Hot or cold, Li-S batteries can take it

Engineers at the University of California San Diego have developed something many of the most extreme temperate locations in the world have long been waiting for, batteries that perform well in freezing cold and scorching hot weather while maintaining high capacity.

Described in a paper published in "Proceedings of the National Academy of Sciences," UC San Diego researchers were able to accomplish the feat of weatherized batteries by developing an electrolyte that is not only versatile and robust throughout a wide temperature range but is also compatible with high-energy anodes and cathodes.

"You need high temperature operation in areas where the ambient temperature can reach the triple digits and the roads get even hotter," said Zheng Chen, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and senior author of the study. "In electric vehicles, the battery packs are typically under the floor, close to these hot roads."

With these batteries, it could allow EVs in cold climates as well, enabling them to travel farther on a single charge, and as anyone in colder weather knows, cold and stored electricity do not mix.

As an added benefit, lithium-sulfur batteries could also reduce the need for cooling systems to keep EV battery packs from overheating in hot climates.

"Also, batteries warm up just from having a current run through during operation," continued Chen. "If the batteries cannot tolerate this warmup at high temperature, their performance will quickly degrade."

In tests, the proof-of-concept batteries retained 87% and 115.9% of their energy capacity at minus 40 and 50 degrees Celsius (minus 40 and 122 degrees Fahrenheit), respectively. They also had high Coulombic efficiencies of 98.2% and 98.7% at these temperatures, respectively, meaning the batteries could undergo more charge and discharge cycles before they would stop working.

The batteries that the team developed are both cold and heat tolerant, thanks to an innovative new electrolyte. Made from a liquid solution of dibutyl ether mixed with lithium salt, a special feature of dibutyl ether is that its molecules bind weakly to lithium ions. In other words, the electrolyte molecules can easily let go of lithium ions as the battery runs – like ionic grease.

Additionally, dibutyl ether can easily take even higher heat because it stays liquid at high temperatures with a boiling point of 141 degrees Celsius (286 degrees Fahrenheit).

Now what makes this new electrolyte special is that it is especially compatible with lithium-sulfur batteries.

Lithium-sulfur batteries

Well before today's EV surge and resultant shortage of lithium-ion battery materials, developing a commercially viable sulfur battery has been an industry goal due to sulfur's natural abundance as well as its chemical structure – more than double the weight-to-energy storage ratio than today's lithium-ion batteries.

Lithium-sulfur batteries, however, tend to lose their capacity very quickly because of a detrimental chemical reaction that occurs between cathode and electrolyte as batteries cycle. The reaction spawns polysulfides, compounds that interfere with a battery's anode and quickly cause the battery to shut down.

Researchers say lithium-sulfur battery cycling stability and slow-charging kinetics need significant improvement before they can be used in practical settings. Improving the charging and discharging functions within the battery cell is critical for promoting lithium transport while simultaneously retarding the movement of polysulfides, which limit a battery's capacity and stability.

To solve these problems, most researchers have focused on working with different electrolytes better able to cycle with sulfur or changes to the separator film keeping the two components apart.

"If you want a battery with high energy density, you typically need to use very harsh, complicated chemistry," said Chen. "High energy means more reactions are happening, which means less stability, more degradation. Making a high-energy battery that is stable is a difficult task itself-trying to do this through a wide temperature range is even more challenging."

Thus comes the dibutyl ether electrolyte developed by UC San Diego scientists, which not only prevents these issues but allows the batteries tested to have much longer cycling lives than even prototype lithium-sulfur batteries.

"Our electrolyte helps improve both the cathode side and anode side while providing high conductivity and interfacial stability," finished Chen.

Obvious next steps are scaling the technology up to viable production means, as well as continuing to optimize the chemistries to work at even higher temperatures and further extending battery life.

Once fully implemented, the chances of a lithium-sulfur battery coming to market could supplant the critical minerals and metals needed for lithium-ion batteries as lithium-sulfur batteries do not rely on metals like cobalt, nickel, and manganese.

Sulfur, an abundant element, is considered a waste or byproduct at many mining operations around the world, and while it may not be pleasant to smell, it could be quite a marketing tactic to tell consumers their EVs are powered by volcanoes.