Construction Of 2D Covalent Organic Frameworks For Fundamental Research In Lithium-Sulfur Batteries

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymer with covalently linked highly ordered structures. With features like large surface area, low density, high thermostability, uniform pore size distribution and adjustable structures, COFs show broad application in gas storage and separation, catalysis and optoelectronic devices.2D COFs are framework materials constructed by the stacking of layers in which building blocks are linked by covalent bonds to form extended 2D sheets with one dimensional nanopores. Recent years,2D COFs have attracted significant attention from electrochemists due to their intrinsic advantages. First of all, the ?-electron coupling between layers provides fine pathways for charge carrier transportation, results in the conductivity of 2D COFs to some degree. Also, with a highly designable skeleton, COF materials with electrochemical properties are able to be synthesized via introducing electro-active groups into the framework or on the side chains linked to the framework. In addition, the nanopores in COFs are able to load guest molecules, as well as facilitate the spread and infiltrate of electrolyte into the electrode.In this thesis, we designed and synthesized several 2D COFs, investigated their structures and explored their fundamental application in lithium sulfur battery. Follows are the main contents:1. We for the first time employed 2D COFs as host material for sulfur impregnation in Li-S batteries to restrict the loss of polysulfide, which improves the cyclability. We synthesized triazine-based COF(CTF-1) to load elemental sulfur and incorparated it into Li-S batteries. The galvanostatic charge and discharge test shows the CTF-1/S@155 ? cathode delivers a specific discharge capacity of 1497 mA h g-1 during the 1st cycle at the current density of 0.1C (1C= 1680 mA h g-1), then decreases to 1197 mAh g-1 at the second cycle, and still retains at 762 mA h g-1 after 50 cycles. While the CTF-1/S@RT composite, which was synthesized by simply mixing CTF-1 and elemental sulfur, exhibits a poor cycling stability. The result indicates that the micropores of CTF-1 can effectively restrict the loss of soluble polysulfide during the discharge process.2. Although we demonstrated that COFs are able to restrict the loss of soluble polysulfide during the discharge process, the content of sulfur in CTF-l/S composite material is relatively low and the cyclability still needs to be improved. In this chapter we designed and synthesized a porphyrin based 2D COF (Por-COF), which possesses large BET-surface area (1095 m2 g-1), large pore volume (0.71 cm3 g-1) and uniform pore size distribution (1.55 nm). We loaded elemental sulfur into Por-COF to form Por-COF/S composite with a sulfur content of 55%, far higher than that of CTF-1/S@155 ? composite. The galvanostatic charge and diacharge test shows a specific capacity of 929 mA h g-1 during second cycle at the current density of 0.5C, and retains at 633 mA h g-1 after 200 cycles. The capacity decay rate is only 0.16% per cycle (starting calculation from the second cycle).3. The above researches showed that COFs could be excellent platform for researching fundamental problems in Li-S batteries. In this chapter we designed and synthesized a pyrene based COF (Py-COF) with larger BET-surface area (1840 m2 g-1), pore volume (1.25 cm3 g-1) and uniform pore size distribution (2.19nm). By loading different contents of sulfur, we discussed the relationship between sulfur content and sulfur electrode properties.