| As one of important sodium ion batteries cathode materials, Na3V2(PO4)3 exhibits a 3D framework, high charge-discharge potential, high theoretical specific capacity, good cycling performance and high rate capability. In this thesis, Na3V2(PO4)3 is used as a main research object. In order to improve its disadvantage of low electronic conductivity, several modification methods are adopted to enhance its electrochemical performance. The detailed research contents are as follows:(1) The porous Na3V2(PO4)3 was synthesized by the sol-gel method combined with a freeze-drying process. The sol of Na3V2(PO4)3 precursor was momentarily frozen by liquid nitrogen; then the frozen sol was adopted by vacuum drying process in the vacuum freeze dryer, during which the porous structure of precursor is well-maintained. The special porous morphology could significantly increase the specific surface area of the material, which greatly enhances the contact area between the electrode and electrolyte, and supplies more active sites for sodium ions. Therefore, The porous Na3V2(PO4)3 exhibits excellent rate performance and cycling stability. It delivers an initial capacity as high as 118,108,105,99,95,92 and 90 mAh g-1 at 0.05,0.1,0.2, 1,3,4 and 5 C in the potential window of 2.7-4.0 V versus Na+/Na, respectively. When the rate returns to 0.1 C, this material can still deliver a discharge capacity of 105 mAh g-1.(2) The p-type B-doping carbon coated Na3V2(PO4)3 composite is synthesized and the modified mechanism is detailed investigated. The modification approach of B-doped carbon coating is initially applied on Na3V2(PO4)3 cathode materials for sodium ion batteries used a facile sol-gel process. It is found that there are four different B-doping species (B4C, BC3, BC2O and BCO2) in Na3V2(PO4)3 samples with different B doping contents; moreover, different B-doping species in the carbon coated layer have different influences on the improvement of the electrochemical properties of Na3V2(PO4)3. Compared to B4C and BC3, due to the introduction of the O atom in the carbon coated layer, BC2O and BCO2 can damage of the carbon skeletons and significantly increase numerous extrinsic defects and active sites, which could accelerate Na+ transport in the carbon layer. Therefore, it is unexpectedly demonstrated that Na3V2(PO4)3/C+B, which consists of the largest total amount of BC2O+BCO2, exhibits the best electrochemical properties. It delivers an initial capacity of 95.8 and 95.2 mAh g-1 at low rates of 0.2 and 0.5 C, respectively, which further demonstrates an excellent rate capability with the value of 93.1,93.0,93.0 and 90.3 mAh g-1 at 1,2,3 and 5 C. When recycled at 3,0.5 and 0.2 C, its can still deliver a discharge capacity of 93.4,93.8 and 93.9 mAhg-1, respectively.(3) The n-type N-doping carbon coated Na3V2(PO4)3 composite is synthesized via an in-situ preparation process and the modified mechanism is detailed investigated. Citric acid and PVP are used as carbon and nitrogen sources, respectively; the N-doping carbon coated Na3V2(PO4)3 is prepared by sol-gel method. Compared to the only carbon coated Na3V2(PO4)3, moderate nitrogen into the carbon coating layer could significantly improve the electrochemical properties of Na3V2(PO4)3. The reason is that there are three carbon-nitrogen species:pyridinic N, pyrrolic N, and quaternary N into the carbon coating layer. Pyridinic N and pyrrolic N could significantly increase the electronic conductivity and create numerous extrinsic defects and active sites. While quaternary N only increases the electronic conductivity without creating extrinsic defects. In consequence, it is unexpectedly demonstrated that the Na3V2(PO4)3/C+N (NVP-C-N142), in which with minimize content of quaternary N or exist most extrinsic defects, exhibits the best electrochemical properties. The NVP-C-N142 electrode could deliver high discharge capacities of about 100 mAh g-1 at 0.2,0.5,1 and 2C, which further demonstrates an excellent rate performance with a value of 93.8 at 3C, 84.3 mAh g-1 at 5 C. When recycled at 3 C,1 C and 0.2 C, its discharge capacity can still reach 90.1,96.4 and 98.2 mAh g-1, respectively.(4) Nitrogen-doped carbon-coated Na3V2(PO4)3 hybriding with multi-walled carbon nanotubes (CNTs) composite, namely double nano-carbon synergistically modified Na3V2(PO4)3 of sodium ion battery, was synthesized by a facile sol-gel method. Based on the systemical analysis of Raman spectra, X-ray photoemission spectroscopy results about this composite structure, it is found that suitable N-doping not only increases the electric conductivity of carbon layer, but also increases its Na-ion migration velocity across the carbon layer. Moreover, due to the intimate contacts between active materials and CNTs, the CNTs 3D conducting network could significantly accelerate the electron transport between multiple-particles of Na3V2(PO4)3. Therefore, the electrochemical properties of this double nano-carbon modified Na3V2(PO4)3 is significantly improved, especially the rate performance and long lifetime. For instance, when the discharging rate increased from 0.2 C to 70 C, its capacity of 94.5 mAh g-1 decreases to 70 mAh g-1 and an unexpective capacity retention of 74% is obtained. Moreover, even at a higher current density of 30 C, an excellent capacity retention of 87% is obtained after 300 cycles.(5) Mn2+-doping carbon-coating was used to modified electrochemical performance of Na3V2(PO4)3 cathode material for sodium ion batteries. Na3V2-xMnx(PO4)3/C composites with different Mn2+ doping contents (x= 0, 0.015,0.025 and 0.035) were prepared by a simple sol-gel method. Based on the precise analysis of crystal structure of Na3V2-xMnx(PO4)3/C, it is found that Mn2+ plays an important role in enlarging the Na-ion migration velocity and in increasing the lattice volume by elongating the a- and b-axis; moreover, suitable Mn2+ doping also could increase the electric conductivity of Na3V2-xMnx(PO4)3/C, thereby improving the electrochemical performance. For example, when the C-rates increased from 0.5 C to 20 C, the discharge specific capacity only decreased from 96.8 mAh g-1 to 71.8 mAh g-1 and an unexpective capacity retention of 74% is obtained. Furthermore, even cycling at a high rate of 15 C, an excellent capacity retention of 93% is maintained from the initial value of 86.7 mAh g-1 to 80.4 mAh g"1 after 100 cycles. |
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