合成橄欖石結構之陰極材料及其電化學性能分析

Synthesis and Electrochemical Performance of Olivine Cathodes

張必聖(Sheng-Bi Chang)

指導教授 ： 謝建德

元智大學/工程學院/化學工程與材料科學學系/碩士(2012年)

摘要

本研究以合成低成本之LiFePO4(LFP)及高能量密度之LiMn0.5Fe0.5PO4(LMFP)為目的。
以碳熱還原法作為低成本之合成方法，選用工業級Li2CO3和FePO4做為前驅物，並以不同含量之葡萄糖做為還原劑及碳源。藉此探討碳熱還原法之反應過程及碳含量最適化。同時以共沉澱法合成LMFP前驅物(Mn0.5Fe0.5)¬¬¬3(PO4)2 (MFP)，利用Mn原子取代橄欖石結構中部分Fe原子，藉此提高材料之能量密度。並藉由TGA、DSC、XRD、SEM及HR-TEM分析反應過程及材料鑑定。
根據研究結果，於氮氣氣氛下240℃恆溫四小時和650℃恆溫十五小時為本研究之較佳煅燒參數，此參數下0.1C充放電速率下電容量達118 mAh g-1，以1C速率下進行三十次充放電循環壽命測試庫倫效率仍然維持99.9 %以上且電容量保持率於5C下可達60%，此結果顯示材料除了擁有良好的晶體結構外也因於材料表面上披覆一層碳層，提供了一個良好的電子傳導路徑以及降低材料之極化現象。
在合成LMFP中，利用共沉澱法可合成具圓球狀之前驅物MFP，並且成功合成出LMFP。此合成方法於0.05C速率下電容量達113 mAh g-1，並且以1C速率下進行三十次充放電循環壽命測試庫倫效率仍然維持99.9 %以上顯示材料之穩定性，經由Ragone plots圖可證明LMFP於能量密度上高於LFP。

關鍵字

橄欖石結構 ； 碳披覆 ； 正極材料 ； 鋰離子電池

並列摘要

Recently, there has been considerable interest in preparing olivine LiMPO4 (M= Mn, Fe, Co, and Ni) as a cathode material for high-power and large-scale applications such as electric vehicles. This study focuses on the carbonthermal reduction method was first employed to prepare LiFePO4/C (LFP/C) cathode materials. The results demonstrated that the optimal calcination process for the synthesis of LFP/C composite material was set at 240°C for 4 h and 600°C for 15 h in N2 atmosphere with the addition of 18.9 wt.% glucose as carbon sources. The as-synthesized LFP/C, having a highly crystalline level, displays high specific capacity of 118 mAh g-1 at 0.1 C and good stability with almost no capacity fading after 30 cycles. The presence of carbon coating significantly is beneficial for rate capability (capacity retention: 60 % at 5C/0.1C) and high Coulombic efficiency (> 99.9%), showing excellent reversibility of insertion/de-insertion of Li ions. This can be ascribed to the fact that well dispersed carbon layer offers an electronic pathway over the LFP/C composite, thus imparting electronic conduction and reducing cell polarization. Accordingly, the deposition of carbon coating, prepared by the carbothermal reduction method, shows a positive effect on the rate-capability improvement of LFP/C cathodes.
As to the second part, one precursor, (Mn0.5Fe0.5)3(PO4)2 (MFP), was prepared by using ammonia as chelating agent and pH adjuster. The as-prepared LMFP was mixing the pre-calcined MFP with Li3PO4, followed by high-temperature calcination. A uniform carbon coating layers was found to coat over the surface of the spherical LMFP. The as-prepared LMFP cathode also exhibited excellent Li-storage performance and outstanding cycleability at 1 C (> 90%). The Ragone plot showed that the LMFP cathode is capable of retaining its higher energy-storage ability at high power density than commercial LFP.

並列關鍵字

Olivine ； Carbon coating ； cathode ； Li-ion battery

參考文獻

26. 費定國，黃楷斌，鋰離子電池陰極材料LiFePO4近年研究進展，工業材料雜誌276期 (2009)。

20. F. Croce, A. D'Epifanio, J. Hassoun, A. Deptula, T. Olczac, and B. Scrosati, Electrochem. Solid-State Lett., 5, A47 (2002).

24. F. Croce, A. D. Epifanio, J. Hassoun, A. Deptul, T. Olczac, and B. Scrosati, Electrochem. Solid-State Lett., 5, A47 (2002).

43. H. P. Liu, Z. X. Wang, X. H. Li, H. J. Guo, W. J. Peng, Y. H. Zhang, Q. Y. Hu, J. Power Sources, 184, 469 (2001).

45. H. Liu, C. Li, H. P. Zhang, L. J. Fu, Y. P. Wu, H. Q. Wu, J. Power Sources, 159, 717 (2001).