The researchers successfully demonstrated the new zinc-air battery in the scorpion and worm toy robots. (Image Credit: University of Michigan)
Current stand-alone batteries are inefficient, too large, add significant mass to any robot design, and need a strong structure for support. Now, researchers at the University of Michigan have developed a new type of zinc-air battery that can be integrated into the robot’s structure. It also reduces weight and takes up less space in addition to providing more energy to the machine than lithium-ion batteries. The team published their findings in Science Robotics.
“Robot designs are restricted by the need for batteries that often occupy 20% or more of the available space inside a robot, or account for a similar proportion of the robot’s weight,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research.
Other structural batteries could not match the energy density of today’s advanced lithium-ion batteries. To make them match, scientists improved an earlier version of zinc batteries on 10 different measures, some of which perform 100 times better. Kotov also believes that energy density and inexpensive materials may allow the battery to double the range of delivery robots.
The battery operates by moving hydroxide ions between a zinc electrode and the air side through an electrolyte membrane. The membrane is comprised of a network of aramid nanofibers, which are carbon-based fibers used in Kevlar vests, and a new water-based polymer gel. The membrane is strong enough to keep dendrites from growing on the electrodes, which can degrade the battery. Additionally, the gel helps to pass the hydroxide ions between the electrodes.
This battery, which is created with cheap, abundant and non-toxic materials, is more environmentally friendly than the ones used today. If the battery suffers from damage, the gel and nanofibers will not catch fire, unlike the flammable electrolyte in lithium-ion batteries. This zinc-air battery could provide up to 72 times more power compared to today’s stand-alone lithium-ion batteries with equal volume.
Researchers have tested their new creation on their worm and scorpion miniaturized toy robots, replacing the original batteries with zinc-air cells. They attached the cells to the motors and wrapped them around the exterior of the robots.
“Batteries that can do double duty—to store charge and protect the robot’s ‘organs’—replicate the multifunctionality of fat tissues serving to store energy in living creatures,” said Ahmet Emre, a doctoral student in biomedical engineering in Kotov’s lab.
However, zinc batteries maintain high capacity for only 100 cycles, which is a disadvantage compared to lithium-ion batteries that have 500 cycles. This is mainly due to the zinc metal producing spikes that puncture the membrane between the electrodes. The aramid nanofiber network helps to maintain a long cycle life for a zinc battery. It’s also easy to replace since the materials are inexpensive and recyclable.
“We don’t have a single sac of fat, which would be bulky and require a lot of costly energy transfer,” Kotov said. “Distributed energy storage, which is the biological way, is the way to go for highly efficient biomorphic devices.”
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