Researchers from Western University, Canadian Light Source, and the Chinese Academy of Sciences have proposed a novel solid-phase Li-S transformation mechanism that enables high energy Li-S batteries in conventional Li-ion carbonate electrolytes. An open-access paper on their work is published in Nature Communications.

Schematic of a lithium sulfur battery in carbonate-based electrolyte. Alucone coating is applied to carbon–sulfur electrodes and the sulfur cathode is in cyclo-S₈ molecule format. Alucone thin film is directly deposited on the C–S electrodes by alternatively introducing trimethylaluminium and ethylene glycol via molecular layer deposition. Blue balls represent aluminium, green ball represent methyl, and gray balls represent hydroxyl. Li et al.

Carbonate-based electrolytes demonstrate safe and stable electrochemical performance in lithium-sulfur batteries. However, only a few types of sulfur cathodes with low loadings can be employed and the underlying electrochemical mechanism of lithium-sulfur batteries with carbonate-based electrolytes is not well understood.

Here, we employ in operando X-ray absorption near edge spectroscopy to shed light on a solid-phase lithium-sulfur reaction mechanism in carbonate electrolyte systems in which sulfur directly transfers to Li₂S without the formation of linear polysulfides. Based on this, we demonstrate the cyclability of conventional cyclo-S₈-based sulfur cathodes in carbonate-based electrolyte across a wide temperature range, from −20 °C to 55 °C.

Remarkably, the developed sulfur cathode architecture has high sulfur content (>65 wt%) with an areal loading of 4.0 mg cm⁻². This research demonstrates promising performance of lithium-sulfur pouch cells in a carbonate-based electrolyte, indicating potential application in the future.

—Li et al.

For most developed Li-S batteries, the well-known reaction mechanism is that solid sulfur first reduces to liquid long-chain polysulfides and then to solid lithium sulfide (solid-liquid dual-phase reaction). However, the well-developed carbonate electrolyte from conventional Li-ion batteries is not compatible with polysulfides since the side-reaction between polysulfides and carbonate electrolyte is fatal to Li-S batteries. Therefore, the most developed sulfur cathodes cannot be used directly in Li-ion carbonate electrolyte.

Prof. Xueliang (Andy) Sun’s team from Western University, Canada developed a novel surface modification strategy to eliminate polysulfide intermediate product in the Li-S electrochemical reaction, consisting of a molecular layer deposition (MLD) alucone coating applied to a conventional sulfur cathode.

This coating material physically separates the direct contact between the electrolyte and the sulfur cathode but still allows Li-ion transmission—resulting in high performance Li-S batteries in carbonate-based electrolyte.

Resources

• Xia Li, Mohammad Banis, Andrew Lushington, Xueliang Sun, et al. (2018) “A high-energy sulfur cathode in carbonate electrolyte by eliminating polysulfides via solid-phase lithium-sulfur transformation.” Nature Communications 9, 4509 doi: 10.1038/s41467-018-06877-9