There-dimensional Self-supported Iron Oxide Electrodes For Sodium Storage

Hematite iron oxide (Fe2O3) is an excellentconversion-type battery material with high theoretical capacity and abundant resource.It has attracted abundant attention in the field of electrochemical energy storage.In particular,hematie is a promising electornde material for sodium storage.The storage through conversion reaction results in a large capacity but a poor activity and cyclability.In practice,the specific capacity of hematite is far below the theoretical value.Therefore,current research focus is to improve it electrochemical activity and structural stability.In this thesis,we report on the design and architecting of three-dimensional (3D)self-supported hematite electrodes.Furthermore,various strategies have been adopted to regulate materials composition,nanostructure and electrochemical performance.The main results and conclusion are summarized as follows:(1) 3D Fe2O3 nanotube array supported on Fe foil was synthesized by anodization,followed by sulfurization treatment to constuct Fe2O3/FeS2 core-shell composite structure(S-Fe2O3).Based on the semiconductors property difference of Fe2O3 and FeS2,the Fermi level bend at heterointerfaces will produce a built-in electric field.Therefore,the diffusion rate of sodium ions in the bulk phase is significantly improved with the positive effect of this electric field.With the synergy effect of porous and stable nanoarray structure,S-Fe2O3 electrode can provide a high acyivity of-400 mAh g-1 at 5 A g-1,and 91% capacity can be maintained after 200 cycles,which is far higher than other nanostructured Fe2O3 electrode material.(2) A 3D self-supporting Fe2O3 nanorod array electrode was prepared on a flexible carbon cloth substrate by hydrothermal method,the as-prepared electrode was then annealed under ammonia gas environment,and finally a surface nitrogen-doped and bulk Fe2+-doped Fe2O3 (N-Fe2O3-x) electrode material was obtained.The Fe-N bond exhibits a stronger covalent bond characteristic than the Fe-O bond,meanwhile,which decreases the sodiumion storage ion barrier,meanwhile,electronic conductivity was improved by bulk Fe2+-doping.Consequently,a high specific capacity of 895 mAh g-1 at 0.1 A g-1 can be provided by N-Fe2O3-x electrode,in addition,a superior capacity retention over 85% is realized after 200 cycles at 0.2 A g-1.