側鍊烷基化之PEO聚輪烷： 合成與其在鋰離子固態電解質之應用

Abstract

本實驗的主要目的是以官能基化的聚輪烷（polyrotaxane, PR）為基材製備固態電解質獨立膜。PR為一主鏈為poly(ethylene oxide) （PEO），並將CD環（α -cyclodextrin）以PEO主鏈串起之特殊高分子，具有特殊的滑環特性而展現出高度的延展性及良好的分子運動性，當應用固態電解質時可以克服PEO在室溫下的結晶性而提升離子傳導度。由於其可透過CD環間的氫鍵作用力進行物理交聯，因此可以在不犧牲機械強度的情形下製備成實現PEO良好的鏈段遷移特性。然而影響其離子傳導性質與機械性質的因素有很多：包括PEO的分子量，CD與EO之間的比例，添加的鋰鹽種類與添加量，以及CD環上所修飾的官能基種類及數目等。為了優化固態電解質的電性，以上影響因素需要仔細的平衡。在此研究中，我們以PR為基材，將四種不同的烷基對CD環進行修飾，並改變修飾的密度，以調控分子間的作用力，並改變鋰鹽的混摻量，嘗試最佳化固態高分子電解質的成膜性、機械性質與離子傳導度。由NMR光譜結果印證，由於線型烷基有較小的立體障礙，其修飾率皆高於分支型烷基。通過溶劑揮發法，可製成約50至100微米厚的獨立膜，並用於其鋰離子傳導度的量測及研究鋰離子的遷移行為。結果顯示，分支型烷基側鏈修飾之PR可以有效改善固態電解質薄膜的柔韌性，並由較低的活化能得到較高的離子傳導度特性。

In this work, we perform alkylation of polyrotaxane (PR) and develop the corresponding solid polymer electrolytes (SPEs) for lithium batteries. Polyrotaxane (PR) is composed of a linear PEO chain threading through multiple cyclic α-cyclodextrin (CD) molecules, which is expected to improve the conductivity at room temperature while retain mechanical strengths due to the unique “mobile crosslinking” of PR, allowing good chain mobility of PEO without sacrificing mechanical strength since CDs contain abundant hydroxyl groups to form strong inter- and intramolecular hydrogen bonds to serve as physical crosslinks. To optimize the performance of SPE, carefully balance the chain mobility and the strengths of hydrogen-bondings is critical to obtain good conductivity and good mechanical strength simultaneously. Hereby, modification of CDs with different alkyl groups in various degree of functionalization is systematically performed, and the resulting alkylated PRs (A-PR) are blended with various concentrations of LiClO4 to fabricate the SPE membranes with thickness of 50-100 μm via solvent casting method. It is found that the degree of CD functionalization of linear alkyl chain is higher than that of branched alkyl group due to less steric hindrance. PR containing alkylated CD with branched ethyl hexyl group and low degree of functionalization of hexyl group would effectively improve the flexibility of the SPE and enhance the ionic conductivity with lowered activation energy for lithium ion transportation.