Researchers at the University of Michigan in Ann Arbor have developed a rechargeable zinc battery that integrates into the structure of a robot to provide more energy.
The approach is especially important as robots shrink to the microscale and below. At smaller scales, today’s stand-alone batteries are too big and inefficient. The solution offers a battery that wraps around the structure of the robot, much like how fat offers energy to living organisms from all parts of the body.
“Robot designs are restricted by the need for batteries that often occupy 20 percent or more of the available space inside a robot, or account for a similar proportion of the robot’s weight,” says Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research.
Applications for mobile robots are growing and include delivery drones, bike-lane take-out bots, robotic nurses, and warehouse robots. On the micro side, researchers are exploring swarm robots that can self-assemble into larger devices. Multifunctional structural batteries can free up space and reduce weight but, until now, they could only supplement the main battery.
“No other structural battery reported is comparable, in terms of energy density, to today’s state-of-the-art advanced lithium batteries,” says Kotov. “We improved our prior version of structural zinc batteries on 10 different measures, some of which are 100 times better, to make it happen.”
The combination of energy density and inexpensive materials means the battery may be able to double the range of delivery robots, Kotov says.
“This is not the limit, however,” says Mingqiang Wang, first author and recently a visiting researcher to Kotov’s lab. “We estimate that robots could have 72 times more power capacity if their exteriors were replaced with zinc batteries, compared to having a single lithium ion battery.”
The new battery works by passing hydroxide ions between a zinc electrode and the air side through an electrolyte membrane. The membrane is partly a network of aramid nanofibers – the carbon-based fibers found in bullet-proof vests – and a new water-based polymer gel. The gel helps move the hydroxide ions between the electrodes.
Made with cheap, abundant, and largely nontoxic materials, the battery is more environmentally friendly than those currently in use. The gel and aramid nanofibers will not catch fire if the battery is damaged, unlike the flammable electrolyte in lithium ion batteries. The aramid nanofibers could be upcycled from retired body armor.
To demonstrate the batteries, researchers experimented with regular-sized and miniaturized toy robots in the shape of a worm and a scorpion. The team replaced their original batteries with zinc-air cells. They wired the cells into the motors and wrapped them around the outsides 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,” says Ahmet Emre, a doctoral student in biomedical engineering in Kotov’s lab.
The downside of the batteries is that they maintain high capacity for about 100 cycles, rather than the 500 or more that are expected of lithium ion batteries in smartphones. This is because the zinc metal forms spikes that eventually pierce the aramid nanofiber membrane between the electrodes. This network is the key to the relatively long cycle life for a zinc battery. The inexpensive and recyclable materials make the batteries easy to replace.
Kotov says the battery’s design could enable a shift from a single battery to distributed energy storage that would come from all over the robot.
“We don’t have a single sac of fat, which would be bulky and require a lot of costly energy transfer,” Kotov says. “Distributed energy storage, which is the biological way, is the way to go for highly efficient biomorphic devices.”
U-M has applied for patent protection and is seeking commercial partners to bring the technology to market. A paper of the research titled “Biomorphic structural batteries for robotics” is slated for publication in Science Robotics.
The research is funded by the Department of Defense, the National Science Foundation, and the Air Force Office of Scientific Research.