Cycling performance of Li–S cells with and without SiO2 electrolyte additive. Evolution of the specific charge (squares) and the Coulombic e ciency (stars) as a function of the number of cycles for Li–S cells cycled with (green symbols) and without (black symbols) fumed SiO2 electrolyte additive. Conder et al. Click to enlarge.

Electrochemistry researchers at PSI gained new insights into the processes responsible for the capacity loss by developing a special examination method using X-rays to track the chemical reactions that take place within the battery. They made directly visible, for the first time, the way lithium-sulfur compounds change and how this leads to the loss of capacity.

The team also observed, for the first time, how ordinary quartz powder (SiO₂)—the principal constituent of sand and the main ingredient of glass—improves the lithium-sulfur battery. Other researchers had previously already determined that quartz powder interacts with the materials in lithium-sulfur batteries. Now the PSI researchers have quantified the gains that quartz powder can deliver.

With this additive, a lithium-sulfur battery’s performance is improved by 25 to 30 percent.We simply added the quartz powder to the electrolyte—that is, the liquid component of the battery—like adding washing powder to the laundry.

—PSI researcher Claire Villevieille, co-author

polysulfides form during the operation of a lithium-sulfur battery. These are a normal constituent of an operating lithium–sulfur battery. A portion of them, though, will be lost to the battery’s liquid component and then travel back and forth between its two electrodes with every charging and discharging cycle—the polysulfide shuttle effect. As a consequence, these rogue polysulfides react with the lithium electrode of the battery, thereby reducing the amount of available sulfur—the active material in the battery—and diminishing the battery’s capacity.

This process can be counteracted through the addition of quartz powder. The researchers found that the quartz binds the polysulfides the way soap binds dirt, Villevieille explained. This increases and preserves the charging capacity, because the interior of the battery stays clean and functional for a longer time. The reversibility of the discharging process—the Coulombic efficiency—improves from around 80 percent to 90 percent, Villevieille said. By comparison, however, the Coulombic efficiency of a conventional lithium-ion battery is more than 99.9 percent.

The positive effect of the quartz was revealed when the PSI researchers, in cooperation with a colleague at the Université Grenoble Alpes, examined the chemical processes within the battery using operando X-ray diffraction. Ordinarily, liquids cannot be observed using this technique, and therefore the processes in the electrolyte remain hidden.

X-ray diffraction works only on ordered, crystalline structures; the polysulfides in the electrolyte, however, normally move around in a disordered fashion, explained Villevieilles's colleague Joanna Conder, first author of the study. To make the polysulfides visible, the researchers added glass fibers to the electrolyte. The polysulfides settled on the surface of the fibres in an orderly manner.

Aligned in this way, the polysulfides diffract the X-rays and thus become visible. This enabled us for the first time to track the accumulation and transformation of the polysulfides inside the battery during charge and discharge.

—Joanna Conder

Unexpectedly, the researchers found that the glass fibers reduced the unwanted accumulation of sulfides. Since glass consists mainly of quartz, the idea to begin using quartz powder as a kind of cleaning agent in the batteries was evident.

The two PSI researchers acknowledge that there are, in principle, other approaches by which polysulfides could be prevented from dissolving and contribute to limiting the battery function. However, these are either very complicated or very expensive or both, especially when the method needs to be implemented on an industrial scale. Quartz, in contrast, is just about the cheapest material there is, Conder said.

Resources

• Joanna Conder, Renaud Bouchet, Sigita Trabesinger, Cyril Marino, Lorenz Gubler and Claire Villevieille (2017) “Direct observation of lithium polysulfides in lithium–sulfur batteries using operando X-ray diffraction” Nature Energy doi: 10.1038/nenergy.2017.69