From my reading of the article, the hexane is used in fabricating the sodium powder, and possibly as a transport mechanism. Does it actually remain in the battery after manufacturing, or does it evaporate after it transports the sodium powder onto the battery components? The article was not entirely clear on the process.
My reading, it is used as a suspension.
So Pol and his team looked to make some tweaks. Using standard ultrasound technology, they melted chunks of sodium down into a milky purple liquid, which they in turn cooled into a powder and suspended in hexane solution, leaving an even spread of powder particles.
Adding just a few drops of this powder material during fabrication of both the anode and cathode electrodes brought much better results. Pol tells us they observed much longer charging and discharging cycles, along with better coulombic efficiency (the efficiency of charge transfer).
And the problem:
One of these is a short life due to the congregation of sodium ions at the electrode made from carbon, called the anode, during the first few charging cycles. Here they build up and form what is known as a solid electrolyte interface, which hungrily eats up the sodium ions and before long, leaves a busted battery.
Given "a few drops", it's a method of doping the electrodes, and the hexane, C6H14, would dissolve into the electrolyte if it is still around. How it does its chemical magic I don't know. C6H14 isn't going to be hilariously toxic either.
From wiki:
The acute toxicity of n-hexane is rather low, requiring grams per kilogram. Inhalation of n-hexane at 5000 ppm for 10 minutes produces marked vertigo; 2500-1000 ppm for 12 hours produces drowsiness, fatigue, loss of appetite, and paresthesia in the distal extremities; 2500–5000 ppm produces muscle weakness, cold pulsation in the extremities, blurred vision, headache and anorexia.[14] The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) for hexane isomers (not n-hexane) of 100 ppm (350 mg/m3 (0.15 gr/cu ft)) over an 8-hour workday.[15]
Basically, it's used as a component in a lot of glues, for degreasing, and other tasks. Given the expected reaction of sodium powder to anything with water in it, this sounds like about as safe of an alternative as practical.
zxcvbob - these seem to be "salt" batteries, using a sodium ion, not a pure metal sodium battery. IE "salt water".
https://en.wikipedia.org/wiki/Salt_water_batteryvs
https://en.wikipedia.org/wiki/Sodium%E2%80%93sulfur_battery