鋰離子電池鋰鐵磷酸鹽正極之高速充放電研究

Study on High-rate Charge/Discharge of LiFePO4 Cathode Material for Lithium-Ion Batteries

張顥薰(Hao-Hsun Chang)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.02395

摘要

鋰鐵磷酸鹽為一新一代之鋰離子電池正極材料。它具有環保、高安全性以及低原料成本之優點，更被證實具有高功率充放電的能力。在本論文中，我們首先以非等溫動力學建立起以固相法合成鋰鐵磷酸鹽的方程式，並以此剖析且獲得一套固相快速製程，可在600oC下，兩小時內合成大量產物，且具有140mAh/g之電容量。產物具有高達1.33g/cc之振實密度，與市售粉末相比，本快速製程之產物的體積能量密度提升了至少20%。
其次，本論文利用同步輻射對鋰鐵磷酸鹽在充放電過程中的結構變化進行了臨場X光繞射分析。結果顯示了結構變化延遲的現象，尤其在1C且55oC的條件下，結構變化更延遲至電流停止後才劇烈發生。此延遲現象可在電池連續充放電，中間電流毫不中斷的狀況下消失，顯示充放電策略亦對鋰鐵磷酸鹽之結構變化有所影響。在循環測試中，發生結構延遲且劇烈變化的電池顯示較差之循環壽命。原因為結構連續的劇烈變化產生了晶格缺陷，在缺陷不斷累積的情形下，結構於是崩解。此結果對於鋰鐵磷酸鹽在極高速充放電的應用具有相當程度的衝擊。
另外，鋰鐵磷酸鹽在高溫進行充放電時，會有因由水與電解液反應產生之氫氟酸腐蝕所導致的鐵溶出，此現象不但影響正極本身之循環壽命，也因為鐵離子穿越電解液而在碳負極上沈積，產生了催化電解液分解形成惰性物質批覆在負極上的現象。為了解決此問題，本論文分別以氧化鈦批覆在鋰鐵磷酸鹽顆粒表面，與在碳負極極版上濺鍍一層金屬層的方法來嘗試改善。前者的結果顯示金屬氧化物之批覆的確有效提升正極之循環壽命，但電解液中的氫氟酸轉而腐蝕金屬氧化物層，使鈦同樣沈積在碳負極上。經測試，鈦金屬催化電解液分解之能力更遠勝於鐵金屬。在眾多文獻中，許多團隊嘗試以類似的方法來解決鋰錳氧化物或鋰鈷氧化物的金屬溶出問題，但皆未測試全電池之效能。此實驗結果顯示所有以金屬氧化物批覆為對策之實驗皆應不可忽略在碳負極上所發生之效應。在碳負極極版上濺鍍金屬層的實驗結果中，以金或銅所形成之金屬層證實具有「過濾」由正極溶出之鐵離子的功能。由於鐵金屬被收集在金屬層之上，並未直接沈積於碳顆粒表面，於是阻止了導致電解液大量分解的催化反應。

關鍵字

鋰離子電池 ； 鋰鐵磷酸鹽 ； 動力學 ； 結構變化 ； 批覆

並列摘要

Lithium iron phosphate (LiFePO4) is a new-generation cathode material for lithium ion batteries. It has the characteristics of environmental benignity, high safety, and low cost, and is proven to have the ability for high-power applications. In this dissertation, the analysis based on a non-isothermal methodology was conducted to establish a model for the synthesis of LiFePO4 via solid-state reaction, thus a rapid synthetic route was acquired which was able to produce a batch of powders at 600oC in merely two hours. The product has the capacity of ~140 mAh/g and the tap density of 1.33 g/cc. Compared with the commercial powders, the volumetric energy density of our product is at least 20% higher.
Moreover, the structural transformation of LiFePO4 during charge/discharge was monitored by using synchrotron X-ray diffraction. The results revealed serious delay of structural change, especially at 1C at 55oC, the phase changed abruptly when current stopped. The structural transformation became “normal” while no rest period was between every charge and discharge process, indicating the effect of charge/discharge protocols on the structural change. In the prolonged cycle test, the cell experienced abruptly structural change every cycle showed higher fading rate after a certain cycle. The reason was the abruptly structural change inducing the defects which, after accumulation along cycling, caused structural collapse. These results impact the applications of LiFePO4 at very high rates.
A vital problem of LiFePO4 cycling at high temperatures is the dissolution of Fe ions which is induced by HF in the electrolyte. The dissolution not only influences the cycle life of positive electrode itself, but also the carbon negative electrode due to the migration of Fe ions through electrolyte and consequently depositing on the surface of carbon particles. The deposited Fe would catalyze the decomposition reaction of electrolyte and cause a thick passivated layer on the carbon electrode, thus reduce the cycle performance of LiFePO4/C full cells. Two methods in this dissertation were proposed to solve this problem. TiO2 coating on LiFePO4 particles was tried to prevent the direct contact between LiFePO4 particles and HF. The results showed that the cycle performance was indeed enhanced in the case of TiO2-LiFePO4/Li half cell, but not of TiO2-LiFePO4/C full cell. It was revealed that the TiO2 layer was alternatively corroded and the dissolved Ti ions also deposited on the carbon electrode. The canalysis ability of Ti was found even stronger than Fe. This problem has never revealed in the literatures trying to coat metal oxides on LiMn2O4 or LiCoO2 for the same reason in that they didn’t look into the performance of full cells. In the other modification, a metal layer was sputtered on the carbon electrode. It was discovered that the Fe ions which diffused from the positive electrode were almost all collected on the top of metal “sieving” layer. The catalytic reaction of the decomposition of electrolyte was prevented because Fe ions were not directly deposited on the surface of carbon particles.

並列關鍵字

Li-ion battery ； LiFePO4 ； Kinetics ； Structural transformation ； Coating

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