磷酸鹽系列電極材料LiMPO4 (M=Fe, Co, Ni, Mn) 由於具有優異的電化學性能、高理論電容量、熱穩定性佳及安全性佳等優點，故成為新一代鋰離子電池之正極材料。其中，磷酸鋰鈷由於其位於4.8V的還原電位在同系列材料中為最高，因此確保了此材料擁有最高的能量密度，故近來受到不少的關注。在本論文中，主要期望發展出具有高電容量及優異循環壽命的磷酸鋰鈷材料，因此需克服其固有的缺點如低導電度、低鋰離子擴散性及充放電循環中結構的不可逆變化等。實驗中主要採用噴霧乾燥法及溶膠凝膠法等兩種方法來合成磷酸鋰鈷，而為了達成前述的目的，我們對材料採取了一連串的改質與修正，如碳材披覆、鐵離子的摻雜及以不同劑量比合成化合物等，以期能改善電化學性能。
首先，我們將材料以不同氣氛燒結以觀察其效應，在氧化氣氛空氣下燒結的樣品，由於碳成份全部被氧化，故形成一個多孔性的二次粒子，有利於鋰離子在材料內部的擴散性。此外，當在前驅物溶液(Precursor solution)中加入過氧化氫水溶液後，由於其易與檸檬酸所含的羧基起反應，以及另一個與Fenton reaction類似的反應發生，故也有助於形成一多孔性的二次粒子。從電性結果中則可發現當合成出多孔性二次粒子時，電性結果受到改善。
為了提升導電度，我們則嘗試將碳材披覆於粒子表面。由於磷酸鋰鈷在充放電過程中會受到分解，造成其電容量衰退速度過快，因此我們利用鐵的摻雜以期改善其結構穩定性。另外，我們也調整不同的熱處理條件如燒結溫度及持溫時間，以及鋰與過渡金屬鈷和鐵的劑量比。透過這些改質與修正，放電電容量及循環壽命等電化學特性則逐步地被改善。
當我們確認透透過碳材披覆以及鐵離子的摻雜可有效改善電性後，我們嘗試在含鐵離子的樣品Li1.1Co0.8Fe0.2PO4外層包覆一層碳材以觀察其效應。從充放電曲線中，我們可發現當含鐵離子的樣品被碳材包覆時，在約3.5V處有一屬於磷酸鋰鐵的反應平台產生。透過X光吸收近邊緣結構(XANES)的結果，則可確認此乃由於碳材會活化磷酸鋰鐵的反應性，故除了由固有的磷酸鋰鈷反應平台所貢獻的電容量外，磷酸鋰鐵也貢獻了一些放電電容量。實驗結果則顯示，在650°C下在空氣中燒結12小時的Li1.1Co0.8Fe0.2PO4樣品擁有最高的電容量以及良好的循環性。

Olivine orthophosphates LiMPO4 (M=Fe, Co, Ni, Mn) is a new-generation cathode materials for Li-ion batteries due to remarkable electrochemical properties, high theoretical capacity, and thermal stability as well as increased safety. Among them, Lithium cobalt phosphate (LiCoPO4) bares highest redox potential at 4.8V versus Li/Li+, which ensures the highest energy density so it receives a lot of attention. In this research, the main purpose is to develop a high capacity cathode material with good cycling stability, consequently the inherent weaknesses of LiCoPO4 like low electronic conductivity, low Li-ion diffusivity and some irreversible structural transformation during cycling have to be conquered. Two synthetic method included spray drying method and sol-gel method was used. To achieve our purpose, a series of modifications like carbon coating, Fe doping and different stoichiometric ratio were adopted to improve the electrochemical performance.
At first, the influence of calcination atmosphere was investigated, and it was found that oxidizing atmosphere Air was helpful for creating porous secondary particle which can boost the diffusivity of lithium ion. Besides, adding H2O2 in the precursor solution also could create the porous structure by reacting with the carboxyl group of the citric acid, and also the Fenton-like reaction is helpful for decomposing the organic compound. With porous secondary particle, the electrochemical performance was improved.
To increase the low electronic conductivity, carbon coating method was adopted. Besides, large capacity fading of LiCoPO4 might result from the decomposition of the structure during charge and discharge process, hence Fe doping was attempted to stabilize the structure. Different heat-treatment conditions like calcination temperature and holding time were tried to find out the most suitable condition. Also, the ratio between Li and transition metal Co and Fe was adjusted. By these modifications, discharge capacity and cycling stability were improved gradually.
As the electrochemical performance could be improved by coating a carbon layer on the particle and by Fe doping, Li1.1Co0.8Fe0.2PO4, which means the sample with Fe doping, was coated by carbon to observe the influence. From charge-discharge curves, a plateau of LiFePO4 at around 3.5V could be observed. By X-ray absorption near edge structure (XANES) tests, it can be confirmed that with carbon coating the reactivity of LiFePO4 was activated. Consequently, except for the capacity devoted by LiCoPO4, LiFePO4 also contributed to some discharge capacity. The experimental results show that carbon coated Li1.1Co0.8Fe0.2PO4 calcined at 650°C for 12 hours in Air delivered highest discharge capacity and good cycling stability.

摘要 I
Abstract III
Table of Contents V
List of Tables VII
List of Figures VIII
Chapter 1 Introduction and Background 1
Chapter 2 Theory and Literature Review 3
2-1 Introduction to Lithium-ion Batteries 3
2-1-1 Historical Developments of Lithium-ion Batteries 3
2-1-2 Basic Concepts of Lithium-ion Batteries 6
2-2 Introduction to Cathode Material for Lithium-ion Batteries 10
2-2-1 Layered Structure 10
2-2-2 Spinel Structure 15
2-2-3 Olivine Structure 17
2-3 LiCoPO4 Cathode Material for Lithium Ion Batteries 21
2-3-1 Basic Features of Lithium Cobalt Phosphate 21
2-3-2 Synthesis of Lithium Cobalt Phosphate 26
2-3-3 Modifications of Lithium Cobalt Phosphate Powders 33
2-4 Experimental Technique: X-ray Absorption Spectroscopy 46
Chapter 3 Experimental 50
3-1 Materials and Chemicals 50
3-2 Synthesis of Lithium Cobalt Phosphate 52
3-2-1 Synthesis of LiCoPO4 by spray-drying method and Sol-gel method 52
3-2-2 The Carbon Coating Process by Glucose 53
3-3 Analysis and Characterizations 57
3-3-1 Thermogravimetry/ Differential Thermal Analysis Thermoanalyzer 57
3-3-2 X-ray Diffraction 57
3-3-3 Scanning Electron Microscopy 58
3-3-4 Elemental Analysis 59
3-3-5 X-ray Absorption Spectroscopy 60
3-4 Electrochemical Characterization 63
3-4-1 Cell Fabricating Process 63
3-4-2 Charge-Discharge Tests 64
Chapter 4 Results and Discussions 67
4-1 The Influence of Reducing and Oxidizing Atmosphere 67
4-2 Effect of Carbon Coating 73
4-3 Effect of Adding H2O2 into Precursor Solution 76
4-4 Effect of Fe Doping 82
4-5 Influence of Different Heat-treatment Conditions 87
4-6 Composition Effect: Different Ratio between Li vs. Co and Fe 95
4-7 Combination of Fe Doping and Carbon Coating 103
4-8 Influence of Different Synthetic Methods 119
Chapter 5 Conclusions 123
References 125

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