RFID & Wireless IoT Global Issue 04-2022
Fraunhofer IZM

Fraunhofer IZM Develops Microbatteries for Smallest Applications

Fraunhofer-IZM produces miniaturized lithium-ion batteries

Fraunhofer IZM

Microbatteries with dimensions of less than 1 x 1 mm are currently being produced at the Fraunhofer Institute for Reliability and Microintegration (IZM) in Berlin.

Batteries of this type are the foundation for the smallest energy-autonomous Sensoren, smart contact lenses with integrated display, and in-ear hearing aids with a new operating principle. The latest materials with high energy density are used.

Integrated Microbatteries from Fraunhofer IZM

Integrated Microbatteries from Fraunhofer IZM Power Contact Lenses, Insect Monitors and Hearing Aids.

Microbatteries are charged over hundreds of charge cycles, enabling innovative, futuristic applications. Microbatteries are manufactured at several thousand pieces per wafer in the clean room.

Microbatteries are manufactured at several thousand pieces per wafer in the clean room.

The Fraunhofer Institute for Reliability and Microintegration (IZM) conducts application- oriented, industry-related research. It was founded in 1993 and carries out development work for the automotive industry, medical and industrial electronics, and even for textile companies.

The "Microbatteries" working group is led by Dr. Robert Hahn and focuses on the extreme miniaturization of electronic systems. The foundation for this is a microbattery developed at Fraunhofer IZM. Dr. Hahn's working group is one of the world's most important research groups in this field.

Microbatteries help to open up new functionalities resulting from the proximity of components to each other and the miniaturization of the overall system. The increasing use of tiny, energy-autonomous wireless sensors, for example, can make the production and use of various products more efficient and thus conserve resources.

Microbatteries are lithium-ion batteries in the submillimeter range that can no longer be produced using conventional battery manufacturing technologies. Production is partially automated in a clean room.

The key to miniaturization is silicon. It enables much higher precision in production compared to other materials. Silicon structuring technology can be used to produce very small and thin-walled battery housings, so that even with dimensions in the millimeter range, a large volume fraction is still available for the active material.

In contrast, the smallest size of conventional batteries and button cells without silicon is in the cm range. At Fraunhofer IZM, silicon serves as a carrier and part of the battery housing.

Lithium is very reactive. If water or oxygen molecules from the ambient air were to enter the battery, the lithium would react with these molecules and would then no longer be available to the battery. With larger batteries, this is sometimes acceptable, however, with micro-batteries, every lithium ion counts. For this reason, the housing of a micro lithium-ion battery must be hermetically sealed.

This is achieved by depositing a metal layer after assembling the base and cover and structuring it lithographically so that the two battery terminals are then accessible for contacting by wire bonding.

A MICROBATTERY

  • Length and width: 0.5–10 mm, Thickness: 0.2–1 mm
  • Amperage: 5–100 μA
  • Battery storage capacity: 20–500 μAh
  • Voltage: 1.8, 2.5, 3.7 V depending on the electrodes used
  • Power per square centimeter: 1–3 mAh/cm2
  • Charging cycles: 500

Microbatteries power contact lenses, insect monitors and hearing aids

A Fraunhofer IZM microbattery is a central component of the electronic backpack worn by bees in the collaborative research project Sens4Bee. The aim of the project is to obtain data on the impact of climate change and intensive agriculture on bees and on the reasons for bee mortality. The battery is charged by a solar cell and energy harvesting. An RFID tag, a data logger and some sensors are also part of the electronic backpack. The size of the battery is 2x2 millimeters and the total weight of the backpack is ten milligrams. It is attached via a biocompatible adhesive directly in the first stage of the bee's development in an animal-friendly manner.

Similar insect monitoring is being carried out in the USA with monarch butterflies. To find reasons for the steady decline in the population recorded for almost 10 years, research teams at the University of Michigan have stuck minicomputers with integrated microbatteries on the backs of individual monarch butterflies. Sensors record light, temperature and air pressure during migration.

The latest hearing aids can be applied directly to the eardrum, similar to a contact lens for the eye. The start-up Vibrosonic has demonstrated this with its hearing contact lens.

It consists of an actuator that acts directly on the eardrum. The vibrations are transmitted to the ossicles without airborne sound and thus in a very efficient and distortion-free manner. The hearing aid has such low energy consumption that a micro-battery can be used. At present, it sits behind the ear with the microphone and is connected by wire to the lens in the ear.

The battery is removed and charged in the evening. In further development, the entire electronic system, including the battery, will be integrated with a solar cell as a hearing contact lens and applied inside the ear. Charging then takes place through an earplug via infrared radiation.

American start-up Mojo Vision has unveiled a prototype contact lens with an integrated 0.5 millimeter diameter display, the Mojo Lens. The display is powered by a battery developed in-house. Also integrated into the lens are a modem, an ARM CoreM0 processor and various sensors. The monochrome display shows information that complements what the user is experiencing. The Mojo Lens is therefore referred to as an augmented reality (AR) contact lens.

Microelectric devices that provide important insights into environmental changes and give people medical assistance only exist thanks to microbatteries.

RFID is a common technology in insect monitoring.
Vibrosonic's hearing contact lens is applied directly to the eardrum. The market launch of the Mojo Lens as a medical product is planned to occur within the next five years.

Battery material research, cost reduction and temperature stability

Miniaturization is only possible if new materials with high energy density and innovative manufacturing processes are continuously researched.

This micro battery was manufactured for a medical application.

This micro battery was manufactured for a medical application.

Microbattery zoom

Although the focus of microbattery technology involves miniaturization of the housing, the latest high-energy density battery materials must be used, or adapted to the needs of the microbattery. This involves a considerable amount of research. A great deal of momentum has been generated in battery and materials research, primarily due to the interest in electromobility. Battery researchers are constantly expanding the range of active materials for batteries.

The requirements for microbatteries, in terms of battery size, are very similar to those for traction batteries: both high-energy and high-power density are required, along with high cycle numbers. In the case of microbatteries, the high current pulses that occur during data transmission and the expectation of a rapid charging process in particular lead to high performance requirements.

The specified performance requirements can currently only be met with a liquid electrolyte. For this reason, Fraunhofer IZM uses liquid electrolyte, which leads to significantly lower internal resistance and higher amperage capability compared to a solid electrolyte. Nevertheless, a solid electrolyte would have great advantages in terms of production technology. The current manufacturing process is quite complex.

In this process, a micro-dispenser with a fine needle goes from one battery to the next and fills tiny nanolitre droplets of liquid electrolyte into each of the batteries.

However, the chemical composition of the liquid electrolyte cannot be the same as that of conventional, large batteries. It must be adapted to the requirements of the microbattery. A very low vapor pressure, for example, is necessary so that the electrolyte in the first battery has not already evaporated by the time the last one is filled.

Indentations are etched into the silicon, into which the battery materials are then inserted. Since the silicon is really only needed as a housing, this is an inefficient approach. Instead, it would be possible to replace the silicon with (even) cheaper plastics with an insulating layer, such as an inorganic sealing layer. However, it is difficult to produce plastics with such precision in the micrometer range next to each other on a substrate. One possibility would be microembossing.

In many applications of microsensors, high ambient temperatures occur and a high temperature stability of the microbatteries is desirable. However, batteries generally do not tolerate heat above approx. 80 °C. Special batteries for use in high temperatures do exist, but they are generally not rechargeable.

Fraunhofer IZM is therefore researching heat-resistant rechargeable microbatteries. Since the final encapsulation of the microbatteries itself involves process steps at elevated temperatures, the researchers hope that this will also simplify the manufacturing process, among other things.

INTERVIEW WITH DR. ROBERT HAHN
Interview

Microbatteries for the Ultra-Low-Power Applications of Tomorrow

Some of the applications that will become possible with microbatteries do not yet exist. Dr. Robert Hahn from the Fraunhofer Institute for Reliability and Microintegration (IZM) talks to RFID & Wireless IoT Global about fascinating projects, difficult challenges and the future of microsystems technology.

Dr. Robert Hahn is Group Leader Microenergy Systems at Fraunhofer IZM in Berlin.

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.

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.

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.

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.

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.

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.

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.

Cookies are necessary to provide you with our services. By continuing your visit on the website, you consent to the use of cookies.
More information Ok