A Swiss company called ReVolt has developed rechargeable zinc-air batteries that can store three times the energy of lithium ion batteries, by volume, while costing only half as much:
James McDougal, the company’s CEO, says that the technology overcomes the main problem with zinc-air rechargeable batteries — that they typically stop working after relatively few charges.
So, how does a zinc-air battery work?
Unlike conventional batteries, which contain all the reactants needed to generate electricity, zinc-air batteries rely on oxygen from the atmosphere to generate current. In the late 1980s they were considered one of the most promising battery technologies because of their high theoretical energy-storage capacity, says Gary Henriksen, manager of the electrochemical energy storage department at Argonne National Laboratory in Illinois. The battery chemistry is also relatively safe because it doesn’t require volatile materials, so zinc-air batteries are not prone to catching fire like lithium-ion batteries.
Making them rechargeable has been a challenge:
Inside the battery, a porous “air” electrode draws in oxygen and, with the help of catalysts at the interface between the air and a water-based electrolyte, reduces it to form hydroxyl ions. These travel through an electrolyte to the zinc electrode, where the zinc is oxidized — a reaction that releases electrons to generate a current. For recharging, the process is reversed: zinc oxide is converted back to zinc and oxygen is released at the air electrode. But after repeated charge and discharge cycles, the air electrode can become deactivated, slowing or stopping the oxygen reactions. This can be due, for example, to the liquid electrolyte being gradually pulled too far into the pores, Henriksen says. The battery can also fail if it dries out or if zinc builds up unevenly, forming branch-like structures that create a short circuit between the electrodes.
ReVolt says it has developed methods for controlling the shape of the zinc electrode (by using certain gelling and binding agents) and for managing the humidity within the cell. It has also tested a new air electrode that has a combination of carefully dispersed catalysts for improving the reduction of oxygen from the air during discharge and for boosting the production of oxygen during charging. Prototypes have operated well for over one hundred cycles, and the company’s first products are expected to be useful for a couple of hundred cycles. McDougal hopes to increase this to between 300 and 500 cycles, which will make them useful for mobile phones and electric bicycles.
ReVolt is also developing a novel battery structure in which one electrode is a liquid — a zinc slurry:
To generate electricity, the zinc slurry, which is stored in one compartment in the battery, is pumped through the tubes where it’s oxidized, forming zinc oxide and releasing electrons. The zinc oxide then accumulates in another compartment in the battery. During recharging, the zinc oxide flows back through the air electrode, where it releases the oxygen, forming zinc again.
In the company’s planned vehicle battery, the amount of zinc slurry can be much greater than the amount of material in the air electrode, increasing energy density. Indeed, the system would be like a fuel-cell system or a conventional engine, in that the zinc slurry would essentially act as a fuel — pumping through the air electrode like the hydrogen in a fuel cell or the gasoline in a combustion engine. McDougal says the batteries could also last longer — from 2,000 to 10,000 cycles. And, if one part fails — such as the air electrode — it could be replaced, eliminating the need to buy a whole new battery.