碳材料的應力釋放機制與作為鋰電池負極之第一原理理論計算研究

First Principles Study of Stress-Relief Mechanism for Amorphous Carbon and the Electrolyte Reduction Reaction at the Graphite Anode for Li-ion Battery

李東昇(Dong-Sheng Li)

指導教授 ： 郭錦龍

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.01857

摘要

碳是一種多樣性的材料，其豐富且截然不同的物理性質主要來自於三種不同鍵結形式組成的各種碳結構所導致。舉例來說，sp2鍵結擁有共振 電子導致石墨烯之導電性質十分優秀，反之由sp3鍵結組成的鑽石則為絕緣材料。由於碳材料在自然界可以豐富的形貌存在，其特殊且多樣的物理性質可廣泛應用於各種工程材料與電子元件的製作。所以了解及掌握碳材料微觀結構變化與其相關重要物理性質的演變便成為一重要課題。 在本研究中，我們嘗試利用第一原理理論計算與分子動態模擬來探討不同形式碳材料的微觀結構與其各種重要性質之間的關聯性。本論文研究主要分為兩個部分。
在第一部分的研究中，我們主要是針對類鑽石非晶碳(tetrahedral amorphous carbon/ hydrogen free diamond-like amorphous carbon, ta-C)的壓應力釋放機制與行為作探討。本文中所使用的非晶碳結構主要是利用分子動態模擬配合液態淬火的方式建構而成，而樣本密度的分布則從2.27 g/cm3至3.45 g/cm3。我們研究結果顯示類鑽石非晶碳中壓應力的釋放機制主要可分成兩種：第一種機制為三配位碳原子轉換為四配位碳原子的鍵結形式轉變，而第二種則為碳原子體積、鍵長與鍵角的最適化調整。針對鍵結形式的轉換，我們分析的結果顯示當系統密度在3.0 g/cm3以上時 (i.e. sp3碳原子比例高於80%)，高溫退火處理後的非晶碳結構中三配位的碳原子會有往四配位鍵結形式轉換的傾向發生，而在轉換過程中也同時會伴隨有內部壓應力釋放的效應產生。在另一方面，我們也發現當非晶碳密度低於2.9 g/cm3時 (i.e. sp3碳原子比例低於60%)，退火之後的結構中三配位碳原子的比例會大幅增加，且同時會伴隨有三配位碳原子群聚與石墨化的現象發生；然而，在此同時我們卻觀察到系統中的壓應力會有不降反升的情形發生，顯示單靠此種鍵結形式的轉變並無法有效降低非晶碳內部的壓應力。我們後續分析的結果顯示，當非晶碳密度在介於2.9 g/cm3和3.0 g/cm3的過渡區間時，初始結構壓應力偏高的樣本其鍵結形式在退火處理後會往四配位比例增加的方向轉變，而初始壓應力偏低者則會傾向往提高三配位碳原子比例與產生石墨化現象的方向演進。這樣的結果告訴我們碳原子間鍵結形式轉換的傾向除了受到本身密度的影響之外，退火處理前結構內部應力的大小與形式亦扮演著關鍵的角色。
第二部分的研究主要是探討以石墨作為鋰電池負極時，不同表面修飾如何影響碳酸乙烯酯(ethylene carbonate, EC)在電極-電解液界面的還原反應。我們嘗試藉由計算石墨負極在不同表面修飾的情形下，如

關鍵字

第一原理理論計算 ； 分子動態模擬 ； 非晶碳材料 ； 應力釋放機制 ； 電解液還原反應

並列摘要

Carbon materials can exist in a wide range of forms exhibiting remarkable properties mainly attributed to its diverse bonding order and hybridization states. Due to its versatile properties, carbon has been extensively applied in the fabrication of microelectronic devices and many applications in engineering materials. Therefore, it would be of great interest to develop a detailed understanding regarding the evolution of microstructures and the associated properties of carbon materials with their chemical bonding states. In this thesis, we have two main focuses:
In the first part of this thesis, we investigated the stress-relief mechanisms in the as-deposited tetrahedral amorphous carbon (ta-C, sp3 > 80%) using first-principles calculations and molecular dynamic simulations. The structure models of amorphous carbon were generated over a wide range of density from 2.27 to 3.45 g/cm3 using ab initio molecular dynamics simulations via the liquid-quench process. Our results show that the intrinsic stresses in ta-C can be released via two different mechanisms: one is bonding conversion from the threefold carbon to the fourfold one, while the other is via the relaxation of bond-lengths, bond-angles, and local atomic volumes. Regarding the bonding conversion in a-C, our calculations show that as the density is higher than 3.0 g/cm3 (i.e. sp3 >80%), the threefold carbon atoms are likely to turn into the fourfold ones accompanied by the relief of internal stresses upon thermal annealing. On the contrary, when the density is lower than 2.9 g/cm3 (i.e. sp3 < 60%), the sp2 bonding was found to increase largely after thermal annealing accompanied with the clustering/graphitization of the threefold carbon atoms and the increase of internal stresses. These results also indicate that the compressive stresses in a-C cannot be released simply by the increase of sp2 fraction, sp2 clustering and graphitization in the amorphous bond network. Furthermore, when the density is in-between 2.9 and 3.0 g/cm3, the bonding conversion was found to be largely dependent on the internal stresses in a-C, i.e. the threefold to fourfold carbon conversion is likely to occur under high compressive stress, while the conversion of fourfold to threefold carbon atom is in favor under low intrinsic stress. This result indicates that bonding conversion in a-C may not only depend on its density but also on the intrinsic stress inside.
In the second part, we studied the effect of terminal groups on the reduction reactions occurred at the interface between the graphite anode and the electrolytes. Here we employed the calculations of electronic reduction barrier, i.e. the energy difference between Fermi level and the LUMO of ethylene carbonate (EC), to predict the effect of various functional groups on the tendency of reduction reaction towards EC decomposition. Our calculated results show that both

並列關鍵字

First principles calculations ； Molecular dynamics simulations ； Amorphous carbon ； Stress-relief mechanism ； Electrolyte reduction reaction

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