分散技術及其在奈米陶瓷和鋰電池電極製備之應用研究

Abstract

本論文探討奈米分散技術及其在鈦酸鋇（BaTiO3，BTO）奈米陶瓷和鋰電池中鋰鈷氧（LiCoO2）之電極製備之應用；鈦酸鋇相關之陶瓷材料包含純鈦酸鋇、鈦酸鍶鋇（Ba0.7Sr0.3TiO3，BST）和添加氧化鑭之鈦酸鍶鋇（La2O3-doped BST，LBST）。奈米陶瓷實驗中使用兩性之氨丙基丙醯胺（□-N，N-2甲基-丙烯乙基氧，Polyacrylamide/(□-N, N-dimethyl-N-acryloyloxyethyl) Ammonium Ethanate，PDAAE）和陰離子型高分子分散劑（聚甲基丙烯酸，Poly(methacrylic Acid，PMAA-Na）為分散劑，本研究另合成一種新型之樹枝狀高分子分散劑（聚（4-4，2苯胺甲基苯氧基鄰苯二甲酐），Poly(4-{4-[di(4-aminophenyl)methyl] phenoxy}phthalic Ether Imide，PEI）以分散奈米碳管（Carbon Nanotube，CNT）並應用於鋰電池鋰鈷氧電極基板 之製備中，以提高鋰電池之效能。 奈米鈦酸鍶鋇及其相關之陶瓷係利用物理研磨加上化學分散進行製備漿料，分散效果之測試包含：黏度、沈降體積、表面電位和高分子分散劑之吸附量來判斷粉體之分散狀況，之後奈米粉體再經過適當之燒結過程而製成電容並測試其相關之介電性質。實驗中發現，控制燒結環境條件可將奈米陶瓷粉末燒結成具有良好之微結構、結晶相和介電特性之電容元件。 奈米陶瓷部份之主要研究成果如下：奈米鈦酸鋇在經由1100□C，6小時之燒結可獲得小晶粒、高密度和良好之介電性質（室溫下介電常數為8000，介電損失為5×10□3）之電容元件。其次，高密度（□ 95%）、高介電性質之奈米鈦酸鍶鋇電容可藉1200□C、6小時燒結製程（晶粒大小約為200 nm，室溫下介電常數為9700，介電損失為0.04）或1300□C、1小時燒結製程（晶粒大小約為220 nm，室溫下介電常數為9800，介電損失為0.075）而獲得。此外，奈米鈦酸鍶鋇電容相較於傳統之鈦酸鋇及鈦酸鍶鋇具有較低之居禮溫度。最後，添加1 mol.%氧化鑭之鈦酸鍶鋇經由1400□C、1小時燒結可得晶粒大小約為200 nm、最佳之介電常數為13800和最低之介電損失2.8×10□4之電容元件。 在關於奈米碳管之分散部分，經由等溫吸附曲線、流變和沈降實驗得知PEI之最佳劑量為2 wt.%（此係相對於奈米碳管重量之比例）。由電化學阻抗光譜（Electrochemical impedance spectroscopy，EIS）之量測得發現電池之交流阻抗由287 □（電極未添加PEI）降至161 □（電極中含2 wt.%之PEI），而其主要之原因為分散良好之奈米碳管在電極中形成一網狀結構並藉此提供良好之電荷轉移路徑。在充放電實驗（Charge-discharge Test）中發現，電極含分散良好之奈米碳管之鋰電池在大電流放電下（8 C）其放電容值仍可維持在74 %（電流密度為101 mAh/g），高放電容量和較長之壽命將有助於大型鋰電池在高電源工具、電動腳踏車、電動摩托車等之應用。

This thesis discusses the dispersion technology and their applications to the fabrication of barium titanate (BaTiO3, BTO)-relating nano-ceramics and LiCoO2 electrode of lithium ion battery (LIB). The BTO-relating ceramics include the genuine BTO, Ba0.7Sr0.3TiO3 (BST) and La2O3-doped BST (LBST). An amphibious dispersant, polyacrylamide/(□-N, N-dimethyl-N-acryloyloxyethyl) ammonium ethanate (PDAAE), and an anionic dispersant, poly(methacrylic acid) (PMAA-Na), were adopted to the dispersion of nano-ceramics. A new dispersant, poly(4-{4-[di(4-aminophenyl)methyl]phenoxy}phthalic ether imide) (PEI), was synthesized for the dispersion of carbon nanotube (CNT) conduction agent in LiCoO2 electrode so as to improve the performance of high power LIBs. BST-relating nano-ceramic powders were prepared via a physical grinding in conjunction with the chemical dispersion. Dispersion properties including viscosity, sedimentation volume, particle size distribution, zeta-potential and isothermal adsorption were investigated so as to determine the optimal dispersion condition. The capacitor samples were then prepared by appropriate sintering process and then dielectric property measurement was followed. It was found that specific control on sintering condition is required for the nano-ceramic powders to achieve desired microstructure, crystalline phase and dielectric properties of devices. The major achievements in BTO-relating nano-ceramics are as follows. BTO sample sintered at 1100□C for 6 hrs possesses relatively small grain sizes (about 140 nm), high density (about 95% T.D.) and distinct room-temperature dielectric properties (dielectric constant = 8000; dielectric loss = 5×10□3). High-density (□ 95%) BST samples with desired dielectric properties could be obtained via sintering at 1200□C for 6 hrs (grain size □ 200 nm; dielectric constant = 9700; dielectric loss = 0.04) or at 1300□C for 1 hr (grain size □ 220 nm; dielectric constant = 9800; dielectric loss = 0.075). Besides, the BST samples exhibited a lower Curie point (TC) in comparison with conventional BTO and BST systems. The 1.0 mol.% La-doped BST sample sintered at 1400□C for 1 hr possesses the finest grain size about 200 nm, the highest dielectric constant about 13800 and lowest dielectric loss about 2.8×10□4. In the part relating to the dispersion of CNT, the optimal amount of PEI (about 2 wt.% relating to the weight of CNT) was determined by means of the studies of adsorption isotherm, rheology and sedimentation behaviors. Electrochemical impedance spectroscopy (EIS) revealed that the ac resistance of electrode decreases from 287 □ for cell without PEI to 161□ for the cell at containing optimal amount of PEI. This is attributed to the network established by dispersed CNTs that effectively promotes the charge carrier migration in electrode. The C-rate test indicated that at the high-rate discharge test results, that the LIB test cell with modified electrode possesses 8 C-discharge capacity of 101 mAh/g, which is about 74% of its 0.2 C-discharge capacity. The results of high rate capability and long cycle life evidences the promising applications of such a LIB to power sources, e.g., power tools, electric bicycle, electric motorcycle, etc.