A substance made from crab and shrimp exoskeletons can serve as an electrolyte in a zinc metal battery. The electrolyte, the layer in batteries that conducts zinc ions from the plus to the minus pole while electrons have to pass through, is therefore biodegradable. That makes the battery more durable than other batteries, write researchers from the University of Maryland, United States, in the magazine Matter. Their hope is that this new material will bring zinc batteries into the picture for stationary storage of sustainable energy.
Batteries are an important link in the transition to sustainable energy. Demand and supply of wind and solar energy do not always match up during the day. At times with peak energy demand, there is often too little power available, while at other times more electricity is generated than is needed. Stationary batteries can offer a solution for those fluctuations within a day.
High energy density
At the moment, lithium-ion batteries are usually used for this. These batteries have a high energy density and the technology has been developed further. But they are actually ideally suited for mobile applications, where low weight and compactness are important. This is less important for large, stationary batteries. That is why a lot of research is being done into other types of batteries, with cheap, common and easily recyclable components. Vanadium flow batteries, for example, or zinc batteries. They are also safer than the highly flammable lithium batteries.
But the chargeability of zinc batteries, so called because zinc is the charge carrier, is a tricky issue, says Marnix Wagemaker, professor of electrochemical energy storage at TU Delft. “In zinc metal batteries, the anode, the positive terminal, dissolves completely during discharge. The zinc metal should return cleanly if you reverse the process. But that doesn’t happen, it forms dendrites, stringy structures.” If the wires become so long that they touch the other electrode, there will be an internal short circuit, which is a safety risk. In addition, the dendrites form a large surface with which the aqueous electrolyte commonly used up to now can react.
It is therefore very important that dendrites are not formed, and the properties of the electrolyte play a major role in this. “It must conduct well, be flexible so that it can counterbalance dendrites and be stable, not break in contact with the zinc metal,” says Wagemaker.
It’s all steps forward
Marnix Wagemaker Tu Delft
The electrolyte that the researchers in Matter demonstrate is a gel consisting of chitosan and zinc. Chitosan is a bio-polymer made from chitin, a substance in the exoskeleton of crustaceans such as crabs and shrimp. To provide resistance to the dendrites, the researchers strongly compressed the porous material. The material also contains little water that can potentially react due to the compression, while the electrolyte still conducts ions well. The researchers note a charging efficiency of 99.7 percent and a lifespan of more than a thousand charging rounds.
“It is a beautiful, new material that combines the three properties to prevent dendrites and is also easily recyclable,” says Wagemaker. “The efficiency of 99.7 percent is a good start. But remember, if you lose 0.03 percent of your capacity every time you charge, you’ll only have 5 percent left after a thousand cycles. That is when you invent a new material, so on a lab scale, already very nice, but it is still a long way from practice. Lithium batteries have a charging efficiency of 99.999 percent.”
“I come across studies like this very often. While reading you think, this is the breakthrough. But then you’ll never hear about it again,” says Wagemaker. Most die on the way to large-scale use. “On the other hand, they are all steps forward, there is a lot of research being done on a lot of different battery chemistries, all of which have specific benefits, and therefore potential. The development of stationary batteries is really still in its infancy. Sustainability is becoming increasingly important in this regard. The non-durable lithium battery is really not sacred anymore.”
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