One of the more promising alternatives to Li-ion batteries are lithium-sulfur (Li-S) batteries, which have an anode of lithium metal and a cathode of sulfur. This electrode pairing promises two to three times higher energy densities and reduced costs, while also using Earth-abundant resources.
But these batteries do not come without their own challenges, including a short cycle life due to the unwanted migration of polysulfide ions and the uneven distribution and occurrence of chemical reactions within the system.
By developing an innovative additive for the electrolyte, researchers at the U.S. Department of Energy's (DOE) Argonne National Laboratory are making progress toward addressing these problems that are limiting the widespread adoption of Li-S batteries.
In Li-ion batteries, lithium ions are stored in the spaces between layers of the cathode material and move back and forth between the cathode and anode during charging and discharging.
Li-S batteries, however, rely on a different process. In these cells, lithium ions move between the cathode and anode by a chemical reaction. Elemental sulfur from the cathode is converted into polysulfide compounds - composed of sulfur atom chains - some of which can dissolve in the electrolyte. Because of this solubility, a ?"shuttling" effect occurs, where the polysulfides travel back and forth between the cathode and the anode. This shuttling results in loss of material from the sulfur cathode because it is deposited at the anode, which limits the overall battery cycle life and performance.
Numerous strategies have been proposed to mitigate polysulfide shuttling and other challenges. One such strategy, using an additive in the electrolyte, has long been thought to be incompatible due to chemical reactivity with the sulfur cathode and other battery parts. Argonne chemist Guiliang Xu and his team have created a new class of additive and found that such additives can actually improve battery performance. By controlling the way the additive reacts with sulfur compounds, researchers are better able to create an interface between the cathode and electrolyte that is necessary to facilitate easy transport of lithium ions.
"The additive, called a Lewis acid additive, is a salt that reacts with the polysulfide compounds, forming a film over the entire electrode," Xu said. ?"The key is to have a minor reaction to form the film, without a continuous reaction that consumes the material and reduces energy density."
The additive forms a film on both the anode and the cathode, suppressing the shuttle effect, improving the stability of the cell and promoting an ion transport ?"highway" throughout the electrode. This electrolyte design also minimizes sulfur dissolution and enhances reaction homogeneity, enabling the use of additives that were previously considered incompatible.
To validate the concept, the researchers compared their electrolyte with the additive to a conventional electrolyte used in Li-S batteries. They observed a significant reduction in polysulfide formation. The new electrolyte showed very low dissolution of polysulfides, which was confirmed with X-ray techniques. Further, they tracked the reaction behavior during battery charging and discharging. These experiments made use of Argonne's Advanced Photon Source (APS) and Brookhaven National Laboratory's National Synchrotron Light Source II, both DOE Office of Science user facilities, which confirmed that the electrolyte design minimized the dissolution and formation of polysulfides.
"Synchrotron techniques provide powerful tools for characterizing battery materials," said Tianyi Li, a beamline scientist at the APS. ?"By using X-ray diffraction, X-ray absorption spectroscopy and X-ray fluorescence microscopy at the APS, it was confirmed that the new interface design effectively mitigates well-known issues including polysulfide shuttle. More importantly, this interface enhances ion transfer, which helps to reduce reaction heterogeneities."
Xu added, ?"With further optimization and development of sulfur electrodes, we believe Li-S batteries can achieve higher energy density and better overall performance, contributing to their commercial adoption."
Another major challenge for Li-S batteries is the stability of the lithium metal - it reacts easily and poses safety concerns. Xu and his team are working on developing better electrolytes to stabilize the lithium metal and reduce the flammability of the electrolyte, ensuring the safety of Li-S batteries.
At the APS, Beamline 20-BM was used for X-ray absorption spectroscopy to probe the solubility of polysulfide. Beamline 17-BM was used for X-ray diffraction imaging to explore the homogeneity or heterogeneity of the entire cell. Beamline 2-ID was used for X-ray fluorescence mapping to confirm solubility of the electrode material and to observe the migration of sulfur in conventional electrolytes.
Other contributors to this work include Chen Zhao, Heonjae Jeong, Inhui Hwang, Yang Wang, Jianming Bai, Luxi Li, Shiyuan Zhou, Chi Cheung Su, Wenqian Xu, Zhenzhen Yang, Manar Almazrouei, Cheng-Jun Sun, Lei Cheng and Khalil Amine.
The results of this research were published in Joule. The study was funded by the Vehicle Technologies Office of DOE's Office of Energy Efficiency and Renewable Energy.
Research Report:Polysulfide-incompatible additive suppresses spatial reaction heterogeneity of Li-S batteries
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Argonne National Laboratory
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