The aim is to store energy in structural components such as the wing of a drone or the bumper of an electric vehicle. Using a cartilage-like solid electrolyte allowed the team to replace the top casings of several commercial drones. The prototype cells can run for more than 100 cycles at 90 percent capacity, and withstand hard impacts and even stabbing without losing voltage or starting a fire.
“A battery that is also a structural component has to be light, strong, safe and have high capacity. Unfortunately, these requirements are often mutually exclusive,” said Prof Nicholas Kotov, Professor of Engineering and material science at Michigan who led the research.
The solid electrolyte uses branched nanofibres that resemble the collagen fibres in cartilage. “Nature does not have zinc batteries, but it had to solve a similar problem,” said Kotov. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries. It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.”
The membranes carry zinc ions between the electrodes and can also stop the development of the dendrites that cause short-circuits. Like cartilage, the membranes are composed of aramid nanofibres interwoven with a softer ion-friendly material, polyethylene oxide and a zinc salt.
The team paired the zinc electrodes with manganese oxide, the same combination found in standard alkaline batteries. In the rechargeable cell, the membrane replaces the standard separator and alkaline electrolyte. As secondary batteries on drones, the zinc cells can extend the flight time by 5 to 25 percent–depending on the battery size, mass of the drone and flight conditions.
The team tested the ruggedness of the cells by deliberately stabbing them with a knife, and the battery continued to discharge close to its design voltage as there is no liquid to leak.
While the batteries are best used as secondary power sources as they can’t charge and discharge as quickly as lithium ion cells, the team s looking at other partner electrodes that could improve the speed and longevity of zinc rechargeable batteries.