Today’s robot-mounted batteries provide electrical power but at the expense of added mass that in turn requires added power to move and use. But a team of researchers from the University of Michigan have devised a clever solution that will enable tomorrow’s batteries to provide power while negating their own weight — it just needs a bit of Kevlar.
Led by Nicholas Kotov, a professor of chemical engineering at U of Michigan, the team has developed a battery system that is strong enough to also serve as a structural support for the rest of the robot. “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,” Kotov told the University of Michigan News
“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, to make it happen,” he continued.
In a study published Wednesday in the journal Science Robotics Kotov points out that not only is the zinc-air battery chemistry his team employed around three times as energy dense as your standard Li-Ion brick but by incorporating the battery into the body of the robot itself, they were able to open up about 20 percent more space in the robot’s interior than if they’d used a conventional power system.
“This 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,” first author Mingqiang Wang, noted.
What’s more, this battery design gets around an other difficult tradeoff seen in other efforts to incorporate batteries into a machines structure: that between the amount of energy the battery can produce and its ability to endure stress. Normally metal-air electrochemical batteries use an aqueous solution to separate the cathode and anode. The U of Michigan battery consists of a zinc electrode and an ambient air cathode separated by a layer of Kevlar fibers suspended in a water-based polymer gel that helps transfer hydroxide ions between the electrodes. Since the separator layers is effectively a solid, it won’t rupture or burst under stress like a liquid separator would. And even if it does rupture, the solution is nontoxic.
What’s more the separator layer so tough that it actually helps inhibit the formation of zinc dendrites, small outgrowths of metal between the electrodes created during the charge-discharge cycle that degrade battery performance and lifespan. Lithium-Ion batteries can go for around 500 cycles without noticeably degrading, however zinc batteries begin to decline after just 100.
Kotov likens the battery system to human body fat. Our fat doesn’t just store energy for us, it also provides cushions for our joints and helps conserve body heat. As such he expects the current single-battery design to eventually evolve into a distributed power storage system. “We don’t have a single sac of fat, which would be bulky and require a lot of costly energy transfer,” Kotov noted. “Distributed energy storage, which is the biological way, is the way to go for highly efficient biomorphic devices.”
Kotov hopes to have a commercial battery system ready within the next 3 to 5 years and expects the first buyers to be drone and robot manufacturers. “And it’s not just about the big Amazon robots but also very small ones,” Kotov told IEEE Spectrum. “Energy storage is a very significant issue for small and flexible soft robots.”