鋰離子二次電池LiFePO4正極材料之合成及其改質研究

Modification and Characterization of LiFePO4 Cathode Material for Lithium-ion Battery

張永昌(Yung-Chang Chang)

指導教授 ： 洪逸明

元智大學/工程學院/化學工程與材料科學學系/碩士(2013年)

https://doi.org/10.6838/YZU.2013.00100

摘要

本研究第一階段為利用水熱法合成出純度高且結晶性佳之LiFePO4粉末並探討粒徑與碳披覆之改質對LiFePO4電化學性能之影響。球磨可使粉體粒徑變小，並使LiFePO4粒徑大小分佈較均勻，而顆粒尺寸減小預計能縮短鋰離子在材料中之擴散距離，提高其電容量。本實驗充放電數據顯示，球磨三天後之樣品其充放電電容量較低，判斷為因為經過球磨後，顆粒被鋯球擊碎，表面披覆的碳層遭破壞，造成粉體導電性能下降，進而降低其放電電容量。故在此利用Ascorbic acid作為碳源，對球磨後的粉末再次披覆碳層，以莫耳比Ascorbic acid : LiFePO4 = 1 : 5，均勻混合後於700 oC煆燒3小時，期望提高LiFePO4材料導電性。藉由FE-SEM觀察材料之表面型態可得知，經過球磨可明顯減小粉體粒徑。由XRD鑑定材料結構，利用TEM觀察碳層披覆與材料表面型態，由循環伏安法分析探討電池內部動力學之表現，利用交流阻抗分析儀(AC Impedance)分析其內部阻抗並計算鋰離子擴散係數。本階段實驗中性能最佳之披覆碳層之奈米級LiFePO4樣品在充放電速率為0.1 C時，其電容量為133 mAh/g。
由第一階段研究結果得知，顆粒細小化後進行碳層再次披覆之樣品電化學性能最佳，所以本研究第二階段利用水熱法在高溫高壓下進行快速化學反應合成出高純度、顆粒小且結晶性佳的LiFe0.95VxNi0.05-xPO4/C粉末，摻雜釩及鎳離子取代晶體中鐵的位置及表面披覆奈米級碳層提升其離子擴散係數及導電性，通過改變不同釩及鎳摻雜比例進行實驗，並進行與第一階段相同之製程對粉末進行粉末細小化與碳層披覆製程，探討其對材料特性和電化學性質之影響。利用X光繞射分析(XRD) 鑑定其結構、場發射式電子掃描顯微鏡分析(FE-SEM) 煆燒過後粉末顆粒大小及型貌、穿透式電子顯微鏡分析(TEM)碳層分佈及能量分散光譜儀-元素分析(EDS-Mapping)觀察元素分佈進行材料特性分析。利用交流阻抗分析儀(AC Impedance)、充放電儀及循環伏安法(CV)進行電池電化學性質測試。
由XRD分析可知，利用水熱法合成之粉末均為單一相斜方晶系橄欖石結構之LiFePO4。由SEM圖可知，摻雜釩與鎳之LiFePO4/C粉末較球型化且減少尖銳的邊角，只有摻雜0.05 mol %鎳的LiFePO4/C粉末些微成長，其餘摻雜比例之LiFePO4/C粉末粒徑尺寸皆降低。由TEM影像可知，碳層皆均勻地包覆於LiFePO4/C粉末外層。EDS-Mapping顯示Fe、P、O、V及Ni在LiFePO4/C顆粒中皆有均勻的分佈。由充放電圖可得知，於充放電速率0.1 C時，摻雜0.05 mol %釩之LiFePO4/C之電容量(141.1 mAh/g)高於未摻雜LiFePO4/C之電容量(132 mAh/g)，且可發現隨著摻雜釩的比例增加可提升LiFePO4/C之電容量。從快速充放電條件下1 C、5 C及10 C的放電曲線圖可發現摻雜0.04 mol %釩與0.01 mol %鎳LiFePO4/C及摻雜0.05 mol %釩LiFePO4/C皆表現較佳之電化學性能。在1 C的速率下對材料進行循環充放電測試，摻雜0.05 mol %鎳會些微降低LiFePO4/C的循環穩定性，其50圈之後剩餘電容量為95 %，而摻雜其他比例的釩與鎳，均能明顯提升LiFePO4/C的充放電循環穩定性，50圈後有電容量並未衰退。由循環伏安法測試結果顯示，摻雜0.04 mol %釩與0.01 mol %鎳及摻雜0.05 mol %釩之LiFePO4/C有較大的極化現象，但電容量都明顯提升，推測極化現象對摻雜0.04 mol %釩與0.01 mol %鎳及摻雜0.05 mol %釩LiFePO4/C之電容量影響不大。利用EIS與CV計算Li+離子擴散係數。由以上結果顯示，適當的摻雜釩與鎳於LiFePO4結構中確實能有效的提高放電電容量，進而增進LiFePO4之電化學性能。

關鍵字

鋰離子二次電池 ； 正極材料 ； 磷酸鋰鐵 ； 水熱法

並列摘要

In this study, well crystalline LiFePO4 powders without impurity phase were successfully synthesized by hydrothermal method. The materials characterization and electrochemical performance of LiFePO4 after ball-milling and carbon coating were investigated. The particle size of LiFePO4 can be reduced, and size distribution is more uniform by ball-milling method. The LiFePO4 with small particle size had a short lithium-ion diffusion distance and increased the capacity of LiFePO4. Charge and discharge data of LiFePO4 show a low charge and discharge capacity after ball-mill three days, due to the coated carbon layer on the surface of LiFePO4 was broken by ZrO2 ball during the ball-mill process, which decreases the electronic conductivity and the discharge capacity of LiFePO4 particles. In order to improve the electronic conductivity of LiFePO4 powder after ball-mill, the LiFePO4 powders were coating carbon again, using ascorbic acid as a carbon source with the molar ratio 1 : 5 of ascorbic acid and LiFePO4. The capacity of coated carbon layer of LiFePO4 with small particles increased to 133 mAh/g.
In the second part of this study, well crystalline LiFe0.95VxNi0.05-xPO4/C powders were successfully synthesized by hydrothermal method. The materials characterization and electrochemical performance of LiFe0.95VxNi0.05-xPO4/C were investigated. The structure, morphology and electrochemical performances of the prepared samples were investigated with XRD, SEM, TEM, cyclic voltammetry and AC impedance. From XRD pattern, it indicates that the LiFe0.95VxNi0.05-xPO4/C powders are single phase with orthorhombic olivine structure. The LiFe0.95V0.05PO4 sample has the highest capacity of 141 mAh/g, which is higher 6% than that of pure LiFePO4 at 0.1 C. As the discharge rate increased to 10 C, the LiFe0.95V0.04Ni0.01PO4 sample has the highest capacity of 100 mAh/g, which is higher 18% than that of pure LiFePO4. The CV results proved that the LFP_V4Ni1 cathode had high capacity and good cyclic performance due to the lithium-ion diffusion transport improved by Ni and V doping.

並列關鍵字

Lithium-ion battery ； Cathode ； LiFePO4 ； Hydrothermal

參考文獻

[30] 鄭如翔、黃炳照， “鋰離子電池正極材料之發展” 化工年會第58卷，第5期，pp. 20, 2011.

[1] R. Koksbang, J. Barker, H. Shi and M. Y. Saidi, “Cathode materials for lithium rocking chair batteries”, Solid State Ionics, vol. 84, pp. 1-21, 1995.

[2] D.W. Murphy, F.J. Di Salvo, J.N. Carides and J.V. Waszczak, “Topochemical reactions of rutile related structures with lithium”, Materials research bulletin, vol. 13, pp. 1395-1402, 1978.

[5] M. S. Whittinghan, “Materials Challenges Facing Electrical Energy Storage” MRS Bulletin, vol. 33, pp. 411-419, 2008.

[6] C. S. Sun, Z. Zhou, Z. G. Xu, D. G. Wang, J. P. Wei, X. K. Bian, and J. Yan, “Improved High-rate Charge/discharge Performances of LiFePO4/C via V-doping”, J. Power Sources, vol. 193, pp. 841-845, 2009.