| 摘要: | 本論文包含六各章節, 前言與結論分別在第一與第六章，所有的結果與討論分別位於第二至第五章。第二章乃用溶液燃燒法，以尿素作為燃料合成奈米結構之錳酸鋰陰極材料；第三章乃以溶膠凝膠法製備3LaAlO3:Al2O3氧化物改質鈷酸鋰陰極材料，研究其於高電壓充電之電化學行為；第四章吾人以高溫固態法，利用具高表面積之碳粉先驅物，改質磷酸亞鐵鋰陰極材料；第五章，吾人利用一新穎的酸性界面活性劑，處理高分子PS球，並將此處理後之PS球作為碳源，改質磷酸亞鐵鋰陰極材料，探討其電化學行為。 第二章: 使用溶液燃燒法，以尿素作為燃料合成奈米結構之錳酸鋰陰極材料，在製程研究上，以700oC煆燒10 hr為最佳製程條件，在4.3 ~ 3.0 V以0.1 C-rate充放電測試下，循環壽命可達229次。由XRD結構分析可發現，所有的錳酸鋰產物皆為純相。TEM圖片顯示所得之產物，具奈米結構，其粒徑約為20-50 nm之間。CV結果顯示，奈米錳酸鋰材料，其幾乎沒有相變化之產生，顯示其可逆性佳，因而循環壽命較長。研究結果顯示，以溶液燃燒法確實可得具奈米粒徑之錳酸鋰陰極材料，並且具有良好之循環穩定性與電池性能。 第三章以3LaAlO3:Al2O3氧化物改質鈷酸鋰陰極材料，由TEM結果發現在鈷酸鋰的表面確實有一層La-Al-O化物薄膜之存在，且其厚度約40 nm左右。電池性能顯示以1.0 wt.% 3LaAlO3:Al2O3氧化物改質鈷酸鋰為最佳，充電至4.4 V時，其循環壽命為182次，相較於未改質之鈷酸鋰材料僅有38次，增加約5倍左右。 第四章乃以高表面積碳粉先驅物改質磷酸亞鐵鋰陰極材料，碳粉之來源為天然廢棄物花生殼經熱裂解後所得。由結果顯示，經過碳塗佈之磷酸亞鐵鋰材料，可以增進其導電度，因而使得材料之電池性能及循環穩定性增加。 第五章乃利用酸性分散劑處理過之PS球為碳源，製備磷酸亞鐵鋰/碳複合材料。由結果顯示，以PS球處理後之磷酸亞鐵鋰材料，其碳層厚度相當均一，約2 nm左右。在4.0-2.8 V，0.2 C-rate充放電條件測試下，初次放電電容量為147 mAhg-1，且經過100循環後，電荷維持率仍為100 %。當碳量增加時，可發現電容量及導電度亦隨之增加，但過多之碳粉會使得電容量降低，原因乃是過多之碳粉會使得電活性物質變少，因而降低整體之電容量。添加適量之碳粉，方有助於電池性能之提升。 This dissertation contains six chapters. Introduction and conclusion are presented in Chapter 1 and Chapter 6, respectively. All results and discussion are divided into the rest of chapters. In Chapter 2, we used urea as the fuel for the solution combustion synthesis of nanoparticulate LiMn2O4 from metal nitrates to use as a cathode for 4 V lithium batteries. Lithium–manganese oxides have received more attention in recent years as high-capacity intercalation cathodes for rechargeable lithium-ion batteries and nanostructured electrodes have been shown to enhance cell cyclability. The optimal synthesis protocol was 10 h calcination at 700 oC, which produced a product that could sustain 229 cycles between 3.0 and 4.3 V at a charge–discharge rate of 0.1 C before reaching an 80% charge retention cut-off value. X-ray diffraction and electron diffraction pattern investigations demonstrated that all the LiMn2O4 products are a spinel phase crystal. TEM micrographs show the prepared products are highly crystalline with an average particle size of 20–50 nm. Cyclic voltammetry shows the absence of phase transitions in the samples ensures negligible strain, resulting in a longer cycle life. This work shows the feasibility of the solution combustion method for obtaining manganese oxides with nano-architecture and high cyclability, and suggests it is a promising method for providing short diffusion pathways which improve lithium-ion intercalation kinetics and minimize surface distortions during cycling. In Chapter 3, we attempted to synthesize different wt. % 3LaAlO3:Al2O3-coated LiCoO2 cathode materials by an in-situ sol-gel process and study their electrochemical performance at higher charging voltages. The LiCoO2 particles were coated with various wt. % of lanthanum aluminum garnets (3LaAlO3:Al2O3) by an in-situ sol-gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirm the formation of a single-phase hexagonal ?-NaFeO2-type structure of the core material without any modification. Scanning electron microscope (SEM) images reveal surface modification of the cathode particles. Transmission electron microscope (TEM) images indicate that the surface of the core material is coated with a uniform compact layer of 3LaAlO3:Al2O3, with an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt. % 3LaAlO3:Al2O3 coated LiCoO2 cathode showed excellent cycle stability of 182 cycles, which was much more than the 38 cycles sustained by the pristine LiCoO2 cathode material when charged at 4.4 V. In Chapter 4, we adopted a solid state method to obtain fine LiFePO4 powders and mixed them with small amounts of a high surface area carbon precursor to prepare the carbon-coated LiFePO4 cathode material. The LiFePO4 particles were embedded in amorphous carbon and the carbonaceous materials were synthesized by the pyrolyzing peanut shells under argon in a two-step process that occurred between 573 and 873 K. The shells were also treated with a proprietary porogenic agent in order to alter the pore structure and surface area of the pyrolysis products. The carbon coating can significantly enhance the electronic conductivity of LiFePO4. The electrochemical properties of the as-prepared LiFePO4/C composite cathode materials were systematically characterized by X-ray diffraction, scanning electron microscope, element mapping, energy dispersive spectroscopy, Raman spectroscopy, and TOC analysis. The specific capacity, cycle property and rate capability were impressive compared to the pure olivine LiFePO4 material. The carbon-coated LiFePO4 cathode demonstrated high capacity and stable cyclability. Chapter 5 presents a novel concept of synthesizing the LiFePO4 by a solid state method using acidic surfactant treated polystyrene spheres that were a uniformly dispersed source, and characterizes its electrochemical behavior. The resultant carbon was entirely coated on LiFePO4 particles as a thin layer of about 2 nm. The LiFePO4/C composite delivered a first discharge capacity of 147 mAhg-1 at a 0.2 C-rate between 4.0 to 2.8 V, and the capacity remains 100% after 180 cycles. The electrochemical behavior and the four-point probe conductivity measurements revealed that the capacity and the conductivity of LiFePO4/C both improved as increased levels of carbon were added. However, too much carbon coating could reduce the capacity of LiFePO4/C because it lowers the ratio of active material in the composite and raises the resistance of lithium ion diffusion on the surface of the material. Adding an optimum amount of carbon increases the utilization of the active material and the electrical conductivity of electrode. |
