Our lives are increasingly battery-powered. From fuelling our mobile phones and electric cars to storing energy for entire households: we need to make more and bigger batteries than ever.
However, to keep up with demand, we need an alternative to lithium ion (Li-ion), since lithium is scarce within the earth’s crust and therefore expensive.
A promising battery for the future could be made with potassium, which is 1,000 times more abundant in the earth’s crust than lithium. However, one technical challenge in developing a potassium ion (K-ion) battery is developing new electrode materials, as those we use for Li-ion batteries would be torn apart by intercalation (insertion) of the larger-sized potassium ions.
Now, a team from researchers from China has developed an anode using a material which not only overcomes the degradation problem by letting K-ions slip in and out of its layers, but also displays excellent storage capacity and requires no energy to move ions back into solution. This is the first time that the porous material – a special class of covalent organic frameworks called covalent triazine frameworks (CTFs) – has been applied for this purpose.
Professor Guangshan Zhu from Northeast Normal University, China, said: "Usually, the battery system is composed of anode, cathode and electrolyte. The lithium ion extracts from the cathode material and intercalates into the anode material with the transmission of electrolyte. In other words, the anode acts as a host to accommodate the lithium ion.
"Our work is mainly focused on the mechanism of alkali ion storage in CTFs, which have rarely been studied in the field of carbon materials of Li-ion battery.
"Particularly, the layer-by-layer assembly of CTF materials is mainly sustained by van der Waals interactions rather than covalent or ionic bonds, which benefits a larger interlayer spacing to accommodate the bigger sized K-ion, with insignificant lattice deformation and contributes to a higher reversible capacity," Professor Zhu added.
The researchers made two versions of the material, varying pore size to see if this had any effect on function. Interestingly, they were able to show that a smaller pore size performed considerably better, having not only a greater K-ion storage capacity but also resulting in an exothermic – and therefore more reversible – reaction.
Professor Zhu said: "The well-defined organic frameworks offer a uniform electrochemical environment which is critical for the investigation of intercalation/deintercalation mechanism in K-ion storage.
"Our results shed important insight into the mechanism of K-ions storage and rational design of CTF anode materials for K-ion batteries with superb energy storage performance."