Preparation Of LiFePO₄/C By Solid-State Synthesis Method And Electrochemical Performance Study

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 LiFe_0.96Co_0.04PO₄/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%å'Œ97.2%.