There are a few things we should demand from a decent power supply. Not least, it shouldn’t require oven-like temperatures to function. That’s just a given.
For decades, fluoride has sat on the sidelines as a potential competitor for lithium-ion batteries. If not for its need to bake at more than 150 degrees Celsius (300 degrees Fahrenheit), fluoride could be ahead in the game. Now, it looks like things are about to shake up in the battery business.
“Fluoride batteries can have a higher energy density, which means that they may last longer – up to eight times longer than batteries in use today,” says Caltech researcher Robert Grubbs, famous for winning a Nobel Prize in Chemistry in 2005.
That means potentially plugging in your smartphone once a week, rather than once a day. Or, if spacecraft are more your thing, packing more power into a smaller cell to save vital weight.
The type of electrochemical technology supplying power to your smart devices makes use of positively charged lithium ‘Li2+’ cations as a kind of chemical ‘piston’ to draw an electrical charge through a circuit.
At full charge, a supply of cations occupy the battery’s anode. Once the circuit is closed, ions surge into the cathode, producing a current that does the all-important work. To reset the cell, all that’s required is a voltage to ‘push’ the lithium piston back again.
Of course, this piston can work in reverse as well. Negative ions such as fluoride (F-) can also create the voltage necessary to draw electrons through a conductor.
In fact, in some ways they can do an even better job, thanks to the lower number of charges per ion.
“For a battery that lasts longer, you need to move a greater number of charges,” says Simon Jones, a researcher at NASA’s Jet Propulsion Laboratory.
“Moving multiply-charged metal cations is difficult, but a similar result can be achieved by moving several singly-charged anions, which travel with comparative ease.”
The difference is a little like sending a waiter with a tray of drinks. One waiter carrying a tray of drinks might seem more efficient, but a pair of lithe waiters with a drink each are far more agile and pack in far more easily.
As such, technology based on small anions could in theory make for a better battery. And fluoride has a low enough atomic mass to have attracted attention as a suitable anion candidate since the 1970s.
“But fluoride can be challenging to work with, in particular because it’s so corrosive and reactive,” says Grubbs.
This isn’t to say nobody has successfully made a functional fluoride ion battery. But the ions are part of a solid structure, which as you can imagine doesn’t let them slip around too easily. Not at room temperature at least.
Beyond 150 degrees Celsius (300 degrees Fahrenheit) this is less of an issue. Of course, now you need to crank your battery up to a temperature that can bake a muffin.
To get around this, the Caltech researchers took a gamble on an electrolyte solvent called bis(2,2,2-trifluoroethyl)ether. Or BTFE for short.
On finding the solvent did a fairly suitable job of allowing fluoride’s anions to shuffle between electrodes at room temperature, the team ran models to find ways to tweak its performance with additives.
The end formula is stable, allows for a high conductivity, and can tolerate operation at a variety of voltages.
Pairing the fluid with copper-lanthanum trifluoride, the researchers found it was possible to make an efficient anion-based battery that can be recharged and discharged without the need to crank the heat.
“We are still in the early stages of development, but this is the first rechargeable fluoride battery that works at room temperature,” says Jones.
So, we might need to wait a bit longer for weekly phone recharges, but this is an exciting step – an we can hardly wait for this exciting technology to come into the market.
This research was published in Science.