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The theoretical energy density of this system is nearly twice that of lithium-ion. Zinc-air cells use oxygen from the atmosphere as the cathode reactant. The cathode is porous, in order to allow diffusion of air to the KOH electrolyte and it is not affected by the reaction. Because the oxygen is not stored in the cell, the zinc anode can occupy a larger proportion of cell, giving a high energy density.
Cells must be stored in sealed containers until needed. When opened, the electrolyte dries out rapidly, or becomes flooded if used in very humid conditions, allowing only a few days' use. However, drying out is less of a problem with some newer button cells, which use thin composite gas-permeable electrodes that include a Teflon-bonded catalyst, metallic mesh and hydrophobic polymer membrane. These have a much longer operating life and are used in hearing aids, watches, paging receivers, etc.
An interesting aspect of this couple is that it is being developed in two separate directions. One type is electrically rechargeable, while the other is mechanically rechargeable (also called a semi-fuel cell). Refuelling takes the form of replacing the zinc electrode plates and the electrolyte, or the addition of zinc pellets as a slurry in electrolyte. Therefore, the battery life depends on the life of the air electrode, which is carbon based with added catalysts. The electrically rechargeable version has historically suffered from a short cycle life, although this has now improved. Refuellable batteries are now being tested in electric vehicles.
