Controllable Nitrogen Doping of High-Surface-Area Microporous Carbons Synthesized from an Organic-Inorganic Sol-Gel Approach for Li-S Cathodes.

Abstract

High-surface-area microporous carbons with controllable nitrogen doping were prepared via a novel organic-inorganic sol-gel approach, using phenolic resol and hexamethoxymethyl melamine (HMMM) as carbon precursors, and partially hydrolyzed tetraethoxysilane as silica template. The pore structures of microporous carbons were completely replicated from a thin silica framework and could be tailored greatly by changing the organic/inorganic ratio. The nitrogen atoms doped into the carbon framework were issued from high-nitrogen-content HMMM precursors, and the nitrogen content could be adjusted in a wide range by changing the phenolic resol/HMMM ratio. Moreover, the porous structure and nitrogen content could be simultaneously controlled, allowing the preparation of a series of microporous carbons with almost the same microstructures (BET surface areas of ca.1900 m(2)·g(-1)and pore volumes of ca. 1.2 cm(3)·g(-1), and the same pore size distributions) but with different nitrogen contents (0-12 wt %). These results provided a general method to synthesize nitrogen-doped microporous carbons and allowed these materials to serve as a model system to illustrate the role of nitrogen content on the performance of the carbons. When used as the supports for sulfur cathodes, only an appropriate nitrogen content of ca. 6.3 wt % was found to effectively improve sulfur utilization and cycle life of the sulfur cathodes. The resulting sulfur cathodes could deliver an outstanding reversible discharge capacity of 1054 mAh·g(-1) at 0.5 C after 100 cycles.