鋰離子電池鍺銅複合負極材料製備與分析

Synthesis and Characterization of Ge/Cu3Ge Composite Anode Materials for Lithium-ion Batteries

王彥霖(Lin-Yan Wang)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02566

摘要

鋰離子電池之高能量與高功率的效能仰賴著電極板結構的穩定性。因此，設計一個能夠定性與定量地提供活物層間結構訊息的檢測技術是必要的。本論文之主要目的為開發以鍺為主體的鋰離子二次電池負極材料。鍺(Ge)擁有優越的理論電容量(~1600 mAh/g)，鍺(Ge)是目前極有可能取代石墨(372 mAh/g)成為新世代高電容量鋰離子電池的負極材料。但由於充放電時伴隨劇烈的體積膨脹，造成極板結構的不穩定性和電性的不可逆，使得鍺在鋰電池上的應用受到限制。為了克服上述問題，我們將從結構穩定性著手，進行研究和討論。
首先，本研究試圖進一步從鍺本身做改善為出發點，藉由提高鍺的孔隙度，在材料本身預留孔洞以及讓材料本身體機先膨脹的方式，來減緩鍺在充放電過程時，所造成的體積劇烈變化。我們利用溶液式的鋰嵌入/嵌出製備多孔性的鍺。實驗結果顯示，此方法確實可以在鍺的表面產生孔洞和裂縫。具有多孔隙及裂縫結構的鍺顆粒能夠在全充全放的模式下於30圈能保持約440 mAh/g的電容量。利用in-situ TXM發現，多孔性的鍺在第一圈的充放電只有~185%的體積變化，大幅改善Ge在充放電時體積劇烈膨脹的問題。經過熱裂解在多孔性的鍺粉體上進行碳的鍍層，經由充放電測試，發現有碳批覆粉體組裝之電池，在全充全放的模式下於30圈能保持約680 mAh/g的電容量。
我們利用高能球磨機將鍺與氧化亞銅依不同比例混合、反應，再經由400℃燒結使其產生鍺銅合金。藉由合金來減緩鍺在充放電過程時，所造成的體積劇烈變化。發現鍺銅合金對於電極的循環壽命有顯著的提升。其中又以鍺和氧化亞銅比例為1:1擁有較佳的電性。經過50圈的充放電測後，其可逆電容量依然有500 mAh/g，優於Ge的表現(<10cycles)。利用臨場穿透式X射線顯微術 (in-situ TXM)發現鍺銅複合材料在充放電時體積並無明顯變化，大幅改善Ge在充放電時體積劇烈膨脹的問題。
再者，我們利用不同黏著劑 (SCMC、PVDF、Na-Alg)，探討不同binder對於鍺銅複合材料的影響。藉此減少SEI膜的形成。發現利用海藻酸鈉 (Na-Alg)為黏著劑的極板擁有較佳的循環壽命。利用SEM觀察極板經過充放電後的體積變化，海藻酸鈉的極板經過70圈充放電後極板膨脹約200%，遠優於SCMC經過90圈充放電後膨脹約600%，Na-Alg大幅減少在充放電過程中SEI膜的形成。借由減少海藻酸鈉的含量以提升極板的機械強度，以提升循環壽命。發現此極板在全充全放的模式下於120圈能保持約400 mAh/g的電容量。

關鍵字

鋰離子電池 ； 鍺 ； 鍺銅複合材料負極 ； 黏著劑 ； 海藻酸鈉 ； 多孔性結構

並列摘要

The performance of high-energy and/or high-power Li-ion batteries depends strongly on the architecture of the electrode over-layers. The main purpose of this research is to explore new anode materials based on germanium for lithium-ion batteries. Germanium possesses a high theoretical capacity (~1600 mAh/g) compared to graphite (~372 mAh/g). However, the dramatic volumetric expansion during cycling result in structural instability and poor cyclability. This study was initiated from the structural stability viewpoints, respectively.
First, a structure design of porous Ge particles has been achieved via solution lithiation/de-lithiation method. Porous Ge particles displayed the pore and crack structure. The electrode of porous Ge displays improved charge capacity retention of ~440 mAh/g after 30cycles in CCCP mode. The particle size is only ~185% expansion during first cycle lithiation/de-lithiation by in-situ transmission X-ray microscopy analysis (in-situ TXM). This structure reduced clearly the volume expansion of Ge electrode. The pitch carbon-coated porous Ge (C-Ge) materials have been synthesized by thermal pyrolysis process. The electrode of pitch carbon coated porous Ge displays improved charge capacity retention of ~680 mAh/g after 30 cycles in CCCP mode.
The Ge/Cu3Ge composites were synthesized with different weight ratio of Ge and Cu2O by high energy ball-milling (HEBM) and 400℃ calcined process. It has been found that the cycle performance is prominently enhanced by Cu3Ge alloy. The structure of alloy is helpful to accommodate the volume expansion during charge/discharge process. For instance, the composites of the weight ratio of Ge and Cu2O is 1:1 that displays improved charge capacity retention of ~500 mAh/g after 50 cycles. This is better than one of using pure Ge. The particle size did not change a lot during first cycle lithiation/de-lithiation by in-situ transmission X-ray microscopy analysis (in-situ TXM). This composite reduced clearly the volume expansion of Ge electrode.
In addition, using different binder (SCMC, PVDF, Na-Alg) to investigate the effect on Ge/Cu3Ge system and reduce the SEI layer formation. The electrode which used Na-Alg as binder exhibited much reduced thickness expansion and remarkably enhanced cycling performance, as compared with that of other binder. The expansion of the electrode using Na-Alg is ~200% after 70 cycles which is better than the one using SCMC which is ~600% after 90th cycle by SEM analysis. The improvement have been attributed to the success in reducing the SEI layer formation after cycling. A better cycle performance is achieved by reducing the amount of Na-Alg. For instance, the capacity still have 400mAh/g and the capacity retention is about 62% after 120 cycles in constant current constant potential (CCCP) mode charge/discharge process. In rate performance test, the composite shows the excellent performance at high charge current.

並列關鍵字

Li-ion battery ； Germanium ； Ge/Cu3Ge composite anode material ； Binder ； Na-Alg ； Porous structure

參考文獻

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