Metal air batteries create a flow of electrons like most batteries but do not need a liquid or electrolyte medium to transfer the electrons during charge and discharge.
Zinc air batteries are somewhat like fuel cells. Air enters the battery through the top copper colored cover. Air passes through the porous air electrode between the blue separator, and into the lower gray zinc electrode. The bottom layer is the battery case.
As the air is drawn into the air electrode, oxygen in the air is converted to hydroxyl (OH negative) ions. The newly formed hydroxyls migrate to the zinc electrode where they cause the zinc to oxidize. Basically: Zn --------> Zn2+ plus 2e-. Those electrons provide the flowing current.
Charging of the zinc-air battery cell converts the hydroxyl ions to oxygen in the air electrode, releasing electrons. On the zinc electrode, the metal ions are reduced to form the metal while the electrons are consumed.
The latest push for the zinc air battery comes from the company ReVolt. Revolt has gained exclusive rights to produce batteries developed by Lawrence Livermore National Laboratory (LLNL). A retired chemist from LLNL has a patent that reportedly provides practical solutions to zinc air problems.
The State of Oregon has provided some $6.8 million in funding and incentives along with $5 million from the US Government. The next year should be interesting for the zinc air battery and its usage in the electric vehicle.
The zinc air battery may have the potential to replace or at least complement the lithium-ion electric car battery. Zinc air batteries are just one type of metal battery. There is also research underway to produce an EV ready lithium-air battery. Thomas Edison was successfully using a rechargeable nickel-zinc battery system in 1901.
Zinc air batteries have the advantages of a possible high energy density, zinc is way more common than lithium, and zinc batteries are easily recycled. Also, the potential cost of a zinc air battery is low and the battery is thermally stable.
Zinc air batteries are used in hearing aids at present but are not yet ready for EVs. Currently, they are restricted to about 3-500 cycles before replacement, and charge and discharge are slow relative to Li-Ion. These issues need to be resolved before the electric vehicle can use them.