提升磷酸鋰鐵陰極材料之電化學特性研究

STUDY ON IMPROVING ELECTROCHEMICAL PROPERTIES OF LiFePO4 CATHODE MATERIALS

柯惟馨(Wei-Hsin Ke)

指導教授 ： 楊木榮

大同大學/工程學院/材料工程學系所/博士(2009年)

摘要

目前在開發中的鋰離子二次電池正極材料中，橄欖石結構相 LiMPO4 (M = Mn, Fe, Co, Ni) 是目前熱門的研究方向。在 LiMPO4結構當中，LiFePO4被認定為最具有發展潛力的鋰離子正極材料，因為其擁有低成本、低的環境汙染性、穩定的熱穩定性、長的循環壽命與高的理論電容量 170 mAhg-1。 然而低的鋰離子擴散速率與低的電子導電率造成磷酸鋰鐵在商業運用上的一大瓶頸。
在此研究中，我們能成功地利用共沉法、微乳法、固態反應法、微波輔助合成法與溶液法製備純相的橄欖石相磷酸鋰鐵材料。在利用共沉法合成LiFePO4/C的研究中，由於在細小且均勻的顆粒表面上能均勻披覆助導性物質，造成粉體的低導電性得到改善，使得電池在50oC環境下，經過1C充放電速率循環試測100圈後，依舊維持143mAhg-1的高放電電容量。
在利用溶液法合成 LiFePO4的研究中，除了嘗試不同碳源前驅披覆外，並探討不同摻雜元素的影響。在不同的碳源前驅物披覆的研究結果，皆可提升粉體的導電度。其中以 PVA分解的披覆碳能得到較佳的電化學特性提升效果，因為 PVA 能提供適量的碳含量批覆與較高石墨化程度的熱解碳。另外在不同的摻雜元素研究結果中，摻雜不同離子(M= Mg2+, Ni2+, Al3+, or V3+)，以半徑小於或相似 Fe2+之大小去置換鐵的位置形成 LiFe0.95M0.05PO4相，以提升其電化學特性。其中以 LiFe0.95V0.05PO4 可達較佳的放電特性， 分別為 C/10 達 152mAh/g 與 1C 達 136mAh/g。這是由於擁有較大的晶格體積與較長的 Li-O 鍵長造成的結果。此外，利用溶液法合成含有 LiFePO4 and Li3V2(PO4)3 兩相均勻混合之複合材料能提升其電化學特性，因為具有細小的顆粒與較佳的電子導電度，使其 LiFePO4/Li3V2(PO4)3複合相表現出極佳的電化學特性，室溫環境下，鈕扣型半電池 10C 放電能力達 100mAh/g水準，然而其他單一相材料在相同的放電速率下僅能達到 80mAh/g以下的水準。

關鍵字

磷酸鋰鐵 ； 鋰離子二次電池

並列摘要

Among several materials under development for use as cathodes in lithium
ion batteries, orthophosphates LiMPO4 (M = Mn, Fe, Co, Ni) structure to olivine are intensively studied as lithium insertion compounds. Among the LiMPO4, lithium iron phosphate LiFePO4 have been recognized as a promising candidate for Li-battery cathode due to the low cost, environmental benignity, cycling stability, and high theoretical capacity of 170 mAhg-1. However, the poor conductivity, resulting from the low lithium-ion diffusion rate and low electronic conductivity in the LiFePO4 phase, has posed a bottleneck for
commercial applications.
In this study, pure olivine LiFePO4 has been successfully prepared with co-precipitation, solid-state, emulsion-drying, microwave assisted synthesis and solution method. In the aspect of co-precipitation, small-size and homogeneous synthesized powders with electronically conductive coatings could be fabricated to improve the poor conductivity of the powders. The cell containing the LiFePO4 cathode prepared by coprecipitation method can achieve high specific capacity (143mAhg-1) even after the 100th cycle with 1C charge/discharge rate at 50C.
LiFePO4 powders prepared by solution method were also investigated in this work. The LiFePO4 particles with carbon coating synthesized from different carbon precursors are attempted to improve electrochemical performance. This result reveals LiFePO4/C prepared from PVA precursor exhibits superior electrochemical performance. The carbon coating synthesized from PVA pyrolysis can provide moderate carbon content and high graphitized pyrolysis carbon. Moreover, displacement ions (M= Mg2+, Ni2+, Al3+, or V3+) with ionic radius similar to or smaller than that of Fe2+ will be attempted to displace Fe2+ into the Fe-site to form LiFe0.95M0.05PO4 samples to improve electrochemical performance. The mean particle size of all samples, independent of doping species, is about 6  0.5m. All samples with carbon content of about 3wt.% carbon coating in this study have similar BET surface area (about 18~20.5 m2g-1). It can be found that the synergetic effect of the supervalent doping and lattice expansion is beneficial to the electrochemical performance of cathode materials, especially under high C rate. Hence the powder (LiFe0.95V0.05PO4) with the V doping of trivalence and largest volume of unit cell (longest Li-O bond length) exhibits highest discharging capacity 152 mAh/g and 136 mAh/g at C/10 and 1C rates.

並列關鍵字

lithium ； lithium iron phosphate

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