| In recent years, with a series of energy crisis and environmental pollution problems have become increasingly prominent, lithium-ion batteries for energy storage devices have been developed rapidly, and lithium-ion batteries gradually replace traditional batteries such as lead-acid, nickel cadmium, nickel metal hydride in fields of hybrid vehicles, mobile communications equipment and medical devices. In the existing cathode material, lithium iron phosphate has received extensive attention in the field of power battery, because of its exhibit many appealing features, such as low cost, environmental benign, good safety, high theoretical capacity, long cycle life, etc. However, it has several defects such as low electronic conductivity and high rate discharge performance is poor, which seriously limits the large-scale application in the field of power battery.This paper aims to overcome these defects on the basis of previous studies, the 7LiFePO4·Li3V2(PO4)3/C composite cathode materials were successfully synthesized via a simple solid-state reaction route using LiH2PO4,FeC2O4·2H2O, NH4VO3 and C6H8O7·H2O as the raw materials, and improve the electrochemical performance of composites by doping metal ions. The samples were characterized by a variety of testing means and methods, and the influence of doping on microstructure and electrochemical performance of composites is discussed. Research shows that:1. The 7LiFePO4·Li3V2(PO4)3/C composite after doping Mg2+, Cr3+ and Ti4+ showed two phase both olivine structure LiFePO4 and monoclinic structure Li3V2(PO4)3, no other impurity was observed in composite, and the crystallinity of composite is higher. The microstructure of the composites was deformation or distortion in different degree after doping metal ions. Meanwhile, the chemical environment of Fe atoms was changed through introducing Mg24, Cr3+ and Ti4+, which partly affected the crystal structure of composites. In addition, the particle size of composites was decreased obviously after doping metal ions, which improved specific surface area of particles and promotes the lithium-ion migration. And the carbon decomposed from citric acid through high temperature which is believed to be present as amorphous carbon coated on the particles surface, leads to a nano-network structure formed between the particles, which inhibit the aggregation of particles on the one hand, and the other hand increases the conductibility of composites.2. Compared to 7LiFePO4·Li3V2(PO4)3/C composite, the discharge capacity and high rate cycle properties of composites have been significantly improved by doping an appropriate amount of metal ions. The composite doped 0.01 Cr3+, Ti4+ has the best electrochemical properties especially the rate capability of which the first discharge capacity was 156.0 and 156.6mAh/g at 0.1C rate, respectively. At 5C rate, the first discharge capacity was 92.8 and 99.9mAh/g respectively, the capacity retention was 98.1% and 98.8% after 50 charge and discharge cycles, respectively, showing good electrochemical properties stability. And the composites exhibits good reversibility and lowest resistance that the value was 121.1 and 108.0? respectively in the process of the electrode reaction, which is conducive to the transfer of charge, thus speeding up the electrode reaction. And the electrical conductivity of composites has been obviously improved by doping an appropriate amount of Cr3+ and Ti4+, the values of electronic conductivity up to 5.37×10-2, 7.16×10-2S·cm-1 respectively, which compared with undoped composite increased 5-7 times. In addition, doping high valence metal ions easily produce vacancies and surplus electrons in the crystal structure, which improving the conductivity of composite and emigration and immigration amount of Li+ within per unit of time, so as to improve the electrochemical performance of composites. |
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