Sodium-and Potassium-Based Batteries Could be Key for Smart Grid of Future

New products entering the market each year such as electric cars that travel several miles on a single charge and chainsaws as mighty as gas-powered versions, take advantage of the latest developments in battery technology. However, that growth has raised concerns that the world's supply of lithium, the metal at the core of numerous new rechargeable batteries, may ultimately be depleted.

Presently, scientists at the Georgia Institute of Technology have found new proof suggesting that batteries based on potassium and sodium have the potential for use as a probable substitute to lithium-based batteries.

"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," he added.

For the research, which was published on June 19^th in the journal Joule and was supported by the National Science Foundation and the U.S. Department of Energy, the team examined how three different ions - lithium, potassium, and sodium - reacted with particles of iron sulfide, also called fool's gold and pyrite.

When batteries charge and discharge, ions continually react with and penetrate the particles that make up the battery electrode. This reaction process causes large volume variations in the electrode's particles, frequently breaking them up into tiny pieces. Since potassium and sodium ions are larger than lithium, it's traditionally been believed that they cause more substantial degradation when reacting with particles.

In their experiments, the reactions that take place within a battery were directly witnessed inside an electron microscope, with the iron sulfide particles acting as a battery electrode. The scientists discovered that iron sulfide was more stable during reaction with potassium and sodium than with lithium, signifying that such a battery based on potassium or sodium could have a lot longer life than estimated.

The difference between how the various ions reacted was plain visually. When exposed to lithium, iron sulfide particles seemed to nearly explode under the electron microscope. In contrast, the iron sulfide expanded like a balloon when exposed to the potassium and sodium.

"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 research also raises doubt on the idea that large volume variations that happen during the electrochemical reaction are always a sign of particle fracture, which causes electrode failure resulting in battery degradation.

The scientists proposed that one probable reason for the difference in how the various ions reacted with the iron sulfide is that the lithium was more likely to focus its reaction along the particle's sharp cube-like edges, while the reaction with potassium and sodium was diffused along the entire surface of the iron sulfide particle. Consequently, the iron sulfide particle when reacting with potassium and sodium formed a more oval shape having rounded edges.

While there's still further work to be accomplished, the new study findings could help researchers design battery systems that use these types of unique 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. 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.

Matthew McDowell