Large-format batteries’ ability to be electrochemically scaled down to smaller, microscale power sources has always been limited, making them difficult to power microdevices, microrobots, and implanted medical devices.
Material science and engineering Professor Paul Braun (Grainger Distinguished Chair in Engineering, Director of the Materials Research Laboratory), Dr. Sungbong Kim (Postdoc, MatSE, current assistant professor at Korea Military Academy, co-first author), and Arghya Patra published a paper describing their findings in Cell Reports Physical Science (Graduate Student, MatSE, MRL, co-first author). They overcome this challenge by developing a high-voltage microbattery (> 9 V) with remarkable performance.
The researchers demonstrated single-, double-, and triple-stacked hermetically sealed (tightly packed to prevent exposure to ambient air) lithium batteries with extremely low package mass fractions, unprecedented operating voltages, high power densities, and energy densities.
Braun contends that “we need powerful tiny batteries to exploit the full potential of microscale technologies” through improving electrode topologies and inventing unique battery designs. The problem is that when batteries shrink in size, the packaging takes up more and more space inside the battery, limiting the electrode surface area. As a result, the energy and power of the battery are dramatically diminished.
In their revolutionary design of potent microbatteries, the researchers developed novel packaging technology that included positive and negative terminal current collectors as part of the package itself (rather than a separate entity). This enabled the batteries’ compact volume (0.165 cm3) and low package mass fraction (10.2%). They also vertically stacked the electrode cells in series to achieve the high operating voltage of the battery (such that the voltage of each cell adds).
Another way to improve these microbatteries is to use highly dense electrodes with high energy density. Polymers and carbon additives account for approximately 40% of the volume of standard electrodes (not active materials). Braun’s group generated electrodes that are entirely thick and free of polymer and carbon additives utilising an intermediate temperature direct electrodeposition approach. These totally dense electrodes have a higher volumetric energy density than their commercial counterparts. The dense electroplated DirectPlateTM LiCoO2 electrodes used in this investigation were manufactured by Xerion Advanced Battery Corporation (XABC, Dayton, Ohio), a company created as a result of Braun’s research.
Current micro-nanoscale electrode topologies and cell designs, according to Patra, are only available in power-dense configurations that sacrifice porosity and volumetric energy density. We were able to create a microscale energy source with a high power density and volumetric energy density.
Insect-sized microrobots powered by these microbatteries can collect critical data during natural disasters, search and rescue operations, and in dangerous areas where direct human access is difficult. “The high voltage is crucial for minimising the electronic payload that a microrobot needs to carry,” says James Pikul, co-author and assistant professor at the University of Pennsylvania’s department of mechanical engineering and applied mechanics. Motors can be powered directly by 9 V, reducing the energy lost during the process of boosting the voltage to the hundreds or thousands of volts required by some actuators. This suggests that, in addition to higher energy density, these batteries provide system-level upgrades, allowing small robots to move farther or relay more essential data to human controllers.
Our work bridges the knowledge gap at the intersection of materials chemistry, special manufacturing requirements for energy-dense planar microbattery configurations, and applied nano-microelectronics, which require a high-voltage, on-board type power source to drive microactuators and micromotors, Kim says.
“Our current microbattery design is well-suited for high-energy, high-power, high-voltage, single-discharge applications,” says Braun, a battery miniaturisation pioneer. The idea will now be applied to all solid-state microbattery systems, which are safer and more energy-dense than liquid-cell counterparts.