‘Packaging-free’ microbattery design quadruples energy density
An increasing need for microbatteries that store more power in less space is being driven by ever-shrinking wireless-enabled electronics. However, say the researchers, energy density gets exponentially harder to improve upon as a battery gets smaller, partially because larger portions of a battery’s footprint must be devoted to protective packaging.
With that in mind, the researchers developed a new kind of current collector and cathode that increase the fraction of materials that store energy while simultaneously serving as a protective shell. This, say the researchers, reduces the need for non-conductive packaging that normally protects a battery’s sensitive internal chemicals.
“We essentially made current collectors that perform double duty,” says James Pikul, assistant professor in the Department of Mechanical Engineering and Applied Mechanics in Penn engineering and a leader of the study. “They act as both an electron conductor and as the packaging that prevents water and oxygen from getting into the battery.”
That extra space efficiency, say the researchers, results in an energy density four times that of current state-of-the-art microbatteries. The researchers’ microbattery design is said to be light enough to be carried by an insect, and opens the door for smaller flying microrobots, implanted medical devices with longer lifespans, and a variety of otherwise impossible devices for the Internet of Things (IoT).
Batteries store energy in the form of chemical bonds, releasing that energy when those bonds are broken. To function properly, this reaction must occur only when power is needed, but then must react rapidly enough to deliver a useful amount of current.
To address the latter requirement, microbatteries have historically required thin electrodes. While this thinness allows more electrons and ions to move quickly through the electrodes, it comes at the cost of having less energy-storing chemicals as well as complex designs that are difficult to manufacture.
The researchers developed a new way to make electrodes that allowed them to be thick while also allowing fast ion and electron transport. Conventional cathodes consist of crushed particles compressed together, a process that results in large spaces between electrodes and a random internal configuration that slows ions as they move through the battery.
“Instead,” says John Cook, Director of R&D at Xerion Advanced Battery Corp., and co-leader of the study, “we deposit the cathode directly from a bath of molten salts, which gives us a huge advantage over conventional cathodes because ours have almost no porosity, or air gaps.”
Pikul adds, “This process also aligns the cathode’s ‘atomic highways,’ meaning lithium ions can move via the fastest and most direct routes through the cathode and into the device, improving the microbattery’s power density while maintaining a high energy density.”
These redesigned components are so efficient at transporting ions, say the researchers, that they can be made thick enough to double the amount of energy-storing chemicals without sacrificing the speed necessary to actually power the devices they’re connected to. Combined with the new packaging, these microbatteries reportedly have the energy and power density of batteries that are a hundred times larger while only weighing as much as two grains of rice.
Looking ahead, the researchers say they will continue to study chemical and physical features that can be tuned to further improve the performance, while also building wearable devices and microrobots that take advantage of these new power sources.