Interview with Dr. Michael Stich

Hello Dr. Stich, you are currently researching lithium-ion batteries. How does a lithium-ion battery work?

As with other batteries, the energy in lithium-ion batteries is stored chemically. When charged, the lithium ions are located in one of the two electrodes. When discharging, they move through the electrolyte to the other electrode and are then stored there. An exchange of charge carriers takes place, resulting in a usable electrical current.

Where are these batteries used?

Lithium-ion batteries were commercialized as early as the 1990s. The first lithium-ion batteries were used to power video cameras. They have since become the standard for cell phone batteries and laptop batteries. The next big step was their use in electric mobility. Nearly all of today's electric cars rely on lithium-ion batteries for energy storage. Lithium-ion batteries are also increasingly being used for stationary storage of renewable energy sources such as wind or solar.

What can be understood by stationary storage?

Due to the change in our energy supply, more and more fluctuating energy sources such as solar or wind energy have to be fed into electricity grids. There is a need to store this energy when it is available at the moment and to take it out of energy storage later - when there is a shortage of energy at the moment. Lithium-ion batteries are used for this intermediate storage.

What does such a battery look like?

Lithium-ion batteries come in all shapes and sizes. The one used in cell phone batteries is usually small and rectangular. In older laptops, interconnections of several cells were installed in a round cell format. This format is similar to commercial batteries used for remote controls, for example.

Are the batteries used in electromobility correspondingly larger?

Yes, battery packs in electric cars are made up of many individual cells. The big challenge is that you want to store large amounts of energy in a small volume and with a low weight. If you look at today's electric cars, they use battery packs that weigh up to 500 kilograms. Of course, the industry is trying to reduce this weight. On the other hand, they want to increase the range of the cars to make them more attractive to customers and increase their acceptance in society. Many people still don't opt for an electric car because its range is limited and charging stations are not available everywhere. Long-lasting lithium-ion batteries can also contribute to the acceptance of electric cars.

How many such batteries are used in an e-car?

If you look at a Tesla model, for example, round cells are installed here that are connected together in so-called modules. Several thousand such cells are used in a car. These can achieve ranges of over 500 kilometers in some cases. In the case of many European manufacturers, these cells are significantly larger and have a prismatic format.

How long does such a battery last on average?

That depends on the chemistry used. If we're talking about a cell phone battery, that's only a few years. In everyday life, you notice that when your cell phone battery runs out quickly. In electromobility, the shelf life of lithium-ion batteries is much longer. This is due to the changed chemistry that is used there as well as the better thermal management. Customers have a guarantee of around eight years, during which the battery may lose a maximum of 20 percent of its capacity.

In your research project, you want to extend the shelf life of batteries. What values are you aiming for?

Since we are conducting basic research in this DFG project, our focus is on better understanding the behavior of the passivation layer. better. In this way, we want to be able to provide valuable information to research and industry. We don't have any specific targets by how many percent the durability will be improved.

Why are you concentrating your research on the passivation layer?

If you look at older types of batteries, they have a voltage of one to two volts. The voltage of a lithium-ion battery, on the other hand, is much higher, close to four volts. In lithium-ion batteries, there is a liquid called electrolyte that is no longer stable at these high voltages. The liquid slowly begins to decompose. Without a protective mechanism, the battery would stop working after just a few charges and discharges. This is where the passivation layer comes in. This stable layer, which is only a few nanometers thick, prevents decomposition.

However, the production of this layer is industrially complex. The cell is assembled, the electrolyte is added and a very long time is needed to form the passivation layer via an electrochemical forming process. The long formation time is a cost driver. We are therefore trying to find ways of improving this layer (solid electrolyte interphase). On the one hand, it should be more stable, but also produced more quickly in order to save costs. As a result, the battery will last longer and be cheaper.

How do you want to make this layer more stable?

There is still an unsolved problem with current lithium-ion batteries: the passivation layer is formed and delays the aging of the batteries, but it is still not perfect. When a lithium-ion battery ages, this is largely due to the fact that this layer continues to grow while the electrolyte slowly but steadily decomposes. This is where our research project comes in. We are investigating different formation conditions and additives and observing how these affect the passivation layer and how the charge carrier transport in the layer can be influenced.

How do you go about this in your research and what challenges does your work involve?

Since the passivation layer is only a few nanometres thick, it is a great challenge to actually make it visible in order to analyse it. To do this, we use so-called in-situ techniques, which allow us to look directly into the lithium-ion battery during operation and observe the passivation layer as it forms. Atomic force microscopy (AFM) is one of the suitable methods we will use in this DFG project. However, this method is metrologically challenging in in-situ operation. As a result of the research project, we hope to be able to provide information about the components that make up the passivation layer and how the conductivity for lithium ions comes about exactly.

You are collaborating on the project with the University of Marburg. What is the TU Ilmenau's focus?

The scientists at the TU Ilmenau are focusing on two investigation methods in particular: The atomic force microscopy (AFM) I just mentioned and the electrochemical quartz microbalance. With the latter, we can determine various properties of the passivation layer, such as its viscoelastic behavior and indirectly its thickness. At the university in Marburg, supplementary investigation methods are used, such as measurements with the scanning transmission electron microscope and electrochemical impedance spectroscopy.

In the course of the energy turnaround, we can expect more electric cars on the road in the next few years. Is the raw material lithium sufficient to produce enough of such cars?

Lithium is available in large quantities and scientific work shows: there is no need to fear that lithium will run out any time soon - even if we assume high sales figures for electric cars.In the future, lithium will mainly be extracted from salt lakes, e.g. the Salar de Uyuni in Bolivia. Unfortunately, this will require large amounts of water, which will then no longer be available to the fragile ecosystem of the salar. However, it should be taken into account that the amount of lithium in a lithium-ion battery is very small (approx. 2%) and that there are definitely alternatives to lithium extraction in salt lakes.

The metal cobalt is also used in the production of lithium-ion batteries. This is currently mined mainly in the Congo, where working conditions are very poor and child labour is widespread. For this reason - and because of the high price of cobalt - attempts are being made to greatly reduce the proportion of cobalt in the cathode materials. This has already been successfully achieved. As a result, only a small proportion of modern cathode materials now consists of cobalt.

The interview was conducted by Eleonora Hamburg