以固態法合成LiFePO4/C正極材料與其電性之探討

Synthesis of the LiFePO4/C Cathode Material by the Solid-State Method and Associated Electrochemical Properties

賴恬郁(Tien-Yu Lai)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/碩士(2009年)

https://doi.org/10.6843/NTHU.2009.00423

摘要

The LiFePO4 powder was synthesized at 750 oC by the solid-state method. LiFePO4/C composites with glucose as the carbon source were also synthesized from 650 oC to 800 oC. The carbon coating method could improve the conductivity of LiFePO4 cathode materials, and also enhance the retention and capacity of LiFePO4. The Fe2P impurity could improve the conductivity of LiFePO4/C composites. The polarization of LiFePO4/C with appropriate amount of Fe2P is reduced at a higher current rate. However, large amount of Fe2P impurities could prohibit the pathway of lithium ions of LiFePO4/C, resulting in the decrease in the capacity of LiFePO4/C at a higher current rate. Therefore, the amount of Fe2P impurities should be carefully controlled when synthesizing LiFePO4 cathode materials.

The structural transformations, the change in the valence state of iron, and the bonding changes of LiFePO4 taking place during charge and discharge were studied by in-situ X-ray diffraction (XRD) and adsorption (XAS). The Fe2P impurities were revealed for the LiFePO4/C samples. The valence state of iron changed from 2+ to 3+ and the bonding length of Fe-O and Fe-P shrank during the two phase transformation (LiFePO4-FePO4) in LiFePO4 synthesized at 750 oC under charging at 0.14 C. On the contrast, the LiFePO4/C synthesized at 800 oC presented a normal structural transformation process, which was due to the existence of Fe2P impurities. However, the delayed structural transformation of LiFePO4/C synthesized at 750 oC was observed during charge and discharge at a lower rate (0.25 C) rather than at a higher rate (0.5 C). It was argued that the nucleation mechanism of LiFePO4 played an important role in the biphasic transformation of the delayed structure.

關鍵字

磷酸鋰鐵 ； 固態法

並列摘要

此研究係使用固態法在燒結溫度750 oC的條件下合成LiFePO4粉末，在製程溫度650到800 oC中並添加葡萄糖做為碳源的情況下合成多種複合材料LiFePO¬4/C以做比較。鍍碳的方式使LiFePO4的導電度增加，同時也提升LiFePO4的電容量與改善其電性穩定度。Fe2P雜相的存在能有效地增加LiFePO4/C的導電性，適量的Fe2P更能降低LiFePO4/C在高速充放電時的極化現象。但是過量的Fe2P會阻礙LiFePO4/C的1D鋰離子進出通道，致使其電容量在高速充放電下有下降的情形。因此，為了避免上述情形發生，合成LiFePO4/C時必須小心地控制Fe2P雜相生成的量。
本研究中使用高能量同步輻射X光觀察LiFePO4/C充放電過程中發生的結構轉變、價數改變與鍵結變化。合成溫度為750 oC的LiFePO4/C在0.14 C的充電兩相轉變過程中，呈現價數改變與鍵結變化趨勢。相較於在較高充放電速率0.5 C的情形，合成溫度為750 oC的LiFePO4/C在較低的充放電速率0.25 C下，會有重疊的兩相結構轉換區間(相遲疑現象)存在。而合成溫度為800 oC的LiFePO4/C在充放電速率0.25 C下，沒有相遲疑的現象發生。此肇因於LiFePO4兩相轉變時的成核機制在相遲疑現象中扮演了很重要的角色。

並列關鍵字

LiFePO4/C ； solid-state method

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