Preparation Of Porous Organic Framework Materials And Their Applications In Lithium-sulfur Batteries

Due to the high theoretical capacity(1675 mAh g-1),energy density(2600 Wh kg-1),low cost and abundance of sulfur,lithium sulfur batteries have been considered promising candidates for next-generation energy storage system.Nevertheless,the poor conductivity of sulfur and the shuttle effect caused by intermediate products polysulfides hinder the practical applications of lithium sulfur batteries.Using conductive porous materials as host materials of sulfur can effectively improve the performance of lithium sulfur batteries.In this thesis,porous aromatic frameworks(PAFs)with different structures for lithium sulfur batteries have been studied.The main results are summarized as follows:1.N-containing porous aromatic framework 41(PAF-41)with hierarchical porous structure has been readily synthesized through AlCl3 catalysed Scholl reaction by using triphenylamine as the monomer.SPAF-41 composite cathode with high sulfur loading of 72 wt%exhibits enhanced rate capability and excellent cycling performance.The cell delivers a high capacity of 725.8 mAh g-1 at 0.5 C in the first cycle and sustains a reversible capacity of 491.4 mAh g-1 after 500 charge-discharge cycles.A low decay rate 0.06%per cycle is achieved.The well conductive PAF-41 support bestows the cell high Coulombic efficiency of 97.36%.2.Using piperazine and cyanuric chloride as monomers,N-containing porous aromatic framework PAF-6 with hierarchical porous structure has been synthesized by catalysis of K2CO3.After heat treatment at 500 ?,the porous carbon material CPAF-6 was obtained.SPAF-6 and SCPAF-6 composites were obtained using PAF-6 and CPAF-6 as host materials of sulfur,respectively.Compared with SPAF-6,SCPAF-6 electrode has better rate capability and long-term cycling performance.The initial discharge capacity of SCPAF-6 with sulfur loading of 49.2 wt%is 751.2 mAh g-1 at 0.5 C.After 300 cycles,the discharge capacity can be maintained at 474.3 mAh g-1.Meanwhile,capacity decay rate 0.123%per cycle and Coulombic efficiency of 96.78%are achieved.3.S1-xSex/CPAF-6(x?0.1)composites were prepared by a melt-diffusion method selecting carbonized PAF-6(CPAF-6)as host material of the active materials.Electrochemical impedance data indicate that as the content of Se in the electrode material increases,the conductivity of the electrode material increases accordingly.S0.9Se0.1/CPAF-6 composites with the active materials of 43.98 wt%have better rate capability and long-term cycling performance.The initial discharge capacity of the composites is 1225 mAh g-1 at 1 A g-1.After 400 cycles,the Coulombic efficiency is 99.86%.Meanwhile,capacity decay rate 0.162%per cycle is achieved.