以石英粉末輔助合成石墨包裹矽奈米顆粒之初步研究

Abstract

石墨包裹矽奈米顆粒(Graphite Encapsulated Silicon nanoparticles, GES)是一種內核為矽或碳化矽晶粒、外殼為非晶質碳或是石墨層之奈米複合材料。GES具有作為鋰離子電池陽極的潛力，內核矽或碳化矽之儲電量約為常見石墨電極的3至10倍，但電池充放電過程中，鋰離子會與矽結成Li15Si4合金，造成體積膨脹3至4倍，而石墨外殼具有良好導電性與延展性，在增加導電度的同時，也能有效限縮矽材料因充放電所造成的體積膨脹，延長電池使用壽命。 本團隊於2020年成功以改良式鎢電弧法合成GES，然而產物中碳化矽比例過高，由於碳化矽之儲電量僅為矽的三分之一倍，會造成GES的儲電量下降；另外，過去製程產量過低，導致難以進行後續量化分析與實務應用之研究。因此，本研究目的在於減少GES產物中的碳化矽比例與試圖提高產量。 從碳-矽二元相圖與文獻回顧發現，控制系統碳含量能有效限制碳化矽的生成，透過比較反應式的自由能高低發現，若系統中含有氧則能降低碳之比例，因此加入一定比例的石英粉末於經過改良之坩堝配置中，於電弧中心提供氧原子，以限制碳化矽的生成反應，並且也能作為矽的來源。根據實驗產物之XRD分析結果，產物中的碳化矽繞射峰強度有明顯下降，顯示石英粉末能有效限制碳化矽生成；透過HRTEM影像分析得知GES的核心為矽單晶或單晶碳化矽，並統計影像中矽核心GES與碳化矽核心GES之數量比為2:1，相對於前人研究之1:9，石英粉末能大幅減少產物中碳化矽比例。此外，透過本研究之製程改良能有效提升GES產量約10倍以上，解決前人產量不足的問題。最後綜合實驗結果提出GES的生成機制模型，說明系統在不同碳含量下，GES可能的生成路徑與結構，且能針對前人研究無法解釋的部分給予合理的解釋。

Graphite Encapsulated Silicon nanoparticles (GES) are core-shell nanocomposite materials with a silicon (Si) or silicon carbide (SiC) core and an outer shell of amorphous carbon or graphite. GES has the potential to be used as the anode material for lithium-ion batteries. The electrical capacity of the Si or SiC core in GES is approximately 3 to 10 times higher than that of conventional graphite electrodes. However, during the charging/discharging process, lithium ions react with silicon to form Li15Si4 alloy, resulting in a 3 to 4 times volume expansion. The graphite shell exhibits excellent electrical conductivity and ductility, which not only increases the electrode’s conductivity but also effectively restricts the volume expansion of the Si core during charging/discharging, thereby extending the battery’s lifespan. In 2020, our group successfully synthesized GES by modified tungsten arc- discharge method. Nevertheless, the product contained a high proportion of SiC, which caused a decreased electrical capacity since the electrical capacity of SiC is only one-third of the electrical capacity compared to Si. Additionally, the low production rate in the past made it difficult to do subsequent quantitative analysis and research on practical applications. Therefore, the purpose of this study is to reduce the proportion of SiC in the GES product and attempt to increase the production rate. Based on the carbon-silicon binary phase diagram and literature reviews, it was found that controlling the carbon content in the system can effectively limit the formation of SiC. By comparing with the Gibbs free energy of the reactions, it was observed that the presence of oxygen in the system can decrease the proportion of carbon. Consequently, by adding a certain proportion of quartz powder to the improved crucible setup, which provides oxygen atoms at the center of the arc limits the formation of SiC and also serves as a silicon source. XRD analysis of the experimental products showed a significant decrease in the intensity of the SiC diffraction peak, indicating that the quartz powder can effectively limit the formation of SiC. HRTEM images revealed that the core of GES consists of single crystal Si or single crystal SiC. Quantitative analysis of the TEM images showed a ratio of 2:1 between GES with Si core and GES with SiC core, which is in comparison to the 1:9 in the previous study. It indicated that quartz powder can significantly reduce the proportion of SiC in the product. In addition, the process improvement in this study increased the production rate of GES by more than about 10-fold, which resolved the problem of low production rate in previous study. Finally, based on the experimental results, we proposed a model to explain the formation mechanism of GES. This model describes the possible formation pathways and structures of GES, providing reasonable explanations for some aspects that couldn’t be explained in previous study.