Solid-state batteries are viewed as the storage technology of the future. In contrast to lithium-ion batteries, they do no contain any liquid components. Their electrolyte consists of a solid. This promises a longer lifetime and a higher level of safety – no leakage and an extremely low risk of fire. In theory, solid-state batteries can also achieve better storage properties than lithium-ion batteries. In practice, however, they are still inferior.

Jülich researchers aim to change this and are particularly focused on the electrolyte. All rechargeable batteries require such a component, through which charged particles (ions) can migrate from one electrode to another. They can typically do this better in liquids than in solid materials. That is why the electrolyte is liquid in most conventional batteries.

This has a number of disadvantages, however. Metals such as lithium, for instance, react very violently with certain liquids such as water. In addition, tiny projections of metallic lithium can form in them during each charging cycle. As soon as these dendrites connect both electrodes, there is a short circuit which can lead to a fire.

The anode in lithium-ion batteries does not therefore typically consist of lithium metal, but usually of graphite, into which lithium ions can be stored. This reduces the storage capacity. Moreover, active materials dissolved in the liquid react with the electrodes, which shortens the lifetime of conventional batteries. The use of a solid electrolyte might therefore offer multiple advantages.

Ion-conducting polymers and ceramics

Jülich scientists conduct research into solid materials that conduct ions well. They are primarily focused on ceramics and polymers. Polymers are easier to produce and process than ceramics. They also react more flexibly to mechanical loads. In contrast, ceramic ion conductors can better withstand higher temperatures and are less susceptible to the influence of moisture and oxygen.

The teams at Jülich’s Institute of Energy and Climate Research that produce and characterize ceramics can mainly be found at the Materials Synthesis and Processing (IEK-1) and Microstructure and Properties of Materials (IEK-2) subinstitutes. The Helmholtz Institute Münster: Ionics in Energy Storage (IEK-12/HI MS) is instead focused on polymers.

In addition to electrolytes, the other components of solid-state batteries, such as electrodes and the separator, are researched at Forschungszentrum Jülich. Scientists from the Fundamental Electrochemistry (IEK-9) and Materials Synthesis and Processing (IEK-1) subinstitutes are trying to find the best combination of materials for solid-state batteries. They also investigate alternatives to the use of lithium for electrodes, for example sodium and silicon, which are inexpensive and available in almost unlimited quantities.

The function of metal–air batteries is based on the reaction of oxygen. However, the oxygen required for this is not found in the battery, but is taken from the air. This type of battery therefore has a high energy density in theory, i.e. a very good storage capacity with a low weight.

Jülich researchers develop, investigate, and improve metal–air batteries whose anodes are composed of pure zinc, aluminium, iron, silicon, magnesium, or lithium. A structure made of carbon or another porous material forms the cathode, which is in contact with the ambient air. The electrolyte can be solid, like in solid-state batteries, or it can be liquid. The electrolyte of the innovative titanium–air battery presented in 2023 by scientists from Forschungszentrum Jülich and the Technion – Israel Institute of Technology consists of an ionic liquid.

Extending battery lifetime

In principle, metal–air batteries are particularly suitable for applications in which a compact design is important. However, their practical application has been limited by one major difficulty: metal–air batteries can often only be charged several times before they are only able to store small amounts of energy or even stop working completely. Using analytical methods and computer simulations, Jülich researchers are helping to explain which processes are responsible for the rapid loss of function. Their aim is to suppress these processes.

Candidate for environmentally friendly large batteries

In addition to their high energy density, metal–air batteries have other crucial advantages if, for example, iron or silicon are used as the anode material: their raw materials are available in large quantities, comparatively inexpensive, and harmless to humans and the environment. They therefore appear to be particularly suitable for storing electricity in large stationary systems.

In order to store several hundred kilowatt hours of electricity, you would need a large number of litihum-ion batteries – or a single redox flow battery with two large liquid-filled tanks. Jülich researchers want to make this type of battery competitive.

A redox flow battery features two tanks, each with a liquid electrolyte, as well as two reaction chambers with one electrode each. 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. In the chambers, electricity is thus converted into energy-rich chemical compounds or vice versa.

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 size of the tanks. In order to store several hundred kilowatt hours, for example, only larger tanks and more powerful pumps are required. In contrast, with litihum-ion batteries, the total number of battery cells must be increased. Redox flow technology thus promises cost advantages for large storage systems – a promise that it has not yet been able to fulfil, however.

Tuning the system

A team from the Helmholtz Institute Münster (HI MS) is researching the technology and focusing on easily accessible, environmentally friendly electrolytes. In particular, they are optimizing the monitoring and control of the redox flow system using customized sensors.

Scientists from the Institute of Energy and Climate Research – Fundamental Electrochemistry (IEK-9) are working on a special form of the redox flow battery, the iron slurry/air storage system. Together with the battery manufacturer Varta and other cooperation partners, they aim to improve the rechargeability of this battery.