Preparation And Study Of Vanadium And Titanium Doping Of Carbon Nanotube-loaded LiFePO₄/C As A Cathode Material

LiFePO₄has a regular structure of olivine and abundant iron resources, thereare many advantages such as environmentally friendly, high thermal stability andexcellent rate characteristics, It is now a keen study of a material which meets therequirement of substituting LiCoO2, it has become one of the material of thehybrid car. However, its conductivity is much lower than the layered cathodematerial, it has a serious attenuation in the large current discharge capacity. But itsperformance can be improved by coating or doping.We used co-precipitation method to prepare LiFePO₄cathode material tooptimize the synthesis conditions. We studied materials synthesis temperature,crystal structure morphology and electrochemical properties by TG-DTA, X-raydiffraction （XRD） and scanning electron microscopy （SEM） and other physicalcharacterization methods, and charge-discharge tests, cyclic voltammetry （CV）and electrochemical impedance （EIS）, etc. electrochemical test methods. Liquidco-precipitation method were studied in different dosage of carbon nanotubes,ultrasonic time and the amount of carbon-coated on the material properties. Theresults showed that when the addition amount of2.0%carbon nanotubes, thesynthesized material showed uniform and smaller particles, and has a goodelectrochemical performance. The material in the magnification of0.1C dischargethe capacity reached130.7mAh/g; when the ultrasonic time was1h, the resultingmaterial particles showed uniform and no obvious agglomeration, and has a goodrate capability and high-current charge-discharge performance. In addition, theproperties of the material by ultrasound for1h after100cycling numbers weremore stable, the capacity retention rate was about107%; To synthesis the LiFePO₄,with Fe（NO₃）₃as an iron source, when the carbon coating amount of10.4%, thecarbon coating layer was uniform, the capacities were131.1mAh/g at0.1Cdischarge capacity, the capacity retention ratio was99.6%,99.0%,98.4%at1C,2C and5C magnification of100cycles, respectively. It can be seen that thematerial in the large current discharge performance under the conditions showedstable and good cycle performance.In order to further improve the performance of LiFePO₄, metal ions V and Ti were doped in the synthetic material. We investigated different molar dopingeffects on the material properties and compared the merits of two metal ions.Studies have shown that doping metal ions both V and Ti can effectively improvethe discharge capacity of LiFePO₄material. When V is incorporated in the molarratio of0.03, the discharge capacity reach147.7mAh/g in the magnification of0.1C, and the molar ratio of0.01when mixed with the resulting material undermagnification showed a good discharge performance, in0.5C,1C,2C and5C,respectively, when the discharge capacity of129.7,110.9,108.9and98.4mAh/g;at0.5C,1C,2C and5C magnification100cycles of discharge capacity retentionrate was99.9%,99.6%,99.0%and98.4%, it can be seen that the materials ofLiFe0.99V0.01PO₄and LiFe0.97V0.03PO₄have a good discharge performance andcycle stability is good. When the molar ratio of Ti incorporation of0.02and0.03were139.8mAh/g or so, discharge capacity of metal ions is greater than theundoped material, and when the discharge rate increased to2C and5C, the dopingmaterials have no longer any advantage, discharge capacity is lower than theundoped material obtained.