| Summary: | 碩士 === 國立勤益科技大學 === 化工與材料工程系 === 100 === The home-made soft package Li-ion batteries are aged by charging/discharging rates of 0.2, 0.5, 1 2 and 3 C-rate at 60, 40 and 25 oC, respectively. The aging properties and mechanisms at the various fading rates are investigated by ac impedance technique. The surface morphologies and crystalline properties of the electroactive materials of aging batteries are analyzed by SEM and XRD, respectively.
The aging conditions are divided into four cases, i.e., high temperature-high rate (60 oC, 3 C-rate; Case I), high temperature-low rate (60 oC, 0.2 C-rate, Case II), low temperature-high rate (25 oC, 3 C; Case III), and low temperature-low rate (25 oC, 0.2 C-rate, Case IV). In Case I, II, III and IV, the capacity fading is found to be 20, 10, 15 and 15 % with the charge/discharge cycles of 40, 90, 60 and 600, respectively. The coulomb efficiencies of home-made batteries for all cases are greater than 97 %, which indicates that the charge/discharge efficiency is very well, and the side reactions in the charge/discharge process are not the main factor for battery aging. For all the cases, the impedances of the devices of equivalent circuit for the negative electrode are generally greater than that for the positive electrode. However, the increase in the impedances of devices at the positive electrode is faster than that at the negative electrode. The results indicate that the aging of the home-made Li-ion batteries is mainly caused by the positive electrode. The main factor caused the fading of positive electrode for all cases is found by increasing Rct in the aging process. In Cases I, II, III and IV, the values of Rct (positive electrode) increases from 0.001 to 0.190, 0.404, 0.050 and 0.152 (increasing by 190, 404, 50 and 151 folds) by increasing the capacity fading from 0 to 20, 10, 15, and 15 %, respectively.
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