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. McDougall hopes to increase this to between 300 and 500 cycles, which will make them useful for mobile phones and electric bicycles.
For electric vehicles, ReVolt is developing a novel battery structure that resembles that of a fuel cell. Its first batteries use two flat electrodes, which are comparable in size. In the new batteries, one electrode will be a liquid–a zinc slurry. The air electrodes will be in the form of tubes. 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. McDougall 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.
As with fuel cells, this system may need to be paired with another type of battery for bursts of acceleration or to capture energy from processes such as braking. Also, Henriksen notes that other experimental zinc-air batteries have already achieved 200 cycles.
Commercial success of the more conventional flat design could depend on other factors, such as whether the new batteries deliver energy at higher rates than other experimental zinc-air batteries, as the company claims, and whether the goals for higher cycle numbers can be met. The new tube-based design is still years away from production.