This fundamental difference in the bond strength of the salt anions in the presence of polysulfide species leads to a large difference in the stability of the anode-electrolyte interface and performance of the Li-S batteries with electrolytes composed of these salts. Therefore, they concluded, anion selection is one of the key parameters in the search for new electrolytes for stable operation of Li-S batteries.

Background. Lithium-sulfur batteries—comprising a sulfur cathode and Li metal anode—are widely seen as a very promising next-generation electric energy storage system due to their very high theoretical specific energy (2550 Wh kg⁻¹) and energy density (2862 Wh L⁻¹). The chemistry faces some fundamental barriers—polysulfide dissolution, low sulfur utilization, large volume expansion, and low Coulombic efficiency. The polysulfide shuttle—the migration of lithium polysulfides formed during charge and discharge from cathode to anode—leads to serious self-discharge, poor efficiency and limited cycle life.

However, a great deal of progress has been made on the cathode side to accommodate sulfur species, mitigate the dissolution of polysulfides, and block the shuttle effect. This, say the PNNL researchers makes addressing the stability of the Li metal anode an even more urgent challenge in the quest for long-term stability for Li-S batteries.

The study. To determine the influence of electrolytes in lithium-sulfur batteries, the team did experiments with both LiTFSI and a similar electrolyte, called LiFSI, which has less carbon and fluoride. After continually measuring the amount of energy that the battery held and released, the team did a post-mortem analysis to study the electrodes. They did this work using instruments at DOE’s EMSL, an Office of Science scientific user facility.

They found that salts used in the liquid in the batteries make a big difference. When LiTFSI salt is packed in the liquid, a test battery can hold most of its charge for more than 200 uses. The LiTFSI helps bind up lithium atoms and sulfur on the electrode but quickly releases them. In contrast, a similar liquid ties up the lithium and sulfur but doesn’t release it. The result is an electrode that quickly degrades; the battery fades after a few dozen uses.

They discovered that with the LiTFSI, the electrode’s lithium atoms became bound up with sulfur. The result is lithium sulfide (LiS_x) forming on the electrode’s surface. With LiFSI, lithium sulfate (LiSO_x) formed. By calculating the strength with which the compounds clung to the lithium, they found that the lithium sulfide easily broke apart to release the lithium. However, the lithium sulfate was hard to separate. The oxygen in the lithium sulfate was the culprit.

By conducting a macroscopic compositional analysis combined with simulations, we can see which bonds are easily broken and what will happen from there. This process lets us identify the electrolytes behavior, guides us to design a better electrolyte, and improve the cycle life of lithium-sulfur batteries.

—Dr. Ji-Guang (Jason) Zhang, study leader

The next step for the researchers is to develop an electrolyte additive that forms a protective layer on the lithium anode’s surface, protecting it from the electrolyte.

This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy, Office of Science, Basic Energy Sciences.

Resources

• Cao, R., Chen, J., Han, K. S., Xu, W., Mei, D., Bhattacharya, P., Engelhard, M. H., Mueller, K. T., Liu, J. and Zhang, J.-G. (2016), “Effect of the Anion Activity on the Stability of Li Metal Anodes in Lithium-Sulfur Batteries.” Adv. Funct. Mater. doi: 10.1002/adfm.201505074