hybrid Capacitors

In recent years,the lithium-ion hybrid capacitors assembled with a lithium-ion battery type anode,a supercapacitor type cathode,and organic electrolyte containing Li⁺combines the advantages of two energy storage mechanisms,which has a good prospect in practical application.The commercial lithium-ion hybrid capacitors adopt activated carbon with high specific surface area as the cathode,but the research and development of anode materials are still difficult to meet the industrialization demands.Among numerous anode materials,iron-based materials have large specific capacity,which can significantly improve the energy density and power density of lithium-ion hybrid capacitors.Also,iron-based materials are non-toxic and environment-friendly.The development of iron-based materials is also in line with China’s national policy of"sustainable development".The iron is extremely rich in resources,only less than oxygen,silicon,and aluminum in the earth’s crust.Therefore,the development of iron-based materials is low-cost and has good economic benefit.Nevertheless,iron-based materials also have their own shortcomings.Because the energy storage of iron-based materials in lithium-ion hybrid capacitors is obtained through the conversion between multivalent states of iron,the conversion process will cause phase transition.It will further result in significant volume change of materials,pulverization,and poor contact with current collector,which will eventually lead to the short cycle life of energy storage devices or even device failure.In view of this,this thesis utilizes cheap hematite as raw material and reuses the abandoned HCl and Na OH in the laboratory to explore efficient,convenient,low-cost,and large-scale methods to prepare a series of iron-based composites and iron derived materials to improve their electrochemical performance.The research contents mainly include the following categories:（1）High-energy flexible quasi-solid-state lithium-ion capacitors enabled by a freestanding rGO-encapsulated Fe₃O₄nanocube anode and a holey rGO film cathodeThe filtration assisted self-assembly method is adopted to convert the Fe₂O₃nanocubes and graphene oxide（GO）aqueous solution to the flexible GO@Fe₂O₃membrane,further annealed to obtain the flexible rGO@Fe₃O₄film that can be used as the anode.GO@Fe₂O₃is etched in an inert atmosphere under high temperature,and the Fe₃O₄is dissolved with HCl to obtain a flexible porous rGO film,which is used as the cathode.A flexible quasi-solid-state lithium-ion hybrid capacitor is successfully assembled with the P（VDF-HFP）based quasi-solid electrolyte,anode,and the cathode.Due to the protection of the rGO nanosheets,the volume change of Fe₃O₄can be effectively buffered,maintaining the microstructure of Fe₃O₄and contributing to its stability during the lithium insertion process.The assembled lithium-ion hybrid capacitor exhibits high energy density,high power density,good cycling stability,and good mechanical flexibility.（2）Nitrogen doped carbon nanofiber encapsulated hexagonal Fe₃O₄nanosheets for high power lithium-ion capacitorsElectrospinning that can prepare materials on a large scale is adopted.Combined with the subsequent annealing process,the precursor of Fe₂O₃hexagonal nanosheets and polyacrylonitrile can be compounded to prepare the necklace like flexible film of Fe₃O₄embedded within carbon fiber matrix（Fe₃O₄@CNF）.Activated carbon（AC）is coated onto carbon fibers to obtain the cathode of AC@CNF.A flexible lithium-ion hybrid capacitor is prepared with the anode and cathode.The results indicate that the carbon matrix can effectively buffer the mechanical stress of Fe₃O₄hexagonal nanosheet during electrochemical reactions,prevent the pulverization of active material,and improve its cycling stability,while the ultra-thin hexagonal nanosheet structure ensures the high lithium storage activity and fast lithium insertion/extraction kinetics.The lithium-ion hybrid capacitor has high energy density and power density.In addition,its superior flexible stability guarantees the lithium-ion hybrid capacitor to be applied in the next generation wearable devices or flexible electronic products.（3）Nitrogen-doped carbon nanotube-buffered FeSe₂anodes for fast-charging and high-capacity lithium-ion hybrid capacitorNatural hematite is used as raw material.After acid treatment and purification,then mixing with melamine and reducing under high temperature,and finally selenizing,the target material of FeSe₂@N-CNT is obtained and used as the anode.Coupled with the cathode of AC,a lithium-ion hybrid capacitor is fabricated.In FeSe₂@N-CNT,the active FeSe₂nanoparticles are tangled within the N-CNT network.The tubular carbon network along with nitrogen doping endow the nanocomposite with enlarged contact area with electrolyte,increased electrical conductivity,shortened Li⁺/e^-diffusion length,and reduced volume variation of the FeSe₂during lithium insertion/extraction.The lithium-ion hybrid capacitor assembled with FeSe2@N-CNT anode and AC cathode has high power density and high energy density.In this work,the simple processing procedures（mixing and annealing）enable FeSe₂@N-CNT composites to easily accomplish mass production in industry.（4）A mechanically flexible necklace-like architecture for achieving fast-charging and high-capacity in lithium-ion hybrid capacitorsThe Fe₂O₃cubes are firstly embedded within polyacrylonitrile（PAN）fibers through electrospinning,with further high temperature reduction and selenization,a necklace structured composite fiber film is obtained（CNF@FeSe₂）and can be directly used as the anode without any additives.AC is coated onto carbon fibers to obtain the cathode of AC@CNF.A flexible lithium-ion hybrid capacitor is assembled using the anode and cathode.In this film,the carbon fiber matrix can not only buffer the volume expansion of FeSe₂during lithium insertion/extraction to maintain its structure stability,but also ensure the ultrafast Li⁺intercalation dynamics.The fiber film of CNF@FeSe₂has high mass loading,high rate capability,and good cycling stability.When CNF@FeSe2 fiber film is utilized as anode to assemble a quasi-solid-state lithium-ion hybrid capacitor,the device shows a high volume energy/power density,excellent cycling stability,low self-discharge rate,and high mechanical flexibility.（5）A high-power lithium-ion hybrid capacitor based on a Fe₂O₃template derived hollow N-doped carbon nanobox anode and its porous analogue cathodePolypyrrole is firstly coated on the Fe₂O₃cube,HCl is then utilized to etch the Fe₂O₃template and obtained the polypyrrole hollow nanobox.Further annealing at high temperature,hollow nitrogen doped carbon nano boxes（HNCNBs）are obtained and used as the anode.Mixing HNCNBs with KOH and then annealing at high temperature,porous hollow nitrogen doped carbon nano boxes（PHNCNBs）are prepared and adopted as the cathode.A dual carbon lithium-ion hybrid capacitor is then constructed with the anode and cathode.Due to the characteristics of high nitrogen doping and hollow nanostructure,HNCNBs show high specific capacity and excellent rate capability.When HNCNBs anode and PHNCNBs cathode are assembled into a dual carbon lithium-ion hybrid capacitor,the hollow structure of both anode and cathode can reduce the Li⁺/e^-diffusion length,contributing to the high energy/power density and long cycling stability of the lithium-ion hybrid capacitor.