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New cathode material for Li-S batteries: sulfur-anchored azulene

A team of researchers from China and Germany have used azulene—a benzene-free and vinyl-free molecule—to polymerize with sulfur to create a cathode material for Li-S batteries. As described in an open-access paper in the RSC journal Chemical Communications,the polymer exhibits high sulfur content and offers longer lifetime stability compared to pure sulfur, providing new protocols to develop new cathode materials for Li-S batteries.

Chen

Cycling performance of Az-S and pure sulfur at 0.3 C. Chen et al.


Among all of the lithium metal batteries, lithium-sulfur (Li-S) battery, using low cost sulfur as cathode with a theoretical energy density of ~2500 Wh kg-1 and 2800 Wh L-1, shows great potential … However, sulfur as cathode material for batteries faces great challenges, including low conductivity, large volumetric expansion (~80%) upon fully charge and discharge, and the dissolution of lithium polysulfide. Tremendous research has focused on development of new compatible host materials as physical barriers to trap the lithium polysulfide species, such as carbon- based materials, conducting polymers, metal oxides and sulfides, etc. Despite the fact that the physical hosts can partially solve the problems mentioned above, it is unavoidable that sulfur escapes the host material due to weak physical adsorption. Another possibility are sulfur-containing polymers or organosulfur polymers, with covalent bond linkage between host and sulfur, which also have drawn a lot of attention.

… Although some sulfur-containing polymers with short -S2-4- chains can avoid “polysulphide-shuttling”, the low sulfur content still limits the performance of Li-S cathodes. Hence, it is urgent to develop new organosulfur polymers with high sulfur content as candidates for Li-S batteries. The reactions of organohalides with sodium polysulfide and sulfur with dithiols seem to be the most effective routes to obtain organosulfur polymers. Furthermore, many reports have focused on inverse vulcanization as a solvent-free reaction to synthesize high sulfur content polymers.

… However, most of these reported molecules are aromatic molecules with alkene and alkyne side groups, which limit the diversity of sulfur-containing materials in the field of Li-S batteries. Herein, azulene, without benzene ring, alkene and alkyne side groups, is used as a new monomer towards an organosulfur polymer for Li-S battery.

—Chen et al.

Azulene and its derivatives have been applied widely in materials science, although little work has been done with azulene and sulfur for Li-S batteries.

The researchers report that the as-produced azulene-based organosulfur polymer exhibits high sulfur content of ~67 wt% and shows typical electrochemical activity for Li-S batteries.

Proof-of-concept Li-S cell with Az-S as cathode material exhibited initial discharge capacity of 1036 mAh g-1 with high Coulombic efficiency. After cycling for 100 cycles, the capacity remained at 515 mAh g-1. Compared with the pure sulfur cathode, the capacity and cycling stability shows an obvious improvement.

Further optimization the Li-S batteries by varying the substitution of azulene is still under investigation.

Resources

  • Zhenying Chen, Jörn Droste, Guangqun Zhai, Jinhui Zhu, Jun Yang, Michael Ryan Hansen and Xiaodong Zhuang (2019) “Sulfur-Anchored Azulene as Cathode Material for Li-S Batteries” ChemComm doi: 10.1039/C9CC04413B

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