Fool's gold gives insight into potassium and sodium batteries

June 21, 2018 // By Nick Flaherty
Fool's gold gives insight into potassium and sodium batteries
Researchers at the Georgia Institute of Technology in the US have found a way that sodium and potassium batteries could be used as an alternative to lithium systems.

"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.

The team looked at how three different ions -- lithium, sodium, and potassium -- reacted with particles of iron sulfide, also called pyrite or fool's gold. As batteries charge and discharge, ions move in and out of the electrodes, changing the volume and often breaking them up. Because sodium and potassium ions are larger than lithium, they cause more significant degradation.

In their experiments, the reactions that occur inside a battery were directly observed inside an electron microscope, with the iron sulfide particles playing the role of a battery electrode. The researchers found that iron sulfide was more stable during reaction with sodium and potassium than with lithium, indicating that such 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 visually. When exposed to lithium, iron sulfide particles appeared to almost explode under the electron microscope. On the contrary, the iron sulfide expanded like a balloon when exposed to the sodium and potassium.

"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 large volume changes that occur during the electrochemical reaction are always a precursor to the fractures that causes electrode failure leading to battery degradation.

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