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Better robot batteries get inspiration from body fat (w/video)

(Nanowerk News) 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 ("Biomorphic structural batteries for robotics").
This approach to increasing capacity will be particularly important as robots shrink to the microscale and below梥cales at which current stand-alone batteries are too big and inefficient.
揜obot 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抯 weight,?said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research.
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.
揘o other structural battery reported is comparable, in terms of energy density, to today抯 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, to make it happen,?Kotov said.
The combination of energy density and inexpensive materials means that the battery may already double the range of delivery robots, he said.
揟his is not the limit, however. 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,?said Mingqiang Wang, first author and recently a visiting researcher to Kotov抯 lab.
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梩he carbon-based fibers found in Kevlar vests梐nd 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.
揃atteries that can do double duty梩o store charge and protect the robot抯 憃rgans挆replicate the multifunctionality of fat tissues serving to store energy in living creatures,?said Ahmet Emre, a doctoral student in biomedical engineering in Kotov抯 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 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.
Beyond the advantages of the battery抯 chemistry, Kotov says that the design could enable a shift from a single battery to distributed energy storage, using graph theory approach developed at U-M.
揥e don抰 have a single sac of fat, which would be bulky and require a lot of costly energy transfer,?Kotov said. 揇istributed energy storage, which is the biological way, is the way to go for highly efficient biomorphic devices.?
Source: University of Michigan
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