Most disposable batteries are technically alkaline batteries. They operate at high pH and often use zinc as a charge carrier. Zinc is a good choice because it is very inexpensive, can be used to make one of the two electrodes and, in the right context, allows the use of air at the other electrode. The following two simplify the battery, allowing it to be more compact and lighter in weight.
But the problem is that these batteries can only be used once because the chemistry theory doesn’t allow things to work in reverse. Carbon dioxide from the air reacts with the electrolyte, forming carbonate salts that block an electrode. Nor does zinc accumulate neatly on the electrodes it produces, instead creating spiky structures called dendrites that can drain the battery.
Recently, however, an international research group has found a way to make rechargeable zinc batteries.
Chemical one-way sugar
On the surface, the chemical description of the alkaline zinc battery is quite simple. The zinc foil acts as an electrode, with each zinc ion releasing two electrons. At the other electrode, oxygen molecules in the air accept four of these electrons, breaking down the molecule and allowing zinc oxide to form. But the “devil” is in the middle of the reaction. In this case, the important intermediate is hydroxide ions, which are naturally formed in the alkaline pH of a water-based electrolyte. It participates in several reactions with zinc, not directly reacting with oxygen in the air.
Those hydroxide ions are also the source of one of the problems with air zinc batteries, as they are also intermediate in the reactions that convert carbon dioxide to carbonate. These carbonate salts coat the electrode where oxygen reacts and eventually block it. This can be avoided to some extent by replacing the air with pure oxygen, but it only extends the shelf life to about 10 cycles or more.
The scientists argue that treating hydroxide ions will not necessarily solve the dendrites formation on the zinc metal sheet, but instead can deal with the problems with the air electrode.
This is not simply a matter of changing the pH of an electrolyte solution, since hydroxide ions form in water at a neutral and even acidic pH. And, under normal conditions, oxygen decomposition at the air electrode occurs via hydroxide intermediates. Thus, the researchers replaced alkaline conditions with an electrolyte that is somewhat hydrophobic or repulsive. The chemical they used, trifluoromethanesulfonate, is essentially a sulfate ion bound to a carbon with triple fluoride attached. The carbon-fluorine portion of the molecule repels water, while the sulfate portion can interact with zinc ions.
The way things change
Switching to this new electrolyte will help block zinc to a certain extent. But more importantly, it has a great effect on the reaction at the air electrode. Here, the normal reaction involves transferring four electrons to break down one molecule of O2 through hydroxide intermediates. With the newly swapped electrolyte, the hydroxide intermediates stop forming. As a result, only two electrons are transferred to the oxygen molecule, creating a peroxide. As a result, ZnO 2 formed when a battery discharges, instead of zinc oxide (ZnO).
More importantly, they discovered the formation of zinc peroxide filaments when discharged and confirmed that they disappear during recharging. Researchers also found pressure changes related to oxygen that are incorporated into the battery during discharge and released when the battery is recharged. When zinc foil was used as an electrode, more than 80% of the zinc was used for electrical discharge. Replacing it with zinc powder increased zinc usage by 94%.
The results are completely different. Instead of dying after a few cycles, the researchers managed to charge a single battery for 1,600 hours. Most of the time dendritic formation is not a problem, and the capacity per weight is twice as high as that of some lithium batteries.
Of course, it’s not that the battery issue is completely resolved. Because batteries depend on air, the water in the electrolyte evaporates over time. The dendritic has formed, which ultimately makes the zinc metal anode unusable. But the biggest problem is probably the charging speed, because a single charge-discharge cycle takes up to 20 hours
If the current density is increased by 10 times, the battery will only run for 160 hours. The more charging density we get, the more we start to break water structures instead of operating the batteries. The team suggests that the catalyst that promotes the formation of peroxide may potentially increase the charge-discharge rate.
But that might not be the problem. Since grid storage does not necessarily require fast discharge rates from individual batteries, it is important as long as there is enough battery to meet the capacity requirements. And here, zinc can be a bonus because it costs a quarter less than lithium carbonate, and that’s only for pure zinc. Plus, having zinc available to meet other needs frees up lithium for other uses, rather than as batteries.
Finally, the researchers note that the same type of chemical theory might work with other metals, including magnesium and aluminum, and both are relatively inexpensive. These could be the best alternatives, given the right balance between pros and cons, and they certainly won’t increase the competitiveness of lithium supplies.