鋰離子電池矽負極材料之製備與特性分析

Preparation and Characterization of Si-based Anode Materials for Lithium-Ion Batteries

劉偉仁(Wei-Ren Liu)

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(2006年)

https://doi.org/10.6342/NTU.2006.03045

摘要

由於具有高理論電容量，極富潛力的矽負極材料有可能取代石墨成為新一代的鋰離子電池材料。然而，目前矽負極材料仍無法商業化地應用的原因在於鋰離子遷入/遷出造成之劇烈體積變化和本身材料的低導電性，進而造成極板結構的不穩定與相當差的循環壽命。

本篇論文首先從純矽材料極板結構之最適化觀點出發，對於矽顆粒大小和助導劑添加量探討對於矽負極材料充放電循環壽命的影響進行研究。研究結果發現助導劑添加量對於循環壽命有相當顯著的影響，隨著助導劑的添加量增加，電池的循環壽命也隨之增加；在另一方面，減小矽負極顆粒的大小可以很有效地增進鋰離子與矽的反應速率。以3微米的矽顆粒搭配30%的助導劑含量下，600 mAh/g的定電容量循環測試可達到超過50次，且庫侖效率可維持在96%以上。

此外，本研究也發現黏著劑的選擇對於矽負極材料在循環壽命上有很大的影響。選用水系複合黏著劑(SBR + SCMC)比傳統且廣泛被使用的有機系黏著劑(PVdF)在電化學的循環測試下有更優越的表現。和PVdF相比，水系黏著劑具有較小的楊氏係數、較大的最大伸長量、和銅箔的強黏著性以及高電解液相容性。無論是純矽或是矽鍍碳的樣品在電容量之表現，在600 mAh/g和1000 mAh/g下皆可達到50次的表現；反觀使用PVdF黏著劑的純矽樣品在600 mAh/g之條件下在不到8次的充放電，電容量急遽地衰退。

另外，本論文亦針對純矽樣品與矽鍍碳樣品在「電容量衰退機制」和「鋰離子遷入機制」分別以充放電測試和交流阻抗法(EIS)進行討論。結果發現「電容量衰退」包含兩種模式，一個是local mode、另一個稱之為global mode。所謂的local mode的崩潰機制起因於個別活性物質之顆粒間與助導劑的接觸隨著充放電次數的增加而變差，進而導致電容量的衰退；global mode崩潰機制的發生導因於整個極板結構的瓦解。結果發現碳批覆層可明顯地抑制兩種衰退機制。而在「鋰離子遷入機制」，交流阻抗結果顯示矽與鋰離子的合金反應為一核殼層結構(core-shell)。

本研究也嘗試以矽化物對矽進行改質，係使用高能球磨法製備具高孔隙度之奈米NiSi/Si複合材料，其孔徑大小約200奈米，孔隙度高達40%。這種預留孔洞的的方法證實確實可以增進矽負極材料的循環壽命，臨場XRD實驗結果觀察出NiSi在充放電過程中可逆地形成NiSi2。

除了矽、矽鍍碳和矽化物等活性物質，本篇論文最後也對其他的負極材料，例如Si/ZrO2、Si/TiO2/C、奈米Si/TiO2/C、SiO、SiO/C和奈米-SiO/ZrO2/C進行特性分析與初步的電化學測試，相信這些具奈米特性的複合材料有機會成為未來鋰離子負極材料的新星。

關鍵字

鋰離子二次電池 ； 矽 ； 流體化床 ； 化學氣相沈積 ； 溶膠凝膠法 ； 高能球磨 ； 鎳矽合金

並列摘要

Silicon is a very promising candidate to replace graphite as an anode material in Li-ion batteries because of its high theoretical capacity. However, the main obstacles to commercialization are dramatic volume changes for lithium insertion/extraction and intrinsically poor conductivity of bare silicon, bring mechanical instability and poor rechargeability during cycling.

From the view point of stabilizing electrode structure, the effects of Si particle size and the content of conductive additives (CA) on the performance of the Si anode are investigated. It is found that CA content has a profound effect on the cycle life of the electrode, which increases with increasing CA content. Reducing Si particle size, on the other hand, effectively facilitates the charging/discharging kinetics. A cycle life, for instance, exceeding 50 cycles with >96% capacity retention at the charge capacity of 600 mAh/g-Si has been demonstrated by adopting the combination of 30 wt. % of CA and 3-um Si particles.

In addition, the choice of binder is also very crucial issue. The cycle-life of the particulate electrode of Si, either with or without carbon coating, has significantly been improved by using a modified elastomeric binder containing Styrene-Butadiene-Rubber (SBR) and Sodium-Carboxyl-Methyl- cellulose (SCMC). Compared with poly-vinylidene-fluoride (PVdF), the (SBR+SCMC) mixture binder shows smaller moduli, a larger maximum elongation, stronger adhesion strength on Cu current collector, and much smaller solvent-absorption in organic carbonate. There are demonstrated cycle lives of > 50 cycles for bare Si at 600 mAh/g or carbon-coated Si at 1000 mAh/g, as contrast to < 8 cycles for PVdF-bound electrode in all cases.

The capacity fading and lithiation mechanisms of Si and C-coated Si particulate have also been studied in this study by cycling tests and electrochemical impedance spectroscopy (EIS) analyses, respectively. The capacity versus cycle number plot was found to serve as a useful guide to elucidating two fading modes, including a local mode arising from loss of electronic contact between individual particles and the conductive network of the electrode and a global mode that results from failure of the entire electrode structure. EIS revealed a core-shell lithiation mechanism of Si. C-coating not only exerts remarkable favorite effects against both fading modes, but also serves as a conduit for Li ions to the reaction with Si particles.

Porous NiSi/Si particles having a pore size distribution peaked at 200 nm and an intra-particle porosity of nearly 40% have been synthesized by high-energy ball milling of mixture of Ni and Si and subsequent dissolution of un-reacted Ni, and the material has been characterized for its microstructures and electrochemical properties for Li ion battery application. The preset intra-particle voids have been shown to help to accommodate volume expansion arising from alloying of the Si component. As a result, upon charge/discharge cycling, the composite electrode exhibits much reduced thickness expansion, as compared with pure Si electrode, and hence significantly reduced capacity fading rate. In-situ synchrotron XRD further indicates that the NiSi component of the composite is active toward Li alloying, and it undergoes reversible transformation to Ni2Si during charge/discharge cycling.

Apart from Si, Si/C, and NiSi/Si composites, the fundamental studies and preliminary electrochemical tests of other active materials, such as Si/ZrO2, Si/TiO2/C, Nano-Si/TiO2/C, SiO, SiO/C, and Nano-SiO/ZrO2/C composite are providing in chapter 6. It is believed that these novel anode composites potentially have opportunities to be promising candidates as anode materials for Li-ion batteries in the future.

並列關鍵字

Lithium-ion batteries ； silicon ； fluidized-bed ； chemical vapor deposition ； sol-gel process ； high energy ball milling ； nickel silicide

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