It is not easy to make batteries that are cheap, powerful, long-lasting, safe, and environmentally friendly all at once. ETH researchers have succeeded with zinc batteries.
The most important in brief -Today's zinc batteries often use toxic salts. -Researchers have now optimized the electrolyte fluid of these batteries: they used harmless salts and less of them. -Batteries with this fluid are faster to charge.
The world needs cheap and powerful batteries. They are supposed to store sustainably produced electricity so that it is available to us even when there is no wind blowing and the sun is not shining. Lithium-ion batteries that power our smartphones and electric cars are quite expensive because there is a global rush for the raw material lithium. Moreover, these batteries are easily flammable.
A promising alternative to lithium-ion batteries are water-based zinc batteries. An international team of researchers led by ETH Zurich has now developed a strategy that decisively advances the development of such zinc batteries, making them more powerful, safer, and more environmentally friendly.
The challenge of durability Zinc batteries have a few advantages: zinc is abundant, cheap, and easy to recycle because there is a well-developed global zinc recycling infrastructure. Moreover, a lot of electricity can be stored in zinc batteries. And above all, zinc batteries do not necessarily require organic solvents that are easily flammable as an electrolyte fluid. Instead, electrolyte fluids that are water-based can also be used.
If only there weren't a few problems that engineers have to deal with when developing these batteries: When zinc batteries are charged with high voltage, the aqueous electrolyte fluid reacts at one of the electrodes to form gaseous hydrogen. This causes the electrolyte fluid to shrink, reducing the battery's performance. This reaction also builds up pressure in the battery, which can be dangerous. Another problem is needle-shaped zinc deposits, called dendrites, that can form in the battery during charging. These can even cause a short circuit and render the battery unusable.
Salts make batteries toxic In recent years, engineers have pursued the strategy of strongly enriching the aqueous electrolyte fluid with salts in order to keep the water content as low as possible. But this also has disadvantages: The electrolyte fluid becomes viscous, and the charging and discharging processes slow down significantly. Moreover, many of the salts used contain fluorine, which makes them toxic and environmentally harmful.
Maria Lukatskaya, professor of electrochemical energy systems at ETH Zurich, has now worked systematically with colleagues from several research institutions in the USA and Switzerland to find the optimal salt concentration for water-based zinc ion batteries. With experiments and computer simulations, the researchers were able to show that optimal is not, as previously assumed, the highest possible salt concentration, but a relatively low one: five to ten water molecules per positively charged salt ion.
Long-lasting and fast-charging The scientists also did not use environmentally harmful salts for their optimizations, but worked with environmentally friendly salts of acetic acid (acetates). "With an optimal concentration of acetates, we were able to minimize the electrolyte shrinkage as well as avoid dendrite formation as well as other researchers had previously done with high concentrations of toxic salts," says Dario Gomez Vazquez. He is a doctoral student in Lukatskaya's group and first author of the study. "In addition, our approach allows batteries to be charged and discharged much faster."
So far, the ETH researchers have tested their new battery strategy in the relatively small laboratory setting, but their results could lead to the development of more powerful and safer zinc batteries on a larger scale.