| 摘要: | 本論文主要探討以燃燒合成法製備LiCoO2陰極材料之製程研究，首先利用XRD鑑定各製程所得材料之結構變化，SEM、TEM及BET鑑定合成材料之表面型態、顆粒粒徑與表面積，接著測試各材料之電池性能，進而求出最佳製程條件。其合成變因有錯合劑與燃料比例、煆燒溫度、時間及鋰計量等。並以循環伏安法測試材料氧化還原行為。 首先以硝酸鋰及硝酸鈷為起始物，以三乙醇氨 (Triethanolamine)為錯合劑，蔗糖(Sucrose)為燃料，以1比1、1比2、1比4、1比8及1比16等不同三乙醇氨與蔗糖比例，並於800℃/10h之煆燒條件，探討最佳錯合劑與燃料之比例。將所得最佳錯合劑與燃料之比例之材料改變600及700℃二種不同煆燒溫度，以及不同煆燒時間2.5及20小時，在空氣氣氛下進行煆燒，藉以求得最佳製程條件。由XRD分析圖譜中可發現在煆燒溫度600℃以上之條件均可合成出純相產物。本實驗最佳製程條件為三乙醇氨與蔗糖比例為1比8，煆燒溫度800 ℃，煆燒時間10小時。其合成材料於充放電截止電壓分別為4.3及3.0伏特時，第一次與第五次放電電容量分別為156與153 mAh/g，電荷維持率為98﹪；當充電電壓升至4.4伏特時，第六次與第十次放電電容量分別為167與165 mAh/g，電荷維持率為98﹪。 為避免高溫熱處理下造成鋰的揮發而造成電容量損失，因此擬藉助加入過量鋰金屬，以改善此一現象。吾人針對x ＝1.05、1.10及1.15進行研究。當鋰計量x＝1.05、1.10以及1.15所合成之LiCoO2陰極材料於充放電速率0.1C，充放電截止電壓分別為4.3及3.0伏特時，第一次放電電容量分別為157、154及155 mAh/g。第六次充電電壓提升至4.4伏特時，其放電電容量分別為166、165及166 mAh/g，且於第十次降為162、163及164 mAh/g。在長循環測試下，各過量鋰計量數之LiCoO2材料其電池性能相差不多，其循環次數約在70次左右，而放電電容量約在133mAh/g。 This dissertation covers the synthesis and lithium-intercalating properties of LiCoO2 prepared by a combustion process with triethanolamine (TEA) as a complexant and sucrose as a fuel-cum-complexing agent. The synthesis parameters – TEA: sucrose mole ratio, and temperature and duration of calcination – as well as lithium stoichiometry were optimized in order to obtain products with the best electrochemical activity. Structural properties of the products were investigated by x-ray diffraction, surface morphology by scanning electron microscopy and transmission electron microscopy, and surface area by the BET method. Lithium intercalation properties were studied by galvanostatic charge-discharge studies at different rates and voltage windows. The various redox regions and phase changes occurring during the charge-discharge processes were studied by cyclic voltammetry. The precursors for the synthesis of LiCoO2 were metal nitrates dissolved in an aqueous solution of TEA and sucrose in various mole ratios: 1:1, 1:2, 1:4, 1:8 and 1:16. Although phase-pure products could be obtained by a 10-h calcination at 600°C, the crystallinity of the product improved with the duration and temperature of calcination. The optimal synthesis conditions were found to be a 10-h calcination at 800°C. The electrochemical properties of the products were correlated with their surface area and R-parameter. Sucrose was first hydrolyzed to glucose and fructose, and subsequently oxidized to gluconic or polyhydroxy acids, which coordinated with the cations and cross-linked with the TEA. TEA complexes with cations and immobilizes them in a carbonaceous matrix formed from sucrose. Thus, upon decomposition of the precursor, the cations find themselves dispersed uniformly in a carbonaceous matrix. Sucrose also acts as a fuel, providing the energy for product formation and sintering. However, a large amount of sucrose in the precursor can also reduce the partial pressure of oxygen in the reaction zone, adversely affecting the product characteristics. At the same time, at low concentrations of TEA, less chelation of the cations means less distribution. The product formation is discussed in terms of the TEA:sucrose ratios. At a 0.1 C rate between 3.0 and 4.3 V, the 10-h 800°C product gave a first-cycle discharge capacity of 156 mAh/g, which faded to 153 mAh/g in fifth cycle, with charge retention of 98%. A subsequent cycling between 3.0 and 4.4 V at a 0.1 C rate gave a discharge capacity of 167 mAh/g in the sixth cycle, fading to 165 mAh/g in tenth cycle, registering a charge retention of 98%. The superior performance of the material compared to the commercial LiCoO2 sample was also demonstrated. For example, at a 0.2 C rate between 3.0 and 4.2 V, not only was the initial capacity of our material higher (137 mAh/g) than that of the commercial sample (132 mAh/g), its cyclability was also higher: 100 cycles versus 68 for the commercial product for an 80% charge-retention cut-off value. Lithium-rich LixCoO2 (where x = 1.05~1.15) phases were also studied. The excess lithium stoichiometric phases were synthesized to compensate for any lithium that might be lost during heat treatment. The first-cycle discharge capacities of these products were 157, 154 and 155 mAh/g, respectively, for x = 1.05, 1.10 and 1.15 at a charge-discharge rate of 0.1 C between 3.0 and 4.3 V. When the voltage window was 3.0~4.4 V in the sixth cycle, the corresponding capacities were 166, 165 and 166 mAh/g, fading to 162, 163 and 164 mAh/g in the tenth cycle. |
