Zinc Air Batteries
An example of Metal Air batteries, cells using zinc-air technology are energized only when atmospheric oxygen is absorbed into the electrolyte through a gas-permeable, liquid-tight membrane. With the removal of a sealing tab, oxygen from the air is introduced into the cell. A zinc-air battery usually reaches full operating voltage within 5 seconds of being unsealed.
The zinc air cell is basically a primary battery however rechargeable designs for high power applications are possible by physically replacing the zinc electrodes.
They use the oxygen content of the air as active mass. The positive electrode (cathode) is a porous body made of carbon with air access. Atmospheric oxygen is reduced at this electrode. The active mass is thus not contained in the electrode but is taken from the surrounding air as it is needed. The initial weight of the battery is reduced accordingly. The negative electrode (anode) consists of zinc. An aqueous solution of potassium hydroxide serves as the electrolyte.
The cell voltage for the chemistry is theoretically capable 1.65 Volts however almost all designs are optimised for less than 1.4 or 1.3 Volts in order to achieve longer lifetimes.
High energy density but low power
The zinc-air system, when sealed, has excellent shelf life, with a self-discharge rate of only 2 percent per year.
In relation to their physical size, Zinc/Air batteries store more energy per unit of weight (in terms of 220 Wh/kg) than almost any other primary type.
Primary cells available in a range of button and coin cell sizes.
Rechargeable high power cells available for traction applications.
Sensitive to extreme temperature and humid conditions.
Carbon dioxide from the air forms carbonate which reduces conductivity.
High self discharge.
After activation, chemicals tend to dry out and the batteries have to be used quickly.
Although recharging is possible it is also inconvenient and is only suitable for high power types.
High internal resistance which means zinc air batteries must be huge to satisfy high current needs.
High power batteries such as those designed for traction applications use mechanical charging in which discharged zinc cartridges are replaced by fresh zinc cartridges. The used cartridges are subsequently recycled.
The system is well known as a primary battery.
Zinc air button cells are commonly used for watches and hearing aids.
Larger types are employed as prismatic or cylindrical cells for telecoms and railway remote signalling, safety lamps at road and rail construction sites or as power sources for electric fences.
Possible traction applications where "Mechanical Charging" cuts down on recharging time but little take up so far.