Researchers are developing new concepts for the next generation of batteries at the Empa. For the purposes of mobile applications, they are using a solid-state battery which does not contain any liquid electrolytes, which means that it cannot burn or even explode. In stationary energy stores, expensive lithium is to be replaced by more cost-efficient substances which are available in abundance.

Maksym Kovalenko, Professor of Functional Inorganic Materials (r.) and Dr Corsin Battaglia, Head of the “Materials for Energy Conversion” Department in the laboratory at the Empa. (Photos: Kellenberger Kaminski Photographie)

The battery market is dominated by lithium-ion batteries. They can be inserted into a smartphone or computer, and can be used in electric cars or for the storage of solar energy at the EPFL. "These batteries have been developed primarily for wearable electronic devices, revolutionising the technology used there," explained Corsin Battaglia, a researcher at the Empa. "We are now looking to develop the next generation of batteries which are better suited to new applications, such as in the area of mobility and for stationary energy storage, for instance in buildings."

The requirements vary greatly. As with the mobile phone, the aim is to keep batteries in electric cars as compact as possible.  "Energy density is key here," said the Head of the "Materials for Energy Conversion" Laboratory. However, batteries that store solar or wind energy have to be cost-efficient above all else; after all, it has to be much cheaper to store the energy than it is to produce it. "And that takes material that is available in abundance," added Maksym Kovalenko, a Professor at ETH Zurich who also conducts research at the Empa. "If we are hoping to store lots of renewable energy for the energy transition in 2050, comparable to that supplied by a nuclear power station nowadays, we will require tons of batteries. That would not work with lithium."

Application filed for a patent

Work is ongoing around the world to replace lithium with sodium or magnesium, for example.  Kovalenko, whose work has received several awards, and his team are developing a battery which is made from aluminium and artificially produced carbon (graphite), as well as chlorine and organic solutions. The researcher pointed out that while the energy density is only half that of a lithium battery, the materials used are much cheaper; "they are among the 15 most common elements on the planet." After three years working on its development, Kovalenko filed a patent for his invention in 2016.

The new concepts are implemented and tested in experiments in the labs at the Empa. Small test batteries power the flight of a drone in a special performance test. However, the batteries developed by Kovalenko's team, which are cost efficient but not particularly powerful, are not destined for use in mobile applications. His colleague Corsin Battaglia is pursuing a different idea for that which also offers greater safety. "Current lithium-ion batteries all have liquid electrolytes inside, connecting the positive and the negative sides," explained the expert. These batteries can leak and, worse still, the liquid may ignite and the battery may even explode. Therefore, cell phone and laptop batteries have repeatedly to be recalled.

Solid state instead of liquid electrolytes

As Battaglia explained, "we develop batteries here at the Empa which do not contain any liquid." The electrolyte is replaced by a solid, non-flammable substance, which means that the entire battery consists of a single block of material. "This increases safety, and we hope to be able to achieve greater capacity or energy density," added the expert. The researchers are continuing to use lithium here, albeit in a different form. In current lithium-ion batteries, the alloy is embedded in graphite as the electrode material. Branch-like growths like on a tree, known as dendrites, form as the lithium ions drift to and fro between the electrodes. This can lead to a short circuit.

In the solid-state battery, the researchers are seeking to use pure lithium metal as the electrode material, thereby achieving greater energy density. While the problem of dendrites is still not resolved there either, "there are ideas out there which are very promising," revealed Corsin Battaglia. Even though lithium is currently the preferred option in the international research community, other elements are also being considered. "There is no principle of physics that says that solid-state batteries could not be manufactured from sodium or magnesium," said the researcher. "We have already achieved individual results that support that idea."

Build upon thin-layer technology

In addition to these projects that examine fundamental questions of research, the Empa has previously been chiefly concerned with developing materials or processes for the production of currently popular types of battery. "However, we have never manufactured whole batteries," revealed Pierangelo Gröning, Head of the "Modern Materials and Surfaces" Department and Head of the Empa Research Focus Area on "Nanostructured Materials". That is set to change now. "We are hoping to develop a separate type of battery in the coming years," said Gröning. The principle of the solid-state battery is a perfect fit for that. "We can concentrate here on solid materials science, which is our forte," indicated the Head of Department.

To enable the ions to penetrate effectively through the solid-state electrolyte, it must be as thin as possible. With that in mind, the researchers are using thin-layer technology which the Empa has been using for years in the manufacture of solar cells. Each layer is applied by means of an evaporation process, creating a sandwich structure. The starting point for the thin-film battery are known concepts and materials.  "It is nothing new," conceded Gröning, "although the production process is every bit as important as the choice of material."

The boundary layers at which the different solids meet are decisive. "Given our expertise, we have a good understanding and command of systems of this nature," said the expert. The project typifies contemporary materials research where substances are examined in interaction with the processing methodology. "A material must make sense, otherwise it is only a substance," revealed Pierangelo Gröning.

Energy for wearable electronics

The end product for the Empa researchers is a flexible film with electrodes. If the thin-film battery is rolled up, it equates to a conventional storage battery. In theory, larger batteries can also be made from this, although the developers are primarily going with the original shape as a film and are seeking to integrate it into textiles, for instances. The thin-film storage battery is particularly suitable for trendy wearable electronics because it is easy to incorporate and is not flammable. Initial specimen batteries have demonstrated that the theory works.  The next step for researchers is to increase the power density and capacity. The Head of Department anticipates that they will have "very telling results" by as early as the end of 2017.

In order to make the product marketable, the entire battery system must be improved. In addition to known evaporation processes, wet-chemical methods, such as those known from 3D printing technology, can be used. "There are lots of questions to answer about the process technology to ensure that every layer has optimum material properties," explained the expert. However, the key thing is that it must be possible for the system to be produced in large volumes at an affordable price some day. "We are not talking here about hundreds of metres of film – hundreds of kilometres will have to be produced for this to have any chance on the market.”