高分子電解質中相行為，作用力機制以及離子導電度之研究

Abstract

近年來，高分子電解質（polymer electrolytes）一直被廣泛的研究，因其同時具有離子導電度與良好的機械性質，可以被應用於離子傳導之電子元件中。然而，完全固態（all-solid-state）的高分子電解質卻受限於低的離子導電度（ionic conductivity），而無法商業化。為了改善此一缺點，許多研究常會添加無機材料或是合成新結構之高分子主體，藉以提高高分子電解質的導電度，但是室溫下所得之離子導電度（＜10-4 S cm-1）卻不盡理想，而無法實際應用於鋰電池（lithium battery）中。在致力於提高導電度的同時，往往忽略了探討離子傳導機制的重要性；在遭遇此種瓶頸時，我們必須轉向更基礎的研究討論，進一步去瞭解高分子主體與鹽類間複雜的作用力情形，從分析過程中，尋求改良離子導電度之途徑。因此，在本篇論文中，我們將藉由熱微分掃瞄卡計（DSC）、紅外線光譜儀（FTIR）、固態核磁共振光譜（Solid-state NMR）以及交流阻抗分析儀（ac Impedance）等儀器，觀察固態高分子電解質中，高分子相容行為（miscibility behavior）與高分子—鹽類間作用力機制（interaction mechanisms）對離子導電度之影響。而本論文可以分為三部分來討論： (1) 由於poly(ethylene oxide)（PEO）具有結構上的優勢，可幫助鹽類解離進而傳導離子，因此廣泛地應用於高分子電解質中。然而PEO中存在著高度結晶，會侷限離子傳導路徑，造成室溫的離子導電度極差。因此，我們加入poly(□-caprolactone)（PCL），藉由PEO與PCL間強大的相容性，降低PEO本身的結晶度，以提高離子導電度。 (2) 根據上述的研究結果得知，PEO與PCL間高分子摻合的相容性極佳，因此可以預期PEO-b-PCL嵌段共聚高分子（monomethoxypoly(ethylene glycol)-block-poly(□-caprolactone) block copolymers）具有更佳的相容性，更有助於PEO結晶的破壞，而達到提高離子導電度之效果。 (3) 由於poly(methyl methacrylate)（PMMA）及poly(vinyl pyrrolidone)（PVP）均可做為高分子電解質中傳導離子的媒介，然而此兩種高分子分別有缺陷存在，使得它們在離子導電度的表現有所限制。所以，我們將擷取雙方面的優點，聚合PVP-co-PMMA無規則共聚高分子（poly(vinyl pyrrolidone-co-methyl metharcylate) random copolymers），並加入LiClO4（lithium perchlorate）組成高分子電解質系統，藉由探討其間複雜的作用力，觀察離子導電度的改變情形。當PVP分子的存在時，其分子鏈上擁有高極性的官能基，可幫助鹽類解離；另一方面，PMMA分子的加入，可破壞PVP分子間強大的偶極—偶極力（dipole-dipole interactions）。上述兩種因素，均反映在離子導電度的提升。

Solid state materials that exhibit high ion transport properties are of interest from both academic as well as applied points of view. Polymer electrolytes are materials of high technological perspective in several electrochemical applications. However, lithium-based polymer electrolytes exhibit several disadvantages that affect the commercialization of such cell; one major drawback is the low ionic conductivity of the electrolyte at ambient temperature. Although great efforts to enhance ionic conductivity have been made over the last 20 years, levels of ionic conductivity are persistently limited to a ceiling of around 10-4 S cm-1 at room temperature, which is insufficient for many lithium battery applications. In the face of such barriers in science, we must direct our attention to the fundamental research of polymer electrolytes, such as the complicated interaction mechanisms within the polymer electrolytes. In this study, therefore, we focused on investigating the effect of miscibility behavior and interaction mechanisms on ionic conductivity of polymer-salt complexes by means of differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), solid-state 7Li NMR, and alternating current (ac) impedance. The experimental work in this dissertation was divided into three sections as follows: (a) The addition of poly(□-caprolactone) (PCL) into poly(ethylene oxide) (PEO)-based electrolytes tends to suppress the crystallization of PEO due to the strong interaction between PEO and PCL, thus resulting in the increase of ionic conductivity for LiClO4/PEO/PCL ternary blend systems-based polymer electrolytes. (b) According to the above research, we subsequently synthesized monomethoxypoly(ethylene glycol)-block-poly(□-caprolactone) (MPEG-b-PCL) block copolymers and studied the miscibility behavior based on polymer electrolytes consisting of LiClO4 and MPEG-PCL. It is reasonable to us to expect that MPEG-PCL may be more miscible than the PEO/PCL binary blend. (c) In the third part, we discussed the interaction mechanisms within the polymer electrolytes composed of LiClO4 and poly(vinyl pyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) random copolymers. The incorporation of MMA moiety tends to play an inert diluent role to reduce the self-association of PVP molecules. The more fraction of dissolved “free” ClO4- of LiClO4/PVP-co-PMMA blends can be detected than that of LiClO4/PVP. Therefore, this factor is responsible for the observed increase in ionic conductivity of LiClO4/PVP-co-PMMA blend.