以第一原理與ReaxFF力場模擬計算探討矽氧碳陶瓷負極材料之微結構及鋰化機制

Abstract

本研究分別利用ReaxFF與第一原理模擬計算探討矽氧碳陶瓷負極材料之微結構，以及其碳氧濃度對其微結構之影響。接著利用第一原理計算進一步進行矽氧碳材料的電子結構以及鋰化機制的分析。 論文的第一部分，本文以Ponomarev等人的參數(UTA1)出發，進行參數的調整，並驗證這些參數對於各一元及二元對系統的結構、機械常數和內聚能的描述。基於以上性質的改善，由我們所調整得到的參數建置的結構中可以看到碳原子相較於UTA1參數所建置的非晶矽氧碳結構來說更傾向進到矽氧碳玻璃相中，因而使聚集碳相(free carbon phase)變得更加平整。另外原來UTA1參數對於系統中配位缺陷過高的容忍度也有所改善。有了以上的參數調整以及改善，利用我們所調整參數建置的非晶矽氧碳結構在熱力學穩定度上明顯優於由UTA1參數所建置的結構。 在第二部分中，本文利用第一原理計算產生六組不同濃度的矽氧碳結構，討論碳與氧濃度對微結構以及電子性質的影響。結果顯示當碳與氧的濃度下降時，聚集碳相的比例會隨之下降。此外，當碳濃度上升或是氧濃度下降時，系統中SiC4與SiC3O四面體的比例會隨之而上升，而SiCO3與SiO4四面體的比例會隨之而下降，致使系統的孔洞體積以及比表面積也因而降低。接著在電子性質方面，結果顯示碳原子的加入有效使矽氧碳玻璃相的能隙縮小，此外隨著氧濃度的降低，Si-C與Si-Si鍵結在系統中的比例逐漸提升，造成能隙逐漸減小的現象。 在最後一部分的鋰化計算中，結果顯現矽氧碳的鋰化過程主要分為兩階段:第一階段電子主要填在聚集碳相的碳原子上而鋰離子吸附在聚集碳相與矽氧碳玻璃相的界面之間的氧原子上；第二階段電子開始填入矽氧碳玻璃相造成系統中明顯的Si-O斷鍵以及Li-O鍵結生成的現象。聚集碳相在鋰化過程中扮演著電子儲存槽以及緩衝系統體積膨脹的角色。最後，我們發現越高的碳濃度將驅使越高的電容量。此外高碳濃度以及低氧濃度將使矽氧碳系統在充放電可逆性上有較佳的表現。

In this thesis, ReaxFF and first principles calculations are employed to explore the the effect of carbon and oxygen contents on nanostructures of amorphous silicon oxycarbide (SiOC). The electronic structures as well as the lithiation mechanism are further studied by first principles calculations. In the first part of this thesis, we modified the Si/O/C ReaxFF parameter set based on the parameters from Ponomarev et al. (UTA1), and validated its performance on lattice constants, elastic constants and cohesive energies of the unary and binary systems. Thanks to the calibrations above, the carbon atoms are more likely to be discovered in the SiOC glass phase, leading to the flatter free carbon phase in structures constructed via our new developed parameters. Moreover, the too-high tolerance of coordination defects in UTA1 is also improved. Finally, our new developed parameter set is able to construct the amorphous SiOC structures that are thermodynamically more stable than the structures built by UTA1 parameters. In the second part of this thesis, we constructed amorphous SiOC structures in six different concentrations by first principles calculations to investigate the influences of carbon and oxygen contents on nanostructures and electronic structures of amorphous SiOCs. The results suggest that the decrease of carbon and oxygen contents will make the proportion of free carbon decrease. Furthermore, the increase of carbon concentration and the decrease of oxygen concentration will both induce the rising in the proportion of SiC4 and SiC3O tetrahedra as well as the drop in the proportion of SiCO3 and SiO4 tetrahedra. In terms of electronic properties, the results show that the introduction of carbon atoms in SiOC glass effectively decrease the band gap. Besides, the decrease of oxygen concentration induces the increase in fractions of Si-C bond and Si-Si bond in the system, leading to the narrower band gaps in the SiOC glass region. In the third part of this thesis, the lithiation calculations show that the lithiation of amorphous SiOCs can be roughly divided into two stages. In the first lithiation stage, the electrons from Li mostly fill the states on carbon atoms in free carbon phase, while the Li ions are absorbed on the oxygen atoms at the interface of free carbon and SiOC glass phase. In the second lithiation stage, Li ions start to interact with the SiOC glass phase with the electron from them fill the Si-O anti-bonding states, leading to the break of Si-O bonds and the formation of Li-O bonds. In the whole process, the carbon atoms in free carbon phase are keeping gaining electrons from Li, suggesting the role of reservoir of electrons that free carbon phase plays. Furthermore, the SiOCs with higher carbon concentrations present smaller relative volumes in the process of lithiation, indicating the function of limiting the volume expansions by free carbon phase. Finally yet importantly, the amorphous SiOCs with higher carbon concentration possess the higher theoretical capacities, and additionally, the high carbon concentration as well as the low oxygen concentration will make better performances of amorphous SiOCs on reversibility.