Research And Exploration Cathode Material For Lithium-ion Batteries

Fe-based phosphates have become new promising cathode materials for lithium ion batteries due to the advantages of crystal structure stability, resource-rich, safety, non-toxic and so on. Recently, researchers have made lots of efforts on LiFePO4cathode material. For LiFePO4, the lithium ions and electrons intercalated/de-intercalated from LiFePO4passes transformed to FePO4phases during the charge/discharge processes. Therefore, the FePO4also can be used as cathode material for lithium ion battery.The FePO4have lots of advantages for cathode materials:Firstly, the synthesis technology is simple, without Fe (II) component, and then no need of protective atmosphere. Thus, equipments and cost can be saved. Secondly, FePO4is binary system, and the synthesis conditions can be controlled and optimized more efficiently. Thirdly, the synthesis of FePO4used the Fe (Ⅲ) compounds as the raw materials, which are easily available. Therefore, FePO4is emerging as the most promising cathode materials for lithium ion batteries due to low lost and mass productions.The samples were prepared via a microemulsion system in H2O/cyclohexane/Triton x-100/n-butyl alcohol and a hydrothermal technique. The physic performances were investigated by TG/DSC, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and the electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge and discharge tests.The nano-sized FePO4cathode material was synthesized by mircoemulsion technique, and the effects of nano-particles on the electrochemical properties were investigated. What’s more, we also made modifications for FePO4cathode material, including doping modifications, coating of conductive agents and the synthesis of FePO4/CNT composites and FePO4/graphene and so on. The diffusion coefficients of lithium ions in FePO4were calculated by EIS and CV.We discovered a new cathode material for lithium ion battery during the experiments——iron hydroxyl phosphate. Studies showed that the theory specific capacity was improved and the electrochemical properties were increased by adding of hydroxyl.The various morphologies of iron hydroxyl phosphate were synthesized via hydrothermal technique, and the relationships between the morphologies and the electrochemical properties were investigated.The iron hydroxyl iron phosphates were coated by Au and carbon.What’s more, the energy band structures, density of states, average potential, diffusion paths and diffusion energy barrier were calculated by first principles.The contents, results and conclusions are as follows:The first chapter, we have made comprehensive introductions, including the developments of lithium ion batteries, the types of cathode materials, the methods of calculating of diffusion coefficients, the synthesis techniques of nano-sized FePO4and the recent research processes of iron phosphate and iron hydroxyl phosphate.The second chapter, the experiment methods, characterization techniques and preparations and assembling of electrodes were investigated in this chapter.The third chapter,(1)the Optimizations of synthesis conditions discovered that synthesis temperatures and surfactants have a great influence on the stability of miroemulsion, for the mircoemulsion systems in a H2O/cyclohexane/Triton x-100/n-butyl alcohol, and the samples synthesized at45℃showed the best performances with a diameter of about20nm.(2)What’s more, the effects of sintering temperatures on the crystal structures, particles sizes, electrochemical properties and morphologies of FePO4cathode materials were investigated. The FePO4samples sintered at380℃and460℃were spherical nano-particles with an average diameter of ca.20nm, which are also homogeneously distributed. The FePO4particles sintered at550℃were slightly large the due to agglomeration of several small primary FePO4particles that form large secondary particles. However, the crystalline FePO4samples sintered at650℃become bulks, which were attributed to the small particles agglomeration and the crystalline growth.The discharge specific capacities of FePO4sintered at380℃,460℃,550℃and650℃are141.4mAh/g,119.6mAh/g,105.6mAh/g and55mAh/g, respectively. The charge/discharge cycle properties showed that the discharge specific capacities of the samples sintered at460℃decreased at the former3cycles and retained about120mAh/g. These results indicated that the particle size and crystal structures have significant influences on the FePO4cathode material. Nano-sized particles and amorphous structure can effectively improve the electrochemical properties, cycle stability and rate properties of FePO4.The forth chapter, the influences of crystal structure and particle size on the diffusion kinetics of FePO4were investigated. The calculations of diffusion coefficients were investigated on the FePO4samples sintered at380℃,460℃,550℃and650℃and were calculated by EIS and CV. The diffusion coefficients of FePO4samples sintered at460℃were1.06×10-13cm2s-1,8.28×10-14cm2s-1. The similar results obtained from EIS and CV deduced that nano-sized and amorphous FePO4facilitate lithium ion diffusion during the charge/discharge cycles.The fifth chapter,(1) the iron hydroxyl phosphate cathode material with a compound formula of Fe1.5(PO4)(OH) were synthesized by a hydrothermal technique. The sample synthesized at150℃with pH2.5for24h was spherical particle with a diameter of1μm. The results obtained from charge/discharge tests showed that the initial discharge specific capacities were176mAh/g and154mAh/g, and the discharge specific capacities retained about165mAh/g and150mAh/g, respectively, at0.1C and0.3C. Results investigated that the Fe1.5(PO4)(OH) cathode material have excellent charge/discharge properties and cycle stability.(2)The morphologies, such as spherical, cubic, multi-armed, cross-like structures and so on, could be easily regulated by adjusting the hydrothermal temperature and pH values.The spherical morphology and small particle size improved the electrochemical performance of Fe1.5(PO4)(OH), which may be due to the larger specific surface area of the small grain size, increasing the reaction interface to enhance the electrochemical activity of Fe1.5(PO4)(OH) and shorten the diffusive distance of the lithium ions in cathode materials. Results investigated that the crystal morphology and particle size play important roles on the electrochemical properties of cathode materials. So the electrochemical properties of cathode materials of lithium ion batteries can be improved by controlling the morphology and particle size.The sixth chapter,(1) Nano-particle FePO4and Fe1-xCexPO4(X=0.02,0.04,0.06,0.08) cathode materials were synthesized by microemulsion method. The results showed that grain size has a great influence on the performances of FePO4, and the sample synthesized at45℃showed the best performances with a diameter of about20nm and a high discharge initial specific capacity of142mAh/g and retaining123mAh/g after25cycles at0.1C. The Ce-doped FePO4, Fe1-xCexPO4(X=0.02,0.04,0.06,0.08), can effectively improve the electrochemical properties of FePO4cathode materials. The Feo.96Ceo.o4P04exhibited an initial discharge capacity of158.2mAh/g and retains152mAh/g after25cycles at0.1C. Hence, Availing Ce3+doping can improve effectively the electronic conductivity of FePO4.(2)The FePO4/CNT composite were synthesized by microemulsion, the TEM, XRD and FT-IR spectra showed that the FePO4particles were loaded on CNTs’ surfaces. The charge/discharge tests showed discharge specific capacities of FePO4/CNT with the loaded mass of20%wt composite were160mAh/g,152mAh/g,148mAh/g and142mAh/g at0.1C,0.3C,0.5C and1C,respectively, and the coulomb efficiency were near98%. Results discovered that the FePO4loaded on CNTs’ surfaces can effectively enhance the electrochemical performances of electrode.(3)At first, the graphene was synthesized successfully by hummer. The FePO4/graphene composite were synthesized by microemulsion, the TEM, XRD and FT-IR spectra showed that the FePO4particles were loaded on graphenes’surfaces. The charge/discharge tests showed discharge specific capacities of FePO4/graphene with the loaded mass of5%wt composite were150mAh/g,148mAh/g,144mAh/g and137mAh/g at0.1C,0.3C,0.5C and1C,respectively, and the coulomb efficiency were near98%. Results discovered that the FePO4loaded on graphenes’surfaces can effectively enhance the electrochemical performances of electrode.(4)Fe1.5(PO4)(OH) coated with Au (mass ratio=0,0.5,2,4,6%) were synthesized via two steps, combining microemulsion with hydrothermal techniques. The charge/discharge tests showed that the discharge specific capacities of Fei.5(PO4)(OH)/Au cathode materials synthesized at150℃with the different contents of Au coating, mass ratio=0,0.5,2,4,6%, are around153mAh/g,174mAh/g,182mAh/g,164mAh/g and165mAh/g, respectively. The results investigated that the Au coating modification can effectively improve the discharge capacity of electrode materials. The modification by conductive materials coating can improve the electronic conductivity due to the closer contact of conductive materials with the electrode materials.(5)The Fei.5(PO4)(OH)/C composites were synthesized by hydrothermal carbonization, and the TEM, XRD and FT-IR spectra showed that the carbon used the dextrin as carbon source were coated on Fe1.5(PO4)(OH) particles’surfaces.The seventh chapter,(41)the results of first-principles calculation were showed as follows:the energy band gap of Fe1.5(PO4)(OH) was0.054eV; and the conductivity of electron of Fe1.5(PO4)(OH) was related with the d orbit of Fe atoms and the s orbit of H atoms; the average potential of Fe1.5(PO4)(OH) was2.86V in accord with the experiment’ values of2.8V; the diffusion barriers were2.18ev and6.079ev, respectively, along the [100] and [010] by Complete LST/QST method, and so the diffusion path of lithium ion in Fe1.5(PO4)(OH) cathode material is [100].(2) The energy band gap of FePO4was0.677eV; and the conductivity of electron of FePO4was related with the d orbit of Fe atoms; the average potential of FePO4was3.08V in accord with the experiment’ values of3.10VThe eighth chapter, the results of this thesis were summarized in this chapter and obtained the relative conclusions.