| 摘要: | 磷酸亞鐵鋰具有低成本、低汙染、良好熱穩定性與長循環壽命等優點，因此成為近年最受矚目的鋰離子電池陰極材料之一。本論文有四大主題，目的是研究LiFePO4商業化發展上遇到的問題，包含材料中磁性雜質的存在、較低的工作電壓與電容量、以及材料吸水劣化等問題。
吾人首先以磁選的方式，製備一系列含有不同磁性雜質含量的樣品，並以新式磁導儀量化磁性雜質對電性的影響，非常適合作為工業上的品管工具，純化後不含磁性雜質的LiFePO4/C，其0.5 C的初始電容量高達151 mAh g-1，自放電率在所有樣品中最低，一週後在0.5 C條件測試下仍有高於97%的電容量維持率，5 C長循環測試下，500次後仍有92%的電容量維持率。
為了提升材料電壓，吾人以流變相法配合錳(Mn)與釩(V)的添加，合成LiFexMnyVzPO4/C複合材料，其中LiFe0.6Mn0.2V0.2PO4/C展現良好的電化學性能，0.5 C充放電條件下，放電電容量達153 mAh g-1。由於電壓的提升，此材料能量密度達520 Wh kg-1，比 LiFePO4/C高出約15%，且碳含量僅3.4 wt.%，比LiFe1-xMnxPO4/C於一般文獻上報導的5~10%低，此可增加工業上電芯製作時的加工性。
承上述V添加的改質經驗，吾人成功合成出Li2Fe0.9V0.1PO4/C複合材料，該材料理論電容量高達220 mAh g-1，吾人於本論文報導的結果中，其0.5 C的放電電容量已達161 mAh g-1，在4.6-2.0電壓範圍內，可承受20 C之充放電速率，並具有良好的熱穩定性，具有成為下世代陰極材料的潛力。
為瞭解水氣對置放不同濕度環境LiFePO4的影響，吾人測試商用LiFePO4含水量、粒徑、比表面積與放電電容量隨時間的變化，含水量與比表面積隨時間有增加的趨勢，且比表面積的增加具有不可逆的特性，粒徑與鈕扣型電池的電容量隨時間幾乎沒有變化，但軟包電芯的放電電容量卻會因受水氣造成材料的脫鋰現象而下降。;Olivine-structured lithium iron phosphates (LiFePO4) become a promising cathode material because of its low cost, low toxicity, remarkable thermal stability and long operation life. In order to overcome the obstacles hindering the commercial development of LiFePO4, there are four topics in this study, including the presence of magnetic impurities, lower work voltage and discharge capacity, and property deterioration upon exposure to humid air.
First, we produced a series of samples containing different amounts of magnetic impurities by magnetic selection, and applied a new alternative current magnetic susceptibility device to quantify the impact of magnetic impurities on LiFePO4/C composites, which is a useful quality control tool for practical lithium battery applications. The magnetic impurity-removed LiFePO4/C composites exhibited the best discharge capacity of 151 mAh g-1 at 0.5 C and excellent cycle performance with a capacity retention of 97% after 500 cycles at 5 C. Self-discharge experiments showed that the purified LiFePO4/C maintains 97% residual capacity after a week at 0.5 C, which is better than the others containing magnetic impurities.
In order to increase the work voltage, LiFexMnyVzPO4/C composites were prepared by a rheological phase method with the addition of Mn and V. The LiFe0.6Mn0.2V0.2/C composites deliver the best initial discharge capacity of 153 mAh g-1 at 0.5 C. Due to the increase of voltage, the energy density of LiFe0.6Mn0.2V0.2/C is 520 Wh kg-1, which is 1.15 times higher than LiFePO4/C. The carbon content is 3.4 wt.%, which is lower than 5~10 wt.% as stated by the previous reports. Lower carbon content leads to better processability of pouch cells in industry.
According to the above modification method, we synthesized Li2Fe0.9V0.1PO4/C composites, whose theoretical capacity is 220 mAh g-1. This work shows that the Li2Fe0.9V0.1PO4/C composites delivered an initial discharge capacity of 161 mAh g-1 at 0.5 C, and can sustain a 20 C-rate between 4.6 and 2.0 V. Besides, Li2Fe0.9V0.1PO4/C shows excellent thermal stability, which has been recognized as the most promising positive cathode.
To better realize the moisture effect on LiFePO4/C in different humidity, we measured the water content, particle size, specific surface area and discharge capacity of commercial LiFePO4/C dependent time. The water content and specific surface area increase with time, and the change of surface area is irreversible. The particle size and discharge capacity of coin cells do not change with time, but the discharge capacity of pouch cells decreases due to the delithiation caused by moisture. |
