New study finds sodium and potassium show promise as battery materials

From electric cars that travel hundreds of miles on a single charge to chainsaws as mighty as gas-powered versions, new products hit the market each year that take advantage of recent advances in battery technology. But that growth has led to concerns that the world's supply of lithium, the metal at the heart of many rechargeable batteries, may eventually be depleted.

Now, researchers at the Georgia Institute of Technology have found new evidence suggesting that batteries based on sodium and potassium hold promise as a potential alternative to lithium-based batteries. The researchers describe this evidence in a paper in Joule.

"One of the biggest obstacles for sodium- and potassium-ion batteries has been that they tend to decay and degrade faster and hold less energy than alternatives," said Matthew McDowell, an assistant professor in the George W. Woodruff School of Mechanical Engineering and the School of Materials Science and Engineering. "But we've found that's not always the case."

For the study, which was sponsored by the US National Science Foundation and the US Department of Energy, the research team looked at how three different ions – lithium, sodium and potassium – reacted with particles of iron sulfide, also called pyrite and fool's gold.

As batteries charge and discharge, ions are constantly reacting with and penetrating the particles that make up a battery’s electrode. This reaction process causes large volume changes in the electrode particles, often breaking them up into small pieces. Because sodium and potassium ions are larger than lithium ions, it's traditionally been thought they would cause more significant degradation when reacting with the particles.

In their experiments, the researchers used an electron microscope to observe directly the reactions occurring inside a battery, with the iron sulfide particles playing the role of the electrode particles. The researchers found that iron sulfide was more stable during reactions with sodium and potassium than with lithium, indicating that a battery based on sodium or potassium could have a much longer life than expected.

The difference between how the different ions reacted was stark. When exposed to lithium, the iron sulfide particles appeared to almost explode under the electron microscope. When exposed to sodium and potassium, however, the iron sulfide particles expanded like a balloon.

"We saw a very robust reaction with no fracture – something that suggests that this material and other materials like it could be used in these novel batteries with greater stability over time," said Matthew Boebinger, a graduate student at Georgia Tech.

The study also casts doubt on the notion that the large volume changes that occur during the electrochemical reaction are always a precursor to particle fracture, which leads to electrode failure and battery degradation.

The researchers suggested that one possible reason for the difference in how the different ions reacted with the iron sulfide particles is that the lithium was more likely to concentrate its reaction along the particles’ sharp, cube-like edges. In contrast, the reaction with sodium and potassium was more diffuse, occurring along the whole surface of the iron sulfide particle. As a result, when reacting with sodium and potassium, the iron sulfide particle developed a more oval shape with rounded edges.

While there's still more work to be done, the new research findings could help scientists design battery systems that can use these types of novel materials.

"Lithium batteries are still the most attractive right now because they have the most energy density – you can pack a lot of energy in that space," McDowell said. "Sodium and potassium batteries at this point don't have more density, but they are based on elements a thousand times more abundant in the earth's crust than lithium. So they could be much cheaper in the future, which is important for large scale energy storage – backup power for homes or the energy grid of the future."

This story is adapted from material from Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.