The Search For The Perfect Solid-State Battery Continues, Self-Healing Sulfur Edition

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Solid-state battery technology seemed far off and out of sight just a few years ago, but research teams all over the world have been hot on the trail of new materials to juice EV sales, meaning higher energy density and lower costs among other improvements. Plain old sulfur is in the running, despite its tendency to fall apart during charging cycles. Nevertheless, sulfur also has advantages in the solid-state battery field, and it looks like the sulfur code has finally been cracked.

Sulfur & The Solid-State Battery Of The Future

Sulfur has been a long, tough slog for solid-state battery researchers. The lithium-sulfur formula came across the CleanTechnica radar in 2013, when we took note of a research project at Oak Ridge National Laboratory, a branch of the US Department of Energy.

“Researchers at Oak Ridge National Laboratory are hot on the trail of a lithium-sulfur battery that packs four times the punch of conventional lithium-ion batteries,” we enthused. “Given the low cost of sulfur, if the technology can be commercialized it would have a huge impact on the market for energy-storing devices powered by wind, solar or other forms of renewable energy.”

“The low cost lithium-sulfur combo could also open up the electric vehicle market,” we added.

Back in 2015 we also checked out next-generation solid-state battery research at Drexel University, involving the use of a high tech, nanoscale ceramic material. We also took note of some challenges.

“The problem is, conventional lithium-sulfur batteries don’t last very long. They start to degrade when they discharge, so their performance drops after just a few charge/discharge cycles,” we observed (see more sulfur battery news here).

The problem is two-fold. Sulfur itself is not particularly adept at conducting electrons to begin with, and its tendency to over-expand during charging cycles presents another hurdle.

Cracking The Lithium-Sulfur Solid-State Battery Code

Much water has gone under the bridge since then. The US Department of Energy has continued to devote considerable energy to new research projects that push the envelope on solid-state battery technology, including the lithium-sulfur formula.

In the latest development assisted by the Energy Department, a research team from the Jacobs School of Engineering at the University of California – San Diego has just published a lithium-sulfur code-cracking study in the journal Nature.

The team has come up with a new self-healing sulfur material for the cathode in a solid-state battery, with lithium metal serving duty as the anode material.

The new material consists of a crystalline form of sulfur, doped with molecules of iodine. “By inserting iodine molecules into the crystalline sulfur structure, the researchers drastically increased the cathode material’s electrical conductivity by 11 orders of magnitude, making it 100 billion times more conductive than crystals made of sulfur alone,” UC – San Diego explains.

That is not a CleanTechnica typo. The school’s exact words are “100 billion times more conductive than crystals made of sulfur alone,” so take it up with them if a more incremental improvement seems more likely.

“These batteries hold promise as a superior alternative to current lithium-ion batteries as they offer increased energy density and lower costs,” UC – San Diego continues. “They have the potential to store up to twice as much energy per kilogram as conventional lithium-ion batteries — in other words, they could double the range of electric vehicles without increasing the battery pack’s weight.”

“Additionally, the use of abundant, easily sourced materials makes them an economically viable and environmentally friendlier choice,” they add.

Next Steps For The Self-Healing Solid-State Battery Of The Future

In addition to leveling up the conductive performance of their sulfur cathode, the team also spotted an opportunity for the new material to overcome sulfur’s over-expansion problem.

The idea is to re-melt the cathode back into shape after the battery is charged. With some materials, that would be complicated by the high level of heat required, but the Jacobs team found that their new iodine-tweaked sulfur cathode melts at around the temperature level of a hot cup of coffee, at just 149 degrees Fahrenheit.

“This means that the cathode can be easily re-melted after the battery is charged to repair the damaged interfaces from cycling,” UC – San Diego explains. “This is an important feature to address the cumulative damage that occurs at the solid-solid interface between the cathode and electrolyte during repeated charging and discharging.”

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Speaking Of Honda…

That’s going to take some doing, so stay tuned for next steps. In the meantime, one of our favorite things to do here at CleanTechnica is to scroll down to the bottom of research news, where all the funders and partners are listed.

UC – San Diego notes that the Jacobs study, published under the title, “Healable and Conductive Sulfur Iodide for Solid-State Li-S Batteries,” is the work of many hands. In addition to the main research team based at Jacobs, and the assist from two Energy Department laboratories, several academic partners are also involved. Honda Research Institute USA makes an appearance as well.

If Honda starts to ring some bells, it should. Last December, CleanTechnica picked up on a report in AutoWeek, regarding a solid-state battery collaboration between Honda, the Ohio State University Institute for Materials and Manufacturing Research, and the firm Schaeffler Americas. The aim is to establish a new EV battery research center in 2025.

In January of 2023, OSU also announced a $3.8 million collaboration with Honda and the Energy Department’s Argonne National Laboratory, aimed at developing “more affordable and efficient electric vehicle batteries.”

Despite the focus on EV battery materials research, Honda also reminds everyone that new battery technology does not exist in a vacuum. The company emphasizes that mass production capability and integration with its electric vehicles are essential factors, too.

“It is possible to develop a small battery while focusing on the performance of materials rather than restrictions of mass-production methods; however, for commercialization, it is important to develop batteries while envisioning future mass-production methods,” the company explains.

In particular, Honda has been exploring new processes for increasing the density of a solid-state battery. The conventional stamping process needs to be finely tuned in order to avoid damaging the material, and that could lead to waste and other excess costs. As an alternative, Honda is looking into something called a roll press.

“Recognizing the high potential of a roll press, which will lead to high production line speed, Honda has been amassing know-how in roll press techniques with an aim to establish production technology of all-solid-state batteries,” Honda has stated.

We haven’t seen Honda pay particular attention to sulfur in the past, but its interest in the Jacobs research indicates that a ride on a roll press could be among next steps for the new solid-state battery. If you have any thoughts about that, drop us a note in the comment thread.

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Photo credit: “This new cathode material for lithium-sulfur materials is structurally healable and highly conductive,” by David Baillot/UC San Diego Jacobs School of Engineering via Eurekalert.

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