| Lithium batteries have been widely used in portable electronic equipments for its advantages such as green, efficient, light and high-capacity. Olivine LiFePO4 has been regarded as a promising cathode material for lithium batteries, because it exhibits a high theoretical capacity of 170mAh/g and a stable redox potential around 3.4V, as well as excellent safety and cycling stability. However, LiFePO4 suffers an intrinsic low electronic and ionic conductivity. Carbon coating and reducing particle size have been adopted to overcome its instinct obstacles. In recent years, with the development of electric vehicles and other large-scale power system, the requirement of ion secondary batteries will be gradually turned to high-powered and low-cost, and the reserves and cost of lithium will become an crucial factor to restrict the large-scale application of lithium batteries. Sodium batteries have similar properties with lithium batteries, and sodium is more abundant than lithium on the earth. In response to reducing the cost, it is significant to develop sodium batteries to replace lithium batteries.In this paper, the main research content includes improving the efficiency of available cathode materials for lithium batteries and developing new cathode materials for sodium batteries.In the first part, we employed cheap industrial products FePO4·2H2O and H2C2O4 as raw materials. In oxalic acid solution, FePO4·2H2O is very simple to form transparent sols. Then we attempted to synthesize LiFePO4/C cathode material by spray drying method and tried to do some tentative explorations on the preparation process. In the following research, we discussed the different influence on the morphology and electrochemical properties of LiFePO4/C when we used spray drying method and sol-gel method, and the pruducts prepared by spray drying method turned out to be better. Afterwards, the effect of sintering temperature on the electrochemical performance of LiFePO4/C had been studied. The preliminary results showed that LiFePO4/C materials synthesized at 600℃ and 700℃ for 10 hours demonstrated good electrochemical performance.In the second part, the main task is to search for high-capacity, safe and stable sodium cathode materials. NaFePO4 exhibits the highest theoretical capacity of 154mAh/g in the sodium cathode materials based on phosphate and it has two different crystal types (maricite-structured and olivine-structured). Depending on the same strategy in the first part, we fabricated the maricite-structured NaFePO4, and during electrochemical test the material exhibited an increasing capacity as the cycle went on. Then olivine-structured NaFePO4 was successfully prepared, while its electrochemical behavior was investigated. Na2FeP2O7 is a new cathode material for sodium batteries. We used HEDP as phosphorus source and chelating agent to fabricate Na2FeP2O7/C cathode material by spray drying method, and the obtained material was characterized by XRD, Rietveld refinement, FTIR and ICP. Comparing spray drying method with soild state method, we analyzed their influence on the morphology and electrochemical properties of Na2FeP2O7/C. Carbon contents were also evaluated, and the products possessed the best rate capability when the amount of the added sucrose was at the molar rate of Na:C=1:2. Moreover, the details of the charge and discharge processes were carefully studied. |
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