Researchers from Rensselaer Polytechnic Institute have demonstrated how to overcome a persistent challenge to potassium metal batteries — dendrites. Their new battery performs nearly as well as a lithium-ion, but has the advantage that potassium is a much more abundant and less expensive element.
Batteries contain two electrodes — a cathode on one end and an anode on the other. If you were to look inside a lithium-ion battery you’d typically find a cathode made of lithium cobalt oxide and an anode made of graphite. During charging and discharging, lithium ions flow back and forth between these two electrodes.
In this setup, if researchers were to simply replace lithium cobalt oxide with potassium cobalt oxide, performance would drop. Potassium is a larger and heavier element and, therefore, less energy dense. Instead, the Rensselaer team looked to boost potassium’s performance by also replacing the graphite anode with potassium metal.
While metal batteries have shown great promise, they have also traditionally been plagued by accumulation on the anode, of metal deposits called dendrites. Dendrites are formed because of non-uniform deposition of potassium metal as the battery undergoes repeated cycles of charging and discharging. Over time, the conglomerates of potassium metal become long and almost branch-like.
If they grow too long, they will eventually pierce the insulating membrane separator meant to keep the electrodes from touching each other and shorting out the battery. Heat is created when a battery shorts and has the potential to set the organic electrolyte within the device on fire.
According to the researchers, their solution to that problem paves the way for practical consumer use. By operating the battery at a relatively high charge and discharge rate, they can raise the temperature inside the battery in a well-controlled manner and encourage the dendrites to self-heal off the anode.
The self-healing process can be compared to what happens to a pile of snow after a storm has ended. The wind and the sun help move the flakes off the mound of snow, shrinking its size and eventually flattening it out.
In a similar way, while the temperature increase within the battery won’t melt the potassium metal, it does help to activate surface diffusion, so the potassium atoms move laterally off the “pile” they’ve created, effectively smoothing the dendrite out.
Their idea is that at night or whenever you’re not using the battery, you would have a battery management system that would apply local heat to cause the dendrites to self-heal .
The team previously demonstrated a similar method of self-healing with lithium metal batteries, but they found the potassium metal battery required much less heat to complete the self-healing process. That promising finding means a potassium metal battery could be more efficient, safe, and practical.