Cornell researchers develop two approaches to improving Li-sulfur performance; spin-off commercializing the tech
|Charge/discharge capacities vs cycle numbers of GO-S-Amy at different sulfur loadings. Credit: ACS, Zhou et al. 2013 a. Click to enlarge.|
Researchers in the lab of Hector Abruña, the Emile M. Chamot Professor of Chemistry and Chemical Biology at Cornell University, have developed two approaches to improve the durability and performance of lithium-sulfur battery cathodes. A paper on one method is published in the journal ACS Nano, while a paper on the other is published in the Journal of the American Chemical Society.
Lithium-sulfur batteries offer a high theoretical capacity of 1673 mAh g-1—about five times that of current commercial cathodes. Although the voltage of Li/S cells is around 2.12.3 V (relative to Li/Li+), the very high capacity and low cost overcome this limitation. However, despite these attractive properties, Li/S batteries suffer from poor cyclability.
After about 50 charge cycles, the energy density of a lithium-sulfur battery decreases rapidly, mainly due to the dissolution of intermediate lithium polysulfide products Li2Sn (4 ≤ n ≤ 8), volumetric expansion and the poor conductivity of sulfur and polysulfide species.
In one approach to combat this problem and stabilize the sulfur, the researchers used amylopectin, a polysaccharide that is a main component of corn starch. An amylopectin-wrapped graphene oxide-sulfur (GO-S-Amy) composite was prepared to construct a 3-dimensionally cross-linked structure.
With the help of this cross-linked structure, the sulfur particles could be confined much better among the layers of graphene oxide and exhibited significantly improved cyclability, compared with the unwrapped graphene oxide-sulfur composite.
With the help of this 3-dimensionally cross-linked structure, the Li/S battery exhibited much improved cycling stability and Columbic efficiency compared with the conventional sulfur electrodes and unwrapped composite. From the comparison of STEM images and EDX data, the branched amylopectin wrapped GO-S successfully confined the sulfur particles among the GO layers, which helps to tether the polysulfides during the charge/discharge processes. Different sulfur loading electrodes were tested and compared as well; a lower sulfur loading electrode showed better capacity and efficiency rela- tive to the higher sulfur loading electrode. While slight capacity fading presents in these and premier studies, we believe that these results provide reliable insights and novel concepts for future Li/S batteries.—Zhou et al. 2013a
In another approach to improving lithium-sulfur battery durability, the researchers reported the synthesis of a polyaniline−sulfur yolk−shell nanocomposite (S-Pani) through a heating vulcanization of a polyaniline−sulfur core−shell structure.
|Two-step synthesis route for a S−Pani composite, with the yellow sphere representing sulfur, the dark green shell representing polyaniline, and the black shell representing vulcanized polyaniline. Credit: ACS, Zhou et al. 2013b. Click to enlarge.|
They observed that this heating treatment was much more effective than chemical leaching to prepare uniform yolk−shell structures. Compared with its sulfur− polyaniline core−shell counterparts, the yolk−shell nanostructures delivered much improved cyclability owing to the presence of internal void space inside the polymer shell to accommodate the volume expansion of sulfur during lithiation.
The yolk−shell material exhibited a stable capacity of 765 mAh g−1 at 0.2 C after 200 cycles.
Similar sulfur-polyaniline composites have previously been synthesized in a “core-shell” structure, but the new method provides an internal void within the polymer shell, called a “yolk-shell” structure.
The polyaniline coating, which chemically is a benzene ring with ammonium and strung into cross-linked chains, is also soft and flexible, and can protect against the “shell” cracking during long cycling.
Provisional patents for these innovations have been filed through the Cornell Center for Technology, Commercialization and Enterprise. A Cornell startup company called Lionano Inc. is now commercializing these technologies for the marketplace.
Lionano is also commercializing advanced lithium-ion battery anode materials with three times the capacity, four times the durability, three times the charging rate and one-fifth the cost of current anode materials.
The papers were supported by the Department of Energy and the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the US Department of Energy (DOE). The researchers used the electron microscopy facility of the Cornell Center for Materials Research with support from the National Science Foundation Materials Research Science and Engineering Centers program.
Weidong Zhou, Hao Chen, Yingchao Yu, Deli Wang, Zhiming Cui, Francis J. DiSalvo, and Héctor D. Abruña (2013a) “Amylopectin Wrapped Graphene Oxide/Sulfur for Improved Cyclability of Lithium–Sulfur Battery,” ACS Nano doi: 10.1021/nn403237b
Weidong Zhou, Yingchao Yu, Hao Chen, Francis J. DiSalvo, and Héctor D. Abruña (2013b) “Yolk–Shell Structure of Polyaniline-Coated Sulfur for Lithium–Sulfur Batteries,” Journal of the American Chemical Society doi: 10.1021/ja409508q