Researchers at Chalmers University of Technology (Sweden) with colleagues at Gyeongsang National University (Korea) have used an ionic liquid (Py_1,4TFSI) as an electrolyte additive to enable longer cycle-life of Li/S batteries.

By tuning the concentration of the ionic liquid (IL), they demonstrated an enhanced stability of the SEI and lower flammability of the solutions—i.e. higher safety of the battery.

A Li/S cell built at high sulfur mass loading (4 mg cm^-2) using the IL-based electrolyte demonstrated a stable capacity of 600 mAh g^-1 for double the cycles of a cell using LiNO₃ additive (300 vs 150). Their paper appears in the journal ChemSusChem.

Lithium/sulfur batteries are attractive due to their high energy density, while being made from materials that are low in toxicity, abundant and low cost.

The Li/S battery, which is based on a conversion reaction, consists of a sulfur-containing cathode and a Li-metal anode; the reaction mechanism involves the formation of a series of Li-polysulfides (Li₂S_X, 3 ≤ X ≤8). The polysulfides, in particular the longer chains, are soluble in the common electrolytes used and risk migrating to and from the Li electrode in a “shuttle” process where they are continuously reduced and oxidized at the electrodes without contributing to the capacity.

This shuttle mechanism causes a number of problems, including loss of active material, parasitic reactions and formation of an insulating interphase on the Li-metal anode.

Some of the strategies adopted to overcome these problems have been based on the modification of the electrolyte by using additives that are able to prevent the shuttle process by the formation of a stable solid electrolyte interphase. The most common of these additives is LiNO₃. However, upon cycling, LiNO₃ is continuously consumed; LiNO₃ is thus a sacrificial salt in the electrolyte.

… the use of Py1,4TFSI as an additive is introduced to stabilize the SEI, reduce the flammability of the electrolyte and extend the cycle life of Li/S cells when using high loading sulfur/carbon composites. The ionic liquid additive suppresses the electrolyte reactivity towards Li- metal and reduce the solvent flammability ensuring higher safety characteristics. Additionally, the mitigation of sulfur deposition through the Li-polysulfide migration to the anode side has been demonstrated. … The significant effect of Py1,4TFSI for building long-life Li/S cells paves the way for bringing this technology to practical applications.

—Agostini et al.

Resources

• Agostini, M., Sadd, M., Xiong, S., Cavallo, C., Heo, J., Ahn, J. H., & Matic, A. (2019) “Designing a safe electrolyte enabling long‐life Li/S batteries,” ChemSusChem doi: 10.1002/cssc.201901770