Robotic designs are frequently inspired by nature, but not when it comes to energy storage solutions. New rechargeable zinc-air batteries developed at the University of Michigan can be integrated into the structure of robots, allowing for a greater surface area for the battery structure while providing additional protection to the internal components of the machine. They also provide additional load bearing capacities just like fatty tissue on an animal.
“No other structural battery reported is comparable, in terms of energy density, to today’s state-of-the-art advanced lithium batteries. We improved our prior version of structural zinc batteries on 10 different measures, some of which are 100 times better,” said Nicholas Kotov, lead researcher on the project.
The fat-like battery works by passing hydroxide ions between a zinc electrode and the air side through an electrolyte membrane that is made up of aramid nanofibers (aka ANFs, carbon-based fibers found in Kevlar) and a new water-based gel that aids the flow of ions between the electrodes. The cartilage-like morphology of the nanofibers improves the efficiency of the batteries while providing structural benefits.
The batteries integrated into “battery-less” miniature robots that have the zinc-air battery wrapped around them. (Image from Science Robotics.)
The biomorphic zinc-air batteries have 72 times greater capacity when compared to a typical lithium-ion battery with the same volume. In its current state, the technology has an energy density that may already double the range of delivery robots. Aside from large-scale commercial applications, this efficiency will be especially crucial when it comes to robots at the microscale and below—there will no longer be any need to devote precious space to an internal battery, which will allow for small models.
“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,” said Kotov.
Compared to lithium-ion batteries, zinc-air batteries only maintain high capacity for about 100 cycles (compared to the 500+ in lithium-ion batteries). This is due to the zinc metal forming dendrites, which eventually pierce the membrane between electrodes. The aramid nanofiber network is key to maintaining the relatively long life cycle of the battery.
The zinc-air batteries are more environmentally friendly than lithium-ion batteries, which currently dominate the market, while the raw materials for the zinc-air batteries are also cheaper and more abundant. An added safety feature of this design is that the aramid nanofibers will not catch fire if they are damaged, unlike the flammable electrolytes in lithium-ion batteries. The researchers have also identified opportunities to upcycle the aramid nanofibers from retired body armor.
The zinc-air membrane batteries can be easily incorporated into existing designs that previously used lithium-ion batteries.
The battery design could enable a shift to a more distributed energy storage approach when it comes to robotics. This mimics nature even more closely, as it would store energy more evenly across the robot, similar to lipids in animals, as opposed to storing it in one location. Not only is there a protective advantage in the system being covered with the membrane, but the distributed energy storage approach also allows the robot to operate when parts of the battery are damaged.
A paper on this research titled “Biomorphic structural batteries for robotics” has been published in Science Robotics.