Future microbatteries could help tiny robots tackle space and time

This projected growth appears to be driven by the expanded use of wearable instruments, the requirement for compact batteries in clinical gadgets, and the rising interest in versatile batteries for the ever-decreasing size of IoT systems.

Equally crucial is the provision of microbatteries to fit into these advancing IoT systems so they can operate. One particular area of research with growing momentum is the development of sub-milliliter scale batteries to power 'smart dust' concepts.

So without further ado, let's take a look.

What is smart dust?

'Smart dust' networks refer to an advanced version of some Internet of Things (IoT) devices. The technology is a wireless system of many sub-millimeter-scale devices with sensors, cameras, and communication functions that can collect and process data.

Also referred to as microelectromechanical systems (MEMS), smart dust can sense and record data about the surrounding environment, such as light, temperature, sound, and the presence of chemicals. This data can then be transmitted wirelessly to larger computer systems for analysis and action.

"Such a tiny size opens up possibilities to access currently unavailable places. You can imagine smart dust distributed in wheat fields to provide more crop growth data or motile smart dust entering into ruins to look for the signal for life.", Dr. Minshen Zhu, a principal materials scientist at TU Chemnitz, Germany, explains to Interesting Engineering (IE).

One challenge in advancing smart dust concepts is a lack of equally small on-chip power sources that can function anytime and anywhere.

Powering 'smart dust' technology

Energy conversion systems that can harvest energy from external sources, such as on-chip photovoltaics or 'solar-powered smart dust,' have been proposed as one solution.

However, as with any solar-powered device, this is likely to result in systems only operating during specific times and in specific localities. In a mobile world which requires on-demand operation 24/7, you can imagine how this approach could considerably limit the operation of smart dust in many environments.

Another solution is onboard energy storage in the form of 'microbatteries' as an alternative solution. For instance, in a future of ubiquitous computing where microelectronics such as miniaturized computers exist, there comes a need for sub-square-millimeter microbatteries.

A study earlier this year, also co-authored by Dr. Minshen Zhu, already demonstrated encouraging results for a prototype microbattery smaller than a grain of salt for powering dust-sized computers. However, the study highlighted the need for on-chip microbatteries to balance achieving both a high energy density and a small footprint.

Serving as motivation to meet this challenge is possibly the most successful design in the bulky battery world- the Swiss-roll cylinder battery.

This battery comprises many layers of electrode material in a limited volume. Cylindrical cells store less energy than prismatic cells, but they also have more power. This means that cylindrical cells can discharge their energy faster than prismatic cells.

One real-world application is the use of Swiss-roll-style cylindrical batteries to power electric cars, as with Tesla's 4680 cells. But, individual 4680 cells measure 46 mm in diameter and 80 mm in length (around 1000 of them may be used in each vehicle), and there is not yet a technique for realizing this at the microscale until now.

Development of on-chip Swiss-roll microbattery

A team of researchers at the Chemnitz University of Technology in Germany recently published a study on Advanced Energy Materials.

They developed an on-chip Swiss-roll-style microbattery. By combining the Swiss-roll structure with an electrode slurry, the team opened doors for high-energy-density micro batteries to enter the sub-square-millimeter era.

"Actually, we use the same architecture of batteries used in Tesla cars: Swiss-roll (jelly roll) because it is one of the most successful battery structures. For the full-sized battery, it is easy to use a mandrel to wind films up, forming Swiss-rolls," says Dr. Minshen.

But at the microscale, no tools are available. Therefore, we use an on-chip self-assembly process driven by inherent strain in layered thin films. This technique is also known as micro-origami," adds Dr. Minshen.

The researchers describe the process as an actuator layer stack made of a non-swellable polyimide film and a swellable hydrogel layer. The two layers roll up a thin metal-layer current collector, forming a micro-Swiss-roll.

The result is a micro-Swiss-roll of 3 mm in length and about 178 µm in diameter- much smaller than a grain of rice. The development is an encouraging result for a solution for constructing sub-millimeter-sized devices.

The study also reveals that such a self-assembly mechanism allows for the parallel fabrication of multiple micro-Swiss-rolls on the wafer in a single run. This could make it much easier and cost-effective to scale up production.

A novel fast-drying electrode slurry

The study points out that one major factor restricting the energy density of microbatteries is the limited choice of electrode materials. This is because the materials used for on-chip microbatteries are obtained mainly by using deposition tools.

However, electrode slurries containing high-capacity materials, binders, and conductive additives offer good stability, high conductivity, and excellent energy storage. Slurries are therefore used for constructing energy-dense bulky batteries.

Still, one problem with standard electrode slurries is in their drying periods - which can be more than 10 hours. And in addition to this time consideration, the temperatures needed are as high as 120°C, which can destroy the microstructure. A quicker drying period is therefore desired.

The team addressed this issue by creating a one-hour-drying electrode slurry. The innovation behind this breakthrough is the use of dispersing magnesium dioxide (MnO2) nanowires in a zincophilic binder, i.e., polyimide.

In simple terms, this polyimide wraps around MnO2 to prevent its dissolution into the electrolyte. The innovative polyimide-MnO2 duo improves the zinc ion transportability and stabilizes the electrode over long-term cycling.

"The successful demonstration of the combination of the on-chip self-assembly process and high-performance battery material slurry provides an alternative way to develop high-performance batteries, which has long been regarded as not feasible. It turns out that it is possible to combine technologies from two fields that are not related to each other," Dr. Zhu tells IE.

Microbatteries for Micro-bots

Miniature wireless-enabled electronics in the form of microrobots have received widespread attention. Future uses could one day include micro-scale machines swimming between our cells, tracking down and treating various diseases.

While this may sound like something for the distant future, some scientists are currently working to create such microscopic devices.

For instance, earlier this year, researchers from the University of Chemistry and Technology Prague developed urea-powered microrobots as a means of treatment for bladder infections.

Like the development of many microrobots, the energy used to power the bots was harvested from an external source, in this case, urea (a nitrogen compound and the end product of the metabolic breakdown of proteins in mammals).

The bots harvested chemical energy using urease enzymes, which convert urea (found in the urinary tract) into chemical energy to provide self-propulsion.

"Integrating [our] microbatteries into micro-robots would free them from operation time and space constraints. Micro-robots integrated with batteries would operate anytime and anywhere," claims Dr. Zhu.

Limitations and the future of microbatteries

There are still a number of challenges facing microbatteries in terms of device design and materials optimization.

One of the main limitations facing microbattery development is the conflict between the high mass loading (defined by the weight of electrode slurry on the current collector in a unit area) of materials and the small battery footprint. A high mass loading allows the microbattery to store more energy.

However, a large amount of materials on a small footprint is unstable and impractical. Dr. Zhu and colleagues' recent development of a microbattery using the micro-origami technology helps to solve this dilemma.

"Microbatteries are also plagued by the same problems that limit the performance of full-sized batteries, such as safety issues, energy decay at low temperatures, heating during the operation, etc.," highlights Dr. Zhu.

"It is hard to define the future of smart dust because it has so many possibilities. First, smart dust can expand the capabilities of current smart devices. For instance, they can enter tiny places and monitor insect movements. More importantly, smart dust can have colony behaviors, like bees and ants, to accomplish more complicated tasks," Dr. Zhu further explains.

The researchers' on-chip Swiss-roll microbattery's next steps are to make the technology as reliable as possible and to improve its readiness.