The increasing popularity of electric vehicles is undoubtedly a good thing for the planet, but there is a problem mounting in the background as more and more take to the road. The lithium-ion batteries used in EVs don't last forever, and a massive influx of depleted units in the coming decade will dwarf our current capacity to recycle them. Scientists have made a breakthrough that could give these efforts a boost, demonstrating how a flotation tank can be used to easily separate some of the precious battery materials for re-use.
The study, which was led by scientists at Michigan Technological University (MUT) as part of the ReCell advanced battery recycling center, seeks to make use of a technique commonly used in the mining industry to separate and purify ores. Called froth flotation, this involves placing materials in a flotation tank and seeing them separate based on whether they repel water and float, or absorb water and sink.
But this approach doesn't easily translate to the world of end-of-life lithium batteries, because the materials that form the cathode component, such as the commonly used lithium nickel manganese cobalt oxide (NMC111) and lithium manganese oxide (LMO), generally just sink. The MTU team has devised a solution to this that involves a mild chemical treatment of the water, which makes NMC111 float instead.
"The separation of battery cathode materials occurs primarily in water," co-author and material scientist Jessica Durham explains to New Atlas. "The process does not necessitate the use of large quantities of hazardous chemicals that would be challenging and expensive to get rid of waste."
With the cathode materials separated, the scientists then ran tests to ascertain their electrochemical performance, with the separating process found to only have a negligible impact in this regard. Both maintained high purity levels, of 95 percent or above, something Durham says will be critical for prospective buyers of the recycled materials.
This technology was demonstrated in a bench-scale flotation tank processing between 20 and 150 g (0.7 and 5.3 oz) of cathode material per liter (0.26 gal) of water. Scientists at the Argonne National Laboratory, where the ReCell team is headquartered, then scaled it up to a 10-liter (2.6 gal) tank capable of processing more than a kilogram (2.2 lb) of cathode material in an hour.
"This involved starting with conditions used in the bench-scale tank and optimizing the conditions to selectively separate cathode materials in the flotation column," Durham tells us. "The flotation column is a continuous operation, similar to what is used in industry, where a slurry of materials and water are constantly fed into the system and separated cathodes are collected from froth and tailings overflow streams."
The breakthrough marks an important step in efforts to efficiently separate these valuable materials, but is just one link in the chain when it comes to the entire recycling process. Materials other than the cathode need to be separated and or recovered too, such as the electrolyte and anode, and all those materials then need to be upcycled into a functional energy storage system. Durham says the ReCell team is tackling each of these steps individually, and the challenge will be stringing them together to form a profitable recycling process.
"Researchers in the ReCell Center are currently scaling different recycling steps and piecing them together to generate recycled material with good performance," Durham says. "Not only will the recycling process have to be worthwhile, but the upcycled material must be able to be accepted and used by industry for lithium-ion battery recycling to be successful."
The research was published in the journal Energy Technology.
Source: Argonne National Laboratory