運用第一原理計算與分子動態模擬探討摻雜元素對矽基負極材料的結構、動力學、熱力學、以及機械性質之影響

本研究使用第一原理計算與分子動態模擬探討可作為鋰電池負極材料之非晶質鋰-矽合金之物理與化學性質如何受組成變化與添加摻雜元素(硼、磷為主)而改變，並進一步探討摻雜元素優化矽基負極材料性能之可能物理機制。此外，我們也對矽基電極操作時表面與電解液反應的機制以及矽基材料表面金屬層對電極性質之影響進行深入的探討。在本研究的第一部分中，我們運用第一原理分子動態模擬的方式建構含有硼、磷摻雜元素的鋰-矽合金系統，並研究其結構、熱力學性質、電子性質、及機械性質隨鋰/矽比之變化。我們發現當矽基材料中摻雜磷原子時系統的最大鋰容量會因此而上升，但硼摻雜反而會使最大鋰容量下降，而此趨勢與實驗結果相符。我們對結構及電子性質進行進一步分析後得知此結果乃由於矽-磷與矽-硼的鍵結型態的差異。在鋰化過程中，摻雜在矽基材料內的磷原子傾向由鋰原子處獲得價電子並以獨立原子的型態存在於合金中，而此特性可使這些磷原子德以在不減損矽基材料固有的鋰容量的狀況下提供額外的鋰容量。反之，當合金中摻雜硼原子時，硼與矽原子間產生的強矽-硼鍵結即使在高鋰濃度下依然可以存在，使得鋰原子之價電子無法轉移至矽的未鍵結軌域，導致最高鋰容量的減損。我們同時也發現無論硼或磷原子皆無法改善矽基材料隨鋰化程度增加伴隨的體積膨脹以及隨鋰含量上升帶來的機械強度弱化，顯示由於充鋰過程中體積膨脹造成的碎裂難以透過化學摻雜改善。第二部分中，我們以第一原理分子動態模擬的方式探討外加應力對鋰-矽合金之結構與擴散行之影響。我們發現，無論在任何鋰-矽組成比例下，鋰-矽合金內的鋰與矽原子之擴散係數皆隨張應力上升而增加且隨壓應力上升而降低。然而，我們同時也發現應力效應會隨著鋰濃度上升而有所減弱。此外，我們的結果也顯示在材料中摻雜磷元素可有效加速在低鋰/矽比合金內原子的擴散，且同時減緩應力帶來的負面效應。第三部分探討的主題為結晶矽基電極表面的摻雜元素對電極鋰化行為之影響。我們首先運用分子動態模擬探討矽基結構中的硼、磷、以及砷等摻雜元素對鋰化速率的影響，而我們的結果顯示在矽基材料中摻雜磷或砷等n型摻雜元素可有效加速鋰化反應的進行，但反之硼摻雜則無法產生明顯的改善效果。我們的結果進一步顯示此乃由於矽基材料中的磷或砷等元素可軟化矽基材料的機械強度並同時提升材料的塑性，使得鋰化過程中結構重構的步驟更容易發生，並因此加速鋰化過程；反之，雖然結構中的硼原子可以幫助鋰原子進入電極，這些硼原子同時也使矽基材料的機械強度上升，而此變化會阻礙結構重構之進行，導致反應速率無法明顯增加。同時，我們的結果也顯示含有n型摻雜的矽基材料進行鋰化反應時所產生的應力較低，而此趨勢可使得電極的結構穩定性提升，減少由於機械破壞造成的不可逆鋰容量流失。本文的第四部份中，我們用第一原理計算與分子動態模擬探討鋰化反應過程中電解質溶劑EC分子在電極表面的化學反應。我們的結果顯示EC分子在電極表面的反應速率隨著鋰含量增加而上升，而此變化與電極的功函數隨鋰含量上升而下降之趨勢相符。此外，我們也發現EC在表面的分解機制隨濃度有相當大的變化。其中，在高鋰含量下透過電極表面鋰原子與EC間靜電作用力誘發產生的還原產物OCOC2H4O2-不僅是反應過程中最有可能產生的產物，也可做為常見SEI成分如低聚物或氧化鋰等之前驅物。我們同時也發現當電解液受到壓應力時會加速EC在表面的分解反應。另一方面，雖然電極內的硼或磷等摻雜元素並不直接參與EC的分解反應，但是摻雜這些元素可以改變矽基材料表面功函數，進而影響EC的反應速率。其中，由於在磷摻雜的鋰-矽合金中帶負電的磷離子會偏析於表面，使表面鋰濃度下降，造成表面功函數上升的，故可抑制EC在表面的分解反應。本論文的最後一部份則探討鋰-矽合金表面的銅或鎳等金屬披覆層對鋰-矽合金之結構、動力學性質、以及表面反應之影響。我們發現，當金屬與鋰-矽合金形成接面時鋰-矽合金內的矽原子與介面上的金屬產生鍵結，形成一層金屬矽化物；反之，合金內的鋰原子並無法穿透金屬層與鋰-矽合金間之介面，而會聚集在介面矽化物層下方，形成一高鋰濃度區。另外，由於銅與鎳皆傾向由鋰-矽合金層獲得電子，他們的存在會造成合金內部鋰-矽間靜電作用力減弱，而此變化亦使合金內原子的擴散加速。另一方面，我們的模擬結果也顯示即使在高鋰濃度的鋰-矽合金表面，金屬矽化層的存在也可使表面功函數大幅增加，抑制EC分子在表面的分解，代表金屬披覆亦有降低SEI造成的影響之功效。

關鍵字

第一原理計算；分子動態模擬；鋰電池負極；矽基材料

並列摘要

In this work, we have employed first-principles density functional theory calculations and ab initio molecular dynamic simulations to investigate the physical and chemical properties of the pristine and doped Si-based anodes during the lithiation processes, including the dopant effects on the properties of the Si-based anodes, stress effects on the kinetic behaviors of Li and Si in the lithiated Li-Si alloys, dopant effects on the lithiation process of the crystalline Si anodes, formation mechanisms solid-electrolyte interface on the Li-Si alloys, and the effects of metal coating layers on the properties of the Li-Si alloys. In the first part of the thesis, B- and P-doped LixSiM1/7 alloys between x = 0.5 and 4.0 were constructed via ab initio molecular dynamics, and their structural, thermodynamics, electronic, and mechanical properties were analyzed. Our prediction for the dopant effects on the thermodynamic properties showed that the Li storage capacity can be increased via P-doping while B-doping causes the decrease in the maximum capacity, consistent with the experimental results. Further analyses for the structural and electronic properties revealed that this discrepancy is primarily attributed to the differences in the bonding characteristics between Si-P and Si-B. Furthermore, our results also suggested that both B- or P-doping cannot reduce the volume expansion of the Si-based anodes during lithiation or increase the mechanical strength of the highly-lithiated anodes, suggesting that doping may not be an effective way to mitigate the cracking of the highly-lithiated Si-based anodes. In the second part, the effects of hydrostatic stress on the kinetic behaviors of Li and Si in pristine and doped Li-Si alloys were investigated via ab initio molecular dynamics simulations. Our results suggested that the diffusivities of both Li and Si in the pristine or doped Li-Si alloys decrease under compressive stress and increase when tensile stress is imposed on the alloy system, but the effects of stress becomes less significant as the Li content increases. We also found that P-doping may reduce the negative effects induced by the compressive stress in less-lithiated Li-Si alloys, but their effects gradually becomes insignificant with the increasing Li content. . In the third part, the lithiation behaviors of pristine and (B, P, As)-doped crystalline Si(001) and Si(011) were examined via ab initio molecular dynamics calculations. Our results showed that the lithiation rate can be enhanced via P- or As-doping while it is not the case for B-doping. Our further analyses for the dopant effects revealed that the improvements brought by n-type dopants are majorly attributed to the mechanical softening effects of the Si matrices, which facilitates the rate-determining structural reconstruction process upon lithiation. On the contrary, despite the existence of B atoms in the Si networks can help the Li insertion, those B atoms also stiffen the Si network, retarding the amorphorization process. Our results also revealed that n-type doping is also effective in the reduction of the induced stress by structural reconstruction and the mitigation of the inhomogeneity in the lithiation rate between different orientations, both of which can improve the mechanical stability of the Si-based anodes. In the fourth part, the chemical reactions between the ethylene carbonate (EC) molecules and the Li-Si surfaces with different Li content were modeled via ab initio molecular dynamics. Our results showed that the reduction of EC molecules on the Li-Si alloys becomes drastic with the increasing Li content, which can be attributed to the decreasing surface work function with the increasing Li content. Furthermore, the changes of the surface configuration with the Li content also caused the dependence of the predominant reduction mechanisms of the EC molecules on the Li-Si surfaces, and the reduction products of EC molecules on highly-lithiated Li-Si alloys OCOC2H4O2- may also be the origin of some common SEI components such as oligomers or lithium oxide. We also found that the EC reduction reactions can be facilitated when compressive stress is imposed on the electrolytes. On the other hand, our results showed that B-doping leads to faster EC reduction while P-doping may inhibit the decomposition, providing a potential route toward reducing the SEI formation of Si-based anodes. In the final part, the effects of Cu and Ni coating layers on the structures, kinetics, and chemical reactivity were investigated via ab initio calculations and molecular dynamics. Our results showed that the Si atoms in the Li-Si slabs exhibits a strong tendency to form a thin silicide layer at the interface, causing the aggregation of the Li atoms underneath the metal silicide interface. We also observed that the metal layers may also accept the valance electrons of Li and compete with the Si atoms, causing an overall decrease in the average negative charge on Si. This trend eventually weakens the electrostatic interaction between Li and Si in the Li-Si alloys, resulting in the acceleration of the Li diffusion. Furthermore, the existence of the silicide layers on the top surface of the Si anodes also inhibits the reduction of EC molecules even on the highly-lithiated Li-Si alloys by greatly increasing the surface work functions.

並列關鍵字

First-principles calculations ； molecular dynamics simulations ； Li-ion battery anodes ； Si-based materials

參考文獻

Chapter 1:

1 Issues and challenges facing rechargeable lithium batteries, J. M. Tarascon and M. Armand, Nature 414, 359 (2001).

2 New Horizons for Conventional Lithium Ion Battery Technology, E. M. Erickson, C. Ghanty, and D. Aurbach, The Journal of Physical Chemistry Letters 5, 3313 (2014).

3 B. Canis, (Congressional Research Service, Library of Congress, 2011).

4 Cost and CO 2 aspects of future vehicle options in Europe under new energy policy scenarios, C. Thiel, A. Perujo, and A. Mercier, Energy policy 38, 7142 (2010).

國際替代計量

運用第一原理計算與分子動態模擬探討摻雜元素對矽基負極材料的結構、動力學、熱力學、以及機械性質之影響

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