Modus operando: Better batteries with X-rays

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  • Published: May 15, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Modus operando: Better batteries with X-rays

Quartz cleanup

PSI researcher Claire Villevieille, head of the Battery Materials Group, at the instrument for X-ray diffraction. Credit: Paul Scherrer Institute/Markus Fischer

Operando X-ray diffraction has been used to elucidate the role of SiO2 (quartz powder) added to the electrolyte (liquid component) in lithium-sulfur battery. Li-S battery is a promising future energy storage system because of it energy density higher than that of conventional lithium-ion batteries. However to date, this system suffers from quickly declining capacity.

Scientists from Paul Scherrer Institute in Switzerland supported by their colleague from Université of Grenoble Alpes, France, have used well known X-ray diffraction (XRD) technique to gain insight into the rapidly declining capacity in lithium-sulfur batteries, a factor that has so far precluded the use of such batteries in electric vehicles and other applications. In their work recently published in Nature Energy, they explain that the use of glass fibres contrained by the special cell design used in XRD experiments led to the idea of adding the quartz powder to the electrolyte as a capacity loss retardant.

Cycling booster

Operando X-ray diffraction allowed the team to observe directly, for the first time, the formation and evolution of lithium-sulfur compounds, lithium polysulfides, during battery operation. Moreover they show that adding ordinary quartz powder, essentially laboratory-grade sand, increases the available energy and reduces the capacity loss. Previous researchers had demonstrated similar effect with silica, but the new research quantifies the gains delivered by adding quartz to the electrolyte. The gain in capacity amounts to 25 to 30 percent, says PSI researcher Claire Villevieille.

Villevieille compares the addition of silica to the electrolyte to an addition of laundry detergent. Indeed, the quartz powder binds to the polysulfides that are formed during the operation of such batteries. The polysulfides are present from the moment the battery is assembled. With each charge-discharge cycle a proportion of these sulfur-based compounds that normally shuttle in the electrolyte between the two electrodes loses their way and react with the lithium electrode, forming solid sulfides. This reduces the amount of available sulfur, quickly lowering the battery's capacity.

The quartz binds the polysulfides precluding their reactions with the lithium electrode, essentially keeping the interior of the battery "clean" and, thus, prolonging its life. The reversibility of the charge/discharging processes is, therefore, improved, explains Villevieille, and the Coulombic efficiency is enhanced from 80 to 90 percent. Of course, the Coulombic efficiency of conventional lithium-ion batteries is more than 99.9 percent, so there is still room for improvement for lithium-sulfur batteries before they can compete with the conventional technology. Yet, the material scientists are getting closer to this value.

Crystalline liquids

In theory, X-ray diffraction only allows us to determine the structure of crystalline solids and is not capable of discerning the processes taking place in a liquid, in the present case in the electrolyte. Team member Joanna Conder explains that in order to make the polysulfides visible to X-rays, the researchers had initially added glass fibres to the electrolyte, providing an ordered platform on which these materials can "crystallize". This facilitated tracking of formation and evolution of the polysulfides inside the battery during the charge and discharge processes, Conder explains. By a chance, they found out that the presence of the glass fibres in the Li-S battery reduces the unwanted accumulation of sulfides and, hence, they came up with the idea of simply adding quartz powder to the electrolyte to bind the polysulfides. 

There are of course other methods by which the polysulfides might be prevented from reacting with the lithium electrode. However, these are complicated to implement and/or expensive upon upscaling. Sand is cheap and as such, quartz powder. Adding it to the electrolyte is easy. This could be the big strengths of this approach as further improvements are made and lithium-sulfur batteries become a viable higher-capacity alternative to conventional rechargeable batteries.

Related Links

Nature Energy, 2017, 2, 17069: "Direct observation of lithium polysulfides in lithium–sulfur batteries using operando X-ray diffraction"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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