Duke researchers, machine learning unveil the secrets of silver-enriched compounds that may enable solid-state batteries.

With help from machine learning, a team of Duke University researchers has discovered the atomic mechanisms that make silver-rich compounds known as argyrodites among the top contenders for a solid-state battery electrolyte that would completely change the electric vehicle landscape.

Whether they use these argyrodites or some other material as the electrolyte, solid-state batteries are expected to store much more energy, charge faster, last longer, and be safer than the lithium-ion batteries currently powering EVs, which contain a liquid electrolyte that is known to degrade and can catch fire.

"Every electric vehicle manufacturer is trying to move to new solid-state battery designs, but none of them are disclosing which compositions they're betting on," said Olivier Delaire, associate professor of mechanical engineering and materials science at Duke. "Winning that race would be a game changer because cars could charge faster, last longer, and be safer all at once."

One of the leading contenders to win the solid-state battery race relies on argyrodites, a group of compounds built from stable crystalline frameworks made of two elements with a third free to move about the chemical structure. While some recipes, such as silver, germanium, and sulfur, are naturally occurring, the general framework is flexible enough for researchers to create a wide array of combinations.

While this group of silver-enriched compounds is not new to the field of solid-state electrolyte candidates, scientists never really understood why.

"This is a puzzle that has not been cracked before because of how big and complex each building block of the material is," said Delaire. "We've teased out the mechanisms at the atomic level that are causing this entire class of materials to be a hot topic in the field of solid-state battery innovation."

The argyrodite discoveries made by the Duke researchers – and the machine learning used to crack the code – could usher in a new era of energy storage that would likely go far beyond the EVs that are driving the race for better batteries.

Surprising silver duality

In a paper published May 18 in "Nature Materials," Delaire and his Duke colleagues look at one promising argyrodite made of silver, tin, and selenium (Ag8SnSe6). The researchers bounced a combination of extremely fast-moving neutrons and X-rays off atoms within Ag8SnSe6 samples to reveal its molecular behavior in real-time.

Mayanak Gupta, a former postdoc in Delaire's lab who is now a researcher at the Bhabha Atomic Research Center in India, developed a machine-learning approach to make sense of the complex data and created a computational model to match the observations.

The results showed that while the tin and selenium atoms created a relatively stable scaffolding, the crystalline structure constantly flexed to create windows and channels for the charged silver ions to move freely through the material.

Delaire says it is like the tin and selenium lattices remain solid while the silver is in an almost liquid-like state.

"It's sort of like the silver atoms are marbles rattling around about the bottom of a very shallow well, moving about like the crystalline scaffold isn't solid," Delaire said. "That duality of a material living between both a liquid and solid state is what I found most surprising."

Olivier Delaire, Duke University

Illustration of the hybrid crystalline-liquid atomic structure in the superionic phase of Ag8SnSe6 – a silver-tin-selenium material that shows great promise for solid-state batteries. The tube-like filaments show the liquid-like distribution of silver ions flowing through the crystalline scaffold of tin and selenium atoms (blue and orange).

As surprising and groundbreaking as this discovery is, the Duke professor believes the advanced experimental spectroscopy and machine learning approach to getting them could be even more important because it should help speed the research and development of solid-state batteries.

"This study serves to benchmark our machine learning approach that has enabled tremendous advances in our ability to simulate these materials in only a couple of years," he said. "I believe this will allow us to quickly simulate new compounds virtually to find the best recipes these compounds have to offer."

Delaire and his team are studying a variety of promising argyrodite recipes for both battery applications and the conversion of heat into electricity.

"Many of these materials offer very fast conduction for batteries while being good heat insulators for thermoelectric converters, so we're systematically looking at the entire family of compounds," Delaire said.

One recipe that replaces the silver with lithium is of particular interest to the group, given its potential for EV batteries.