Researchers develop new lithium battery

Researchers at the USC Loker Hydrocarbon Research Institute just bypassed a major roadblock in the development of a new kind of rechargeable battery — the lithium-sulfur.

Their findings, which were published in the January issue of the Journal of the Electrochemical Society, might lead to the introduction of the lithium-sulfur battery to markets as opposed to the industry standard lithium-ion battery.

Sri Narayan, the lead author of the study, has been studying batteries since graduate school, for 30 years now. However, he and his research assistant Derek Moy began working on this particular project five years ago with the goal of improving the life cycle of the lithium-sulfur battery.

“Lithium-sulfur batteries are the next step from lithium-ion in that you have about twice the energy density [than] the lithium ion or even more,” Narayan said. “Unfortunately, that technology has suffered many limitations because of the processes that happen inside the battery.”

The lithium-sulfur battery has many advantages over the lithium-ion, according to Narayan. The material it is composed of — sulfur — is common and affordable. These batteries are also smaller due to their great density, thus making them better at storing power and a worthy alternative to the larger lithium-ion batteries.

Despite these advantages, lithium-sulfur batteries are not usually seen in markets because their life cycles are inferior to those of the lithium-ions. Also, the lithium-ion batteries being sold today can be recharged as many as 1,000 times, the lithium-sulfur battery can only be recharged 50 to 100 times, making them unreliable alternatives despite their affordability and superior density.

“Lithium-sulfur can double the energy content of the lithium-ion battery in its current configuration,” Narayan said. “That means you get twice the run time, which also means that you have more energy at your disposal. But you also want cycle life. Lithium-sulfur offers you currently energy content, but does not offer you cycle life.”

However, Narayan and Moy have devised a solution to the lithium-sulfur battery’s limited life cycles — a thin, semi-permeable membrane they dubbed the “Mixed Conduction Membrane.” They placed this membrane between two porous separators, which were then placed between the two electrodes.

Narayan said that the idea for this membrane arose from the need to inhibit the movement of sulfur molecules and allow for the movement of lithium ions.

“The solution was actually in the lithium battery space,” Narayan said. “If you take the cathode materials, the positive electrode materials of the lithium ion battery, they have the ability to move around lithium ions, which we need to go between the two electrodes of the lithium-sulfur battery. But they will not let the sulfur molecules that are bad for the lithium-sulfur battery move.”

The purpose of this mixed conduction membrane is to halt polysulfide shuttling. In a normal lithium-sulfur battery, the shuttling of dissolved polysulfides between anode and cathode results in the cycle strain that has inhibited energy storage in lithium-sulfur batteries. Use of a mixed conduction membrane, on the other hand, still allows for the kind of flow of lithium ions seen in lithium-ion batteries, while harnessing all the advantages of the lithium-sulfur battery.

“The shuttling process causes some permanent changes at the other electrode when things go from one electrode to the other,” Narayan said. “The simple goal was: Can we stop this shuttling process? If we [do], then immediately we should be able to increase the life of this battery.”

Narayan and Moy found that lithium-sulfur batteries with the mixed conduction membrane had up to four times longer life compared to batteries without the membrane. This led them to believe that, once a few other issues involving the lithium electrode are fixed, this battery may possibly make its way into the market in a few years.

“The way we stopped the shuttling process is just one of the solutions,” Moy said. “People can now focus on the negative electrode, which is the next big step. If they can stabilize the negative electrode, which is the lithium electrode, then we could very well be seeing lithium-sulfur batteries on the market.”

Narayan and Moy are continuing their work with lithium-sulfur batteries, looking to study the electrodes further and make improvements to their mixed conduction membrane.