Researchers at the University of Pennsylvania’s School of Engineering and Applied Science have developed new current collector and cathode designs that increase the fraction of materials that store energy while simultaneously serving as a protective shell. This reduces the need for non-conductive packaging and boosts the energy density by a factor of four.
These microbatteries are built by the electroplating of 130 µm-thick 94 percent dense additive-free and crystallographically oriented LiCoO2 onto thin metal foils, which also act as the encapsulation layer. These devices weigh 50 to 80mg and have a volume of 20 to 40 µL, giving them an energy density of 430 Wh/kg and 1050 Wh/l.
“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.”
The 840mg weight opens up smaller flying microrobots, implanted medical devices with longer lifespans, and a variety of otherwise impossible devices for the Internet of Things (IoT).
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.
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“Instead 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,” said John Cook, Director of R&D at Xerion Advanced Battery and co-leader of the study,
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.
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