鋰離子電池矽碳複合負極材料製備與分析

Synthesis and Characterization of C-coated Si Composite Anode Materials for Lithium-ion Batteries

嚴佑展(Yu-Chan Yen)

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(2011年)

https://doi.org/10.6342/NTU.2011.10318

摘要

本論文之主要目的為開發以矽為主體的鋰離子二次電池負極材料。擁有絕佳的理論電容量(~ 3600 mAh/g)、蘊藏量豐富、安全無毒性、價格低廉等特性，矽(Si)是目前極有可能取代石墨（372 mAh/g）成為新世代高容量鋰離子電池的負極材料之一。但由於充放電時伴隨劇烈的體積膨脹與低導電度，以及固相電解液介質(SEI)的負面影響，進而造成極板結構的不穩定和電性的不可逆性，使得矽在鋰電池上的應用受到限制。為了克服前述的問題，我們分別從物理的觀點(結構穩定性)和化學的觀點(SEI的生成)著手，進行研究和討論。
首先，我們利用SEM與XPS的分析，探討經過充放電於純矽以及矽碳負極材料上所形成的SEI膜的形態與組成 (以LiPF6 為鋰鹽，EC/EMC 為電解液)。結果顯示，於純矽極板上經過一圈的充放電之後就有明顯的表面沉積物，是源自矽表面氧化層的親水性所造成電解液的分解。然而經過表面改質鍍碳的矽，能有效的阻絕矽氧化物與電解液的接觸，改變電解液分解的反應，降低氟化合物的生成。
再者，我們以熱裂解及流體化床化學氣相沉積兩種不同方法在矽粉體上進行碳的鍍層，發現碳層的覆蓋有很好的均勻性，而且對於電極的循環壽命表現有顯著的提升。經由充放電測試，發現有雙層碳批覆粉體組裝之電池，在1000mAh/g的充電深度下有95 cycles的循環壽命，優於Si的表現(<10 cycles)，而且減少了在充放電的不可逆(irreversibility)和極化(polarization)現象。
除了利用碳鍍層的表面改質外，本研究試圖進一步從矽本身作改善為出發點，藉由提高矽的表面積，在材料本身預留孔洞的方式，來緩衝矽在充放電過程時，所造成的體積劇烈變化。我們應用半導體製程中化學蝕刻技術，利用氧化劑以及氫氟酸水溶液來製備多孔矽。實驗結果顯示，此方法確實可以在矽表面上產生許多的孔洞。結合碳批覆後，此多孔性矽碳複合材料在定電容(1000mAh/g)充放電測試中，展現100圈以上的循環壽命。接著，將此一複合材料導入全電池的測試和失效機制分析，發現負極的充放電深度對電池穩定性有顯著的影響。而第一圈不可逆電容量和SEI膜的生成還仍待進一步改善。
最後，利用改良式的化學蝕刻增進多孔性結構，此一結構大幅提升電池循環穩定性。負極的預鋰化(prelithiation)可以補償SEI的生成，降低電容量損失進而增加充放電的可逆性。此結果對矽負極在全電池的應用，有明顯的進展。

關鍵字

鋰離子電池 ； 矽 ； SEI膜 ； 碳批覆 ； 多孔性結構

並列摘要

The main purpose of this research is to explore new anode materials based on silicon for lithium-ion batteries. Silicon, which is the second abundant in earth and environmental friendly, possesses a high theoretical capacity (~3600 mAh/g) compared to graphite (372 mAh/g). However, the dramatic volumetric variations during cycling and intrinsic low conductivity result in structural instability and poor cyclability. Moreover, the irreversibility caused by solid electrolyte interphase (SEI) formation fastens the capacity fading rate as well. The study was initiated from the physical (structural stability) and chemical (SEI formation) viewpoints, respectively.
First, the SEI layers formed on the electrodes of pristine Si and carbon-coated Si (C-Si) particles in Li cells have been studied. The counter electrode is Li, and the electrolyte is LiPF6 in the mixture of ethylene carbonate and ethyl methyl carbonate. Other than those, such as Li carbonates and fluoride, already known to the SEI of graphite electrode, there were detected significant amounts of SEI species unique to each of the Si electrodes. On the pristine Si electrode, there was concurrence of abundance of C and Si fluorides after long cycles. Coating the Si particles with a graphitized carbon layer has significant effects on the SEI formation. It helps to keep the Si particles remaining integrated after cycling, resulting in a smooth superficial SEI layer. It removes the native oxide layer not only to reduce humidity contamination but also to significantly change the SEI compositions. The SEI of the C-Si electrode shows the absence of Si and C fluorides but the presence of siloxane species. Reaction mechanisms leading to the formation of the fluoride and siloxane species have been proposed, elucidating an important role played by the native Si oxide layer.
Second, carbon-coated Si (C-Si) materials have been synthesized by a chemical vapor deposition (CVD) reaction or a thermal pyrolysis process. It has been found that the cycle performance is prominently enhanced by means of double layer C-coating from the two methods. The carbon coatings obtained from different procedures gives various donation of morphological stability. Being a compact coating layer, the carbon form CVD process can tolerate the volume change of Si particles during lithaition/ delithiation. On the other hand, the loose-structured C-coating from pyrolysis reaction can act as a cushion for the volume expansion of Si upon cycling. A better cycle performance is achieved by constructing double-layer structure and much less amount of carbon was required. For instance, a cycle-life of over 95 cycles with > 95% capacity retention at the discharge depth of 1000 mAh/g has been demonstrated by double layer C-Si electrodes.
In addition, a structure design of porous Si particles has been achieved via Ag-assisted chemical etching of Si particles. As prepared porous Si powder possesses randomly distributed intraparticle voids with sizes in the nano-meter range and a bulk porosity of > 20%. Carbon-coating of porous Si particles by a fluidized-bed chemical vapor deposition (CVD) process not merely covers the Si surface but also fills the pores onto the surface. Therefore, carbon can serve as the conductive and cushion matrices more efficient upon lithiation/delithiation cycling. The electrode made of the porous Si/C particles exhibited much reduced thickness expansion and remarkably enhanced cycling performance, as compared with that of pristine Si particles. The improvements have been attributed to the success in introducing the preset voids to partially accommodate volume expansion arising from Si lithiation. The full cell evaluation involving the porous Si/C anode has been carried out and the fading mechanism has been explored as well. Research revealed that the cycle stability is greatly affected by the depth of charge of Si anodes. And irreversibility issue of Li-ion cells needs to be further alleviated.
Finally, a new structure design of Si and prelithiation of Si anodes was employed in Li-ion batteries. Impressively enhanced cyclability and reversibility were achieved. For example, with prelithiation, the cell exhibits reduced irreversibility of the 1st cycle from 25% to 14% and 5% increment of capacity retention after 200 cycles.

並列關鍵字

Li-ion batteries ； Silicon ； SEI film ； Carbon coating ； Porous structure

參考文獻

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