Applications for mobile robots are exploding, from delivery drones and delivery robots to nursing robots and robots for warehouses. At the micro level, researchers are looking at very small swarm robots that can self-assemble into larger units. Here, conventional individual batteries would be too large and inefficient. Biomorphic batteries, which are integrated into the structure of the robot like fat pads, can create space and reduce weight. So far, however, they have only been able to supplement the main battery, not replace it.

"When designing robots, developers are constrained by the need for batteries, which often take up more than twenty percent of the available space inside a robot or make up about the same proportion of its weight," explained Nicholas Kotov, Professor of Chemical Engineering and Materials Science at the University of Michigan, who led the research.

He adds: "No other structural battery that has been reported is comparable to current lithium-ion batteries in terms of energy density. We have improved our previous version of structural zinc-air batteries by ten different measures, achieving a factor of 100 improvement in individual parameters." The combination of energy density and cost-effective materials means that the battery can already double the range of delivery robots, he said.

"But this is not the limit. We estimate that robots could have 72 times more capacity if their outer shell were replaced with zinc batteries, compared to a single lithium-ion battery," said Mingqiang Wang, first author and recent visiting researcher in Kotov's lab.

"The batteries serve two purposes at once," added Ahmet Emre, a biomedical engineering PhD student in Kotov's lab. "They store energy and protect the inner workings of the robot. In this way, they replicate the multifunctionality of fatty tissue, which serves as an energy store in living beings."

Structure of the new zinc-air battery

The new battery is based on passing hydroxide ions between a zinc electrode and the air side through an electrolyte membrane. This membrane consists partly of a mesh of aramid nanofibers - carbon-based fibers found in Kevlar vests - and a new water-based polymer gel. The gel supports the exchange of hydroxide ions between the electrodes.

The battery is made from inexpensive, abundant and largely non-toxic materials and is more environmentally friendly than those currently in use. The gel and the aramid nanofibers do not ignite if the battery is damaged - unlike the flammable electrolyte in lithium-ion batteries. In addition, the aramid nanofibers can be reused from discarded body armor.

To demonstrate their batteries, the researchers tested them with normal-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 to the drives and wrapped them around the outer surfaces of the creepy crawlies.

The downside of zinc batteries is that they retain their high capacity for around 100 cycles, rather than the 500 or more we expect from the lithium-ion batteries in our smartphones. This is because the zinc metal forms needles that eventually pierce the membrane between the electrodes. The strong mesh of aramid nanofibers between the electrodes is the key to the relatively long life cycle of a zinc battery. And thanks to the inexpensive and recyclable materials, the batteries are easy to replace.

Beyond the benefits of the battery's chemistry, Kotov says the design could enable a shift from a single battery to distributed energy storage.

The University of Michigan has applied for patent protection and is looking for partners to commercialize the technology.