請用此 Handle URI 來引用此文件:
http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/90178
標題: | 以退火實驗探討石墨包裹矽奈米顆粒之合成機制與模型 Discussion of the Synthesis Mechanism and Model of Graphite Encapsulated Silicon Nanoparticle by Annealing Experiments |
作者: | 李晨瑜 Chen-Yu Lee |
指導教授: | 鄧茂華 Mao-Hua Teng |
關鍵字: | 奈米顆粒,碳化矽,矽,石墨,核-殼結構,鋰離子電池, nanoparticles,silicon carbide,silicon,graphite,core-shell structure,lithium-ion battery, |
出版年 : | 2023 |
學位: | 碩士 |
摘要: | 石墨包裹矽奈米顆粒(Graphite Encapsulated Silicon Nanoparticle, GES)是一球狀奈米複合材料,粒徑大小約20-100 nm,並具有核-殼之層狀結構。其外殼由石墨片以及無定形碳堆疊而成,核心則由奈米級的矽與碳化矽晶粒組成。本研究室利用矽和碳容易取得、用途廣泛、成本低廉等優勢,於2020年首次合成出石墨包裹矽奈米顆粒。GES有應用在鋰離子電池負極的潛力,相較業界如今主要採用的石墨材料,GES核心中矽及碳化矽的電容量約為石墨電容量的十倍及三倍之多;而殼層石墨擁有良好的導電性,並且其穩定的物化性質可以隔絕核心與電解液,同時限制矽在充電時的體積膨脹三至四倍的情況,提高鋰離子電池循環穩定性和長期的使用壽命。
雖然目前已可以成功合成GES,然而GES殼層大多以非晶質碳組成,其平均層厚度約為5-7 nm,且並非所有核心皆被完整包裹,造成矽質裸露。非晶質碳層亦有保護核心與穩定結構的作用,但仍需建立在完整包裹的基礎。另外,非晶質碳導電度遠不及石墨,且結構也不如石墨具有良好的物理伸縮性,因此本研究目的為提升殼層包裹矽質的完整度以及所含之石墨量。 本研究利用退火的熱處理方法,希望能夠提升GES殼層的石墨含量,並了解GES在不同溫度下可能發生的反應,反推GES於電弧中的合成機制。實驗分別以500℃至900℃(每次增加100℃)的氮氣氣氛,以及1200℃至1400℃(每次增加100℃)的氮氣及真空環境進行退火,再對熱處理後的樣本進行分析。本研究利用X光粉末繞射儀 (x-ray powder diffractometer, XRD)、穿透式電子顯微鏡(transmission electron microscope, TEM)、高分辨解析率穿透式電子顯微鏡(high resolution transmission electron microscopy, HRTEM)、熱重分析儀(thermogravimetric analysis, TGA)等儀器分析產物。初步結果顯示,初產物在900℃以下的溫度退火,殼層厚度會隨著退火溫度上升而小幅度增加,然而在XRD圖譜中沒有出現石墨的訊號。當退火溫度提高至1300℃時,XRD分析出現明顯的石墨訊號;經由TEM影像則發現附著於殼層外側的非晶質小顆粒消失,而殼層厚度大幅提升。其中,由人造鑽石粉合成之GES殼層可增厚至約20 nm。然而當退火溫度高至1400℃後,殼層厚度再次降低至5-8 nm,石墨繞射峰也消失。 根據研究結果可知,1300℃為最適合GES的退火溫度,能夠同時增加殼層的包裹完整度與石墨含量。附著之非晶質顆粒在此溫度下成為GES殼層的一部分,造成其平均厚度增厚而提升包裹完整度;石墨則由原本位於殼層的無定形碳重新排列形成,抑或是晶質碳化矽熱裂解為矽與石墨。 本研究透過分析結果提出GES之初步退火結論,並藉由GES在不同溫度區間發生的反應,提出改良式石墨包裹矽奈米顆粒合成機制模型。期望在了解GES之合成機制並提升殼層之性能後,能夠進一步強化其特殊核-殼結構之優勢,提高GES應用於鋰離子電池的潛力,也期待未來能真正將GES應用到電池產業當中,幫助提升鋰離子電池的性能。 Graphite Encapsulated Silicon Nanoparticles (GES) is a spherical nanocomposite material with a particle size of about 20-100 nm and a core-shell layered structure. The outer shell is made of layered graphite flakes and stacked amorphous carbon, while the core is composed of nano-scale silicon or silicon carbide grains. Due to the wide range of uses and low cost of silicon and carbon, our laboratory successfully synthesized GES for the first time in 2020. GES has the potential to be applied in the negative electrode of lithium-ion batteries. Compared with graphite, which is mainly used in negative electrodes today, the capacitance of silicon and silicon carbide in the core of GES is about ten times and three times that of graphite. The graphite shell exhibits good conductivity and its stable physicochemical properties can isolate the core from the electrolyte, while limiting the volume expansion of silicon by three to four times during charging. This enhances the cycle stability and long-term lifespan of lithium-ion batteries. Although GES has been successfully synthesized, the shell of GES is mostly composed of amorphous carbon, with an average layer thickness of about 5-7 nm, and not all cores are fully encapsulated, resulting in exposed silicon. While the amorphous carbon layer provides some protection and stabilizes the structure, it still needs to be based on complete encapsulation. Furthermore, the conductivity of amorphous carbon is far lower than that of graphite, and its structure lacks the good physical flexibility of graphite. Therefore, the purpose of this study is to improve the encapsulation efficiency of the shell and the amount of graphite it contains. To achieve this, an annealing heat treatment method was employed. The temperature range for annealing experiments was from 500°C to 900°C (increased by 100°C each time) in nitrogen atmosphere and from 1200°C to 1400°C (increased by 100°C each time) in nitrogen and vacuum environments. The heat-treated samples were then analyzed using instruments such as X-ray powder diffractometer (XRD), transmission electron microscope (TEM, high-resolution transmission electron microscopy (HRTEM), and thermogravimetric analysis (TGA). The experimental results show that when the initial product is annealed at a temperature below 900°C, the shell thickness will increase slightly with the increase of the annealing temperature, but there is no graphite signal in the XRD pattern. When the annealing temperature was increased to 1300°C, XRD analysis showed obvious graphite signals. Through TEM images, it was found that the small amorphous particles attached to the outside of the shell disappeared, and the thickness of the shell increased greatly. For GES shells synthesized from diamond powder, the thickness can reach approximately 20 nm. However, when the annealing temperature is further increased to 1400°C, the shell thickness decreased again to 5-8 nm, and the graphite diffraction peak also disappeared. Based on the results, 1300°C is the most suitable annealing temperature for GES, which can increase the package integrity and graphite content of the shell at the same time. At this temperature, the attached amorphous particles become part of the GES shell, resulting in an increased average thickness and improved encapsulation. The graphite is formed through the rearrangement of amorphous carbon originally located in the shell, or the thermal decomposition of crystalline silicon carbide into silicon and graphite. This study proposes a preliminary annealing model for GES through the analysis results, and presents an improved synthesis mechanism for graphite encapsulated silicon nanoparticles based on the reactions occurring at different temperature ranges. It is expected that after understanding the synthesis mechanism of GES and improving the performance of the shell layer, the advantages of its special core-shell structure can be further strengthened, and the potential of GES applied to lithium-ion batteries can be improved. It is also expected that GES can be truly applied to the battery industry in the future. Helps improve the performance of lithium-ion batteries. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/90178 |
DOI: | 10.6342/NTU202304021 |
全文授權: | 未授權 |
顯示於系所單位: | 地質科學系 |
文件中的檔案:
檔案 | 大小 | 格式 | |
---|---|---|---|
ntu-111-2.pdf 目前未授權公開取用 | 9.57 MB | Adobe PDF |
系統中的文件,除了特別指名其著作權條款之外,均受到著作權保護,並且保留所有的權利。