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  • 學位論文

利用臨場穿透式X射線顯微術觀察鋰離子電池含錫負極粒子內部結構變化

Study on Microstructural Deformation of Working Sn-containing Anode Particles for Lithium-ion Batteries by In-situ Transmission X-ray Microscopy

指導教授 : 吳乃立

摘要


鋰離子電池之高能量與高功率的效能仰賴著電極板結構的穩定性。因此,設計一個能夠定性與定量地提供活物層間結構訊息的檢測技術是必要的。本論文之主要目的是將同步輻射穿透式X射線顯微術(TXM)之技術應用在檢測電池電極板在充放電過程中電極材料內部微結構的變化。在本研究中,我們首次利用臨場穿透式X射線顯微術並結合臨場X射線繞射之技術揭示出含錫之負極材料,包括錫(Sn)、錫銻合金(SnSb)和錫氧化物(SnO),在前幾圈鋰遷入、鋰遷出充放電過程當中內部微結構演化的過程。 結果顯示微結構與變形的速度為粒子組成、尺寸和與鋰形成合金比例的函數。在第一次的鋰遷入過程中,鋰錫與鋰錫銻合金粒子皆表現出核(金屬)-殼(含鋰化合物)內部架構。對於錫顆粒而言,初期於錫顆粒表面形成之低鋰計量比之結構緊實的鋰錫相阻礙了鋰繼續遷入的過程,導致尺寸較大之錫顆粒延後膨脹的現象。相較之下,錫銻合金中的銻金屬在較高的反應電位下即立即地與鋰反應形成具多孔結構的鋰銻相(Li3Sb)之表面層,加速了鋰遷入的過程並排除了鋰遷入過程取決於粒子尺寸的關係。當第一次鋰遷出後,顆粒僅些微收縮,並其內部結構因金屬再結晶行為逐漸發展成多孔的結構。由於此多孔結構的產生,使得在之後的充放電當中能夠快速地進行鋰遷入以及舒緩活物尺寸上的變化。 含有板狀結構之錫氧化物(SnO)二次粒子則在第一次鋰遷入時能夠均勻的膨脹,其第一次鋰遷入包含兩個階段,首先第一階段鋰氧化物(Li2O)會產生並保持原本錫氧化物顆粒的形貌,接著第二階段鋰會與在第一階段產生的奈米錫顆粒繼續反應。其後之第一次鋰遷出過程與之後的充放電過程當中,僅有第一次鋰遷入中的第二階段過程是可逆的。結果清楚地顯示出,因擁有快速的鋰遷入、遷出反應,較小的體積膨脹變化,與因Li2O的存在而具有較強的顆粒機械強度的錫氧化物(SnO),比結構緊實的純錫更加適合做為負極材料。 此外,根據我們在臨場穿透式X射線顯微術所觀察到現象,錫在經過第一圈再結晶的過程後,會形成多孔結構的錫顆粒。我們即利用溶液法成功製備出具多孔性的錫。具有41.4%孔隙度之多孔錫顆粒能夠在全充全放的模式下於49圈能保持約480 mAh/g的電容量。

並列摘要


The performance of high-energy and/or high-power Li-ion batteries depends strongly on the architecture of the electrode over-layers. Devising a proper characterization technique that is capable of providing qualitative/quantitative information of the architecture of the active over-layer is essential for such research. The main purpose of this research is to establish the protocol of synchrotron transmission x-ray microscopy (TXM) for studying the electrode architecture, in conjunction with other electrochemical characterization techniques to understand the interplay between the electrode architecture and cyclic performance of the electrode. For the first time, the evolution of interior microstructures of three types of Sn-containing particles, including Sn, SnSb, and SnO, during initial cycles of electrochemical lithiation/de-lithation has been revealed by in-situ transmission x-ray microscopy, complemented by in-situ x-ray diffraction to provide phase information. The microstructures and deformation rates are shown to depend on particle composition, size and alloy stoichiometry with Li. During first lithiation, both Sn and SnSb particles exhibit core (metal) -shell (lithiated compounds) interior structures. Initial formation of a dense surface layer containing LixSn phases of low Li-stoichiometry on the Sn particle hinders further lithiation kinetics, resulting in delayed expansion of large particles. In contrast, Sb in SnSb is readily lithiated to form a porous Li-rich (Li3Sb) surface layer at higher potential than Sn, which enables to accelerate lithiation and remove the size dependence of the lithiation process. Both lithiated particles only partially contract upon de-lithiation, and their interiors evolve into porous structures due to metal re-crystallization. Such porous structures allow for fast lithiation and mitigated dimensional variations upon subsequent cycles. A SnO secondary particle consisting of plates of primary particles has been shown to homogeneously expand during the first lithiation in two stages, including the first producing Li2O matrix that bears most original particle morphology and the second involving full lithiation of the precipitated Sn nano-particles from the first stage. Only the second stage is reversible upon de-lithiation, and the particle undergoes the reversible second-stage deformation during subsequent cycles. The results indicate clear advantages of using such a porous secondary SnO as the anode material in comparison with dense Sn particle previously revealed, including fast lithiation/de-lithiation kinetics, reduced overall volume expansion and enhanced mechanical robustness of the particle, supported by the Li2O backbones. Moreover, according to the observation of porous Sn resulting from the local re-crystallization of Sn after first cycle, research in fabricating porous Sn anode particles via solution lithiation/de-lithiation method has been studied. Porous Sn particles with a porosity of 41.4% displays improved charge capacity retention of ~480 mAh/g after 49 cycles.

參考文獻


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