Lithium-Sulfur Batteries

11 Battery Engineering

Gregory J. Offer

Imperial College London, Exhibition Road, London, SW7 2A, UK

11.1 Mechanical Considerations

As described in the section on degradation, the volume and therefore the thickness of a lithium–sulfur pouch cell can change significantly during operation [1]. Upon a normal charge–discharge cycle a lithium–sulfur cell can vary by 5% or more, known as “breathing.” This change is driven by both the increase in the number of species dissolved in the electrolyte as the long chain polysulfides become oxidized to lower chain polysulfides, and the plating and dissolution of the lithium electrode that seems to contribute the most to irreversible thickness increase. This irreversible thickness increase as the cell degrades (i.e. loses usable capacity) can lead to a 30–40% increase in thickness at the end of life. Cells will also undergo thermal expansion as the temperature changes, which is comparable to the “breathing” caused by cycling.

From a systems engineering perspective, this presents a challenge, as space must be left to allow cells to both “breathe” during normal cycling or to account for changes in temperature, and irreversibly expand as they age. This can impact on the achievable volumetric energy density for the pack even at the beginning of life.

11.2 Thermal and Electrical Considerations

These two considerations are so intrinsically intertwined that it is not recommended to consider them in isolation. Previous work on lithium ...