Penn State team develops highly crumpled nitrogen-doped graphene sheets as high-performance cathode for Li-sulfur batteries
Researchers at The Pennsylvania State University have synthesized highly crumpled nitrogen-doped graphene (NG) sheets with ultrahigh pore volume (5.4 cm3) and large surface area (1158 m2/g), which enable strong polysulfide adsorption and high sulfur content for use as a cathode material in Li-sulfur batteries. The wrinkled graphene sheets are interwoven rather than stacked, resulting in rich nitrogen-containing active sites.
Lithium–sulfur battery cells using these wrinkled graphene sheets as both sulfur host and interlayer achieved a high capacity of 1227 mAh/g and long cycle life (75% capacity retention after 300 cycles) even at high sulfur content (≥80 wt %) and sulfur loading (5 mg sulfur/cm2). A high capacity of 1082 mAh/g was still achieved with an ultrahigh sulfur content of 90 wt %, and a capacity of 832 mAh/g was retained after 200 cycles. Areal capacity was 5 mAh/cm2. A paper on their work is published in the ACS journal Nano Letters.
|Schematic illustration of the synthesis of highly crumpled NG sheets. Credit: ACS, Song et al. Click to enlarge.|
… practical applications of Li−S batteries are highly hindered by the low electrical conductivity of sulfur and the diffusion of soluble lithium polysulfides intermediates generated during cycling, which lead to lower utilization of sulfur, loss of active material from the cathode, and polysulfide shuttle phenomenon. As a result, Li−S cells experience a fast capacity fading, low Coulombic efficiency, and poor rate capability. To address these issues, various types of cathode materials, including porous carbon−sulfur, low-dimensional conducting material (such as carbon nanotube and graphene−sulfur), and conducting polymer−sulfur composites, have been exploited to improve the overall electrochemical performance of the Li−S cells.
Despite the promising advances, the preparation of sulfur cathodes with long-cycle-life and high Coulombic efficiency still presents challenges, especially for the cathodes with high sulfur content in the cathode and high areal sulfur loading in the electrode, which is necessary toward high-energy-density Li−S battery.—Song et al.
Nitrogen-doping of carbon has been shown to be effective for improving the overall electrochemical performance of sulfur cathodes in Li–S batteries.
Nitrogen-doping can promote chemical bonding between carbon scaffold and sulfur chains upon heat treatment during the sulfur loading process. This helps enable uniform distribution of sulfur in the carbon host initially as well as after redeposition, improving cycling performance.
Nitrogen-doping can also greatly enhance the adsorption of soluble lithium polysulfides intermediates. This effectively retards the diffusion of the lithium polysulfides and traps them within the cathode, thus improving both cycling stability and Coulombic efficiency.
Nitrogen-doped carbon is also highly conductive and thus allows for direct redox and utilization of adsorbed material rather than requiring desorption and diffusion of polysulfides to an electrochemically active surface. However, current nanoporous nitrogen-doped carbons suffer from limited pore volume, resulting in relatively low sulfur content (≤70 wt %) and sulfur loading (2 mg sulfur/cm2)—less than ideal for practical application.
Nitrogen-doped could combine the advantages of graphene and nitrogen-doped carbon. However, graphene sheets tend to form irreversible agglomerates or even restack to form graphite, resulting in the loss of specific surface area. This lowers the polysulfides adsorption capacity due to a decrease of accessible active sites.
Although the advances have been made in the pursuit of nitrogen-doped graphene as sulfur host for Li–S batteries recently, the electrochemical performance such as the cycling stability and sulfur utilization is still unsatisfied. This indicates that preventing graphene aggregation to maintain the accessible active surface and constructing adequate pore volume to host sulfur are of particular importance when graphene sheets are used as electrode materials for Li–S batteries.—Song et al.
The Penn State team synthesized their highly crumpled nitrogen doped-graphene sheets through a facile thermally induced expansion approach with cyanamide serving as both nitrogen source and porogen. This obviated the need for other templates.
The performance of the material presents a promising path for further development in Li−S batteries and can be potentially exploited for other applications such as supercapacitors and oxygen reduction reaction catalysts, the researchers said.
Jiangxuan Song, Zhaoxin Yu, Mikhail L. Gordin and Donghai Wang (2015) “Advanced Sulfur Cathode Enabled by Highly Crumpled Nitrogen-Doped Graphene Sheets for High-Energy-Density Lithium–Sulfur Batteries” Nano Letters doi: 10.1021/acs.nanolett.5b03217