纖維素基高分子固態電解質之材料設計與電池效能測試

Abstract

近幾十年來科技產品的快速發展，大幅提升了人們對儲能裝置的需求。由於具有高能量密度、高工作電壓、優異的循環壽命和重量輕盈等特性，鋰離子電池被認為是最具潛力的儲能裝置。在新世代的鋰離子電池發展中，固態電解質因具有高機械強度、良好的熱穩定性、電解液不易洩漏等優點而備受關注。在目前的固態電解質發展中，高分子固態電解質因額外具有高柔韌性、與電極表面的高相容性、高加工便利性等優勢，而成為了此領域近年來的研究熱點之一。 因此，本研究嘗試以新穎的生質材料奈米纖維素TOCN搭配小分子量的聚乙二醇(PEG)作為高分子基質，發展一複合式高分子固態電解質。得益於TOCN在水溶液中的高分散性與小分子量PEG的高水溶性，我們開發了一簡易水溶液製程，僅需於加熱蒸發水份後，即可透過一步驟製備電解質膜。因TOCN具有奈米級的尺寸大小，加上其與聚乙二醇的良好親和性，所製備的TOCN高分子電解質複合薄膜具有高機械強度與良好的柔韌性。此外，我們進一步透過添加塑化劑、改變高分子基質、與進行TOCN表面改質等不同方式，探討這些變因對於此類固態高分子電解質電化學性能影響。 在此系統中以PEGDME取代PEG作為高分子基質後，得益於PEGDME末端的甲氧基對比PEG末端的羥基與鋰金屬間的反應性較小，我們觀察到了充放電容量的上升與電解質和鋰金屬介面穩定性的改善。此外，我們發現TOCN上的羧酸鈉官能基中的鈉離子會與鋰鹽中的鋰離子進行離子交換，使電池放電時產生不正常的放電平台，藉由將TOCN上的羧酸鈉官能基改質成羧酸鋰官能基後便能解決此問題，且因消除了鈉離子在正負極處產生的不良反應，使製備薄膜的電化學性能得到進一步的提升。本研究的成果與發現可望為纖維素基固態高分子電解質的發展有所貢獻。

The rapid development of technological products in recent decades has significantly increased the demand for energy storage devices. Lithium-ion batteries (LIBs) are considered to be the most promising energy storage devices due to their high energy density, high operating voltage, excellent cycle life and light weight. In the new generation of LIBs, solid-state electrolytes have attracted increasing attention because of their high mechanical strength, good thermal stability, and the fact that the electrolyte is not easy to leak. In the current development of solid electrolytes, solid polymer electrolytes (SPEs) have become on of the hot research topics in this field in recent years because of their additional advantages such as high flexibility, high compatibility with the electrode surface, and high processing convenience. Therefore, in this study, we attempt to develop composite SPEs by using a novel biomaterial, TEMPO-oxidized cellulose nanofiber (TOCN) and small molecular weight (MW) poly(ethylene glycol) (PEG) as the polymer matrix. Thanks to the high dispersibility of TOCN in aqueous solution and the high water solubility of small MW PEG, we develop a simple aqueous process to prepared electrolyte membranes in a single step by heating and evaporating water. Due to the nanoscale size of TOCN and its good affinity with PEG, the prepared composite TOCN SPE membrane has high mechanical strength and good flexibility. In addition, we further investigate the effects of different variables on the electrochemical performance of the prepared SPEs by adding plasticizers, changing the polymer matrix, and modifying the TOCN surface. After replacing PEG with PEGDME as the polymer matrix in this system, we observe an increase in the charge/discharge capacity and an improvement in the stability of the interface between the SPE and the Li metal due to the lower reactivity between the methoxy group at the end of PEGDME and the Li metal compared to the hydroxy group at the end of PEG. In addition, we found that the sodium ions in the sodium carboxylate functional group on TOCN would exchange with the lithium ions in the lithium salt, resulting in an abnormal discharge platform when the battery was discharged. This can be solved by modifying the sodium carboxylate functional group on TOCN to a lithium carboxylate functional group, and the electrochemical performance of the prepared films was further improved by eliminating the undesirable reaction of the sodium ions at the anode and cathode. The results and findings of this study are expected to contribute to the development of cellulosed-based SPEs.