A research team led by Claire Villevieille of the Paul Scherrer Institute used operando x-ray diffraction (XRD) to observe soluble polysulfide intermediates in a lithium-sulfur battery. This study, which expands the battery-characterization capabilities of operando XRD, was published recently in Nature Energy (doi:10.1038/nenergy.2017.69).

Lithium-sulfur batteries are a potential replacement for the ubiquitous lithium-ion batteries that are found in cell phones, computers, and electric cars. That is, if the formation of parasitic polysulfide intermediates can be characterized and controlled in these batteries. “To date, we haven’t found a proper solution to the problem in lithium-sulfur batteries: when the sulfur, which is a solid, starts to be cycled in the battery, it turns into a liquid, forming various polysulfides,” Villevieille says. The continuous loss of sulfur material during this process results in decreased capacitation over time, and a reduction in lithium-sulfur battery performance. To combat this, researchers have engineered many ways to trap the polysulfide intermediates, developing trapping layers or separators to keep the intermediates on the sulfur cathode. Understanding how these intermediates form and how they interact with the materials used to contain them could lead to the development of better trapping layers.

X-ray diffraction (XRD) contour plot of the XRD patterns (right), with the corresponding galvanostatic curve (left) recorded during the first cycle of the lithium-sulfur battery. The intensity chart is given at the very right. The asterisk refers to a peak arising from a cell part. α-S₈ and β-S₈, the two polymorphs of sulfur formed at the cathode, are represented by white vertical lines with diamond and oval symbols, respectively. The positions of the peaks labeled PS1 and PS2, attributed to the polysulfide–silicon dioxide interactions, are indicated by vertical black dashed lines. Li₂S refers to the solid end-of-lithiation product lithium sulfide, and is symbolized by a dashed-dotted black vertical line. Overall, the horizontal dotted black lines are used as guides for following the changes in the peaks intensity as a function of the potential/time. Credit: Nature Energy.

“Characterizing the polysulfides, which is required to figure out ways to mitigate their formation or trap them, has been a challenge,” says Michael Toney, of the Stanford Synchrotron Light Source. Toney also uses XRD and imaging techniques to characterize electrochemical materials, but was not connected with this work. Scientists have used x-ray and UV-visible absorption spectroscopy to try to capture the formation and migration of these intermediates, but such methods lack precise characterization of their location and quantity in the cell. XRD has been used to study the structure of the solid components of the battery in great detail, but it would not have been possible to see the polysulfides, which lack long-range order when they are dissolved in the electrolyte.

Because of this, Villevieille’s group anticipated only observing the solid electrodes in the XRD experiment and seeing how changing the separator layer between them would alter their structure. They started using silica fibers as a simple separator material. They were surprised to see two unknown peaks appearing in their XRD diffractograms when they expected to see none.

“If the liquid [polysulfides] are visible,” Villevieille says, “[this means that] it’s deposited as a layer somewhere.” Further characterization of the separator using electron microscopy revealed that the polysulfides had adsorbed onto the silica fibers of the separator. Suspecting an interaction between the silica and the polysulfides, Villevieille’s group used a polymer separator. The peaks disappeared when the polymer separator was used but reappeared when fumed silica was added to the electrolyte solution. “When there is silicon dioxide, we see the signature,” Villevieille says.

Villevieille does not think that this effect is limited to silicon dioxide alone and is currently testing other oxides to scavenge the polysulfide intermediates. Her next step is to demonstrate how this technique can be used to optimize the battery cycling stability. These future measurements and others like it are “going to be valuable for informing the community on how to move forward,” Toney says.