1. Dr. Hahn, you and your working group manufacture microbatteries. How wide is the range of applications for these microbatteries?
As a matter of fact, it is not yet that extensive. Our batteries are about 1 x 1 mm or 0.5 x 2 mm, so they are really very small. They can only be loaded with a few microamperes, maybe 50 μA, for a short period of time. There are not that many applications for this range yet.
2. What is the reason for this?
There are actually hardly any chips today that manage with such low power requirements. As we see it, we are a pioneer in battery development, but we are also currently largely limited to the field of research in terms of applications.
Other research institutes and larger companies that conduct research are in the same position. They commission us with battery developments for their own research and are preparing for future applications in large quantities. So far, we are producing small quantities for very interesting applications.
3. Which of the projects that use one of your batteries excites you the most?
That would certainly be Sens4Bee, the joint project with the Helmholtz Center for Environmental Research in Leipzig and the company Micro Sensys in Erfurt. In this project, bees are fitted with a data backpack that integrates sensors and our battery in addition to an RFID tag. As they fly around, the bees generate data that allows conclusions to be drawn about the effects of climate change and intensive bee agriculture, helping us to understand and ultimately prevent bee mortality.
4. Were you able to use an existing battery for this or did you have to make modifications, for instance, because of the weight of the battery on the bee?
The electronic backpack weighs about 10 mg. This does not bother the bee and, according to the experts, does not interfere with its behavior. Bees do not have their own sense of weight. They can collect a maximum of about 60 mg of nectar, and they do so even with the electronic backpack.
However, we did make changes to an existing battery for Sens4Bee. This is because normally, the battery housing is metalized, which has a very unfavorable effect on the function of the antenna coil that is part of the system. For this reason, we did not metallize the entire surface of the battery for this project, but rather only around the perimeter in the area of the actual joint. The battery is charged using multiple solar cells connected in series. Therefore, we used battery electrode materials that result in a low nominal voltage. The normal battery voltage of 4 volts would simply be too high here, and we would then have needed a voltage converter. This way, we get by without one.
5. How do specific changes to a battery for a project affect pricing?
Rather unfavorably. The smaller the quantities we produce, the more expensive the individual battery. Our batteries for hearing aids, for example, cost around 10 euros. That is still acceptable, but there are applications for which this is simply too much. For this reason, we are constantly researching ways to produce microbatteries at lower cost. In addition to our ongoing efforts to qualify the latest active materials for our technology, this is a very important part of our work.
6. Take us briefly into your everyday work: What materials do you research and why?
There are materials with higher energy density, so that the amount of energy in the same volume increases. This means that the battery becomes smaller while the power remains the same or increases. We would like to use such materials. A very suitable material in this respect is metallic lithium. We would like to use it for the anode because it has the highest energy density.
The difficulty with it, however, is recharging, because dendrites are created during discharge. They are one of the biggest problems in battery reliability. Dendrites are narrow metal needles that grow on the negative terminal during charging and eventually penetrate the separator, causing a short circuit.
Lithium is simply very reactive. In connection with solid-state electrolytes, however, it could work with metallic lithium, because dendrite growth can be better suppressed there. However, the internal resistance of solid-state batteries is still significantly higher than that of batteries with liquid electrolytes, especially at room temperature or at lower temperatures. As a result, the current conductivity is insufficient for many applications. Lithium titanate, on the other hand, has a lower energy density but a high stability over many thousands of cycles, which we also often use as a negative electrode.
7. It sounds like you have to find just the right compromise between energy density, conductivity and stability of the active materials for each application, for one of your batteries.
Yes, this is indeed the case. Lithium titanate, for instance, remains stable during deep discharges. This is particularly favorable for use in insect monitoring, and that is where we use it. The battery in the Sens4Bee bee project charges via a solar cell, but it might not work for several days in a row if it is cloudy.
A lithium-ion battery with graphite as the negative electrode would then break down, while a lithium titanate battery would last a few days at 0 volts. This is a special advantage for sensor applications where not everything can be controlled. There is no one battery for every solution. You have to consider the options for each application.
Thank you very much!
