Controllable Synthesis And Electrochemical Properties Of Iron Fluorides Cathode Materials

Over the past few decades,rechargeable lithium-ion batteries?LIBs?have been widely developed and used in various portable electronics devices,and are further expanded to be utilized in electric vehicles and energy storage system.As the most important component in LIBs system,cathode materials play an important roles on the energy density,cost and safety of LIBs.Iron fluorides have drawn tremendous attention because of their high electromotive force,huge theoretical capacity,abundant sources,and safety.Despite these advantages,the practical application of iron fluorides for LIBs has been retarded by the slow diffusion of Li⁺and poor electron conductivity.In order to solve these issues,we have developed a new method to prepare nanostructured iron fluoride and nanocomposites in this thesis.The main obtained achievements are listed below:In this thesis,nanostructured iron fluorides were prepared through a facile reverse micelles method,using iron?III?acetylacetonate?Fe?acac?₃?as iron source,hydrogen fluoride aqueous solution as fluorine source and tetraethylene glycol?TEG?as the solution.Fe?acac?₃ has two parts:hydrophbic acetylacetonate groups and hydrophilic Fe³⁺cation.After Fe?acac?₃ was dissolved in TEG,reverse micelles will be formed with the inside pool of hydrogen fluoride aqueous in the two-phase system.Monodispersed iron fluoride nanospheres?³00 nm?were obtained by tailoring the content of HF aqueousin the reaction system.Their electrochemical performances in LIBs were examined and characterized:a reversible discharge capacity of 101 mAh g^-1 can be obtained after 50 cycles at 1000 mA g^-1,and the retention is about 84%.Based on the method as above,iron fluorides with hierarchical mesoporous structure were successfully synthesized by tunning the temperature of synthetic process.This unique hierarchical mesoporous structure is beneficial for electrolyte accessibility and rapid lithium-ion diffusion.The hierarchical porous structured iron fluoride exhibited excellent rate capability(115.6 mAh g^-1 at 2000 mA g^-1)between 1.8 V and4.5 V versus Li/Li⁺,and a high reversible capacity of 143.2 mAh g^-1 after 100 cycles was obtained at 1000 m A g^-1.In addition,the charge-transfer resistance does not change obviously during the cyclic process,and the nanostructure maintains intact after 100cycles.FeF₃·0.33H₂O@rGO composites were prepared via an in-situ synthesis method to improve the conductivity.FeF₃·0.33H₂O nanoparticles were uniformly loaded onto the surface of graphene oxide?GO?,due to the interactions between the Fe³⁺and the oxygen groups of GO.At the same time,GO could offer more sites for the nucleation of FeF₃·0.33H₂O nanoparticles.GO is further reduced to rGO after heat treatment to act as a two dimensional conductive network.The Fe F₃·0.33H₂O@rGO composites exhibited excellent cyclic capability with a high reversible discharge capacity of 133.1 mAh g^-1after 100 cycles at 1000mA g^-1,and the capacity retention is about 99%.