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New graphene-wrapped sulfur-carbon nanofiber cathode for Li-S batteries shows increased capacity and long-cycle stability
23 May 2013
Researchers at Duke University have developed a novel graphene–sulfur–carbon nanofibers (G-S-CNFs) multilayer and coaxial nanocomposite for use as the cathode of Li–sulfur batteries with increased capacity and significantly improved long-cycle stability.
Electrodes made with the G-S-CNFs were able to deliver a reversible capacity of 694 mA h g–1 at 0.1C and 313 mA h g–1 at 2C—both substantially higher than electrodes assembled without graphene wrapping. More importantly, the long-cycle stability was significantly improved by graphene wrapping, they noted in their paper published in the ACS journal Nano Letters.
The cathode made with G-S-CNFs with a initial capacity of 745 mA h g–1 was able to maintain 273 mA h g–1 even after 1500 charge–discharge cycles at a high rate of 1C, representing an extremely low decay rate (0.043% per cycle after 1500 cycles). As a comparison, the capacity of an electrode assembled without graphene wrapping decayed significantly with a 10 times high rate (0.40% per cycle after 200 cycles).
They attributed the improved rate capability and cycle stability to the unique coaxial architecture of the nanocomposite, in which the contributions from graphene and CNFs enable electrodes with improved electrical conductivity, better ability to trap soluble the polysulfides intermediate and accommodate volume expansion/shrinkage of sulfur during repeated charge/discharge cycles.
...sulfur based lithium batteries are of particular promise for next-generation energy storage systems. However, despite of these promises, several challenges exist for Li−S batteries that hindered their commercial applications. Such challenges include the inherent low electrical conductivity of sulfur (5 × 10−30 S cm−1), which results in limited active material utilization efficiency and rate capability, and shuttling of high-order polysulfides between cathode and anode and high solubility of polysulfide intermediates in the electrolyte, both of which lead to limited cycle stability. Additionally, sulfur experiences severe volumetric expansion/ shrinkage during charge and discharge (∼80%), which gradually decreases the mechanical integrity and hence stability of the electrode over cycles.
...Here we report an approach to assemble graphene−sulfur− carbon nanofiber (G-S-CNF) coaxial structured nanocomposite with sulfur sandwiched between graphene and CNFs that can be used as cathode for Li−S batteries with significantly improved cycle stability and capacity.
In our approach, sulfur was first coated uniformly on the surface of CNFs and then graphene was used to wrap around the whole structure. In this well designed structure, both CNFs and graphene serve as good conducting fillers to improve the overall conductance of the film. More importantly, graphene over-coated on S limited the dissolution of polysulfide intermediate, while CNFs provided much needed mechanical stability of the film.—Lu et al.
The work was supported in part by a research grant from the National Science Foundation (NSF) and the Environmental Protection Agency (EPA).
Songtao Lu, Yingwen Cheng, Xiaohong Wu, and Jie Liu (2013) Significantly Improved Long-Cycle Stability in High-Rate Li–S Batteries Enabled by Coaxial Graphene Wrapping over Sulfur-Coated Carbon Nanofibers. Nano Letters doi: 10.1021/nl400543y
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