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Title: | 鋰離子電池正極過量鋰鎳錳氧化物與磷酸鋰鐵混和材料之電化學行為評估 Assessment on Electrochemical Behavior of Li1+x(MnNi)1-xO2 and LiFePO4 Blended Cathode Materials for Lithium-ion Batteries |
Authors: | Chung-Hsien Chuang 莊忠憲 |
Advisor: | 吳乃立(Nae-Lih Wu) |
Keyword: | 鋰離子電池,層狀過量鋰材料,磷酸鋰鐵,電位櫥窗測試,混合材料, Li-ion batteries,Layered Li-excess materials,LiFePO4,Potential window test,Blended materials, |
Publication Year : | 2017 |
Degree: | 碩士 |
Abstract: | 近年來隨著環保意識的抬頭,電動車逐漸成為的市場上注目的焦點,相較於常見的行動式裝置,電動車所需要的鋰離子電池不論在品質的要求、市場的規模都更勝以往,至於鋰離子電池發展最大的問題在於正極材料的瓶頸有待突破。橄欖石結構磷酸鋰鐵(LiFePO4)具有低成本、低污染性與優異的快速充放電能力,使其成為電動車動力來源的首選材料之一;然而,該材料因充放區間電位較低、電化學反應平台變化甚小等問題,使得在實際應用上仍有進步的空間。另一方面,層狀過量鋰錳基過渡金屬氧化物(Li-excess oxide) 具有高電容量、高工作電位等優勢,恰巧能和磷酸鋰鐵達到截長補短的效果,若將上述兩種材料的特性完美結合,相信能更進一步推升電動車領域發展。
本研究中,以固相法將兩種材料在不同比例下混合,並且結合數學模型進行模擬,探討混合材料在實際與模擬狀況下的電化學表現,以期能夠找到最適化的混合材料條件。本實驗中,從最初的過量鋰前驅物合成條件開始,到過量鋰與磷酸鋰鐵最適化充放電區間的各別討論,至最後的實際混合與模擬,進行完整系統性的研究。 第一部分從共沉降法(co-precipitation)合成層狀過量鋰前驅物著手,探討濃度效應與攪拌效應對於沉降粒徑分佈與形貌影響,確立最適化的合成條件;再以濕式球磨的方式對層狀過量鋰氧化物與前驅物個別處理,以期能改善層狀過量鋰本身快速充放電上表現的不足。 第二部分則測試商用磷酸鋰鐵與層狀過量鋰的充放電區間,並確立最適化的工作電位範圍。兩者材料因本身的特質造就差異甚大的電位櫥窗,因此在結合前細部的探討與測試可避免混合材料過度充放電的可能;另外,在這部分的實驗也意外發現調整層狀過量鋰材料的電化學活化上限截止電位後,在犧牲相對少的電容量下,能夠有效提升快速充放電、循環壽命與降低電池內阻抗,這在目前有關層狀過量鋰材料的文獻中尚未相關的討論。 第三部分,基於前面最適化充放電區間得到磷酸鋰鐵與層狀過量鋰的各別電化學表現後,以數學模型進行各種比例混合之電化學模擬;接著再以最直觀的固相法將兩種材料混合,探討實驗與模擬間的差異。本實驗中,發現到特定比例下的混合材料呈現出超出預期的電性表現,在循環壽命測試也觀察到上述兩種材料的搭配能夠減緩磷酸鋰鐵的極化現象。從結果顯示,磷酸鋰鐵/層狀過量鋰系統之混合材料在能量密度、電量監控管理上有顯著的提升,並且在材料的製備上相對簡易,對於目前商用電動車的電池材料提供一個優異的方案。 In recent years, with the raising of environmental awareness, electric vehicles have gradually become the focused spotlight in the market. Compared with the mobile devices in our daily live, the requirement of Lithium-ion batteries regardless of the quality and quantity are better than ever in electric vehicles. Nowadays, the biggest bottleneck for developing Lithium-ion batteries is the breakthrough of cathode material. The olivine structure lithium iron phosphate (LiFePO4) has low cost, eco-friendly and excellent rate capability, make it one of the best candidate for electric vehicles power source material. However, there are some problems need to be solved, such as relatively low potential window and the flat electrochemical plateau, before more extensively utilizing in real application. On the other hand, layered structure Lithium-excess manganese base transition metal oxides (Li1+x(M, Mn)1-xOy) possess high specific capacity and working potential window, coincidentally can be complementary with LiFePO4. If the characteristics of these two materials can effectively combine, we can further facilitate the development of electric vehicles market. In this research, we adopt solid phase method to mix LiFePO4 and Li-excess oxide into blended materials in different ratios, along with the simulation to investigate the electrochemical behavior of the electrodes. In the experiment, we start from the synthesis condition of Li-excess precursor, to separately discuss of the optimized operating potential window of Li-excess oxide and LiFePO4, then certainly blending the materials and make a comparison with mathematical simulation. This thesis conduct a systematical study. In first part, co-precipitation method is employed to synthesize layered structure Li-excess precursor, the concentration effect and mechanical stirring have significant influence on size distribution and surface morphology. Besides, wet ball milling is manipulated on both Li-excess oxide and precursor for improving the rate capability performance. In second part, we detailed test the optimum working potential window for commercialized LiFePO4 and Li-excess oxide materials, and establish the appropriate range. Because the instinct characteristics of two materials bring about quite different potential window, it is necessary to eliminate any possibility of overcharge before blending two components. Furthermore, we also accidentally discover a feasible approach to enhance the rate capability, improve cyclic retention and reduce the impedance with acceptable sacrifice in capacity by turning the cutoff potential in formation cycle for Li-excess oxide material. This discovery has not reported in any literature yet so far. In last part, based on the optimum potential window, we utilize the mathematic model to simulate the electrochemical behavior of the blending materials in different ratios, followed by physically mixing two compounds into blended electrodes, to investigate the discrepancy between experiment and simulation. We find the blended electrodes exhibit far beyond expectation in electrochemical performance in specific mixed ratios. In addition, cyclic capability also reveals that blended electrodes are conducive to retard polarization phenomenon. From a series of discussion, LiFePO4/Li-excess oxide system blended materials can promote the energy density, and state of charge monitor system. Moreover, this blended system is easily achieved in manufacturing, which indicates it offers an excellent solution for commercial electric vehicles batteries materials. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/77951 |
DOI: | 10.6342/NTU201702192 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 化學工程學系 |
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ntu-106-R04524052-1.pdf Restricted Access | 15.92 MB | Adobe PDF |
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