Researchers at the Massachusetts Institute of Technology (MIT) have developed groundbreaking zinc (Zn)-based micro-batteries that deliver impressive energy output in volumes as small as two picoliters each. These microscopic power sources are poised to revolutionize the functionality of tiny sensors and robotic components.

Remarkably, a single 2-inch silicon wafer can produce up to 10,000 of these micro-batteries, each with the capacity to power minuscule devices. The batteries harness oxygen from their surroundings to trigger a zinc oxidation reaction, achieving an energy density between 760 and 1,070 watt-hours per liter. Despite their minuscule size—less than 100 micrometers wide and just 2 micrometers thick—these batteries pack a powerful punch.

While technological advancements have consistently miniaturized robotic devices, reducing the size of their energy sources has been a significant challenge. Traditional battery materials and manufacturing processes typically do not scale down well to match the needs of microelectronics, often leaving micro-batteries confined to millimeter-sized scales—far larger than the devices they are meant to power. Alternative power sources, like solar cells, are also limited by their reliance on light, restricting their use in low-light conditions, such as underground or within pipelines.

To address these challenges, the MIT researchers developed an innovative Zn-air picoliter battery that can be photolithographically etched onto a silicon wafer and subsequently released into a solution. This process allows for the creation of high-energy-density micro-batteries at the picoliter scale.

Using photolithography, the research team created tiny zinc/platinum/SU-8 batteries that leverage oxygen in the air or dissolved in solutions to drive the zinc oxidation reaction. These batteries achieve an energy density of 760 to 1,070 watt-hours per liter, with each battery measuring under 100 micrometers wide and 2 micrometers thick. The efficiency of the photolithography process enables the production of 10,000 batteries on a single silicon wafer, which can then be suspended in liquid as colloidal particles, each storing energy onboard.

Each of these tiny batteries, just 2 picoliters in volume, produces voltages around 1.05 volts, with a total energy output of 5.5 to 7.7 microjoules and a maximum power close to 2.7 nanowatts. Tests demonstrated that these batteries are capable of powering extremely small devices, such as micrometer-sized memristor circuits used for nonvolatile memory. The batteries were also used to power microscale actuators that bend at a rate of 0.05 hertz, essential for robotic movement, as well as two types of nanosensors and a clock circuit.

Although these colloidal batteries are designed to function in large electrolyte reservoirs, some applications may require them to operate in dry environments or without available ionic species. To explore these scenarios, the researchers conducted two proof-of-concept experiments.

In the first experiment, a micro ink-jet printer was used to deposit 500 picoliters of ionic liquid electrolyte onto the batteries. Despite the higher viscosity and lower conductivity of the electrolyte, the batteries delivered 0.82 microjoules of energy at 0.1 mA cm−2. In the second experiment, 20 picoliters of salt were dried onto the battery. When immersed in deionized water, these batteries released salt, producing between 0.6 and 1.2 microjoules of energy. These experiments demonstrated that Zn-air batteries could function in different environments, even without a large electrolyte supply.

Thanks to their high energy density, compact size, and efficient design, these picoliter zinc-air batteries hold great promise for mass production and integration into tiny, independently functioning robots. As technology continues to shrink, these micro-batteries could play a crucial role in powering the next generation of miniature devices.

By Impact Lab