鋰離子二次電池LiNi0.5Mn1.5O4正極材料其性質研究與表面改質

The Properties and Surface Modification of LiNi0.5Mn1.5O4 Cathode Material for Lithium-ion Battery

陳彥廷(Yan-Ting Chen)

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.2013.00161

摘要

近年來，鋰離子二次電池正極材料LiNi0.5Mn1.5O4 因其具有4.7 V的高工作電壓，時常與LiCoO2 及LiFePO4 等正極材料相互比較。本研究可分為製程探討及表面改質兩階段，第一階段為利用溶膠凝膠法合成LiNi0.5Mn1.5O4 粉末，在不同的酸鹼條件 (pH值) 、螯合劑與金屬離子莫耳比、煆燒時間與溫度參數下，探討不同製程對LiNi0.5Mn1.5O4 粉末其純度及電化學性能之影響，尋找製備LiNi0.5Mn1.5O4 粉末的最佳條件。第二階段則延續第一階段，以最適化的製程製備LiNi0.5Mn1.5O4 粉末後，進行表面改質，改善LiNi0.5Mn1.5O4 本身導電性不佳及高溫下HF腐蝕正極材料降低其循環穩定性的缺點。本研究利用X光繞射分析 (X-ray diffraction) 鑑定其結構並比較其結晶性； 利用場發射式電子掃描顯微鏡 (Field Emission Scanning Electron Microscope) 及穿透式電子顯微鏡 (Transmission Electron Microscopy) 分析粉末大小及表面形貌，並利用充放電儀 (Galvanostatically Cycled)、循環伏安法 (Cyclic Voltammetry) 與交流阻抗分析 (Electrochemical Impedance Spectroscopy) 分析其電化學性能、模擬鋰離子擴散阻抗、電子傳遞阻抗與鋰離子擴散係數。
經由XRD 分析結果，對照至LiNi0.5Mn1.5O4 其標準圖譜 (PDF#802162) 可得知，所有合成的LiNi0.5Mn1.5O4 粉末皆為標準的立方尖晶石結構，而以5wt% La0.6Sr0.4Co0.8Fe0.2O3- 改質後的LiNi0.5Mn1.5O4 粉末，則可發現有額外La0.6Sr0.4Co0.8Fe0.2O3- 標準峰 (PDF#480124) 產生，說明La0.6Sr0.4Co0.8Fe0.2O3- 同樣也能藉由溶膠凝膠法製備出。由SEM 及TEM 影像可得知，隨著pH 值、煆燒溫度及時間的增加，粉末顆粒大小及結晶性皆會隨之成長，在粉末表面則有團聚現象及雜質的產生；改質過後之LiNi0.5Mn1.5O4 粉末表面可觀察出La0.6Sr0.4Co0.8Fe0.2O3- 以顆粒與薄膜的形式披覆在其粉末表面。藉由循環伏安法及交流阻抗分析計算鋰離子擴散係數及模擬其阻抗值，交叉比對後可得知較低的電子傳遞阻抗及較高的鋰離子擴散係數，能表現出優異的電容量。由連續充放電結果可得知，以pH 7、螯合劑與金屬離子莫耳比為2及煆燒條件為850 oC、12小時所製備而成的LiNi0.5Mn1.5O4 粉末在充放電速率為0.2 C、2 C、5 C時，具有較優異的電容量，分別為130.8 mAh/g、112.1 mAh/g及84.1 mAh/g。在以2 C速率進行100圈充放電過後，其電容量仍維持初始電容量的100%，顯示其循環穩定性極佳，對照XRD 及SEM 結果，可發現此製程能製備出高結晶性及高純度的LiNi0.5Mn1.5O4 粉末，為最佳化製程。
以La0.6Sr0.4Co0.8Fe0.2O3- 表面改質過後的LiNi0.5Mn1.5O4 粉末，在完成所有的電化學分析以後可發現其電化學性能皆優於改質前之粉末，其電容量在室溫下以5 C速率充放電後，由84.1 mAh/g提升至107.8 mAh/g，起因於La0.6Sr0.4Co0.8Fe0.2O3- 為一高導電性之氧化物，將其披覆於LiNi0.5Mn1.5O4粉末表面，能改善其本身導電性不佳之缺點，有效降低其電子傳遞阻抗，並能於LiNi0.5Mn1.5O4粉末表面形成一保護層，防止在充放電過程中，電解液所產生之HF 對活性物質之腐蝕，進而提升快充速率之電容量。

關鍵字

鋰離子二次電池 ； 鋰鎳錳氧 ； pH 值 ； 煆燒溫度 ； 煆燒時間 ； 螯合劑莫耳比 ； 表面改質

並列摘要

The spinel LiNi0.5Mn1.5O4 cathode material has been successfully prepared by sol-gel method. The effects of pH conditions, calcining temperature, time and the mo-lar ratio of citric acid on the structural and morphological characterization and elec-trochemical performance of the products were investigated by XRD, SEM, cyclic voltammetry and galvanostatic charge-discharge tests in detail. The results show that all the parameters in this study have a remarkable impact on the crystallinity, the purity, the Mn3+ amount and the particle size distribution. Among the all of samples, LiNi0.5Mn1.5O4 synthesized by pH 7; calcined at 850 oC for 12h and molar ratio = 2 of citric acid / metal demonstrated the most excellent electrochemical performance. It delivers high initial capacity of 130.8 mAhg-1 at 0.2 C, and high rate discharge capa-bility of 112.1 mAhg-1 and 84.1 mAhg-1 at 2 C and 5 C, respectively. Therefore, the optimal method to prepare LiNi0.5Mn1.5O4 was observed in first part of this study.
In the second part, we want to improve the electrochemical performances of LiNi0.5Mn1.5O4. Therefore, La0.6Sr0.4Co0.8Fe0.2O3- (LSCF)－coated 5 V spinel LiNi0.5Mn1.5O4 as cathode was prepared by same route with ball-milling. The electro-chemical properties of coated－LiNi0.5Mn1.5O4 were also investigated by galvanostatic charged / discharged tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) ,too. It was found that the LSCF－coated LiNi0.5Mn1.5O4 samples showed lower lithium-ion diffusion resistance and charge transfer resistance than uncoated LiNi0.5Mn1.5O4. After surface modification, the discharge capacity at high C-rate was enhance from 58.8 mAhg-1 to 107.8 mAhg-1. The improved electro-chemical performance of the surface modification samples can be attributed to the de-creasing contact area between the electrode and electrolyte during electrochemical cycling and enhance the electronic conductivity by LSCF－coated layer. As the re-sults, the surface coating with LSCF is an effective approach to improve the electro-chemical performance especially at the high C－rate.

並列關鍵字

Lithium-ion battery ； LiNi0.5Mn1.5O4 ； pH value ； calcining temperature ； calcining time ； citric acid ； Surface modification

參考文獻

[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 Lithi-um”, Materials Research Bulletin, vol. 13, pp. 1395-1402, 1978.

[5] R. Santhanam, and B. Rambabu, “Research progress in high voltage spinel LiNi0.5Mn1.5O4 material”, Journal of Power Sources, vol. 195, pp. 5442-5451, 2010.

[6] S. Patoux, L. Daniel, C. Bourbon, H. Lignier, C. Pagano, F. Le Cras, S. Jouanneau, and S. Martinet, “High voltage spinel oxides for Li-ion batteries: From the material research to the application”, Journal of Power Sources, vol. 189, pp. 344-352, 2009.

[7] M. Kunduraci, J.F. Al-Sharab, and G.G. Amatucci, “High-Power Nanostructured LiMn2-xNixO4 High-Voltage Lithium-Ion Battery Electrode Materials:  Electrochemical Impact of Electronic Conductivity and Morphology”, Chemistry of Materials, vol. 18, pp. 3585-3592, 2006.