| With the increasing global crisis of energy and environment, the development of lithium ion batteries has attracted intensive attention from both governments and research institutes. The currently used LiCoO2 is an unsuitable cathode material for future advanced lithium ion battereis because of its high cost and toxic properties. Therefore, it is desired to explore some high performance cathode materails with low toxic and price. Among numorous candidates under consideration, compounds with NASICON structures have been regarded as promising ones in recent years. In this study, two cathode materails with NASICON structures, Na3V2(PO4)2F3 and Li3V2(PO4)3 were synthesized, and the structure and electrochemical properties of the materials were studied in detail. The major conclusions of this thesis are summaried as below:The Na3V2(PO4)2F3 cathode material was synthesized by sol-gel method using NaF, NH4VO3, NH4H2PO4 as the starting materials. Citric acid was used as both a gelating agent and the carbon source. The proper synthetic temperatue of the material was 600 oC. The amount of residural carbon in the Na3V2(PO4)2F3/C composite was determined to be 8.5 wt%. TEM image shows that the residural carbon was distributed uniformly around the Na3V2(PO4)2F3 particles. The average particle size of the material was about 20 nm. Since carbon has a high electronic conductivity. The residural carbon in the composite material was helpful to enhance the electronic conductivity of the materail, which was about 1.0×10-3 S·cm-1. The electrochemical lithium insertion mechanism of Na3V2(PO4)2F3 was studied by combinative ICP and EIS studies. It is proposed that the electrochemical insertion mechanism of Na3V2(PO4)2F3 was a continous progress from Na+ insertion to Li+/Na+ hybrid ion insertion, and finally to Li+ insertion. Charge-discharge cycling experiment was carried out at 0.1 C rate in the potential window on 3.0 - 4.5 V. The initial discharge capcity of the materail was 117 mAhg-1, which decreased to 106 mAhg-1 after 40 cycles.Li3V2(PO4)3 compounds was synthsized by Pechini method using Li2CO3,(NH4)2HPO4,NH4VO3 as the raw materails. Citric acid was used as the gelating agent, and PVA-2000 was served as the solvent. The use of Pechini method made it possible to obtain homogeous mix of the cations, and also derease the synthetic temperature. XRD study showed that phase pure Li3V2(PO4)3 compounds could be obtained in the temperature region of 700 900 oC with a heating time of 8 h. The crystallite size increased with the increasing of heating temperature. SEM showed that the materail prepared at 800 oC had an average particle size of 150 nm. Raman scattering analysis showed that the graphite phase in the residural carbon of the material prepared at 800 oC was the largest in all of samples. This indicates that the material had the largest electronic conductivity, which is good for electrochemical performance. The electrochemical performance of the 800 oC sample was the best in all of samples. When the material was cycled in the potential window of 3.0 4.3 V at 0.1 C rate, the material showed an initial charge/discharge capacity of 126 and 124 mAhg-1, which could be maintained at 119 and 118 mAhg(-1 after 20 cycles. In the 3.0 4.8 V window, the material showed an initial discharge capacity of 161 mAhg-1, which decreased to 122 mAhg-1 after 20 cycles. EIS analysis showed that the lithium diffusion coefficient of the material between 3.0 and 4.3 V were about 1.28×10-8 1.26×10-9 cm2s-1, which were three times of magnitute higher than those between 3.0 and 4.8 V. In addition, the charge transfer resistence in the wider potential region was also much higher. Both of these were attributed to the relatively poor cycling performance of Li3V2(PO4)3 in the potential window of 3.0 4.8 V.
|
PDF Full Text Request