Structure,Morphology And Property Of LiFePO₄/C Composite Cathode Materials Prepared By Hydrothermal Method

Because of its advantages,such as high theoretical specific capacity,high voltage,low cost of raw materials,non-pollution,good cycle performance and stability,etc.,olivine LiFePO4 is considered as the most potential cathode materials for the next-generation lithium ion batteries.In this study,LiFePO4 were prepared by the hydrothermal method,and LiFePO4/C composite cathode materials were prepared by mixing sucrose and then a heat treatment.The crystal structures and electrochemical performances of the prepared samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),gal vano static charge-discharge and electrochemical impedance testing.The effects of the types of the reactive autoclave,reaction temperature,reaction time,stirring speed,feeding sequence and reducing agent on the structures and electrochemical properties of LiFePO4/C composite cathode materials were investigated in detail.The results indicated that LiFePO4 that had been synthesized by the hydrothermal method showed olivine crystal structure,high purity,good crystallinity and uniform particle size distribution.Furthermore,the specific capacity of the sample that was prepared by the big autoclave was about 42.9 mAh/g larger than that of the sample prepared by the small one.Also,under the conditions of the reaction temperature at 150 ? and the reaction time of 20 hours,the initial discharge specific capacity of the prepared sample reached 144.4mAh/g.With the increasing of the stirring speed,the particle size distribution of the LiFeP04/C composite was becoming more and more uniform.When the stirring speed was at 500 r/min,the particle size prepared by the teflon-lined stainless steel autoclave was about 200 nm,and the cycle performance of the battery was also improved with the increasing of the stirring speed.The effects of feeding sequence on the structures and electrochemical properties of LiFePO4/C composite cathode materials were also studied,and the results showed that the specific capacity of the sample that was prepared by neutralization method was 144.4 mAh/g,while those of other samples prepared by non-neutralization method were 155.3 mAh/g.Meanwhile,in order to improve the specific capacity of the samples,the reducing agent was used to avoid Fe2+ to be oxidated to Fe3+ during the synthesic precess.Furthermore,the effects of iron sources on the structures and electrochemical properties of LiFePO4/C composite cathode materials were also investigated.The results showed that the initial discharge specific capacities of the samples,which were synthesized by three different kinds of iron sources,e.g.ferrous sulfate,iron(?)D-gluconate dehydrate and iron(?)acetate,were 124.3 mAh/g,155.3 mAh/g and 139.5 mAh/g,respectively.After 30 cycles,the specific capacity of samples synthesized using iron(?)D-gluconate dehydrate as the iron source did almost not decrease.The SEM images showed that the morphology of the sample that was synthesized using iron(?)D-gluconate dehydrate as the iron source was a spindle-like structure formed by the aggregation of the small spherical particles with the diameter at about 10-20 nm,and the particle size distribution was relatively uniform.Meanwhile,the electrochemical impedance spectra indicated that the sample using iron(?)D-gluconate dehydrate as the iron source had the lowest charge transfer resistance at the interface.