Like biological fat reserves store energy in animals, a new rechargeable zinc battery integrates into the structure of a robot to provide much more energy, a team led by the University of Michigan has shown.
This approach to increasing capacity will be particularly important as robots shrink to the microscale and below—scales at which current stand-alone batteries are too big and inefficient.
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.
Applications for mobile robots are exploding, from delivery drones and bike-lane take-out bots to 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 potentially free up space and reduce weight, but until now they could only supplement the main battery.
The researchers 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. That membrane is partly a network of aramid nanofibers—the carbon-based fibers found in Kevlar vests—and a new water-based polymer gel. The gel helps shuttle 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 their batteries, the 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 creepy crawlers.
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.
The downside of zinc batteries is that they maintain high capacity for about 100 cycles, rather than the 500 or more that we expect from the lithium ion batteries in our smartphones. This is because the zinc metal forms spikes that eventually pierce the membrane between the electrodes. The strong aramid nanofiber network between the electrodes is the key to the relatively long cycle life for a zinc battery. And the inexpensive and recyclable materials make the batteries easy to replace.
A paper on this research is published in the journal Science Robotics, titled, “Biomorphic structural batteries for robotics.”
News Source: University of Michigan
