Unlocking Superionic Transport: How Liquid-Like Molecular Dynamics Illuminate Solid-State Battery Materials

Scientists at Duke University have revealed the molecular mechanics of a substance that may serve as the foundation for future rechargeable batteries.

In contrast to the widely used lithium-ion batteries with a liquid interior, this lithium-based compound exists as a solid at working temperatures. However, even with its firm internal framework, charged ions can still swiftly navigate through it, qualifying it as a “super ionic” material. While this compound has piqued researchers’ interest for some time, the mechanism allowing lithium ions to traverse its solid crystalline architecture with ease remained unclear.

The newfound findings address numerous unresolved queries, exhibiting unexpected liquid-like properties at the atomic scale. Armed with these revelations and the machine learning models that facilitated them, researchers are poised to pursue other similar formulas to tackle various enduring challenges in the discipline.

The findings were published online on January 6 in the journal Nature Physics.

Generally, rechargeable batteries operate by relocating atoms with an electric charge referred to as ions through a chemical substance known as an electrolyte. The speed and ease with which these ions can voyage play a pivotal role in the rate at which a battery can charge and supply energy.

For a long time, lithium-ion batteries have been the prevailing technology utilized across a wide range of commercial uses, from compact smartwatches to massive grid energy storage units. Although they have been remarkably effective, lithium-ion batteries possess several limitations, rendering newer technologies more appealing for specific applications.

For instance, lithium-ion batteries typically utilize a liquid electrolyte that, while incredibly efficient in enabling swift ion movement, is also highly flammable. Additionally, this liquid electrolyte is corrosive, constraining engineers’ options when designing other essential components of the battery.

“A significant effort is underway to create enhanced batteries that can power electric vehicles to go further, charge more rapidly, and be safer from unexpected fires,” stated Olivier Delaire, associate professor of mechanical engineering and materials science at Duke. “Solid-state batteries are highly attractive due to their stability; however, determining how to configure them for rapid ion flow is a fundamental issue in the discipline.”

One promising solid-state electrolyte receiving attention from researchers is a variant of argyrodite based on lithium. Previously identified as a superionic substance, it occupies a space between crystalline and liquid states of matter. While being solid at room temperature, it still permits lithium ions to move through its molecular structure with the same speed as its liquid counterparts. However, the mechanics and reasons behind this ion behavior have long been enigmatic.

To explore this lingering mystery, Delaire and his team scrutinized samples of the material at the Spallation Neutron Source at Oak Ridge National Laboratory. By directing neutrons at the atoms at exceptionally high speeds, researchers captured a sequence of snapshots detailing the atoms’ exact movements within the atomic framework of the argyrodite compound.

“Working with this material posed some unique challenges, as lithium readily absorbs neutrons, complicating this kind of investigation,” remarked Delaire. “We had to rely on our collaborators from the University of Münster to obtain a rare variant with an additional neutron known as Lithium 7 to gather the necessary data.”

Once the atomic movements were recorded, Delaire’s team secured time at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory to interpret the data. After executing simulations and models that typically require weeks, even on these extraordinarily powerful computers, the team successfully compared their computational outcomes with the recorded measurements.

The researchers found that the ions’ capacity to swiftly navigate this substance arises from its molecular vibrations. At lower temperatures, molecular structures within solids usually oscillate at a limited set of intrinsic frequencies, making it challenging for ions to traverse them. However, this specific variant of lithium argyrodite boasts a greater diversity of vibrational modes similar to a liquid.

An analogy could be drawn to a basketball team trying to pass the ball during a play. If the players generally remain still and inactive, it becomes more difficult to navigate the ball through the defense. On the other hand, if all players are dynamic and moving around, the ball can easily find pathways between them.

“Understanding how these dynamics function will aid us in designing materials leveraging these principles,” Delaire stated. “For instance, keeping these concepts in mind, the materials don’t necessarily have to include lithium. Lithium is costly and not widely produced, so if we could devise equivalent materials using sodium, for instance, these batteries could potentially become significantly cheaper and simpler to manufacture.”