Last updated December 8, 2017 at 11:20 am
A new battery technology offers the chance for a cheaper, greener and safer replacement to lithium batteries. The key is replacing the lithium with the far more common, and less flammable, sodium.
Recharge your battery facts
To understand this technology, let’s quickly refresh the basics behind batteries.
All batteries convert chemical energy into electrical energy by moving metal ions from one side of the battery to the other. In a battery, a metal ion reacts from the electrochemical high point at the anode, and flows to the chemical low point at the cathode, just like water from from a high point to a low.
The start of the reaction at the anode involves oxidising the metal into a positively-charged ion by giving up an electron. The ions travel across the battery through an electrolyte to the cathode on the other side. To chemically join the cathode, that same metal ion requires an electron to return it to its neutral state. But where does it get this electron from? Well, a wire is placed from the anode to the cathode, providing an external path for the electrons to flow across. This ‘electrical river’ enables the chemical reaction so that we can power modern technology.
To charge the battery just reverse the flow of electrons which causes the metal to move in from the cathode to the anode and revert to the initial state.
Sodium batteries require different electrolyte
In a battery, like what you might find in your phone, the metal ion that travels internally from the anode to cathode is lithium. In a new sodium battery, the lithium has been replaced with the cheaper and more environmentally-friendly sodium.
However, when you change one component of a battery, you need to change the others. Swapping the metal ion from lithium to sodium means that a different electrolyte is required for the ions to flow through.
Depending on the strength of the sodium chemical reaction these flowing electrons from anode to cathode have an electrical height, or potential difference also known as voltage. A higher voltage, just as with a higher waterfall, can be used to drive more powerful equipment.
Solid electrolyte solution
New research has proposed a new electrolyte in sodium batteries to maximise their power output – a solid piece of closo-borate.
There are many considerations for the perfect electrolyte.
Firstly, it has to conduct, or carry, the ion charge quickly. For closo-borate this means allowing positive sodium ions to rapidly move across it, which it does well.
Secondly, the electrolyte has to be stable at high temperatures, as the batteries heat up due to resistance or the object they power does, you don’t want it melting. In this case, the closo-borate material was found to be thermally-stable up to at least 300oC.
Thirdly, the material has to be stable – there’s no point building the battery out of material that will chemically react at lower voltages. Once more this new battery works well.
Being a solid electrolyte, there’s a particular phenomenon that might offer an even more impressive future for this cheaper material to replace lithium. In lithium a microscopic stalagmite can form in the fluid electrolyte that bridges between the anode and cathode, leading to a short circuit which can cause fires. To try to prevent this, the anode is made out of graphite instead of lithium, however this means the battery doesn’t carry as much charge as it could. If the research team’s hopes are right, and that these bridges from anode to cathode don’t form in their solid sodium battery, then more of the battery can be used to store charge.
Making a solid electrolyte sodium battery possible
You might ask how do you get a solid electrolyte to connect as well with the anode and cathode as a fluid? Well for that the team first dissolved the battery electrolyte, layered it between anode and cathode, then let it evaporate, leaving behind the solid and now tightly-fitting electrolyte. Repeat this layering like a lasagne and you have your solid battery.
For these batteries to find widespread adoption they need to be reusable, and that means they have to keep their ability to charge after multiple recharges. For your smartphone, repeated discharge and recharge really does damage the battery and cause it to hold less charge at ‘full’ capacity over time. (Software updates aside, my phone really does last for less time between charges than it used to years ago when brand new. Yes, it’s also time I got a new phone.)
In theory, sodium should make for a more robust battery, able to be fully discharged and recharged many times. However, this is one area where the sodium battery still lags behind lithium, so don’t throw out your lithium ion batteries just yet. After 250 cycles, the sodium battery’s capacity dropped to a maximum of 85%. For release on the market it would need to have similar performance after 1200 charging cycles. This magic number corresponds to regular usage, for example, I charge my phone every night plus the odd time in the day, that lasts two or three years. But since the team haven’t begun to optimise the battery for performance or reliability, there is much more improvement to come.
That means more charge in ever smaller devices, which you could be seeing in a smartphone near you soon.
The research was published in the Journal for Energy and Environmental Science
To find out more about the challenges of a battery-led future, read Start planning now for a lithium-battery future