Controllable Construction Of Iron-based Nanomaterials@Carbon Nanoreactors For Electrochemical Energy Storage Applications

Given the huge energy demands and environment pollution issues,the development of alternative new energy resources becomes significant and urgent.Great deal of research has been devoted to the development of green,cheap,non-toxic,efficient and sustainable energy storage devices.Recently,iron-based nanomaterials have attracted great interest in energy storage systems because Fe is low-toxic and earth-abundant when compared with other counterparts like Co,V,Mn,etc.More importantly,the iron-based materials can exhibit a high theoretical capacity when used in lithium/sodium ion or aqueous batteries.However,detrimental pulverizations and agglomerations of electrode materials during repeated phase conversions would reduce their cyclic life and rate performance,which greatly hinders their widespread applications.Therefore,the reasonable design of safe,low-cost electrode materials has become a great challenge for the current scientific and application research.In this dissertation,we herein develop an in-situ evolution method to make FeF₃,Fe,Fe₂F₅nanoparticles confined in carbon nanoreactors.The main contents are listed as follows:To best reuse such rusty wastes,we herein propose a smart and applicable method to convert them into uniform?-Fe₂O₃ nanospheres.Only after a simple and conventional hydrothermal treatment in HNO₃ solution,nearly all of iron rust can evolve into sphere-like?-Fe₂O₃ nano products with a typical size of³0 nm.After a quick chemical-vapor-deposition?CVD?process,the crystallized carbon shells?10-20layers?are in situ catalytically grown on each nanosphere surface,giving rise to the generation of core-shell Fe₃O₄@C hybrids.Particularly noteworthy is that,rather than gaseous hydrocarbons?ethylene,acetylene,etc.?commonly serving as carbon sources,the ethylene glycol was alternatively employed as the carbon feedstock.Such core-shell Fe₃O₄@C intermediates are further transformed into FeF₃@C nanoreactors?size:⁶0nm?when adequately exposed in HF atmosphere followed by an annealing treatment?all this operation proceeds in a sealed and safe container?.When used in Li-ion battery cathode,such core-shell FeF₃@C nanoreactors configurations can exhibit outstanding cathode behaviors involving excellent reversible capacity retention and superior cyclability.After 600 cycles,the discharge capacity of FeF₃@C hybrids still remains at?260.8 mAh g^-1.However,pure FeF₃ cathode exhibits poor electrochemical behaviors;its specific capacity decreases quickly to a low capacity value of less than?70 mAh g^-1 after 170 cycles.The superior cyclability and capacity of core-shell hybrids may be tightly attributed to the outer C armors with good mechanical stability and electrical conductivity.Such C nanoreactors can play a vital role in protecting FeF₃ from adverse aggregations into bulks and accommodating volume variations in repeated electrochemical reactions.Furthermore,we develop an in-situ evolution method to make single-phased Fe nanoparticles confined in thick C nanoreactors?Fe@TCNRs?using gas-phase reactions.The raw materials in total fabrication only involve the cheap solvent ethanol and Fe₂O₃nanospheres made from useless iron rust.In particular,note that the as-made Fe nanoparticles in the core regions are of purity.These derived nanohybrids exhibit prominent anodic performance in terms of both stored capacity and cyclic durability when serving as anodes for Ni-Fe batteries.We choose metallic Fe as an electrode material for the following reasons:?i?it has superb physicochemical properties and overwhelming advantages(good electronic conductivity,high theoretical capacity over other ferruginous species?Fe₂O₃,Fe₃O₄?.?ii?The thick C nano shells can act as robust“self-adapting”nanoreactors,helping to accommodate volume changes of the active materials via graphitic stacking layer slipping during the insertion/deinsertion process of OH^-.In addition,the hybrid of Fe₂F₅@C nanosacks are synthesized via a one-step CVD method using the mixture of cheap ethylene glycol and ethanol as carbon sources and pre-made?NH₄?₃FeF₆ nanoparticles as initiating materials.During the CVD evolution process,?NH₄?₃FeF₆ salts are decomposed and transformed into Fe₂F₅,which can further act as catalysts to launch the in-situ growth of C nanosacks on their surface,giving rise to the formation of an intriguing yolk-shell hybrid nanostructure.The unique yolk-shell hybrid nanostructure can provide huge electrode/electrolyte contact areas and reduce the ion diffusion path for fast electrochemical kinetics.The in situ formed Fe₂F₅@C nanosacks product shows prominent anodic performance with high electrochemical activity and capacity,obviously prolonged cyclic lifetime,and outstanding rate capabilities.Besides,by pairing with the cathode of?-Co?OH?₂nanowire arrays@carbon cloth,a full device of rechargeable aqueous batteries has been developed,capable to deliver both large specific energy and power densities(Max.values reaching up to¹63 Wh kg^-1 and¹4.2 kW kg^-1).