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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/64991

    Title: 鋰離子電池磷酸亞鐵鋰陰極材料之製程改質研究
    Authors: 林逸全;Lin,Yi-chuan
    Contributors: 化學工程與材料工程學系
    Keywords: 鋰離子電池;陰極材料;磷酸亞鐵鋰;Lithium ion battery;Cathode;Lithium iron phosphate
    Date: 2014-07-25
    Issue Date: 2014-10-15 14:37:41 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 摘 要

    一、 提高粉體振實密度,以改善電芯加工性;
    二、 金屬摻雜磷酸亞鐵鋰,以增強電化學性能;
    三、 引入高電壓磷酸基團材料於磷酸亞鐵鋰,以提升平均工作電壓;
    第一部分吾人將結合碳熱還原法與融鹽法,製備出橄欖石結構之LiFePO4陰極材料。為了提高粉體的振實密度,利用碳熱還原法所合成的LiFePO4/C複合材料,將其壓成錠後置於氧化鋁杯中,並且錠周圍以KCl包覆;最後再將氧化鋁杯置於管狀高溫爐中,於755 ℃高溫下進行煆燒1.0小時。KCl融鹽介質將能有效地影響材料的單位晶格體積,以及粉體的表面型態與振實密度,並且進一步地改變LiFePO4/C的電化學性能。經由融鹽法改質前後的LiFePO4/C複合材料,將使用X光繞射圖譜(XRD)、掃描電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、動態光散射儀(DLS)、拉曼光譜、振實密度測試儀等進行一系列鑑定。最後,經過融鹽法改質的LiFePO4產物具有高振實密度為1.5 g cm-3,含有4.58 wt.%碳含量,並且於2.8-4.0 V截止電壓與0.2 C-rate充放電速率下,其放電電容量為141 mAh g-1。因此,藉由融鹽法改質,將使得LiFePO4/C複合材料不僅具有高振實密度,同時展現出不錯的放電電容量。
    第二部份吾人利用高溫固態法,以癸二酸為碳源,摻雜鈣離子(Ca2+),製備出橄欖石結構之LiFe1-xCaxPO4/C複合材料,其晶體結構與電化學性質將作進一步地鑑定。吾人採用同步輻射X光繞射圖譜分析材料結構,發現鈣離子摻雜於LiFePO4材料,不會影響本身的晶體結構變化,但是單位晶格體積卻些微地增大。電化學性能測試結果顯示,LiFe0.99Ca0.01PO4/C複合材料於2.8-4.0 V截止電壓與0.2 C-rate充放電速率下,具有最佳放電電容量為149 mAh g-1,其電容量的提升來自於材料導電度與鋰離子擴散速度的增加。此外,LiFe0.99Ca0.01PO4/C複合材料將可承受20 C-rate大電流充放電,相當於充放電僅需要3分鐘。
    由於磷酸釩鋰(Li3V2(PO4)3)、磷酸氟鋰釩(LiVPO4F)、磷酸錳鋰(LiMnPO4)等磷酸基團化合物具備高工作電壓特性,近期已成為鋰離子電池陰極材料之熱門研究主題,第三部份將利用共沉澱法與碳熱還原法,嘗試引入LiVPO4F於LiFePO4材料中,製備出xLiFePO4‧(1-x)LiVPO4F (LFP-LVPF)複合材料,以提升LiFePO4材料之工作電壓。將採用X光繞射圖譜(XRD)、掃描電子顯微鏡(SEM)、總有機碳含量分析儀(TOC)等鑑定分析一系列LFP-LVPF複合材料之材料特性。當莫耳數比LFP : LVPF (即x : (1-x))為1:0、0.99:0.01、0.75:0.25、0.5:0.5、0.25:0.75、0:1時,LFP-LVPF複合材料之0.2 C放電電容量將分別為153、160、132、106、92與78 mAh g-1,其放電電容量將隨著LVPF莫耳數增加而減少。此外,混掺較多LVPF莫耳數之LFP-LVPF複合材料,即x : (1-x)為0.75:0.25、0.5:0.5、0.25:0.75之樣品,其工作電壓高於LiFePO4材料,而且位於4.35/4.15 V之充電/放電電壓平台,將隨著LVPF莫耳數的增加而變長。

    Globally, the development of high-power lithium-ion batteries is focused on lithium manganese oxide batteries, lithium cobalt nickel manganese batteries and lithium iron phosphate batteries. Lithium iron phosphate is regarded as a practical and popular cathode material for high power lithium-ion batteries due to its high capacity, high rate capability, long cycle life, superior thermal stability and good high-temperature performance. However, the intrinsic disadvantages of pure LiFePO4 material are its poor electronic conductivity, low tap density and low operating voltage. Therefore, we combined a number of innovative concepts and techniques in the fabrication processes to alleviate the aforementioned problems. The main purpose of this study has been divided into three sections:
    (1) To improve the manufacturing process of battery cells by enhancing the tap density of LiFePO4 powders.
    (2) To enhance the electrochemical performance by doping the metal ions into LiFePO4 crystal.
    (3) To increase the average operating voltage by introducing the phosphate-based compounds with high working voltage into LiFePO4 material.
    Olivine-structured LiFePO4 cathode materials were prepared via a combination of carbothermal reduction (CR) and molten salt (MS) methods. To enhance the tap density of powders, the LiFePO4/C composite was pressed into pellets and then sintered for at least 1 h at 1028 K in the reaction environment of KCl molten salts. The use of molten salt can effectively influence unit cell volume, morphology and tap density of particles, and consequently change the electrochemical performance of LiFePO4/C. The composites were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), Raman spectroscopy and tap density testing. The final product with high tap density of 1.50 g cm−3 contains 4.58 wt.% carbon and exhibits good discharge capacity of 141 mAh g−1 at a 0.2 C-rate in the potential range of 2.8-4.0 V.
    These olivine LiFe1-xCaxPO4/C composites (x = 0 ~ 0.014) were synthesized by a solid-state method using sebasic acid as a carbon source. The structure and electrochemical properties of the LiFe1-xCaxPO4/C compounds were studied. The X-ray diffractometer (XRD) results indicated that Ca2+ doping did not affect the structure of the samples, but the unit cell volume of the doped samples was slightly increased. Electrochemical measurements showed that the LiFe0.99Ca0.01PO4/C composite delivered a discharge capacity of 149 mAh g-1 at a 0.2 C-rate between 4.0 and 2.8 V, probably due to the significant improvement in electronic conductivity and Li+ ion diffusion. The cell could also sustain a 20 C-rate, and this rate capability is equivalent to charge or discharge in 3 min.
    Phosphate-based compounds with a high working voltage, such as Li3V2(PO4)3, LiVPO4F and LiMnPO4, have been proposed as a new class of cathode materials for lithium-ion batteries. To improve the operating voltage of LiFePO4, we introduced LiVPO4F to the preparation of xLiFePO4‧(1-x)LiVPO4F (LFP-LVPF) composites through an aqueous precipitation and carbothermal reduction method. A series of LFP-LVPF composites have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and total organic carbon (TOC) analysis. The discharge capacities of LFP-LVPF composites for x:(1-x) = 1:0, 0.99:0.01, 0.75:0.25, 0.5:0.5, 0.25:0.75 and 0:1 at a 0.2 C-rate were 153, 160, 132, 106, 92 and 78 mAh g-1, respectively. The discharge capacity decreased with increasing mole fraction of LVPF. Moreover, the operating voltage of LFP-LVPF composites for x:(1-x) = 0.75:0.25, 0.5:0.5 or 0.25:0.75 is higher than that of LFP, and the charge/discharge plateaus around 4.35/4.15 V for LFP-LVPF composites become longer as the value of x decreases.
    Appears in Collections:[化學工程與材料工程研究所] 博碩士論文

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