Biomorphic batteries could provide more energy for robots

Similar to how biological fat reserves store energy in animals, a new rechargeable zinc battery integrates into the structure of a robot to provide much more energy.

That’s according to a new study led by the University of Michigan (U-M) and published in the journal Science Robotics. This approach to increasing capacity will be particularly important as robots shrink to the microscale and below — scales at which current standalone 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,” said U-M Professor Nicholas Kotov, who led the new research.

Applications for mobile robots are exploding, from delivery drones and takeaway 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.

“No other structural battery reported is comparable, in terms of energy density, to today’s state-of-the-art advanced lithium batteries,” Prof Kotov said. “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 researchers’ 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 nanofibres — the carbon-based fibres 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 non-toxic materials, the battery is designed to be more environmentally friendly than those currently in use. The gel and aramid nanofibres will not catch fire if the battery is damaged, unlike the flammable electrolyte in lithium-ion batteries. The aramid nanofibres could also be upcycled from retired body armour.

To demonstrate their batteries, the researchers experimented with regular-sized and miniaturised 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,” said Ahmet Emre, a doctoral student in Prof Kotov’s lab.

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 nanofibre network between the electrodes is the key to the relatively long cycle life for a zinc battery, while the inexpensive and recyclable materials make the batteries easy to replace.

The combination of energy density and inexpensive materials means the new battery may already double the range of delivery robots. “This is not the limit, however,” said Mingqiang Wang, recently a visiting researcher to Prof 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.”

Beyond the advantages of the battery’s chemistry, Prof Kotov said the design could enable a shift from a single battery to distributed energy storage, using a graph theory approach developed at U-M.

“We don’t have a single sac of fat, which would be bulky and require a lot of costly energy transfer,” Prof Kotov said. “Distributed energy storage, which is the biological way, is the way to go for highly efficient biomorphic devices.”

The university has applied for patent protection and is now seeking commercial partners to bring the technology to market.

Image credit: ©stock.adobe.com/au/phonlamaiphoto

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