Scientists have, for the first time, watched how liquid electrolytes move and interact inside working lithium-sulfur (Li-S) batteries—without opening or damaging them.
Using a technique called operando neutron tomography, a research team at Helmholtz-Zentrum Berlin (HZB) was able to see, in real time, how electrolytes spread, shift, and wet the electrodes during charging, discharging, and resting.
The findings, published in Advanced Energy Materials, could help design lighter, more powerful Li-S batteries for use in aerospace, robotics, and long-range electric vehicles.
Lithium-sulfur batteries are considered one of the most promising next-generation energy storage technologies.
They can achieve extremely high energy densities—more than 700 watt-hours per kilogram, compared to around 250 Wh/kg for today’s lithium-ion batteries.
They also use sulfur, an abundant and low-cost material, instead of critical metals like cobalt and nickel. This makes them attractive not just for their performance, but also for their potential to reduce supply chain risks.
However, turning that theoretical potential into real-world performance is challenging. One of the biggest issues lies in the electrolyte—the liquid that moves lithium ions between the electrodes.
In commercial battery packs, a significant portion of the weight comes from this liquid.
To make Li-S batteries lighter and more energy-dense, manufacturers try to use less electrolyte.
But with less liquid, it’s harder to completely wet the electrodes, and dry spots can cause the battery to age quickly or even fail.
The problem is that until now, researchers couldn’t easily see what was happening inside a sealed battery. “It’s extremely difficult to observe electrolyte wetting without cutting the battery open,” explains HZB chemist Prof. Yan Lu, who led the study.
To solve this, Lu’s team designed special multilayer Li-S pouch cells at HZB’s Pouch Cell Assembly Lab, using an industry-relevant design and a small amount of electrolyte.
Then, in collaboration with imaging experts Dr. Ingo Manke and Dr. Nikolay Kardjilov, they used neutron tomography at the Institut Laue-Langevin in France to watch the electrolyte in action.
Neutrons are especially good at detecting light elements like lithium and hydrogen, making them ideal for this kind of study.
The images revealed something remarkable: during resting phases, unwetted areas appeared and shifted around, showing that the electrolyte was redistributing itself. Short rests improved wetting, but longer rests made little difference.
Charging and discharging cycles, on the other hand, made the electrolyte more evenly distributed and activated the sulfur, boosting the battery’s capacity.
The team even discovered a unique “breathing” behavior—electrolyte wetting patterns that expanded and contracted in sync with the chemical reactions inside the battery. This is caused by sulfur compounds dissolving and re-precipitating, a process very different from what happens in conventional lithium-ion cells.
“These observations give us new clues about why Li-S batteries age and fail,” says Yan Lu. “With this knowledge, we can design cells that use less electrolyte, weigh less, and still last longer.”
