| For its rich source of raw materials, high specific capacity, high voltage plateau, good safety performance, monoclinic Li3V2(PO4)3 has been regarded as the most promising cathode material to replace commercial material LiCoO2, LiMn2O4 and LiFePO4 becoming the next generation of lithium-ion battery. But the material has low electronic conductivity because of its three-dimensional structure, and that makes it display poor rate capacity and cycling performance. Meanwhile, there is series of problems at the synthesis of Li3V2(PO4)3 such as complicated preparation, too big particle size and impurities of products and so on. Thereby, solving these two main problems is critical to achieve the commercialization for this material.In this dissertation, a fast simple sol-gel method has been developed to synthesize monoclinic Li3V2(PO4)3,which shortened the total time of preparing process to 16h. The results showed that Li3V2(PO4)3 calcinated at 850℃for 4h displayed best performance with a first discharge capacity of 138mAh/g,97% for coulombic efficiency at 0.1C. AC impedance measurements showed that a Rct of 670Ωfor this Li3V2(PO4)3 sample.The Li3V2(PO4)3 was modified by carbon surface treatment. PEG (M=10000) & Super p(1:1)were firstly chosen as a co-carbon source also as reaction precursor in the Li3V2(PO4)3/C composite synthesis. The results showed that carbon sources with a content between 2.29%-10.44% won't effect the structure of the compound. The Li3V2(PO4)3/C samples displayed a spherical-like morphology with minimal agglomeration, an average size of 600nm with nanometer sized carbon particles dispersing uniformly around which is smaller than pure Li3V2(PO4)3 with an average size of 900nm. Meanwhile, as a conductive source surrounding the Li3V2(PO4)3 crystallines, the nano-carbon particles are aggregated to form chained structure, which may play a certain role in improving the conductivity.The Li3V2(PO4)3/C sample with a carbon content of 3.79% exhibited the best performance.When measured at 0.1C between 3.0-4.8V, the sample displayed a highest discharge capacity of 173mAh/g, a capacity retention rate of 90% after 30 cycles, a Rct of 200Ω. which is much smaller that of than pure Li3V2(PO4)3. When measured at 0.25C,1C, the capacity retention rate can both be 80% after 60 cycles.At the high rate of 5C,the capacity retention rate can also be 78% after 60 cycles.In the voltage range of 3.0-4.3V, the highest discharge capacity can be 138.2mAh/g and 106mAh/g, the capacity retention rate can be 93% and 92% at high rate of 1C and 5C, respectively.B was chosen as doping element to synthesize Li3V2-xBx(PO4)3/C(x=0,0.1,0.15, 0.2,0.25,0.3)samples.The results indicated that the samples won't affect the monoclinic structure of Li3V2(PO4)3 when x<3.But the Li3V2-xBx(PO4)3/C(x=0.25, 0.3)samples displayed impurities according XRD patterns.After modified by B doping, materials showed much less agglomeration, but there is appearance of large particles when x=0.25,0.3.Most of the modified materials showed Rct lower than 530Ωexcept L13V1.7B0.3 (PO4)3/C sample which exhibited a Rct of 1200Ωmuch larger than that of pure Li3V2(PO4)3.Among Li3V2-xBx(PO4)3/C(x=0,0.1,0.15,0.2,0.25,0.3) samples, Li3V1.85B0.15(PO4)3/C displayed the best electrochemical performance.When measured at 0.1 C and 0.5C rates, the capacity retention rate could both be higher than 70%. |
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