石墨烯負極和離子液體電解液於鈉二次電池之應用;Applications of graphene-based anodes and ionic liquid electrolytes for sodium secondary batteries

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Title:	石墨烯負極和離子液體電解液於鈉二次電池之應用;Applications of graphene-based anodes and ionic liquid electrolytes for sodium secondary batteries
Authors:	羅旭峯;Luo, Xu-Feng
Contributors:	材料科學與工程研究所
Keywords:	鈉離子電池;石墨烯;離子液體;固態電解質介面;陰離子插層;sodium secondary battery;graphene;ionic liquid;SEI film;anion intercalation
Date:	2019-08-22
Issue Date:	2019-09-03 14:50:58 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究以鈉二次電池為主軸，分別針對石墨烯負極、離子液體電解液、SEI膜特性、安全性議題、石墨正極等各個議題進行探討。微電漿輔助製程之石墨烯(graphene nanosheets obtained by microplasma-assisted chemical vapor deposition technique，MPGNS)具有高結晶性和低缺陷密度。相較傳統RGO (reduced graphene oxides)，MPGNS負極具有更優異之電化學特性，包括首圈庫倫效率(45%)、電容值(250 mAh g－1 at 0.03 A g－1)、高速充放電能力 (44% at 5 A g－1)。導入NaFSI/PMP-FSI離子液體電解液後，石墨烯負極之充放電可逆性進一步獲得提升，化成後之庫倫效率近100%。相較於傳統有機電解液，本研究結果顯示NaFSI/PMP-FSI離子液體所生成之SEI膜具有更理想的電化學穩定性和熱穩定性，因而提升了庫倫效率、循環穩定性和安全性。此外鈉離子濃度顯著地影響SEI膜之化學組成，2 M NaFSI/PMP-FSI電解液可使SEI膜中的有機/無機質成分比例達到理想平衡，可穩固SEI膜之結構並兼具良好的電化學穩定性。除了電解液配方之外，負極材料之表面特性同樣也影響著SEI膜的生成和其效能。以MPGNS為例，適量的表面官能基可做為SEI膜的成核點，提升SEI膜的生成效率，迅速地鈍化充電態負極。於電池的安全性議題方面，本研究透過DSC系統性地分析有機電解液、離子液體電解液、和石墨烯之含氧官能基對於SEI膜之熱穩定性以及各類電解液對於充電態負極之熱反應性。於離子液體之應用方面，本研究提出以NaFSI/PMP-FSI離子液體做為電解液之陰離子插層石墨正極，其平均4.5 V (vs. Na/Na+)之工作電位能提供高能量密度。石墨正極同時能使鈉離子電池脫離對於過渡金屬礦產之依賴，實現高經濟效益之鈉二次電池。;This thesis focuses on sodium secondary batteries, including investigation on graphene-based anodes, ionic liquid electrolytes, SEI chemistry, safety issues, and graphite cathodes. A microplasma-assisted chemical vapor deposition technique is used to produce graphene nanosheets (denoted as MPGNS). The obtained MPGNS has higher crystallinity and less defects, compared to those of conventional reduced graphene oxides. MPGNS is able to deliver a superior Coulombic efficiency of 45%, a high capacity of 250 mA h g－1(@ 0.03 A g－1), and show excellent rate capability (44% @ 5 A g－1). Using NaFSI/PMP-FSI ionic liquids as electrolytes, the electrochemical reversibility of graphene anodes was improved. After formation cycles, the Coulomic efficiency is close to 100%. This study demonstrates a superior SEI film derived by NaFSI/PMP-FSI ionic liquids, which exhibits a better stability against charge-discharge process and elevated temperatue, compared to those of conventional organic electrolytes. In this way, Coulombic efficiency, cyclic stability, and safety are enhanced. Moreover, Na ion fractions considerably impact on SEI chemistry. 2 M NaFSI/PMP-FSI generates the most robust SEI film with the ideal balance between organic and inorganic ingredients, improving the adhesion and electrochemical steadiness. Apart from electrolyte compositions, the surface characteristic also determines the formation and effectiveness of SEI films. Take MPGNS for example, the oxygen-containing functional groups can act as nucleation sites, and thus rapidly passivate charged anodes. Concerning the aspect of safety, this study systematically analyzes the thermal behaviors between charged anodes and electrolytes using DSC. Finally, anion intercalation into graphite cathodes using NaFSI/PMP-FSI as electrolytes shows the high working voltage above 4.5 V (vs. Na/Na+), which promisingly enables high energy density and replacement of rather expensive transition metal-based cathode materials.
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