Supercapacitors and batteries are energy storage types with different advantages. While batteries score with high storage capacities, supercapacitors impress with their short charging time. Are there intersections in the underlying technologies? Can the advantages from both worlds be combined? The team of authors led by Prof. Volker Presser from the Saarbrücken Leibniz Institute for New Materials (INM) and Dr. Simon Fleischmann, Helmholtz Institute Ulm (HIU), addresses this issue in their perspective article in the renowned scientific journal Nature Energy.

Time is a precious commodity. This is especially true for electrochemical energy storage devices: the almost empty cell phone battery shortly before leaving home or the e-car that has to stay plugged in for a few more hours before you can set off to visit relatives. In such cases, you want recharging times to be as short as possible. However, rapid charging and discharging processes are extremely stressful for the electrode materials in batteries and shorten their service life. Supercapacitors do not have this problem: unlike batteries, no ions are incorporated into crystal lattices here, but are merely deposited on the enormously large surface of activated carbon. This means they store significantly less energy than batteries, but a few seconds are enough to recharge the cell.

To combine the best of both worlds, scientists are conducting intensive research into so-called pseudocapacitors. These are electrochemical energy storage devices that behave electrically like a capacitor and can therefore be charged particularly quickly. Their energy storage mechanism, on the other hand, works like that of a battery: energy is stored by ion intercalation in crystal lattices. These special properties can often be achieved by using 2D materials as electrodes. Dr. Simon Fleischmann, a former INM employee and doctoral student at Saarland University and now a junior research group leader at the Helmholtz Institute in Ulm, explains: “The special thing about 2D materials is their flexible interlayer space. By selectively adjusting the interlayer spacing in the range around 1 nanometer, we can observe interesting nanoeffects in the so-called confinement.” What is meant by this is that ions and electrolytes, which are needed for ion transport, behave quite differently in such small nano-spaces than they do in a large volume or on a surface. Proper “matching” of ion size, electrolyte and nanospace of the electrode lattice can enable significant increases in energy storage capacity and fast-charge capability.

The storage mechanism of pseudocapacitors has so far been assigned to either capacitors or batteries. Current research by an international team led by Prof. Veronica Augustyn of North Carolina State University has now established a unifying approach to this. “We see a continuous transition from very classical lithium-ion battery materials to ideal activated carbon,” explains Volker Presser, head of the Energy Materials program area at INM. “It is important to understand this gradual transition from electrosorption to intercalation as a spectrum. Depending on the size and geometry of the nanospace, ions will (partially) shed their electrolyte shells and can undergo redox processes.” Which brings us back to 2D materials like MXenes or layer-structured metal oxides. “The interlayer space of 2D materials in particular is a great playground for us in materials science. Here we can combine fast ion transport and high energy storage capacity through reversible redox processes by means of targeted material design,” adds Simon Fleischmann.

The Perspective Paper “Continuous transition from double-layer to Faradaic charge storage in confined electrolytes” in the current issue of Nature Energy is part of the long-standing American-German-French cooperation of the INM and an important subject of the BMBF-funded NanoMatFutur project of Dr. Fleischmann at the Helmholtz Institute Ulm.