Challenges in Making Tiny Batteries for Smart Dust Applications

Tiny batteries lead to big challenges.

(Image courtesy of The Michigan Micro mote. Source: Martin Vloet/University of Michigan.)

Many applications today, from medical diagnostics to surgery, biology, and agriculture, require very small monitoring electronics. Smaller than a millimeter in size and just hundreds of micrometers thick, these devices must sense and process information and wirelessly communicate. These sensors are known as smart dust. 

Smart dust devices are wireless micro-electro-mechanical sensors (MEMS) able to detect everything from light to sound, movement and vibrations. For example, Hitachi has successfully made an RFID IC chip with a size of 0.15 x 0.15 mm and a thickness of 7.5 µm. Designing these small devices is a real challenge. While the available technologies, such as etching and deposition, are quite successful in creating microelectronic devices based on semiconductor wafers, making millimeter-sized batteries remains a challenging and attractive researching topic. This article will present the technologies and methods that could lead to designing a powerful millimeter-sized battery.

The main challenge is that reducing battery size means reducing capacity as well. Millimeter-sized batteries have insufficient capacity to power the device for the required time. This implies that small devices need an external source of energy, e.g. solar panels, which involves the issue of additional size and continuity of energy supply. Creating such a small but powerful battery requires a new approach and several new developments:

• Battery materials that will improve storage capacity
• Improvements in battery energy density
• Innovative design and architectures to shrink and combine battery components
• Fabrication methods

This challenge involves different branches of electrical engineering and material, battery and polymer science. Developing a millimeter-sized battery can be grouped into two main tasks: finding an effective battery design and finding the right battery material.

Every battery contains electrodes, the anode and cathode, which are physically separated and immersed in an electrolyte. A separator prevents the electrodes from coming into contact and short-circuiting, which would destroy the battery. The ions from the electrolyte react with the electrodes. This chemical process causes the electron flow—the electricity. There are also metal current collectors connected to the electrodes that enable the battery to supply power. The larger the electrodes, the more charge they can store.

A real battery is made as a sandwich with many such layers. Deformation, defects and cracks in the battery disturb the electron and ion flow, increase resistance and cause the battery to fail.

Basic battery parts.

The basic battery design is presented here to understand the challenges in making a small but effective battery. First, the smaller the electrodes, the lower the battery capacity. Second, the electrodes must not have physical contact. Third, no part should have any physical defect.

Researchers from the Institute for Integrative Nanosciences in Germany have proposed four different battery design approaches to store more charge that could result in a powerful millimeter-sized battery. Their work is presented in Nature 589, 195-197 (2021).

The first approach is to build rows of magnetic particles inside thick electrodes. These lanes create conductive channels that should help charges move smoothly through electrodes. However, it is a challenge to add magnetic particles accurately on a very small scale. Additionally, a risk of cracks remains.

The research and development of negative electrodes that could accept a higher charge rate without metal deposition is not only of interest for smart dust applications. Growing demand for fast‐charging batteries in EVs has the same problem. The fabrication process is demonstrated for LiCoO2 and mesh carbon microbead graphite and presented in Fabrication of Low‐Tortuosity Ultrahigh‐Area‐Capacity Battery Electrodes through Magnetic Alignment of Emulsion‐Based Slurries. The experiment confirms that the LiCoO2 cathodes have low tortuosity via DC‐depolarization experiments and deliver high areal capacity. By using an emulsion‐based, magnetic‐alignment approach, thick electrodes with ultra-high areal capacity have been made. The electrodes are less than 400 µm thick with an areal capacity of up to 14 mAh/cm². It is significantly higher when compared to conventional Li-Ion electrodes with 2–4 mAh/cm² areal capacity. The researchers think that this fabrication method has the potential to enable thick, low‐tortuosity electrodes with many other active materials.  

Installing conductive channels enables smooth charge flow in fat electrodes. (Source: Nature/M. Zhu & O. G. Schmidt.)

This approach involves building many thin battery sandwiches together, thus increasing the area for collecting charge. Since it is not simple to deposit the layers correctly and keep them even, this approach also has real challenges. Annealing one electrode layer by using a high temperature can damage other layers, and different materials do not overlap well on top of others. As expected, a chance for malfunction increases with the number of layers.

The stacking of thin multiple battery layers increases the charging area. (Source: Nature/M. Zhu & O. G. Schmidt.)

The third idea is to build current collectors in arrays using pillars instead of sheets to increase the contact area of electrodes and electrolytes. These 3D structures can be made by engraving them into a silicon wafer. However, working on microscales is a challenge and has not been demonstrated. Coating the electrode materials would present an especially challenging issue.

Building current collector arrays increase the charge storage. (Source: Nature/M. Zhu & O. G. Schmidt.)

This design provides more power but filling materials into pillar arrays on a microscale remains a challenge.

The fourth design idea could increase storage capacity with more film windings (rolls) or folds using the so-called “micro-origami” method. This method means self-assembling 2D patterned nanomembranes into 3D microarchitectures, enabling for parallel and scalable manufacturing of microelectronic devices or micro-batteries. This can be done either by rolling or folding the battery films. This method is already used in cylinder batteries. On a large scale, the rolling can be done by hand. For a microscale, it becomes a challenge to roll films many times perfectly without inconsistencies. The final geometry can be plagued by structural inaccuracies. The potential energy landscape forces the rolling structures toward misassembly.

Roll (left) or fold (right) design approach. Battery film materials should be suitable to roll or fold. (Source: Nature/M. Zhu & O. G. Schmidt.)

A promising alternative approach assembles patterned nanomembranes into microelectronic devices by remotely programming the assembling under the influence of external magnetic fields. Magnetic guiding can be used to provide proper curling. Magnetic particles can be incorporated into the battery film, and an external magnetic field can be used to keep proper curling. Magnetostatic forces provide stabilization in the assembly process. The magnetic field can be achieved using axial or radial magnetic field approach methods.

Self-assembly of a “Swiss-roll” provided by using an axial magnetic field-assisted (left) and radial magnetic field-assisted (right) assembly process in a rotating external magnetic field. (Source: Institute for Integrative Nanosciences.)

This Institute for Integrative Nanosciences research group has used tiny capacitors—sheets of dielectrics placed between metals—to demonstrate this approach. However, the researchers believe that battery stacks are more challenging because they are thicker than capacitors, and mechanical behavior is difficult to predict.

The folding approach requires high force to bend the growing stack, and hinges are subject to stress and crack. It is estimated that folding a thin-film battery 30 times into an area suitable for the smallest computer, 0.14 mm², could power it for at least 100 days with one charge. However, many tiny applications require more-powerful batteries with hundreds of folds. More folds or windings always mean more energy stored.

To enable these micro-battery designs, it is important to provide advanced thin battery films using materials that provide better charge storage and are suitable for the micro-origami fabrication process. These battery materials should be compatible with semiconductor technologies allowing on-chip fabrication.

The most commonly used cathode materials, such as metal oxides LiMn2O4 and LiCoO2, allow microscale building by using etching or lifting off redundant materials. However, making microscale anodes and electrolytes remains a challenge. The anode is usually made of graphite, and electrolytes are liquid organic compounds dipped into a matrix or separator. Several solutions have been presented:

• To provide more charge storage, the anodes could be made of silicon and lithium, which have to be stabilized because they react to each other. When a battery is charging, it swells, destroying the electrode. As a solution, nanotechnology could be used to wrap silicon in graphene nano-sheets. Polymers that absorb the volume change could also be used.
• Metallic lithium anodes should be improved to have longer life cycles. When the battery discharges, lithium is removed from the anode and rebuilt while charging. With every cycle, the anode gradually wears down. To better manage lithium in an anode, a lithium electrode should be built from ions still in the electrolyte during charging instead of using a sliver of metal.
• Zinc anodes in aqueous zinc batteries can give better results if acidic instead of alkaline electrolytes are used. Anode has to be coated with an anti-corrosion layer because zinc dissolves in acid, releasing hydrogen. An electrolyte-decoupling strategy provides the optimal redox chemistry of both the Zn and MnO2 electrodes enabling the full potential of ZnMnO2 batteries. Those decoupled ZnMnO2 batteries have an open circuit voltage of 2.83 V, which is significantly higher than the typical voltage of 1.5 V in conventional ZnMnO2 batteries, and better cyclability. This approach can be used for other high-performance Zn-based aqueous batteries such as Zn–Cu and Zn–Ag batteries.
• Instead of a liquid electrolyte, solid ceramics can be used in tiny batteries. The issue with very thin ceramics is losing conductivity and becoming brittle. To solve that issue, polymers can be shaped. However, the processes for doing so (ion etching and photocuring) need to be improved by building links into their molecular chains that are easy to form or break. Other methods, such as spin coating or depositing polymer electrolytes in the gas phase, need to be improved. The liquid electrolyte still has better conductivity, so the polymer electrolytes’ conductivity must be improved.

Making micro-batteries that will supply microelectronic devices requires improvements in both battery design and materials. Materials and micro-electronics engineers must work hand in hand to develop micro-batteries with a high storage capacity that can be built into micro-devices. Design and mechanical engineers have to understand how changes in materials, such as crystallinity, thickness and synthesis routes, influence the stability of designed films. The self-assembly process is also influenced by the electrochemical and mechanical properties of the materials. Computer modeling and machine-learning algorithms are important to simulate and understand the process.