| Lithium Ion Capacitor (LIC) is a type of hybrid electrochemical capacitor that has been developed in recent years. An LIC, in which an activated carbon electrode with electric double layer capacitor combined with an electrode of lithium ion battery, exhibits high power density of supercapacitors and high energy density of secondary batteries, so research and development of LICs is one of the hot topics in the research field of supercapacitors. Compared with organic electrolytes, aqueous electrolytes are safer and more environment-friendly. No stringent environment is required for the production of LICs when using aqueous electrolytes, which facilitates the cost control. In order to deepen the understanding about the relationship among the electrodes, operating condition and the performance of LICs, LiFePO4/AC hybrid capacitors with aqueous electrolyte were developed and the potential window, electrode matching and high temperature performance of aqueous LiFePO4/AC LICs were studied. In order to improve the conductivity of LiFePO4, LiFePO4/C composites were prepared and the electrochemical capacitive performances were investigated systimatically in this thesis.The potential window of LiFePO4/AC LIC and the mass ratio of the positive and negative electrode are optimized as0-1.7V and1:1, respectively, by cyclic voltammogram, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The influence of operating temperature on the capacitive behavior of LiFePO4(LFP) electrode, AC electrode and LiFePO4/AC LIC using the same electrolyte were investigated. It was found that the capacitive behavior of the LFP electrode in the aqueous electrolyte was affected by the temperature obviously. The specific capacitance of the LFP electrode decreases gradually with cycling at various temperatures. The decay was more serious at70℃and the specific capacity decreased from134.03mAh/g to25.38mAh/g after charging/discharging for200cycles at this temperaure. However, AC exhibited good stability in aqueous electrolyte. The activity of the electrolyte ions were improved at high temperature, which facilitated the migration of electrolyte ions and the formation of the electric double layer at the interface between AC and the electrolyte, thus improved the capacitive behavior of AC electrode in aqueous electrolyte. AC electrode exhibited good cycling performance at various temperatures as well. The capacitive behavior of LiFePO4/AC LICs using aqueous electrolyte is controlled by the LFP electrode. At30℃, the specific capacity of LiFePO4/AC LIC kept stable at30.1-30.5mAh/g during cycling. The specific capacity of the LIC fluctuated slightly at50℃. Obvious decay for the specific capacity of the LIC was observed at70℃during the charge/discharge cycling. So the aqueous LiFePO4/AC LIC should be operated at a temperature lower than50℃.Composite electrode materials of LiFePO4/C were prepared by sol-gel method. The influence of lithium source, iron source, calcination temperature, calcination time and calcination atmosphere on the physical properties and electrochemical performances of the composite materials were studied. The phase composition and electrochemical performance of the LiFePO4/C composites were characterized by X-ray diffraction, cyclic voltammogram, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The optimum lithium source and iron source were LiAc and Fe(NO3)3and the ideal conditions for the preparation of LiFePO4/C composite materials were as follows:calcination temperature700℃, calcination time3h and calcination atmosphere N2+5%H2. The particle size of LiFePO4/C composite material prepared under the optimum condition is small. This composites exhibited high electronic conductivity, high ionic diffusivity and good electrochemical performances. The specific capacity of LiFePO4/C in2M LiNO3aqueous electrolyte reached as high as128.70mAh/g at the current density of0.1A/g, and a capacity of100.19mAh/g still retained at2.5A/g with a capacity retention ratio of77.85%. |
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