Researchers from the US Department of Energy's (DOE) Argonne National Laboratory are moving closer to solving the lower energy density issue impeding the broad use of Li-S batteries by creating a novel electrolyte additive. The research was published in the journal Joule.
A schematic depiction of the synchrotron X-ray experiment used in this research at the APS to study the Li-S battery cell. Image Credit: Argonne National Laboratory/Guiliang Xu
Lithium-ion (Li-ion) batteries are used in everything from electric cars to laptops and cell phones. As the world becomes more electrified, scientists worldwide are battling to create even better “beyond Li-ion” batteries, even though Li-ion batteries have been a huge success.
Commercial Li-ion batteries use relatively costly materials like cobalt and nickel compounds. They are also highly dependent on unstable supply chains and have a lower energy density than alternative batteries.
Lithium-sulfur (Li-S) batteries, which have a sulfur cathode and a lithium metal anode, are among the more promising substitutes for Li-ion batteries. This electrode combination uses readily available resources on Earth and promises two to three times higher energy densities and lower costs.
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.
Guiliang Xu, Chemist, Argonne National Laboratory
However, these batteries have drawbacks, such as a short cycle life caused by undesired polysulfide ion migration and the system's uneven chemical reaction distribution and occurrence.
Lithium ions are kept in the gaps between the cathode material layers in Li-ion batteries, and they alternate between the cathode and anode when the battery is being charged and discharged.
However, Li-S batteries work differently. In these cells, lithium ions travel chemically between the cathode and anode. The cathode's elemental sulfur is transformed into polysulfide compounds, which are made up of chains of sulfur atoms, some of which dissolve in the electrolyte.
This solubility causes a “shuttling” effect, in which the polysulfides move back and forth between the cathode and the anode. This shuttling limits the battery's overall cycle life and performance, as material from the sulfur cathode is lost because it is deposited at the anode.
Several approaches have been proposed to address polysulfide shuttling and other issues. Because of its chemical reactivity with the sulfur cathode and other battery components, one such tactic using an additive in the electrolyte has long been believed to be incompatible.
Guiliang Xu, a Chemist at Argonne, and his colleagues have developed a novel class of additives and discovered that they can enhance battery performance. By manipulating the additive's reaction with sulfur compounds, researchers can better create the cathode-electrolyte interface required to enable 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. The key is to have a minor reaction to form the film, without a continuous reaction that consumes the material and reduces energy density.
Guiliang Xu, Chemist, Argonne National Laboratory
By creating a film on the anode and cathode, the additive reduces the shuttle effect, increases cell stability, and encourages an ion transport “highway” across the electrode. This electrolyte design also improves reaction homogeneity and reduces sulfur dissolution, making it possible to use previously thought to be incompatible additives.
To verify the idea, the researchers compared their electrolyte with the additive to a standard electrolyte used in Li-S batteries. They highlighted a notable decrease in the production of polysulfides. X-ray techniques confirmed that the new electrolyte exhibited very low dissolution of polysulfides. They also monitored the behavior of the reactions when the batteries were being charged and discharged.
Utilizing Argonne’s Advanced Photon Source (APS) and Brookhaven National Laboratory’s National Synchrotron Light Source II, both DOE Office of Science user facilities, these experiments verified that the electrolyte design reduced the dissolution and formation of polysulfides.
Synchrotron techniques provide powerful tools for characterizing battery materials. 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.
Tianyi Li, Beamline Scientist, Argonne National Laboratory
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 significant issue for Li-S batteries is the stability of the lithium metal, which reacts readily and raises safety issues. To ensure the safety of Li-S batteries, Xu and his team are working on improving electrolytes that will stabilize the lithium metal and reduce the electrolyte's flammability.
X-ray absorption spectroscopy was performed at the APS using Beamline 20-BM to examine the solubility of polysulfide. X-ray diffraction imaging was performed using Beamline 17-BM to investigate the homogeneity or heterogeneity of the entire cell. Beamline 2-ID was utilized for X-ray fluorescence mapping to observe the migration of sulfur in conventional electrolytes and to verify the electrode material's solubility.
Additional authors 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 Vehicle Technologies Office of DOE’s Office of Energy Efficiency and Renewable Energy supported the research.
Journal Reference:
Zhao, C., et al. (2024) Polysulfide-incompatible additive suppresses spatial reaction heterogeneity of Li-S batteries. Joule. doi.org/10.1016/j.joule.2024.09.004.