新型聚輪烷衍生物在鋰電池類固態電解質與膠態電解質之應用

Abstract

本研究的目的是以聚輪烷(Polyrotaxane, PR)衍生物為鋰電池開發類固態聚合物電解質(QSPEs)與膠態聚合物電解質(GPEs)，並探討QSPEs與GPEs的熱性質、機械性質、電化學性質和介面性質。聚輪烷(PR)由一長鏈狀的線性聚環氧乙烯 (poly(ethylene oxide), PEO)作為主鏈，在主鏈上套上多個由6-8個醣類所組成的環糊精(Cyclodextrin, CD)，最後再由封端所組成之超分子聚合物材料。由於CDs與PEO主鏈之間不存在化學鍵結，因此CDs可以沿PEO鏈滑動，形成所謂「滑環材料」，提供QSPE與GPE良好的分子運動性，在加入少量電解液即具有良好的離子傳導度，並與電極間有良好的貼合，以達到同時兼顧電池安全性與效能的目標。 在QSPE的研究中，我們開發了一新型聚輪烷(PR)衍生物hep-PR，將α-環糊精(α-CD)進行修飾得到具有末端雙鍵的1-庚烯(1-heptene)長碳鍊接枝於CD上。我們以hep-PR為基材，透過同時進行的溶劑揮發與硫醇烯反應(thiol-ene reaction)製備hep-PR獨立交聯膜，爾後將電解液加入乾膜製備成QSPEs。透過調整hep-PR的接枝率、製膜方法學、交聯劑當量與電解液添加量，並加入奈米纖維素(CNF)以提升QSPE的尺寸安定性、機械性質與離子傳導特性。當QSPEs含有100wt%的電解液時其室溫離子傳導度可達2.57✕10-4 S cm-1，在60℃時則可達1.07✕10-3 S cm-1。 在GPE的研究中, 我們對hep-PR進行環氧化反應(epoxidation)將雙鍵轉換為環氧基團得到e-PR，並將e-PR加入1,3-二氧環戊烷(DOL)的系統中作為原位陽離子開環聚合(in-situ cationic ring-opening polymerization)中的交聯劑。e-PR的CD滑環特性使e-PR成為可移動的交聯劑，提升無添加共溶劑GPEs的表現。相較於使用固定式交聯劑的GPE，e-PR GPEs最柔軟且具有最低的介面阻抗，顯示其與電極有更好的貼合；而在-20℃下其仍具有低體阻抗與適中的介面阻抗，展現其在低溫下操作的潛力。

The objective of this work is to develop quasi-solid polymer electrolyte (QSPEs) and gel polymer electrolyte (GPEs) for lithium batteries containing polyrotaxane derivatives, and to study the thermal, mechanical, electrochemical and interfacial properties of the obtained QSPEs and GPEs. Polyrotaxane (PR) is a supramolecule comprising of an end-capped linear poly(ethylene oxide) (PEO) main chain threading through multiple cyclodextrins (CDs) composed of 6-8 glucose units. As no chemical bonding exists between CDs and the PEO main chain, allowing CDs to slide along the PEO chain, usually recognized as “slide-ring” effect; PR should provide high molecular mobility to benefit the obtained QSPEs and GPEs with good ionic conductivity and intimate contact with electrodes upon limited liquid electrolyte uptake. Therefore, the safety and the performance of the lithium batteries could be concurrently satisfied. In the QSPE studies, we develop a new custom made polyrotaxane derivative hep-PR by modifying α-cyclodextrin (α-CD) of PR to allow the CD tethered with 1-heptene side chains functionalized with terminal double bonds. The free standing matrix of QSPE comprised of crosslinked hep-PR is prepared from in-situ thiol-ene reaction upon solvent casting and which are subjected to liquid electrolytes uptake to afford the QSPEs. The properties of QSPEs are tailored by adjusting the degree of functionalization of hep-PR, the membrane preparation methodology, the crosslinking density, and the amount of liquid electrolyte uptake. Cellulose nanofibers (CNF) are incorporated to further enhance the dimensional stability and the mechanical properties. With 100 wt% liquid electrolyte uptake, the ionic conductivity of the QSPE could reach 2.57✕10-4 S cm-1 and 1.07✕10-3 S cm-1 at 20℃ and 60℃, respectively. In the GPE studies, the double bonds of hep-PR is further converted into epoxide to afford e-PR, a PR derivative with CDs to serve as a polymeric crosslinker in gel polymer electrolytes (GPEs) prepared from in-situ cationic ring-opening polymerization of 1,3-dioxolane (DOL). The slide ring characters of CDs allow e-PR act as a “moveable crosslinker” to improve the performance of GPEs without the use of co-solvents. Comparing to reference GPEs using “fixed crosslinkers”, the e-PR GPEs are the softest and exhibit the lowest interfacial resistance, suggesting better interface compatibility with the electrodes. It is worthy to address that low bulk resistance and moderate interfacial resistance could be achieved even at -20℃, suggesting good potential of e-PR GPEs to operate at low temperatures.