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Preparation of LiFePO4/C by Solid-State Synthesis Method and Electrochemical Performance Study

Author: Yang
Tutor: RenQiang
School: Shaanxi University of Science and Technology
Course: Materials Science
Keywords: LiFePO4/C solid-state synthesis method carbon coating ion doped electrochemical performance
CLC: TM912
Type: Master's thesis
Year: 2012
Downloads: 45
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Abstract


Olivine-structured LiFePO4is subject to more and more interest as apotential candidate cathode material for rechargeable lithium-ion batteries fromboth economic and environmental points of view. However, the pure LiFePO4with olivine-type has the low conductivity, poor electrochemical performanceand poor rate capability, especially in high charge/discharge rates, resulting fromthe low lithium-ion diffusion rate in the LiFePO4phase. Thus, LiFePO4ismodified with the methods of providing carbon cladding and doping metallic ionto improve its conductivity and cycle performance. Carbon cladding can controlgrain size of LiFePO4powder, inhibit its growth and is good for improving itselectrochemistry properties; Doping metallic ion for LiFePO4, crystal structure ofLiFePO4powder can be changed, lattice defect can be increased, internalconductivity of LiFePO4can be improved, charge-discharge capacity and cycleperformance of LiFePO4can be improved.With a summary of the progress of solid-state synthesis method in LiFePO4,the modification in LiFePO4including surface modification and doping areintroduced in this paper. The prospect of LiFePO4in the future and the problemsit contains are also presented. LiFePO4/C composite material was prepared bysolid-state synthesis method using Fe2O3as starting material. And the effects ofcarbon content, the ratio of doped ion, and the description of doped ion oncrystal structure, morphology and electrochemistry property of LiFePO4/C wereinvestigated.Y3+-doped LiFePO4/C composite material was prepared by solid-statesynthesis method using Fe2O3as starting material. The reaction processes,morphology of powders and electrochemical performance of samples werestudied by TG-DSC, XRD, SEM and constant current charge-discharge method.The results show that the Fe3+is reduced to Fe2+at300-550℃, and thecomposites synthesized at650℃own olivine structure. Its initial discharge capacity is151.6mA·h/g at0.2C rate.LiFePO4/C composite material was prepared by solid-state synthesismethod using Fe2O3as starting material. The structures, morphology ofpowders and electrochemical performance of samples were studied by XRD,SEM and constant current charge-discharge method. The results show that thecarbon coating doesn’t change the crystal structure of LiFePO4. The sampleswith carbon content of10wt%show the best electrochemical properties. Itsinitial discharge capacity is144.6mA·h/g at0.2C rate. After30cycles, thecapacity remaines131.4mA·h/g, and the capacity retention rate is91%.Co2+-doped LiFePO4/C composite material was prepared by solid-statesynthesis method using Fe2O3, Li2CO3and NH4H2PO4as starting material. Thestructures and electrochemical performance of samples were studied by XRD,SEM and constant current charge-discharge method. The results showed that theCo2+doping doesn’t change the crystal structure of LiFePO4. The volume ofcrystal cell changed wth the increase of Co2+,and reached the maximum atx=0.04. The LiFe0.96Co0.04PO4/C sample proved the best electrochemicalproperties. Its initial discharge capacity was138.5mA·h/g at1C rate. After30cycles, the capacity remained127.7mA·h/g, and the capacity retention rate was92.2%.LiFe0.98M0.02PO4/C composite material was prepared by solid-state synthesismethod using Li2CO3, NH4H2PO4, Fe2O3, MgO, NiO, Co（CH3COO）2·4H2O andSrCO3as starting material. The results shows that the ion doping didn’t changethe crystal structure and morphology of LiFePO4. cell volume graduallyincreased, which was good for embedding and dis-embedding of Li+andimproving conductivity and charge-discharge performance. The initial dischargecapacity of Ni2+doped sample reached156.6mA·h/and149.4mA·h/g at0.2Cand1C rate. After30cycles, the capacity retention rate was still98.8%和97.2%.

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