What causes the aging in lithium-sulfur batteries?

As the transformation to electromobility gathers pace, so too does the search for alternatives to the lithiumion batteries used today. One candidate is the lithiumsulfur battery. Its advantages include the fact that sulfur is inexpensive, environmentally friendly and abundant. Moreover, the theoretical energy density of these cells is considerably higher than that of current lithium-ion batteries. On the other hand, the lithiumsulfur battery is not yet able to reach its maximum potential capacity and lifetime. To find out why, researchers at PTB developed a measuring system that can analyze the battery during charging and discharging. This work revealed one possible reason for the undesired aging that lowers battery capacity: the formation of chain-like molecules of lithium and sulfur (so-called polysulfides) that accumulate at the negative electrode, steadily reducing the amount of lithium and sulfur available for energy storage. Using two ultramodern techniques based on X-rays produced at the BESSY II synchrotron radiation source in Berlin, the formation, movement and accumulation of these polysulfides were analyzed and linked to the state of charge. The measurement results suggest that the development and use of polysulfide-impermeable separators can increase the lifetime of these batteries.