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New Rechargeable Zinc Battery Integrated in Robots Provides More Energy

A team of researchers, under the guidance of the University of Michigan, has demonstrated a novel rechargeable zinc battery that combines with the structure of a robot to deliver relatively more energy—similar to how biological fat reserves preserve energy in animals.

Structural, rechargeable zinc battery

Video Credit: University of Michigan.

This novel method to boost the capacity will be specifically significant as the size of the robots reduce down to the microscale and even below this—scales at which existing stand-alone batteries are inefficient and too bulky.

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.

Nicholas Kotov, Study Lead and Joseph B. and Florence V. Cejka Professor of Engineering, University of Michigan

Mobile robots are increasingly being used in many different fields, right from bike-lane take-out bots and delivery drones to warehouse robots and robotic nurses. On the micro-level, scientists are investigating swarm robots that are capable of self-assembling into larger devices.

With multifunctional structural batteries, space can be saved and weight can be reduced, but to date, these batteries could only complement 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. We improved our prior version of structural zinc batteries on 10 different measures, some of which are 100 times better, to make it happen.

Nicholas Kotov, Study Lead and Joseph B. and Florence V. Cejka Professor of Engineering, University of Michigan

According to Kotov, the combination of low-cost materials and energy density implies that the battery may already increase the range of delivery robots by two-fold.

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.

Mingqiang Wang, Study First Author and Visiting Researcher, University of Michigan

Wang has recently joined Kotov’s laboratory.

The latest battery operates by transmitting hydroxide ions between a zinc electrode and the airside via an electrolyte membrane. The electrolyte membrane is partly a new water-based polymer gel and a web of aramid nanofibers—the carbon-based fibers used in Kevlar vests. The polymer helps transmit the hydroxide ions between the electrodes.

The battery is made with abundant, inexpensive, and largely innocuous materials, and therefore, it is eco-friendly when compared to those being used today.

Even if the battery is damaged, the aramid nanofibers and gel will not catch fire, making it different from the flammable electrolyte used in lithium-ion batteries. Moreover, the aramid nanofibers from retired body armor could be upcycled.

To prove the performance of the new batteries, the team tested miniaturized and regular-sized toy robots in the shape of a scorpion or a worm. They used zinc-air cells in the place of original batteries, then wired the cells into the motors, and finally enclosed them around the exteriors 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,” stated Ahmet Emre, a doctoral student in biomedical engineering in Kotov’s laboratory.

The drawback of zinc batteries is that they sustain high capacity for around 100 cycles, instead of the 500 or more anticipated from the lithium-ion batteries used in smartphones. The reason is that the zinc metal develops spikes that ultimately penetrate the membrane between the electrodes.

The network of robust aramid nanofiber between the electrodes is integral to the comparatively long cycle life of a zinc battery. The recyclable and inexpensive materials also make it easy to replace the batteries.

Apart from the benefits of the battery’s chemistry, Kotov added that the design could help move from a single battery to distributed energy storage, utilizing the graph theory method designed at the University of Michigan.

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

An article on this study, titled “Biomorphic structural batteries for robotics,” will be published in the Science Robotics journal.

The study was financially supported by the Department of Defense, the National Science Foundation, and the Air Force Office of Scientific Research. The battery was tested at the Energy Institute of the University of Michigan.

Kotov is also a professor of chemical engineering, materials science and engineering and macromolecular science and engineering, and Wang is a postdoctoral researcher at Harbin Institute of Technology in China.

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


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