Scientists from Gwangju Institute of Science and Technology (GIST), Korea, have found that a new catalyst material can improve lithium–sulfur battery life significantly. A paper on the work is published in the journal ChemSusChem.

Unlike conventional Li-ion batteries (LIBs), the Li-sulfur battery (LSB) reaction pathway leads to an accumulation of solid lithium sulfide (Li₂S₆) and liquid lithium polysulfide (LiPS), causing a loss of active material from the sulfur cathode and corrosion of the lithium anode. To improve battery life, scientists have been looking for catalysts that can make this degradation efficiently reversible during use.

The GIST research team, led by Prof. Jaeyoung Lee, introduced cobalt oxalate (CoC₂O₄) as an electrochemical catalyst in a lithium-sulfur battery and succeeded in identifying the electrochemical catalyst reaction during the charging and discharging process.

While looking for a new electrocatalyst for the LSBs, we recalled a previous study we had performed with cobalt oxalate (CoC₂O₄) in which we had found that negatively charged ions can easily adsorb on this material’s surface during electrolysis. This motivated us to hypothesize that CoC₂O₄ would exhibit a similar behavior with sulfur in LSBs as well.

—Prof. Jaeyoung Lee

The team used a LSB cell with a dual-layer structure for cycle performance testing. The dual-layer cathode structure had already demonstrated advantages in the team’s prior previous studies. In this approach, a carbon layer is placed directly on the conventional sulfur cathode.

The carbon interlayer serves as a filter to trap the diffused LiPS physically, and also as an additional electrode in which the retained LiPS undergoes further electrochemical reaction. This improves the specific capacity by enhancing the sulfur utilization rate and long-term cycle stability.

In the current study, the team fabricated and tested an LSB cell using a carbon interlayer with 15 % CoC₂O₄ placed between the separator and the cathode containing CoC₂O₄.

CoC₂O₄’s ability to adsorb sulfur allowed the reduction and dissociation of Li₂S₆ and LiPS. Further, it suppressed the diffusion of LiPS into the electrolyte by adsorbing LiPS on its surface, preventing it from reaching the lithium anode and triggering a self-discharge reaction. These actions together improved sulfur utilization and reduced anode degradation, thereby enhancing the longevity, performance, and energy storage capacity of the battery.

Long-term cycle performance and Coulombic efficiency of 15% CoC₂O₄-containing carbon layer on a sulfur cathode with CoC₂O₄ under various current densities. Source: Kim et al.

In addition, it was confirmed that cell performance continued without performance degradation due to self-discharge even if the battery was left for about a week at about 1.5 times the level of conventional lithium-sulfur batteries.

Resources

• J. W. Kim, G. Seo, S. Bong, J. Lee, (2021) “Improved Redox Reaction of Lithium Polysulfides on the Interfacial Boundary of Polar CoC₂O₄ as a Polysulfide Catenator for a High‐Capacity Lithium‐Sulfur Battery” ChemSusChem 14, 876 doi: 10.1002/cssc.202002140