Study On Structural Design And Li-Ion Storage Properties Of Open Framework FeF₃·0.33H₂O As Conversion-Type Cathode

Iron-based fluoride,as a kind of insertion/conversion-type cathode material,has attracted extensive attention of researchers because of its high theoretical specific capacity,high operating voltage,low-cost and environmentally friendly.However,its application is limited by severe dissolution of active substances,poor rate and cycling performance,and serious voltage hysteresis caused by the poor intrinsic electronic conductivity,large volume expansion,sluggish kinetics.In order to address the above problems,the open framework Fe F3·0.33H2O with relatively good structure stability and conductivity is selected as the research object in this thesis,with the aim of improving the lithium storage performance of Fe F₃·0.33H₂O as the guidance.A series of Fe F3·0.33H2O cathode materials with unique morphology and structure,heter-atomic doping and surface coating were designed and prepared by three strategies of nano engineering,band-gap engineering and surface engineering.In addition,the crystal structure,morphology feature,element distribution,valence state,electronic structure,electrochemical properties and mechanism characteristics of the as-prepared materials were systematically investigated and analyzed by means of different physical and electrochemical measurements combined with first-principles calculation.The specific research contents are as follows:1.The Fe F₃·0.33H₂O microspheres with hierarchical micro/nano structure was successfully fabricated by a simple solvothermal method and annealing treatment to improve the sluggish conversion kinetics and serious voltage hysteresis of Fe F₃·0.33H₂O.Through designing process route and optimizing the reaction conditions,e.g.,temperature,reaction time and the concentration of reactants,a series of Fe F₃·0.33H₂O with various morphology and crystal type is also obtained.The experimental results show that the prepared material is pure Fe F₃·0.33H₂O with high crystallinity,and exhibits the spherical morphology of micron scale by self-assembly of primary nanoparticles.When this material is used as the cathode for lithium-ion batteries,it shows good lithium storage performance,which is mainly attributed to its three-dimensional hierarchical micro/nano structure,which can shorten electron/ion transport paths and accelerate conversion kinetics.2.To improve the poor electronic conductivity of Fe F₃·0.33H₂O caused by the strong electronegativity of F,multiwalled carbon nanotubes（CNTs）wiring and bowl-like carbon（BC）confining of Fe F₃·0.33H₂O nanocomposites（Fe F₃·0.33H₂O/CNTs/BC）were synthesized by precise temperature control combined with polyethylene glycol assisted deposition method.Wherein,the CNTs provides an omni-directional conductive network for electron transport,while the BC can confine the part of Fe F₃·0.33H₂O in the"bowl",which can improve the overall conductivity of the material and alleviate the dissolution and volume expansion of the active material.Benefiting from the unique structural design described above,Fe F₃·0.33H₂O/CNTs/BC exhibits reduced charge transfer impedance and rapid ion diffusion,resulting in a significant improvement in electrochemical performance.3.The Ni-doped Fe F₃·0.33H₂O cathode material was prepared by simple solvothermal method to improve its electrical conductivity and reaction kinetics of Fe F₃·0.33H₂O.Through the first principles calculation,comprehensive physical/chemical characterization and electrochemical measurements for the doped materials,the results show that the Ni-doped samples present a porous spherical shape composed of ordered self-assembly of primary particles at about 10 nm,and part of Fe at 4b and 8h sites were replaced by Ni atoms accompanied by the generating of F vacancies and without changing the crystal structure.Moreover,Ni-doping can reduce the band gap,improve the intrinsic conductivity and broaden the ion channels of Fe F₃·0.33H₂O.Thus,it is beneficial to the rapid transfer of ions/electrons and obtain excellent electrochemical performances.4.To further enhance the bulk phase conductivity of Fe F₃·0.33H₂O and alleviate the dissolution of active substances during cycling,a 3D sandwich structure of r GO-encapsulated Co/Ni dual‐doped Fe F₃·0.33H₂O nanoparticles（NC-FF）was successfully by coprecipitation and subsequent calcination.The flower-like Co/Ni dual‐doped Fe F₃·0.33H₂O nanoparticles can be stably encapsulated in highly conductive r GO sheets by M-O-C（M=Fe,Co,Ni）bonding to form a robust electron/ion network and a good protective buffer layer to prevent large volume expansion and side reactions.Additionally,it has been found that can facilitate to the capacity improvement by expanding the cell volume,and Co-doping can promote the rate capability and cycling stability enhancement by accelerating the migration of Li⁺.Benefiting from the synergies of Ni/Co dual-doping and encapsulation of r GO,the NC-FF exhibits excellent high-speed performance and cycle stability.5.In order to comprehensively improve the intrinsic conductivity and structural stability of the materials,a rod-like architecture Nb-substituted Fe F₃·0.33H₂O（Nb-Fe F₃·0.33H₂O@C）nano-crystals with in situ carbon coating derived from carbonization of ionic liquid are deliberately designed and prepared.It was found that the prepared materials have abundant active sites,fast electronic/ionic transfer paths and stable hierarchical structure.Based on the theoretical calculation and electrochemical kinetic analysis,it can be proved that Nb-substitution into partial Fe sites at 4b and 8h can dramatically reduce the total energy of system and weaken ionic bond properties between Fe and F,thus boosting the structural stability and electronic conductivity of the entire Fe F₃·0.33H₂O.While in situ carbon coating derived from carbonization of ionic liquid can form a uniform N-and F-doped conformal carbon coating on the surface of Nb⁵⁺-doped Fe F₃·0.33H₂O,which can obviously enhance the conductivity of Fe F₃·0.33H₂O.Thus,the Nb-Fe F₃·0.33H₂O@C composite exhibits high discharge capacity,good rate performance and excellent long-term cyclic stability due to the dual modification strategy of Nb substitution and conformation carbon coating.