鋰離子電池的電子結構與空間分佈之X光吸收能譜學與影像顯微術研究

Abstract

層狀鋰鎳錳氧化物是極具潛力的正極材料，有希望從眾多高能量的可充電電池中脫穎而出，成為下一個世代的明日之星。但是在正極中，每個元素之間的交互作用與運作機制仍是缺乏完整的理解。本論文中，我們分別利用軟X光吸收光譜、掃描式穿透X光顯微術、X光螢光顯微術來研究存在於Li2MnO3、Li1.2Ni0.2Mn0.6O2、LiNi0.5Mn0.5O2中的元素。並配合著三種狀態：未充電、於第一次充放電循環中分別抵達0% (放電)和60% (充電)的電位，來瞭解元素間複雜的運作機制。 我們的軟X光吸收光譜是在X光吸收近邊緣結構的範圍內量測到的，以用來解析材料中電子結構的基本性質以及變化。於氧K-edge的吸收光譜中，其光譜內的變化特徵反映了鎳對此材料的影響力以及電子結構中錳的變異程度。然而，LiNi0.5Mn0.5O2的這個樣品，其氧的吸收能譜結果，跟我們從錳的吸收結果所推測的預期並不一致。所以我們推測鎳和錳之間仍有其他的相對關係存在，進而在電池效能的表現上扮演著舉足輕重的角色。因此，我們利用掃描式穿透X光顯微術和X光螢光顯微術來呈現Li1.2Ni0.2Mn0.6O2與LiNi0.5Mn0.5O2中，過度元素在空間上的分佈關係。由於元素散佈的均勻程度會影響到鎳和錳之間的替代性還原作用，因此我們預測這可能是導致鎳含量增多，卻不一定使可逆電容量提升的原因。

The layered lithium nickel manganese oxide cathode materials show potentiality for the next generation of high-energy rechargeable batteries. However, the mechanism between each element in the cathodes is still lack of complete explanation. In this thesis, we performed two ex situ soft X-ray measurements, soft X-ray absorption spectroscopy (XAS) and scanning transmission X-ray microscopy (STXM) as well as a hard X-ray fluorescence microscopy (XFM) to survey the complicated relationships among the elements within Li2MnO3, Li1.2Ni0.2Mn0.6O2 and LiNi0.5Mn0.5O2 at three charge states, pristine, state of charge (SOC) 0% and 60%, during the first cycle. The soft X-ray absorption spectra are obtained in X-ray absorption near-edge structure (XANES) region giving the message of basic properties and transformation in electronic structure of the materials. From the oxygen K-edge spectra, the characteristic changes reflect the influences of nickel and the degree of electronic structural changes in manganese. Furthermore, since the result of LiNi0.5Mn0.5O2 conflicts with our speculation from the manganese L-edge spectra, we suggest that the correlation between nickel and manganese plays an important role in the batteries performance. For that reason, we employed STXM and XFM to illustrate the spatial distribution of transition elements upon Li1.2Ni0.2Mn0.6O2 and LiNi0.5Mn0.5O2. Because the uniformity in the distribution of elements will affect the substitution reduction between manganese and nickel, it might be the reason why increasing the amount of nickel does not always guarantee higher reversible capacity.