Nano-batteries may enable mega possibilities

Bigger batteries are getting a lot of attention these days, where “bigger” is defined in terms of capacity, density, charging times, lifetime cycles, and other desirable attributes.

However, all this “big-battery” attention tends to obscure the significant but literally nearly invisible activity at the other end of the physical and energy scale with ever-smaller batteries. These could be used to power the electronics associated with microsensors, tiny actuators, and even nano-robots. If the batteries were small and light enough yet offered adequate capacity, they could be power medical micro-implants or free those swarming robo-insects from tethers or the need for laser beams focused on their minuscule solar cells for transmitted power (interestingly, those configurations are known as “marionettes” because they are powered by an external source).

Creating such batteries is the project undertaken by an MIT-led multi-university research team. They have developed and fabricated a battery which is 0.1 millimeters long and 0.002 millimeters thick that can capture oxygen from air and use it to oxidize zinc, creating a current at a potential of up to 1 volt.

Their battery consists of a zinc electrode connected to a platinum electrode, embedded into a strip of a polymer called SU-8, a high-contrast, epoxy-based photoresist designed for micromachining and other microelectronic applications where a thick chemically and thermally stable image is desired. When these electrodes interact with oxygen molecules from the air, the zinc becomes oxidized and releases electrons that flow to the platinum electrode, creating a current.

To fabricate these batteries, they photolithographically patterned a microscale zinc/platinum/SU-8 system to generate the highest energy-density microbattery at the picoliter (10⁻¹² liter) scale, Figure 1.

Figure 1 The fabrication and release of Zn/Pt/SU8 picoliter Zn-air batteries. (a) Side view schematic of a Zn-air picoliter battery placed in a droplet of electrolyte. (b) Height profile and (c) optical micrograph of an open-circuit Zn-air picoliter battery after fabrication. Scale bar: 40 μm. From a to c, the SU-8 base has a side length of 100 μm. d) Image of a Si wafer with a 100 × 100 array of picoliter batteries. (e)(f)(g) (h) Optical micrographs of picoliter batteries at different stages of the fabrication, as indicated by the annotation. (i) Optical micrograph of picoliter battery arrays patterned for Cu etching. Scale bar: 200 μm. (j) Schematics of batteries with loads (memristors in this case) released into solution. (k) Image of a bottle of dispersion containing 100 μm batteries. (l) Optical micrographs of open circuit and short-circuited Zn-air picoliter batteries, both are 100 μm. (m) Central image: optical micrographs of picoliter batteries deposited onto a glass slide. Scale bar: 200 μm. Side images: optical micrographs of individual batteries that were facing down (left), and up (right). Scale bar: 50 μm. (n) Optical micrographs of short-circuited batteries with various sizes. Scale bar: 50 μm. (o) Optical micrographs of 20 μm batteries after releasing and re-depositing onto a glass slide. (The dust in the leftmost image was residual from the sacrificial substrate.) The rightmost image showed a 20 μm battery that was facing downward.

The device scavenges ambient or solution-dissolved oxygen for a zinc oxidation reaction, achieving an energy density ranging from 760 to 1070 watt-hours per liter at scales below 100 micrometers in the lateral direction and 2 micrometers thickness in size. Similar to IC fabrication, the inherent “parallel” nature of photolithography processes allowed them to fabricate 10,000 devices per wafer.

Within a volume of only 2 picoliters each, these primary (non-rechargeable) microbatteries delivered open-circuit voltages of 1.05 ± 0.12 volts, with total energies ranging from 5.5 ± 0.3 to 7.7 ± 1.0 microjoules and a maximum power of nearly 2.7 nanowatts, Figure 2.

Figure 2 Performance summary and comparison. (a) Ragone plot of energy and power of individual batteries with 2 pL volume. The theoretical Gibbs free energy of the cell reaction is shown as the red dashed line. (b) Ragone plot of the average energy and power densities under 4 current densities. The error bars represent the standard deviation across multiple devices. The red squares are data of Li-MnO2 primary microbatteries from literature. (c) Master plot of the energy density versus cell volume for various microbatteries reported in the literature (electrolyte volume excluded for all entries). This work is shown in red asterisk.

While this doesn’t sound like much energy or power—and it isn’t, clearly—it’s enough for the diverse applications with which they tested it, such as powering a micrometer-sized memristor circuit for providing access to nonvolatile memory. They also cycled power to drive the reversible bending of microscale bimorph actuators at 0.05 hertz for mechanical functions of colloidal robots, powered two distinct nanosensor types, and supplied a clock circuit. In this study, the researchers used wires to connect their battery to the external powered device, but they plan to build robots in which the battery is incorporated into a device, analogous to an integrated circuit.

I could go into details of what they have done, how they did it, and their tests and results, but that would be duplicative to their paper “High energy density picoliter-scale zinc-air microbatteries for colloidal robotics” published in Science Robotics; while that paper is unfortunately behind a paywall, an identical preprint is fortunately posted here.

For their next phase, the researchers are working on increasing the voltage of the battery, which may enable additional applications. The research was funded by the U.S. Army Research Office, the U.S. Department of Energy, the National Science Foundation, and a MathWorks Engineering Fellowship.

Will these microbatteries become meaningful in the real world? Do they provide adequate useful power with enough energy capacity for projects you might like to explore? Can you think of situations where you would use them? Could they lead to new types of powered devices that are so tiny that new applications become realistic? Or are they just another eye-catching, head-turning topic which is well-positioned to get more research grants?

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