While lithium-ion batteries (LIBs) remain the most widely used rechargeable batteries worldwide, energy engineers have been testing various alternatives with different underlying chemistries. These include lithium-sulfur batteries, which store and release energy by moving lithium ions between two electrodes on opposite sides of a cell, while sulfur undergoes reactions in the cathode (i.e., positive electrode).

Lithium-sulfur batteries could offer several advantages, including high energy densities and lower production costs, because sulfur is abundant and can store significant charge relative to its mass. Despite their potential, these batteries often exhibit low operating voltages (i.e., the pressure driving current), slow electron-transfer reactions, and energy losses caused by the migration of sulfur compounds between electrodes.

Researchers at the University of Maryland, Vanderbilt University, the Brookhaven National Laboratory, and other institutes recently introduced a new ionic liquid electrolyte that could improve the performance of lithium-sulfur batteries. This electrolyte, presented in a paper published in Nature Energy, was found to increase both the voltage and energy storage of lithium-sulfur batteries.

“Our starting question was simple: could we get more energy from sulfur, an abundant and inexpensive material?” Prof. Chunsheng Wang, senior author of the paper, told Tech Xplore. “Earlier work in my group on chlorine- and bromine-based battery materials inspired us to use halogens to oxidize sulfur at a high potential, further increasing the energy density.”

Overcoming the limits of conventional lithium-sulfur batteries

In conventional lithium-sulfur batteries, every sulfur atom gains or releases two electrons at the cathode during charging and discharging, while these electrons flow through the battery’s external electrical circuit. Prof. Wang and his colleagues wanted to make a third electron available for transfer, thereby allowing the same amount of sulfur to store more charge at a higher voltage.

“Crucially, this extra reaction needed to work repeatedly without continually consuming the liquid inside the battery,” explained Dr. Nan Zhang, the paper’s first author and postdoctoral researcher in Dr. Wang’s group.

As part of their study, the researchers tested various electrolytes with different proportions of lithium salt and an ionic liquid containing chloride. Professor De-en Jiang and his PhD student Jinyi Zhang at Vanderbilt University then ran computer simulations to model the movement and interactions of atoms and molecules in batteries with these electrolytes. Ultimately, the team identified the best-performing electrolyte and used it to design a lithium-sulfur battery with a different internal chemistry.

“We combined lithium metal on one side of the battery with sulfur, porous carbon, and lithium chloride on the other,” said Dr. Jijian Xu. “We designed it so that chloride ions, a charged form of chlorine, could react with sulfur during charging to form disulfur dichloride. This opens up the additional energy-storing reaction.”

Notably, the disulfur dichloride produced while lithium-sulfur batteries are charging does not mix readily with the team’s electrolyte. This helps keep the compound within the porous carbon cathode, preventing it from shuttling across the battery and reacting with lithium metal, which would waste stored energy.

“We tested how much charge the batteries could store, how they performed at different charging and discharging speeds, and how they performed over repeated use,” said Prof. Wang. “We also tested a flat, pouch-shaped prototype. X-ray and laser-based measurements tracked the chemical changes. Computer calculations and simulations, including density functional theory (DFT) and molecular dynamics (MD), helped us understand the reactions and how the electrolyte worked at the atomic level.”

Towards lithium-sulfur batteries with higher voltages and capacities

In initial evaluations, the team’s proposed electrolyte and battery chemistry achieved very promising results. Their design successfully produced a redox reaction involving three instead of two electrons per sulfur atom, boosting the resulting battery’s capacity and average operating voltage.

“Our main achievement is getting more energy from sulfur by using this additional reaction, not simply by putting more sulfur into the battery,” said Prof. Wang. “In our tests, the charge delivered per gram of sulfur increased by about 58%, and the average voltage rose from 2.05 to 2.54 volts, compared with the conventional lithium-sulfur battery used in our study.”

The researchers also estimated that increasing the sulfur cathode’s areal capacity (i.e., the amount of charge stored per unit area) could improve the battery’s stack-level specific energy. They used their approach to create an initial pouch-shaped prototype and assessed its performance in the lab.

“The pouch-shaped prototype retained 78% of its original charge-storage capacity after 100 charge-and-discharge cycles,” said Prof. Wang. “The sulfur-carbon materials delivered more than 1,700 watt-hours per kilogram.”

In the future, the team’s proposed battery chemistry and the electrolyte they introduced could contribute to the realization of more reliable lithium-sulfur batteries with high capacities and higher voltages. Meanwhile, Prof. Wang and his colleagues plan to continue refining their electrolyte’s underlying chemistry to achieve even better results.

“These findings demonstrate that high-valent sulfur redox is viable, opening a new route toward ultra-high-energy-density batteries,” added Prof. Wang. “We now want to improve the electrolyte so the battery lasts longer and works better at faster charging and discharging speeds. We also want to reduce the unwanted reactions that gradually consume this liquid. The next challenge is to turn the chemistry we have demonstrated into a more practical rechargeable battery.”

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Ingrid Fadelli

Freelance journalist with BSc Psychology and MA International Journalism. Covers AI, robotics, neuroscience, and astrophysics since 2018.

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Robert Egan

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