Nickel metal hydride
About
- Anode: metal hydride
- Cathode: nickel oxide hydroxide
- Electrolyte: potassium hydroxide
- Type: Rechargeable
- Nominal cell voltage: 1.2 volts
- Energy density: 140–300 W·h/L
- Monthly self-discharge rate: 30% pending storage temperature
Pros:
- Versatile - NiMH cells serve many functions - from cameras to electric vehicles
- Superior capacity - compared to NiCd cells
- Low toxicity - NiMH contains no heavily toxic metals
Cons:
- Limited raw materials - REEs like lanthanum are in short supply
- Memory effect - NiMH cells suffer from memory effect (though less than NiCd)
- Low cell voltage - 1.2 volt cells, meaning many cells are needed to make up high voltage batteries.
General information on nickel metal hydride
NiMH batteries first came onto the market in 1989, quickly replacing NiCd batteries due their superior capacity (up to 3 times better) and energy density. They share the same cathode and anode materials as NiCd cells, but use a hydrogen absorbing alloy instead of toxic cadmium for the anode. The relative downside to this chemistry is that NiMH cells are more delicate than NiCd and have a much higher rate of self-discharge. They also suffer from a type of memory effect just like NiCd.
Like many other battery technologies, NiMH is losing popularity in the face of lithium-ion cells. In Japan, NiMH batteries accounted for half of all rechargeable battery sales in 2000, dropping to 22% by 2010. Innovations in NiMH technology in 2015 improved the durability and weight of the batteries by modifying their microstructure, effectively improving the energy density of the battery.
Why recycle NiMH batteries?
NiMH batteries are considered relatively non-toxic. The only safety concern is proper treatment of the nickel (which is toxic to some plants) and alkaline potassium hydroxide electrolyte - a corrosive substance that can lead to burns, blindness, and other health issues. However, they also contain high amounts of economically and technologically significant materials, such as nickel, cobalt, and rare earth elements, also known as REEs. Around 87% of spent nickel metal hydride batteries can be recycled, of which 18% is nickel, 17% is REEs, and 4% is cobalt.
Today, NiMH recycling is even more profitable than lead-acid battery recycling - another great incentive driving recycling efforts.
Most common appliances
Taking over many positions previously held by nickel cadmium, NiMH batteries are durable and low-cost rechargeable batteries used in power tools, cameras, and other portable electronics such as electric toothbrushes. They have also been used in the past for electric vehicles, and still today for hybrid vehicles. In 2008 over two million were manufactured that ran on NiMH batteries.
Process highlights
Comminution
Used NiMH batteries (delivered from sorting facilities) are fed into a shredder, such as a cross-flow shredder, for comminution. During their life cycle, NiMH batteries produce small amounts of hydrogen which become trapped within the cell. When broken apart in a shredder, the hydrogen is released from the batteries and immediately extracted by vacuum to avoid build up and volatile reactions from taking place.
The shredded contents are then left to stabilise for a short period and ventilated to remove any last traces of hydrogen along with the electrolyte - potassium hydroxide.
Pyrometallurgic and hydrometallurgical processes
Many responsible recyclers of NiMH batteries combine pyrometallurgy and hydrometallurgy to get the best results. For example:
Pyrometallurgical distillation under vacuum can be carried out in an induction furnace, much the same as with NiCd cells. As the temperature rises water and oils are evaporated off and collected first. As the heat increases to between 1200 - 2000°C, slag rises to the top of the molten mix which can then be drawn off and further treated to extract rare earth elements for refining. The metal alloy mix left behind resembles that of the NiCd process. This is poured into ingots and can either be refined by specialised metal facilities or used directly for steel production.
The slag contains rare earth metals like cerium, lanthanum and neodymium, which can be extracted using pyrometallurgy or hydrometallurgy. Extracting REEs is a recent and ongoing development as they are deemed increasingly important and recycling them is increasingly legislated by law.
Hydrometallurgy is arguably better suited for this task, as it is able to fully recover the REEs with a high purity while using less energy and generating less waste. Acid leaching oxidises the metallic REEs, followed by precipitation and solvent extraction processes to end up with rare earth elements in the form of salts and pure metals.
Some processes in use today use only hydrometallurgical or pyrometallurgical routes (not both), which affects the final output streams and purities.
Rare Earth Elements in the Slag
Hydrometallurgical Recovery
End products
- Polymer plastics
- Papers
- Steel
- Nickel, iron, and cobalt alloy
- Rare earth elements
- Copper
- Zinc
- Slag
*(Some of these products may be consumed to produce energy in a pyrometallurgical process or end up in the slag).
Future trends
Like many other slightly older battery types, NiMH is seeing a slow, steady decrease as it is replaced with newer technologies such as lithium-ion. They are no longer really a competitive choice for use in the electric vehicle industry, and have been phased out of many portable electronic devices in favour of lithium-ion technology (which offers a number of favourable properties including higher energy density, which leads to longer range). However, there are still many NiMH cells in production and circulation around the world, and these will eventually need to be recycled.
Within the EU, rare earth elements such as those found in NiMH cells have been classified as critical raw materials (CRM). Legislation such as this has created a lot of interest in refined recycling methods that can efficiently extract these materials from used batteries. Hydrometallurgical or a combination of pyrometallurgical and hydrometallurgical routes will most likely be adopted by recycling facilities around the world in order to meet regulations.
