Lithium-ion batteries are currently the dominant battery type on the market. However, some requirements for stationary and mobile storage systems may be better met by other types that are not yet as technologically mature. Scientists from Jülich are researching various types - three examples that could help to satisfy our hunger for energy.
The metal–air battery theoretically promises a high energy density, as it uses oxygen from the air. This makes the battery type lighter and leaves more space for the metal anode, which can therefore be larger. The metal anode consists of sodium, iron, aluminum, or zinc, and the metalloid silicon is also used.
“We are investigating nearly all members of the metal–air battery family. It is important that the metals for the anode have a high energy density and that the raw materials are available in large quantities and are harmless to humans and the environment,” says Dr. Emre Durmus from the Institute of Energy Technologies (IET-1). However, the metal–air batteries still deliver significantly less energy than expected and the number of charge–discharge cycles is still far from sufficient for commercial use. The researchers are working on finding out the fundamental causes of this. “We use, for instance, in operando techniques that allow us to observe the battery components in operation on a microscopic level,” explains Durmus. “Gaining a better understanding of the complex charging and discharging processes is an important step on the path to harnessing the theoretical potential of this type of battery.”
In batteries, an electrolyte enables ions to move between the two electrodes. In current lithium-ion batteries, the electrolyte is liquid. But it does not necessarily have to be. Batteries with a solid electrolyte are considered to be particularly safe, since nothing can leak and there is an extremely low risk of fire. However, ions sometimes migrate more slowly in solids than in liquids.
Jülich scientists are therefore working on improving the ionic conductivity of solid materials. They are also developing construction concepts that fully exploit the potential advantages of solid-state batteries. For example, a team led by Dr. Frank Tietz from the Institute of Energy Materials and Devices (IMD-2) has found a way for ions in sodium–sulphur solid-state batteries to migrate quickly enough between the electrodes at room temperature. This type of battery, which has been known about for decades, has so far only exhibited a satisfactory performance at temperatures above 250 °C. This severely restricts its potential application. The team led by Dr. Tietz has produced a ceramic electrolyte which is so thin that its area-specific resistance is about ten times lower than usual.
Prof. Dina Fattakhova-Rohlfing from IMD-2 is improving ceramic lithium and sodium batteries, or more precisely the manufacturing process for them. “These solid-state batteries are robust and safe, but manufacturing them using conventional methods is still energy-intensive,” says the scientist. To produce the cell components of these batteries, ceramic powder is typically heated for hours at high temperatures in a sintering process to compact and solidify it. “In addition to high energy consumption, this leads to high production costs and undesirable material degradation, which has an adverse effect on battery performance,” says Fattakhova-Rohlfing. Researchers in her department have therefore developed advanced processing and sintering techniques that allow ceramic batteries and battery components to be manufactured at lower temperatures and in shorter production times. “This is crucial for the future market development of this type of battery,” Fattakhova-Rohlfing stresses.
Redox flow systems occupy a special position among batteries. They feature two tanks, each with a liquid electrolyte, as well as two reaction chambers with one electrode each. The chambers are separated by a membrane. If required, the electrolyte solutions are pumped in two separate circuits through the reaction chambers where they then absorb or release electrons at the electrodes while consuming or producing electricity.
The advantage of this construction method is that in order to expand the storage capacity of redox flow batteries, it is only necessary to increase the volume of the tanks. Redox flow technology therefore promises cost benefits for large storage systems. It has not yet been able to fulfil this promise, however, in part because the electrolyte solution of commercially available redox flow systems is based on expensive vanadium compounds. A team led by Mariano Grünebaum at the Helmholtz Institute Münster (IMD-4, HI MS) is therefore looking for easily accessible, environmentally friendly electrolytes. The team recently published a digital blueprint that any research group can use to produce small redox flow batteries themselves using 3D printing. The costs involved amount to € 230 – commercially available redox flow systems are at least ten times more expensive.
Text: Frank Frick | images: Forschungszentrum Jülich/Ralf-Uwe-Limbach, Forschungszentrum Jülich/Sascha Kreklau; illustrations: Jens Neubert
