含有亞胺及環硼氧烷的共價有機骨架：合成與在鋰二次電池的應用

Abstract

共價有機框架 (COFs) 是具有高度有序和多孔結構的有機聚合物材料，其功能可以通過預先設計單體和在合成過程中改變反應參數調整，COFs通常表現出高結晶度、良好的水熱穩定性且不溶於有機溶劑，具有合適的官能團和立體結構，已被證明具有廣泛的應用性，包括催化劑、偵測器、氣體存儲和分離以及鋰電池。 我們設計並合成了一種包含亞胺和環硼氧烷鍵的雙功能COF，以捕獲鋰鹽中的陰離子，從而改善鋰離子的傳導，可以通過一鍋法合成，我們系統地研究了溶劑、進料比和催化劑對所得 COF 結構的影響，並通過 FTIR 和粉末 X 射線衍射鑑定，從 SEM、TEM 和 HR-TEM 中鑑定其形貌變化，孔隙率由 BET 測量計算。後將COF作為添加劑用於製備以PEO為基材的複合固體高分子電解質，具有不同的COF負載量，觀察到添加少於 2 wt%的COF可以顯著提高室溫下的離子電導率1~2個數量級，有趣的是，複合 SPEs 的離子電導率高於純PEO組成的SPE，即使在高於SPE熔點的溫度下也具有較低的活化能，這表明COF本身可能具有鋰離子傳導能力。 我們還通過一鍋法製備COF改性天然石墨 (COF@G) 作為負極活性材料。PXRD和HRTEM證實結晶COF可以在NG表面生長。與原始NG相比，當半電池以10 C的速率運行時，COF@G的充電容量增加了25 %，表示COF可以幫助提高負極的rate performance，並且穩定的庫倫效率接近到100 %。SEM 圖像顯示，NG 上的 COF 可以顯著抑制高充電速率下不希望的SEI 增厚，提高快速充電期間的循環穩定性和安全性。

Covalent organic frameworks (COFs) are organic polymeric materials having highly ordered porous structures, which could be tailored by pre-designed monomers and by changing the reaction parameters during syntheses. COFs generally exhibit high crystallinity, good hydrothermal stability, and insolubility in organic solvents. With suitable functional groups and stereoscopic structures, COFs have been demonstrated to be useful in a wide range of applications, including catalysts, sensors, gas storage and separation, as well as lithium batteries. In this work, we design and synthesize a bifunctional COF containing imine and boroxine bondings to entrap the anions in lithium salts and thus to improve lithium ion transport. The COFs could be feasibly synthesized via one pot chemistry. We systematically study the effect of solvents, feed ratios and catalysts on the structure of the resulting COFs, which are identified by FTIR and powder X-ray diffraction. The morphological information is retrieved from SEM, TEM and HR-TEM. The porosity is calculated from BET measurement. The COF is employed as an additive to prepare composite solid polymer electrolyte based on Poly(ethylene oxide) with various COF loadings. It is observed that less than 2 wt% COF addition could significantly improve the ion conductivity at room temperature by 1~2 order of magnitude. Interestingly, the ion conductivity of the composite SPEs is higher than the pristine PEO SPE with lowered activation energy even at temperatures above the melting point of the SPE, suggesting COF itself might have lithium ion conducting capability. We also prepare COF modified nature graphite (COF@G) via one pot chemistry to serve as anode active materials. It is confirmed that crystalline COF could grow on the surface of NG by PXRD and HRTEM. The COF could help the rate performance of the anode as a 25% increase in the charging capacity for COF@G is observed in comparison with the pristine NG when the half cell is operated in a rate of 10C, alone with a steady Columbic efficiency close to 100. The SEM images show the COF on NG could significantly inhibit the undesired SEI thickening at high C rate, enhancing the cycling stability and safety during fast charging.