On one hand, pyrometallurgy, which uses extremely high temperatures, produces greenhouse gas emissions. On the other hand, hydrometallurgy, which uses acids and reducing agents for extraction, creates wastewater that needs to be processed and handled.
In contrast, the University of Toronto group is using supercritical fluid extraction to recover metals from end-of-life Li-ion batteries. This process separates one component from another by using an extracting solvent at a temperature and pressure above its critical point — where it adopts the properties of both a liquid and a gas.
To recover the metals, the scientists used carbon dioxide as a solvent, which was brought to the supercritical phase by increasing the temperature above 31 Celsius, and the pressure up to 7 megapascals.
They were able to show that this process matched the extraction efficiency of lithium, nickel, cobalt and manganese to 90% when compared to the conventional leaching processes, while also using fewer chemicals and generating significantly less secondary waste. The main source of energy expended during the supercritical fluid extraction process was due to the compression of the carbon dioxide.
“The advantage of our method is that we are using carbon dioxide from the air as the solvent instead of highly hazardous acids or bases,” head researcher Gisele Azimi said in a media statement. “Carbon dioxide is abundant, cheap and inert, and it’s also easy to handle, vent and recycle.”
A caffeinated connection
Supercritical fluid extraction is not a new process. It has been used in the food and pharmaceutical industries to extract caffeine from coffee beans since the 1970s. Azimi and her team’s work builds on previous research in the lab to recover rare earth elements from nickel-metal-hydride batteries.
However, this is the first time that this process has been used to recover metals from lithium-ion batteries.
“We are now moving towards commercialization of this method to increase its technology readiness level. Our next step is to finalize partnerships to build industrial-scale recycling facilities for secondary resources. If it’s enabled, it would be a big game changer,” Azimi said.
In her view, if more batteries are recycled, it is possible to sustain the constrained supply chain for battery metals and help bring down the cost of EV batteries, making the vehicles more affordable.