Even though sodium-ion batteries would be physically heavier than lithium-ion technology, researchers have been investigating sodium-ion batteries because they could store energy for large solar and wind power facilities at lower cost.
The problem is that sodium ions stick to the hard carbon end of a battery, called an anode, during the initial charging cycles and not travel over to the cathode end. The ions build up into a structure called a “solid electrolyte interface.”
“Normally the solid electrolyte interface is good because it protects carbon particles from a battery’s acidic electrolyte, where electricity is conducted,” Pol said. “But too much of the interface consumes the sodium ions that we need for charging the battery.”
Purdue researchers proposed using sodium as a powder, which provides the required amount of sodium for the solid electrolyte interface to protect carbon, but doesn’t build up in a way that it consumes sodium ions.
They minimized sodium’s exposure to the moisture that would make it combust by making the sodium powder in a glovebox filled with the gas argon. To make the powder, they used an ultrasound – the same tool used for monitoring the development a fetus – to melt sodium chunks into a milky purple liquid. The liquid then cooled into a powder, and was suspended in a hexane solution to evenly disperse the powder particles.
Just a few drops of the sodium suspension onto the anode or cathode electrodes during their fabrication allows a sodium-ion battery cell to charge and discharge with more stability and at higher capacity – the minimum requirements for a functional battery.