Button cells
About
- Anode: various
- Cathode: various
- Electrolyte: various
- Type: primary / secondary
- Nominal cell voltage: 1.4 - 3 volts
- Energy density: various
- Monthly self-discharge rate: various
Pros:
- Size - Button cells are the smallest commercial batteries in the world
- Energy density - Button cells have a high energy density
- Price - these batteries are typically inexpensive and last quite long
Cons:
- Non-reusable - most button cells are primary batteries and can’t be recharged
- Confusing - button cells look identical but vary greatly in chemical composition
- Choking hazard - Every year small children ingest or choke on button cells
General information on button cells
Button cells are a unique category in this list as they are not defined by the battery chemistry but rather by the shape. ‘Button cell’ refers to flat cylindrical batteries like those commonly found in watches, pocket calculators, and hearing aids. They can range from around 5 to 25mm in diameter and 1 to 6mm in height. These batteries are single cells, using the metal casing as the cathode and an insulated metal top as the anode.
The chemistry within these cells varies hugely. Traditionally they are primary batteries, but rechargeable versions exist as well. Some of the cell chemistries are:
- Alkaline
- Mercuric oxide
- Zinc air
- Silver oxide
- Lithium
The button cell battery is also known as a coin cell, or watch battery. The first button cells had a mercury oxide anode and zinc cathode in an alkaline electrolyte. They were used for military applications during World War 2, though the format was adopted by the watch industry soon after to power the world’s first electric watches (the first being the Hamilton 500).
As mercury became less popular and was eventually banned due to its toxicity, other cell chemistries were implemented in this button format to continue powering small, discrete electronics.
Most common appliances
Button cells can mainly be found in small electric devices, such as watches, pocket calculators, digital thermometers and hearing aids. They are also hidden in musical greeting cards.
Why recycle button cells?
Legislation starting in the EU in 1991 successfully banned the production of mercury batteries, but some are still in circulation in the form of old button cells. These present a threat to us and our environment, and must be disposed of through responsible recycling.
Button cells also present another, unique threat. Small and shiny, they are easily ingested by infants as well as curious animals. In landfills these batteries can get mixed in with organic waste and find their way into the food chain, poisoning animals, the environment, and humans too.
Process highlights
Sorting
Button cells are initially separated out at sorting facilities as their own stream, but then they often go through a further screening process and are sorted based on battery chemistries, to be added in with the appropriate recycling process.
Pyrometallurgical process
However, button cells are also sometimes recycled en masse. The first step in this process is to extract any mercury in the mix due to its toxicity. This is most often done through heat treatment in a distillation unit which evaporates mercury out of the cell (as mercury evaporates at a very low temperature - around 357°C. These vapours are collected by ventilation and condensed into pure liquid mercury for re-use.
In some facilities this is basically the entire process. Whole button cells are loaded into a furnace and heated gradually to 1100°C, pausing at specific temperature intervals to evaporate off any mercury, zinc and alkaline electrolyte within the mix. The remaining mix turns molten and forms a metal alloy mix which is then sent to external partners to extract silver (from silver-based button cells) and separate out other valuable metals in the mix.
Hydrometallurgical process
Several hydrometallurgical approaches exist as well, though are not commonly used at an industrial scale. Hydrochloric acid can be used to consume the zinc, iron and nickel casing while leaving behind silver oxide. This can be recovered through precipitation and reduction to metallic silver with an efficiency greater than 99% (by reaction with metallic zinc powder).
In short, there are many routes used in the recycling of button cells. The key goal is to extract all mercury (for safety) and silver (for economic value), while more sophisticated processes also target the various other alloys contained within.
End products
- Mercury
- Silver
- Various alloys of nickel, zinc, and iron
Future trends
Button cells continue to play a vital role in powering a variety of small electronic devices, from medical equipment to laser pointers. It is quite a stable corner of the battery industry without much change in either direction, due to the specific size and structure involved.
With environmental and resource management concerns shaping the recycling industry, it’s likely that many recycling methods currently in use for button cells will be done away with in favour of more closed-loop solutions that recover as much of the battery content as possible. This would best be achieved by separating button cells into their respective chemistries and sending these to specialised recyclers.
