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.
Materials researchers of the Paul Scherrer Institute PSI in Switzerland have, in collaboration with the Université Grenoble Alpes (France), developed a simple method that can improve the performance of lithium-sulfur batteries by 25-30%. In a study published in the journal Nature Energy, the team reported that the additional of silicon dioxide (SiO2, quartz) powder to the liquid electrolyte slows the rapid capacity loss that can plague lithium-sulfur batteries.
The lithium-sulfur battery is considered a promising candidate for future high-energy storage devices. The materials required are inexpensive, environmentally friendly, and readily available, and the battery theoretically can deliver around three times as much energy as today’s widely used lithium-ion battery. In practice, however, there are still several hurdles. For example, the lithium-sulfur battery rapidly loses capacity with repeated charging. Present-day prototypes manage far fewer charging cycles than conventional lithium-ion batteries – and besides that, they deliver only a fraction of the theoretically possible energy.
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.
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.
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.