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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/3979

    Title: 以不同有機酸為碳源製備LiFePO4/C複合鋰離子電池陰極材料;Carboxylic Acid-Assisted Solid-State Synthesis of LiFePO4/C Composites and Their Electrochemical Properties as Cathode Materials for Lithium-Ion Batteries
    Authors: 呂東霖;Tung-Lin Lu
    Contributors: 化學工程與材料工程研究所
    Keywords: 磷酸亞鐵鋰;;鋰離子電池;有機酸;複合物;Composite;Carboxylic acid;Carbon;LiFePO4;Li-ion Battery
    Date: 2007-05-02
    Issue Date: 2009-09-21 12:27:35 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 磷酸亞鐵鋰(LiFePO4)因具結構穩定、原材料價格低廉、環境危害低、優良的安全性能及循環壽命等優點,近年來已成為熱門的鋰離子電池陰極材料;然而此材料不易量產、導電度及鋰離子擴散速率不佳,為目前所遭遇最大的瓶頸。在本研究中,利用高溫固態法之製程,分別加入不同結構之有機酸作為碳源,製備LiFePO4/C複合陰極材料,期能改善材料之導電度,進而提升電池性能。 本製程所製備之LiFePO4/C複合材料,經X光粉末繞射分析結果顯示皆為純相。各種有機酸添加量中,以添加60 wt.%丙二酸有最佳之電池性能,在充放電截止電壓為4.0~2.8 V及0.2 C的測試條下有150 mAh g-1左右之電容量,藉由總有機碳分析儀(TOC)進行碳含量之分析,最佳樣品碳含量為1.9 wt.%。另外,吾人亦使用拉曼光譜儀分析樣品中碳的成份及組成,並藉由特徵峰強度之比值(ID/IG)估算碳的石墨化程度多寡,添加各種有機酸碳源於60 wt.%添加量下之樣品,其ID/IG值介於0.907~0.938之間,顯示使用本製程能獲得含有較多石墨化碳比例之LiFePO4/C複合陰極材料。 在製備LiFePO4之過程中添加有機酸作碳源,可以抑制LiFePO4晶粒成長,同時達到縮小粒徑及提升電池性能之效果,並可經由FE-SEM觀察材料之表面型態可知,碳源添加量過少,則抑制晶粒成長之能力亦降低,但若添加量過多,則易造成材料團聚,同時發生因電活性物質之比例降低,致使電容量下降的情形;另外,為了解材料中碳之塗佈情形,吾人以TEM/SAED/EDS進行分析,其中半透明網狀之碳膜包覆於灰黑色LiFePO4材料表面,經SAED分析後發現樣品中同時具有結晶型及不定型結構之碳,並可與拉曼光譜之測試結果相互佐證;綜觀來說,添加各種有機酸碳源進行改質後,可提升材料導電度至10-4 S cm-1左右,但導電度亦受碳含量影響,適當之碳含量可於導電度、材料粒徑及電活性物質的比例中取得平衡,進而使材料能展現最佳之性能。 In order to enhance the capability of LiFePO4 materials, we attempted to coat carbon by incorporating various organic carboxylic acids as carbon sources. These acids include (a) mono-acid containing a ring structure, (b) straight-chain di-acids, and (c) tri-acids. The LiFePO4/C composite was synthesized using lithium carbonate, iron (II) oxalate dihydrate, and ammonium dihydrogen phosphate in a stoichiometric molar ratio (1.03:1:1) by a high temperature solid-state method. The purity of LiFePO4 was confirmed by XRD analysis. Galvanostatic cycling and conductivity measurements were used to evaluate the material’s electrochemical performance. A galvanostatic charge-discharge study was carried out between 2.8 and 4.0 V. The best cell performance was delivered by the sample coated with 60 wt.% malonic acid. Its first-cycle discharge capacity was 149 mAhg-1 at a 0.2 C rate or 155 mAhg-1 at a 0.1 C rate. The presence of carbon in the composite was verified by total organic carbon (TOC) and Raman spectral analysis. The actual carbon content of LiFePO4 was 1.90 wt.% with the addition of 60 wt.% malonic acid. The LiFePO4/C samples sintered with 60 wt.% various carboxylic acids were measured by Raman spectral analysis. The intense broad bands at 1350 and 1580 cm-1 are assigned to the D and G bands of residual carbon in LiFePO4/C composites, respectively. The peak intensity (ID/IG) ratio of the synthesized powders is from 0.907 to 0.935. Carbon coatings LiFePO4 with low ID/IG ratios can be produced by incorporating carboxylic acid additives before the final sintering process. The product morphology was analyzed by SEM and TEM/SAED/EDS. The particle size of the product decreased as the added amount of malonic acid increased. However, adding too much malonic acid caused a dramatic increase in particle growth. The EDS carbon map shows a uniform distribution of carbon in the sample on the surface of the composite particles. The TEM micrograph consists of two parts: a dark region and a grayish region which surrounds the dark region. It is interesting to know that both SAED patterns indicate that the materials of interest were in a crystalline phase. The TEM/EDS results unambiguously show that the particles in the dark region are LiFePO4 with a trace of carbon and those in the grayish region are carbon only. To produce LiFePO4 with carboxylic acid as a carbon source not only increases the overall conductivity (~ 10-4 Scm-1) of the material, but also prevents particle growth during the final sintering process.
    Appears in Collections:[化學工程與材料工程研究所] 博碩士論文

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