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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/43431


    Title: 鋰離子電池陰極材料表面/摻雜改質之製程研究;A Process Study on Surface/Doping Modification of Cathode Materials for Lithium Ion Batteries
    Authors: 卓永達;Yung-Da Cho
    Contributors: 化學工程與材料工程研究所
    Keywords: 陰極材料;碳塗佈;鑭摻雜;釩酸鈷鋰材料;磷酸亞鐵鋰材料;複合材料;鋰離子電池;Cathode material;carbon coating;La doping;LiCoVO
    Date: 2010-07-30
    Issue Date: 2010-12-08 13:37:49 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 本論文包含五個章節,介紹與結論分別在第一與第五章,所有的結果與討論分別於第二章至第四章。 第二章中吾人以檸檬酸-尿素聚合法製備純LiCoVO4與La2O3表面改質LiCoVO4材料,以及藉由高溫固態法合成La2O3塗佈之LiCoVO4/C複合材料。使用檸檬酸-尿素聚合法可成功地合成出具奈米晶粒之LiCoVO4粉體,並從Roman與XPS分析可確定碳與La2O3塗佈於LiCoVO4¬顆粒表面。FTIR光譜分析顯示,在低溫下合成過程中,可完全去除剩餘有機物質,並可獲得LiCoVO4材料。從TEM圖顯示LiCoVO4顆粒相當均一,且粒徑約95 nm,而在LiCoVO4材料表面上的塗佈層厚度約15 nm。從Li/LiCoVO4的電化學特性發現,當電壓充電至高電壓4.5 V時,複合材料還能具有良好的電容量維持率,以檸檬酸-尿素聚合法所合成的純LiCoVO4材料,經30次充放電後,電池性能為10 mAhg-1,而經0.5 wt.% La2O3塗佈之LiCoVO4材料,經110次充放電後,電容量維持37 mAhg-1。然而,當0.5 wt.% La2O3表面改質LiCoVO4與60 wt.%丙二酸碳源一同煆燒後,所形成的複合材料,可表現出更佳的電池性能與熱穩定性,其具有71 mAhg-1的初始放電電容量,經30次循環後還有60 mAhg-1電容量。在熱穩定性方面,吾人以 0.1 C-rate充電至4.5 V維持10小時之測試條件,發現經La2O3與碳塗佈於LiCoVO4顆粒表面後,其熱分解起始溫度升高至475 K,而純LiCoVO4材料為452 K。另一方面,複合材料的總反應熱明顯地下降到35 Jg-1,純LiCoVO4材料為176 Jg-1。從以上的結果顯示,經La2O3與碳表面改質LiCoVO4粉體,可明顯地改善其循環壽命、熱穩定性與初次放電電容量。 第三章中吾人嘗試透過La3+晶格摻雜與碳非晶格摻雜合成LiFe1-xLaxPO4/C複合材料。從四點探針導電度儀測試發現,LiFePO4材料經碳表面改質與鑭摻雜改質後,可大幅地改善材料導電度,因此吾人有系統地研究經高溫固態法所合成的鑭摻雜LiFePO4/C複合材料之物理與電化學性質。經鑭摻雜後並不會影響材料結構,但可明顯地改善其電池性能與循環穩定性。從電池性能測試結果顯示,以LiFe0.99La0.01PO4/C複合材料在0.2 C-rate、2.8~4.0 V條件下,擁有最高放電電容量156 mAhg-1,相較於純LiFePO4材料的104 mAhg-1。此複合材料在80%電荷維持率下,能維持497 次的循環壽命,此電化學性質明顯改善的原因,乃因提高複合材料的電子導電度(從5.88×10-6 提高至2.82×10-3 Scm-1),以及提升其鋰離子擴散性質。 在吾人碳塗佈過程中,發現碳塗佈層厚度與結構對LiFePO4材料的電池性能扮演著重要的角色。因此,在第四章中,吾人研究碳塗佈層厚度與均勻性對LiFePO4/C複合材料的電化學性質之影響,並佐以碳蒸氣沉積技術,與使用聚苯乙烯與丙二酸當作雙重碳源,吾人有系統地研究此複合材料的物理、結構與鋰離子的擴散動能。藉由碳蒸氣沉積技術,可發現碳含量與均勻度對於LiFePO4/C材料的電化學特性是一決定性關鍵參數。當LiFePO4顆粒表面塗佈一層薄且均勻的碳層,在0.2 C-rate、4.0~2.8 V測試條件下,擁有151 mAhg-1的最高初次放電電容量,且在80%電荷維持率下,具有415次的循環壽命。經研究發現,為了降低極化現象,碳塗佈層厚度必須是均勻地分佈於活性物粒子表面,可獲得較高的可逆電容量。 This dissertation contains five chapters, introduction and conclusions are presented in Chapter 1 and Chapter 5, respectively. All results and discussion are divided into the rest of chapters. In Chapter 2, we prepared the pristine LiCoVO4 powder and La2O3-coated LiCoVO4 materials by a citric acid-urea polymeric method, and La2O3-coated LiCoVO4/carbon composite cathode materials by a solid state high temperature method. A citric acid-urea polymeric method was successfully applied to synthesize nanocrystalline LiCoVO4 cathode materials. Raman and XPS analyses confirmed the presence of carbon and La2O3 coating on the surface of LiCoVO4 powders. FTIR spectral results confirmed the complete removal of organic residues at a low temperature and the formation of LiCoVO4. TEM images revealed that the particles were a uniform nanosize of about 95 nm and the coated layer was about 15 nm thick on the LiCoVO4 material. The electrochemical performance of the Li/LiCoVO4 cell demonstrated good capacity retention when charged to a high voltage of 4.5V. The cell performance of pristine LiCoVO4 synthesized by a citric acid-urea process sustained 10 mAhg-1 for 30 cycles and 0.5 wt.% La2O3-coated LiCoVO4 sustained 37 mAhg-1 for 110 cycles. However, the sample obtained from a 0.5 wt.% La2O3-coated LiCoVO4 calcined with 60 wt.% malonic acid demonstrated the best cell performance and thermal stability of the materials we studied. It had an initial capacity of 71 mAhg-1 and reached 60 mAhg-1 at 30 cycles. The onset temperature of thermal decomposition of this composite cathode material was 475 K compared to 452 K for bare LiCoVO4. The total reaction heat was reduced significantly by La2O3 and malonic acid coatings and its heat evolution was 35 Jg-1 vs. 176 Jg-1 for bare LiCoVO4, after they were charged to 4.5 V at a 0.1 C-rate and then potentiostated at 4.5 V versus Li+ for 10 h. These results demonstrate a remarkable improvement of the LiCoVO4 cathode material in terms of capacity, cycle life and thermal stability. In Chapter 3, we synthesized LiFe1-xLaxPO4/C composite materials through lattice doping with La3+ cation and non-lattice doping with carbon. The conductivity of LiFePO4 was enhanced significantly via carbon coating and La-doping. The physical and electrochemical properties of La-doped LiFePO4 cathode materials synthesized via a high temperature solid-state method were systematically investigated. The La doping did not affect the structure of the cathode material, but considerably improved its capacity performance and cyclic stability. Among the materials studied, the LiFe0.99La0.01PO4/C composite demonstrated the best cell performance with a maximum discharge capacity of 156 mAhg-1 cycled between 2.8 and 4.0 V at a 0.2 C-rate, compared to 104 mAhg-1 for pure LiFePO4. This composite electrode can sustain 497 cycles based on 80% charge retention. Such a significant improvement was mainly attributed to enhanced electronic conductivity (from 5.88×10-6 to 2.82×10-3 Scm-1) and high Li+ mobility in the doped samples. In our carbon coating process, we found that carbon coating thickness and structure play important roles in determining the capacity of LiFePO4 cathode materials. Therefore, in chapter 4, we investigated the effect of carbon coating thickness and its homogeneity (uniformity) on the electrochemical properties of LiFePO4/C composite materials prepared by a carbon vapor deposition technique, and used both polystyrene and malonic acid as carbon sources. The physical, structural, and Li-ion diffusion kinetics of LiFePO4/C composites were systematically investigated. Using a carbon vapor deposition technique, we have shown that the amount of carbon and its coating thickness and uniformity in LiFePO4/C materials are all crucial parameter in determining the electrochemical performance. LiFePO4 coated with a thin and uniform carbon film can deliver maximum discharge capacity of 151 mAhg-1 at a 0.2 C-rate and sustain 415 cycles at 80% of capacity retention. In order to minimize the polarization, a carbon coating layer must be uniformly distributed around each active particle, because the lower polarization leads to a higher reversible capacity.
    Appears in Collections:[化學工程與材料工程研究所] 博碩士論文

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