New lithium-sulfur battery chemistry boosts energy storage by 58%

New battery chemistry could help lithium-sulfur batteries store more energy

Lithium-sulfur batteries are an experimental alternative to today’s lithium-ion batteries that could store considerably more energy for their weight but have proved difficult to make durable and practical.

Now, researchers at Vanderbilt University and the University of Maryland have demonstrated a new approach that could push their energy-storage potential further.

In a study published in Nature Energy, the team developed a battery chemistry that allows sulfur to participate in an additional energy-storing reaction. The result is a sulfur-based battery that operates at a higher average voltage while also storing more charge.

De-en Jiang, the H, said:

The traditional way of thinking about lithium-sulfur batteries leaves some of sulfur’s redox power untapped,

Eugene McBrayer Professor of Chemical Engineering and Professor of Chemistry. He is also a co-corresponding author of the study. “We wanted to see whether we could expand sulfur’s redox capability and use it to store more energy.”

Getting more energy out of sulfur

Sulfur has long attracted battery researchers because of its abundance and lower cost. Plus, it can store far more charge by weight than the materials commonly used in lithium-ion battery cathodes.

The challenge has been that most lithium-sulfur batteries operate at a relatively low voltage. And the amount of energy a battery can store depends on both how much charge it holds and the voltage at which it operates.

The researchers found a way to improve both functions. In a conventional lithium-sulfur battery, each sulfur atom can exchange two electrons as the battery charges and discharges. The new system extends that chemistry by introducing chlorine, allowing sulfur to exchange a third electron.

That additional reaction also takes place at a higher voltage. In laboratory tests, the new chemistry increased sulfur’s charge-storage capacity by about 58% and raised the battery’s average operating voltage from about 2.05 volts to 2.54 volts compared with standard lithium-sulfur chemistry tested under the same conditions. The combination of the higher charge-storage capacity and the higher voltage allowed one kilogram of sulfur to store more than 1,700 watt-hours of energy in their experimental cell.

Designing an electrolyte to make it work

Making the new reaction reversible presented another challenge. The team needed chlorine to interact with sulfur during charging without allowing the resulting sulfur-chlorine compound to travel through the battery and react with its lithium-metal electrode.

Researchers used molecular simulations to understand how lithium and chloride ions behave inside different electrolytes. Those calculations led to the design of an electrolyte in which chloride remains available to react with sulfur.

The team also selected an electrolyte in which the sulfur-chlorine reaction product does not readily dissolve. That helps keep it where it belongs—inside the battery’s positive electrode—so the reaction can be reversed when the battery discharges.

Experiments using several different spectroscopy techniques confirmed that sulfur repeatedly moved through the newly accessible chemical state as the cells were charged and discharged.

An early demonstration

The researchers ultimately built a small, single-layer pouch cell to show that the chemistry could work outside the coin cells commonly used for early battery experiments. The pouch cell retained 78% of its initial capacity after 100 charging cycles.
Considerable work remains before the approach could be used commercially. The prototype uses a lithium-metal electrode, relatively modest amounts of sulfur and more electrolyte than would be desirable in a commercial battery.

But Jiang said the larger significance is the possibility of expanding the chemistry available to sulfur batteries rather than simply improving the conventional reaction.

The researchers estimate that, after accounting for major battery components rather than the electrode alone, the approach could enable a stack-level specific energy of 477 watt-hours per kilogram, about 37% higher than the conventional lithium-sulfur system used for comparison in the study.

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