| Lithium-ion batteries have been widely used in various portable electronic devices because of their high energy density, high power density and long cycle life. However, with the rapid development of electric vehicles, we look for more potential electrode materials with better performances for lithium ion batteries. Considering that the energy density of cathode materials is determined by the specific capacity and discharge voltage, and the power density is determined by the rate capability, this thesis first focuses on the synthesis and characterization of lithium vanadium phosphate (Li3V2(PO4)3) with high theoretical specific capacity (197 mAh/g) and high voltage (3.0-4.8V). Doping, coating and nanocrystallization are adopted to improve the performances of Li3V2(PO4)3. On the other hand, the development of large-scale energy storage is limited by the unevenly distributed and inadequate amount of lithium resources. Thus the research on room temperature sodium-ion batteries has some strategic significance. This thesis also focuses on sodium vanadium phosphate (Na3V2(PO4)3) with a stable structure. The as-prepared nanoparticles are tested in half-cells (vs. Na), full cells and symmetrical cells, respectively.Chapter 1 briefly introduces the structure, working principle of lithium-ion battery and typical cathode materials, anode materials for lithium ion batteries and sodium ion batteries. The common preparation methods of lithium/sodium vanadium phosphates are also described. Finally, the background and research are presented briefly.Chapter 2 describes the experimental reagents, equipment, methods and materials characterization techniques in this thesis. The details of preparations of 2032 type coin-cell and electrochemical test are also presented.In Chapter 3, Li3V2(PO4)3/C composite is synthesized by a sol-gel method and characterized by XRD and SEM. In the voltage range of 3.0-4.3V, the initial discharge capacity is 121 mAh/g with an initial coulombic efficiency of 94% and after 100 cycles,91.7% of the capacity retains. At IOC rate, the material delivers 56 mAh/g, 46.3% of the initial discharge capacity. In the voltage range of 3.0-4.8V, the initial discharge capacity is 166 mAh/g with its initial coulombic efficiency dropping to 87%. It delivers a capacity of 53 mAh/g at IOC rate,32.0% of the initial discharge capacity.In Chapter 4, nano-Li3V2(PO4)3/C is prepared by a thermal polymerization method and its crystal structure, carbon content and morphology are characterized. The particle sizes of the intermediate product powder and the final product are all less than 200 nm. The carbon is partially coated on the surface of Li3V2(PO4)3 particles and the rest exists between particles with a total carbon content of 4.6 wt%. This nano-Li3V2(PO4)3/C sample shows a discharge capacity of 124 mAh/g without capacity fading after 100 cycles at 0.1 C in the voltage rang of 3.0-4.3 V. Excellent rate performance is also achieved with capacities of 110 mAh/g at 10C and 80 mAh/g at 20C. In the voltage range of 3.0-4.8V, the sample delivers a discharge capacity of 166 mAh/g at 0.1 C and 82.5% capacity retention after 100 cycles. When discharge at 10C, the capacity reaches to 100.4 mAh/g. This excellent performance is mainly due to its particle size and coating carbon. After that, Ni-doping in V lattice site is introduced to improve the performance of Li3V2(PO4)3 and the amount of Ni is optimized.In Chapter 5, Li3V2(PO4)3/graphene is prepared by a freeze-drying method with graphene content optimized. Two different cooling methods, i.e. slowly frozen in a refrigerator and quickly frozen in liquid nitrogen, are discussed by comparing the morphologies and electrochemical properties of the obtained materials. The results indicate that the samples rapidly cooled in liquid nitrogen are significantly better than those slowly cooled in the refrigerator. The morphology of the rapid-cooled samples (LVPGN) exhibits a microstructure of graphene with wrinkles on the particles surface. The Li3V2(PO4)3 nanoparticles (30-150nm) are strongly adhered to the surface of the graphene and enwrapped into the graphene sheets uniformly. The LVP nanoparticles can decrease the lithium ion diffusion length and the graphene can effectively improve the electronic conductivity, thus the material shows great rate performance with a capacity of 105.7 mAh/g at 20C for 3.0-4.3V, and a good cycling performance with a discharge capacity of 123.2 mAh/g (96.7% of the first discharge capacity 127.4 mAh/g).In Chapter 6, Na3V2(PO4)3/C is synthesized by thermal polymerization method with the amount of concentrated nitric acid and the temperature of xerogels treatment are optimized based on the morphologies and properties. The results show that a temperature of 450℃ for xerogels treatment is suitable to form a uniform morphology and result in great performance. Three kinds of electrolytes, NaClO4/PC, NaBF4/PC and NaPF6/PC, are used to assemble half-cells and the results show that they have significant influences on the electrochemical properties of sodium vanadium phosphate and NaBF4/PC electrolyte shows the best performance. In the voltage range of 2.7-3.9V, the sample NVPC450 delivers an initial discharge capacity of 113 mAh/g,96.6% of its theoretical specific capacity and an initial coulombic efficiency of 98.5%. After 80 cycles,96% of the initial capacity retains. When discharged at 5C rate, it delivers a capacity about 85 mAh/g. In the voltage of 1.0-2.0V, NVPC450 delivers an initial charge capacity of 57 mAh/g,98.2% of its theoretical specific capacity and an initial coulombic efficiency of 85.1%. After 50 cycles,94.7% of the initial capacity retains. When charged at 5C rate, it delivers a capacity about 39 mAh/g,68.4% of the initial capacity. Full sodium ion cells are assembled using NVPC450 both as cathode material and anode material. The results show that the anode-limited full cell has higher capacity and more excellent performance than the cathode-limited full cell. In each side of a symmetric cell, the role of NVPC450 continues to convert in the voltage range of-2-2V, indicating that NVPC450 can be used as both a cathode material and an anode material.Finally, in Charpter 7, an overview of innovations and deficiencies in this thesis is summerized. Some prospects and suggestions in the future research are presented as well. |
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