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Zinc air battery technology comes in a variety of forms offering high energy density, low cost and varieties including non rechargeable (primary cells) as well as a rechargeable version of the technology and also a mechanically rechargeable version.
Zinc air battery technology lends itself to used from small hearing aid batteries to larger ones for cameras (replacing the older mercury batteries that are not longer available) to the possibility of much larger batteries for electric vehicles
This form of battery technology has many advantages in terms of size, energy density and also the environmental impact - even though a strong hydroxide is normally used as the electrolyte, other materials are benign and cause no major issues.
Development of zinc air batteries - history
The first effects of air on batteries and cells was noted soon after the original wet Lechalnche cells were used. It was seen that when atmospheric oxygen was absorbed by the carbon cathode, the performance improved.
It was then in 1878 when carbon air electrodes plated with platinum were found to work very well instead of the manganese dioxide found in most cells of the time.
It took many years for products using this principle to be manufactured successful, so it was not until 1932 that the first cells using this principle were made.
This type of cell using the platinised carbon cathode formed large zinc–air cells that were used for navigation aids and rail transportation. However, the current capacity was low and the cells were bulky, so their use was very limited.
During the 1970s research into thin electrodes based on fuel-cell research enabled the development of small button and prismatic primary cells for hearing aids, pagers, and medical devices, especially cardiac telemetry.
In fact today, miniature zinc air cells are now very cheap and widely available for applications like hearing aids.
Types of zinc air battery
Development of zinc air batteries is on-going and a number of different types are either available or emerging from development.
Broadly there are three types of these batteries and cells:
Primary - non-rechargeable: The non-rechargeable type of zinc air battery is the most common. The small versions of these batteries or cells can be bought very cheaply and they are widely used for applications like hearing aids.
The primary or non-rechargeable zinc air cells and batteries are the most widely used form of the zinc air technology at the moment.
Secondary - rechargeable: Development of secondary or rechargeable zinc air cells is ongoing and it is hoped that these could be more available for general use in the near future.
These rechargeable zinc air cells and batteries could be used for a variety of situations and it has even been proposed they might be used in electric vehicles.
Mechanically rechargeable: Another way of extending the life of these batteries is to replace the spent fuel, zinc and electrolyte. This is a physical action where and therefore it is referred to as being mechanically rechargeable.
How zinc air primary batteries & cells work
Like all batteries, the zinc air cell converts chemical energy into electrical energy. As such they are electrochemical generators.
In essence the operation of zinc air cells is based around the fact that the two electrodes are zinc and a porous cathode separated by a high pH electrolyte.
Oxygen from the air enters the cathode region and it reacts with water from the electrolyte to form a hydroxide. The hydroxide formed at the cathode surface travels to the anode and it reacts with the zinc in the anode to release energy that provides the power.
In more detail, a porous zinc anode is saturated with electrolyte. Oxygen from the air reacts at the cathode to form hydroxyl ions. Then these migrate into the zinc anode and form zincate Zn(OH)2.
This reaction releases electrons which travel to the cathode, thereby creating a potential difference and causing a current to flow.
The zincate at the anode decays to form zinc oxide and also water which returns to the electrolyte.
The water and the hydroxyl from the anode are recycled at the cathode, so the water is not consumed.
The theoretical output from the cell is 1.65 volts, but in practice the maximum output voltage is only about 1.35 to 1.4 volts per cell.
It is found that zinc air cells have many of the properties of fuel cells, and are a half way house between batteries and fuel cells. The zinc is actually the fuel and as would be expected for a fuel cell, the output rate of the cell can be controlled by the amount of air-flow through the cell. The zinc and electrolyte paste can also be mechanically replaced to give the cell a new lease of life.
It should be remembered that zinc air cells cannot operate in a sealed battery compartment, and even if the air flow is restricted, then performance will fall. This is why any equipment using zinc air cells needs to ensure that there is an unrestricted flow of air within the battery compartment.
Hints & tips for using zinc air batteries
Although not all zinc air cells and batteries are the same, there are some general hints and tips that can be applied to most, and certainly to the types that are widely available today that are used for hearing aids and other electronic circuits where small but effective batteries are needed.
Remove the tab: Unlike other forms of cell and battery, zinc air batteries will have a tab over the air vent or vents when new. This covers the vent to prevent air entering before it is required for use, and in this way it means that none of the active elements of the cella are activated before use. This prevents ageing of the cell. Typically the primary cells can last for two or three years without any noticeable degradation in performance, provided the tab is kept in place. However it is imperative that the tab is removed before use otherwise the cell will not work.
For many batteries, it is best to take the tab off the battery and leave it for a short while before use. Check the manufacturers instructions for the exact usage as optimum operation will vary between manufacturers.
Let batteries breathe: When in use, the cells and batteries must have access to air as their operation is dependent upon air as it is one of the primary constituents for the process. This can even run as far as opening the battery compartment when the unit is turned off.
Battery storage: Before use, the cells or batteries will need to be stored. They are best stored in in a cool dry place, or at room temperature. Very cold or elevated temperatures can reduce the shelf life and capacity.
Use oldest batteries first: Zinc air cells have a long shelf life when the tab sealing the air holes is covered, they still lose some of their capacity over time. Accordingly it is always best to use the older cells and batteries first. This is also the case for other types of battery and cell as well.
General care and battery common sense can help in ensuring that zinc air batteries can provide the optimum operation. Ensuring they do not become damp and that they have an adequate air supply, along with not being used or stored at temperature extremes can provide significant advantages.
Zinc air cells offer some significant benefits. They have a very high energy storage density and the chemicals used are generally quite benign, especially when compared to those used in other batteries and cells.
There is considerable research and development being taken into the technology for zinc air batteries and cells and this means that they are likely to be seen increasingly in the years to come.
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