| 摘要: | 本論文之研究內容是以花生殼作為先驅物，經由適當之製孔劑處理後，於600?900℃下煆燒後合成無次序碳材以作為鋰離子電池陽極材料，論文共分三個主題進行：(1)比較不同比例製孔劑與煆燒溫度效應對花生殼處理所得碳粉之影響；(2)探討花生殼以製孔劑處理前後煆燒所得碳粉之電化學差異；(3)不同電流速率電解製孔劑處理花生殼碳粉之電池性能影響。 首先是利用可大幅增加表面積的製孔劑，處理台灣自有的農業廢棄物—花生殼，改變煆燒條件，期望得到具有高可逆電容量之碳材料。 花生殼以不同比例製孔劑處理(P=1~5)，500℃下煆燒製得之碳粉進行充放電測試，以P=5所合成的碳粉具有較佳之可逆電容量，第一次放電電容量為4765 mAh/g，充電電容量為1385 mAh/g。當P=1或2時，幾乎無電容量。 花生殼以P=5之製孔劑比例處理，改變不同煆燒溫度所製得碳粉，由測試結果可知所有樣品之第一次循環不可逆電容量均相當大，尤以500℃所合成的碳粉為最大，隨著煆燒溫度增加，不可逆及可逆電容量同時減少。當煆燒溫度為600℃時則有最佳之電容量，其第一次放電電容量為3504 mAh/g，充電電容量為1650 mAh/g，至第10次循環時，可逆電容量仍高達1504 mAh/g。 第二部份比較花生殼未經處理及利用製孔劑處理，比較兩者製得碳粉之電化學性質差異，未處理的碳粉在電壓範圍為1~2 V之間有一寬闊的還原峰存在，當電壓低於0.7V以下，為一般文獻中所報導陽極材料提供電容量的電壓範圍；如以製孔劑處理後，從電壓1.3伏特開始處即有明顯還原峰存在。並且我們可由CV圖形中看出兩種材料之循環穩定性，經處理所製得之碳粉自第二次循環後還原峰就無變化，故循環穩定性較未處理者佳。 我們亦可利用交流阻抗分析結果計算交換電流密度，以表示材料之反應速率，而當我們利用製孔劑處理後，交換電流密度皆比未處理者高，表示該系統具有較佳之反應速率。 比較花生殼未處理與花生殼以製孔劑處理所得碳粉之阻抗，Re係數與鋰的嵌入量多寡無關， Rp則是先增加再降低，增加的原因是由於電解質液產生還原反應所致，表示材料表面結構鬆散，容易與電解質液發生反應，而未處理花生殼在低於0.32V下材料表面結構則相對較穩定，是由於有鈍化膜產生保護的緣故。 最後一部份則以不同電流速率，電解製孔劑處理花生殼，600℃下煆燒所得碳粉並組裝成電池測試，由長循環數據可知，利用0.4C rate電解後，其電池性能為最佳，電池以特徵曲線方式測試，當測試的電流速率為0.2、0.4與0.8C-rate，可逆電容量數值分別約為900、700及500mAh/g，甚至在1.6C-rate高速率的充放電條件下，測試了約130次的循環後，可逆電容量仍有300mAh/g以上。 如依照第一次循環可逆電容量對電解速率大小作圖，不同速率電解處理過後，以0.4C rate的效果為最佳，高或低於0.4C-rate其電解效果皆不好，電容量對電解速率作圖呈一個具有極大值的曲線。 This thesis describes the structural and lithium-insertion properties of pyrolytic carbons derived from peanut shells. Peanut shells were treated with different weight ratios of a proprietary porogenic agent and carbonized between 600 and 900°C. The work covers three areas: (1) optimization of the porogen-to-peanut shell weight ratio (P) and the pyrolysis temperature, (2) comparison of the lithium-insertion properties of carbons obtained from untreated and porogen-treated peanut shells, and (3) charge-discharge studies with pre-lithiated carbons. Porogen treatment was implemented in order to alter the pore structure and effect a manifold increase in the surface area of the carbonaceous product. Both the untreated and porogen-treated shells yielded carbons with poor crystallinity, but the pore diameter of the latter was twice as large and the surface area was 66 times greater than the untreated carbon. Both types of products were primarily non-parallel single sheets of carbons, as determined by the values of their R factors. While porogen can increase the number of uncorrelated graphene fragments, leading to more lithium accommodation sites, the pyrolysis temperature can induce breakage of the links between adjacent sheets and encourage their parallel alignment. The products obtained with P = 5 at 500°C gave a first-cycle lithium insertion capacity of 4765 mAh/g, which is the highest value reported for any lithium-insertion material so far. At a pyrolysis temperature of 600°C, the P = 5 product gave the optimal insertion and deinsertion capacities, their values in the first cycle being 3504 and 1650 mAh/g, respectively. The deinsertion capacity of this sample in the tenth cycle was very high at 1504 mAh/g. However, the irreversible capacities of these carbons, especially in the first cycle, were too large to be practical. The large irreversible capacities were reflected in the cyclic voltammograms of the carbons, where the absence of a significant anodic peak indicated that only part of the inserted lithium could be retrieved. In the case of the P = 0 carbon, lithium insertion was observed below 0.7 V vs. Li+/Li, while in the P = 5 carbon, the insertion process commenced from about 1.3 V. Moreover, the decrease in the insertion current with cycle number was lower in the case of the porogen-treated carbon than with the untreated carbon, suggesting the former had better capacity retention. No distinguishable current peaks were seen in the cyclic voltammograms, indicating lack of any long-range ordering, which precludes staging behavior during the insertion and deinsertion processes. The P = 5 carbon also exhibited higher exchange current densities, which would imply that the kinetics of the insertion reaction was faster than when the carbon was untreated. Electrochemical impedance studies showed that the resistance due to the formation of surface film increased when the carbon was charged. However, the slight increase in resistance suggests that the products of the surface reduction are either soluble in the electrolyte or are loosely held to the surface. Charge-discharge studies with the porogen-treated carbon, pre-charged and discharged prior to use in coin cells, indicated that the first-cycle reversible capacity was the greatest when the charge-discharge rate was 0.4 C. At this rate, the carbon maintained capacities of about 325 mAh/g for 20 cycles, and then stabilized at around 380 mAh/g for over 70 cycles. Signature curves of the carbon showed that the deliverable capacities at charge-discharge rates of 0.2, 0.4, 0.8 C were 900, 700 and 500 mAh/g, respectively. Even at the 1.6 C rate, more than 300 mAh/g could be tapped from the carbon after 130 cycles. |
