Controllable Nitrogen Doping of High-Surface-Area
Microporous Carbons Synthesized from an Organic–Inorganic Sol–Gel
Approach for Li–S Cathodes
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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<sup>2</sup>·g<sup>–1</sup>and pore volumes of ca. 1.2 cm<sup>3</sup>·g<sup>–1</sup>, 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<sup>–1</sup> at 0.5 C after 100
cycles