Zinc sponge makes batteries safer

Porous metal structures are the basis for a light and economical alternative to lithium-ion batteries

Nickel-Zinc Battery: A porous sponge structure of the zinc anode (right) allows multiple recharge, since no dendrites (left) lead to short circuits.

Washington (USA) - As powerful lithium-ion batteries are now, they are exposed to an explosion risk, depending on the processing. Thus, the Korean company Samsung had to recall a tablet series only 2016 because of overheating danger of the batteries. A reliable alternative is now proposed by American scientists with a renaissance of the nickel-zinc battery, which could not be charged up to now. In the trade magazine "Science", they report on the first prototypes, which even provided a storage capacity comparable to lithium-ion batteries. This was due to the spongy structure of the zinc electrode, which prevented self-destruction of the nickel-zinc batteries.

"The key to rechargeable zinc batteries lies in the controlled behavior of the metal during charging cycles," says Joseph F. Parker of U.S. Naval Research Laboratory in Washington. Up to now, these batteries had anodes of zinc powder, in which elongated, sharp crystal structures, so-called dendrites, grew during the charging process. These penetrate the separating layer between the electrodes and cause short circuits with the nickel cathode. Therefore, Parker and his colleagues exchanged the zinc powder with a porous metal sponge of zinc. Thanks to this three-dimensional structure, no destructive dendrites arose during the charging cycles.

Parker and colleagues tested the loading properties of several prototypes consisting of stacks of metal electrodes and separation membranes surrounded by a nylon mantle. Thus, they discharge a nickel-zinc battery as low as possible and obtained a specific energy comparable to values ​​of commercial lithium-ion batteries with more than 100 watt hours per kilogram at a constant voltage of 1.93 volts. As a result, the battery consumed a good 90 percent of its theoretical capacity. However, the current storage of this heavy load could withstand just under 100 charging cycles. The stability would have to be increased to several thousand charging cycles in order to replace lithium ion batteries.

Nickel-zinc batteries held up considerably longer if they were only partly discharged. The stability culminated in over 50,000 charging cycles with a specific energy of up to 40 watt hours per kilogram. With this value, it can nevertheless compete with conventional lead acid systems, which are used as starter batteries in cars. In addition, they would be significantly lighter than lead batteries and would only have to be replaced every ten years.

In further experiments, Parker and colleagues, in collaboration with the EnZinc company in San Anselmo, want to further examine the great potential of rechargeable zinc-based batteries. Also a higher stability with deep discharge could be achieved. If this step is successful, a technology that uses inexpensive and globally readily available materials with zinc and nickel can be recycled efficiently.