Recovering critical metals from spent lithium-ion batteries is not a particularly environmentally friendly process. But a pair of University of Toronto chemical engineers have recently shown how to reduce the impact by tweaking a method used since the 1970s to extract caffeine from coffee beans.
Currently, lithium-ion batteries are recycled in one of two ways. The first uses pyrometallurgy, which requires extremely high temperatures, and the second uses hydrometallurgy, which requires acids and reducing agents for extraction. Both are problematic – pyrometallurgy produces greenhouse gas emissions, while hydrometallurgy creates wastewater that must be treated.
Instead, Professor Gisele Azimi and PhD student Jiakai (Kevin) Zhang are studying the use of supercritical fluid extraction to recover metals from end-of-life lithium-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.
“It has low viscosity and high diffusibility, which are gas properties and it has high solvency, like liquid,” says Azimi. “This creates very strong solvents that can substitute for hazardous acidic or basic solvents.”
In a paper published in September in Resources, Conservation, and Recycling, Azimi and Zhang describe how Zhang used carbon dioxide as a solvent. He brought the CO2 to the supercritical phase by increasing the temperature above 31 C and the pressure up to seven megapascals.
Working with lithium-ion batteries from electric vehicles, Zhang tweaked the supercritical fluid extraction process by using adducts – tributyl phosphate–nitric acid, and hydrogen peroxide. In this way, he was able to effectively separate out the batteries’ lithium, nickel, cobalt, and manganese.
“The process had an extraction efficiency of 90 per cent when compared to the conventional methods,” says Zhang. It also used 30 to 40 per cent less energy than pyrometallurgy and generated less chemical waste than hydrometallurgy.
“Plus, you can recycle the solvent,” adds Zhang.
As both he and Azimi point out, mining these metals from raw ore takes a lot of energy. But if we treat spent batteries as ores, their metal concentrations of roughly 30 per cent (versus one to two per cent in natural ores) means less energy is required to “mine” these critical metals.
This is important because as part of Canada’s commitment to reach net-zero emissions by 2050, it has required all new light-duty cars and passenger trucks sold in the country to be electric by 2035.
The life expectancy of electric vehicle batteries ranges from 10 to 20 years, but most car manufacturers only provide a guarantee for eight years or 160,000 km — whichever comes first. While the batteries can be refurbished or recycled to recover metals, many are simply thrown out and end up in landfills where they can corrode and leach out contaminants.
Once extracted from used EV batteries, lithium, nickel, cobalt, and manganese can be recombined in a way that recreates their crystal structures so that the battery can once more store and release energy.
Azimi says since publicizing the process, several companies have expressed interest in working with her lab to commercialize the process. “If it works, it could be a game-changer,” she says.