Discarded lithium-ion batteries are a problem that is only getting worse with their increasing demand for cellphones, electric vehicles, and other electronic devices that utilize them. For this reason, many including the U.S. Department of Energy are looking for ways of recycling lithium-ion batteries.
Rice University researchers have devised a method that could serve as an important part of the lithium-ion battery recycling process. The research team including materials scientist Pulickel Ajayan created a deep eutectic solvent to extract valuable elements from the metal oxides frequently used as cathodes in lithium-ion batteries. (Rice University research scientist Babu Ganguli and graduate student Kimmai Tran show test tubes with their eutectic solvent and varying concentrations of cobalt drawn into the solution in the photo above by Jeff Fitlow).
A deep eutectic solvent is a mixture of two or more compounds that freeze at temperatures that are considerably lower than each of its precursors. In that way, she pointed out, one can get a liquid from a simple combination of solids.
The goal, according to the researchers, is to diminish the use of harsh processes to recycle batteries and keep discarded batteries out of landfills.
The solvent, made of commodity products ethylene glycol and choline chloride, extracted over 90% of cobalt from powdered compounds. It also extracted a smaller but still significant portion of cobalt from used batteries.
“It’s important to recover strategic metals like cobalt that are limited in supply and are critical for the performance of these energy-storage devices,” Ajayan said. “Something to learn from our present situation with plastics is that it is the right time to have a comprehensive strategy for recycling the growing volume of battery waste.”
The team reported their results in Nature Energy.
Drawbacks of Other Approaches
“This has been attempted before with acids,” noted Rice graduate student and lead author Kimmai Tran. “They’re effective, but they’re corrosive and not eco-friendly. As a whole, recycling lithium-ion batteries is typically expensive and a risk to workers.”
Other processes also have disadvantages, she said. Pyrometallurgy for example, involves crushing and mixing at extreme temperatures, and the harmful fumes require scrubbing. Hydrometallurgy uses caustic chemicals, while other “green” solvents that extract metal ions often demand additional agents or high-temperature processes to completely capture them.
Advantages of New Solvent
“The nice thing about this deep eutectic solvent is that it can dissolve a wide variety of metal oxides,” Tran said. “It’s literally made of a chicken feed additive and a common plastic precursor that, when mixed together at room temperature, form a clear, relatively nontoxic solution that has effective solvating properties.”
Solvent Discovered While Developing Supercapacitor Electrolytes
When Tran joined, the Rice group was already testing a eutectic solution as an electrolyte in next-generation high-temperature supercapacitors.
“We tried to use it in metal oxide supercapacitors, and it was dissolving them,” said Rice research scientist and co-corresponding author Babu Ganguli. “The color of the solution would change.”
The eutectic was pulling ions from the supercapacitor’s nickel.
“Our team was discussing this and we soon realized we could use what was thought to be a disadvantage for electrolyte as an advantage for dissolving and recycling spent lithium batteries,” Ganguli said.
The clear solvent produced a wide spectrum of blue-green colors that showed the presence of cobalt dissolved during tests with lithium cobalt oxide powder.
At 180oC (356oF), the solvent extracted almost 90% of lithium ions, and up to 99 percent of cobalt ions from the powder when certain conditions were satisfied.
Solvent Disolved Cobalt and Lithium in Prototype Battery
They constructed small prototype batteries and cycled them 300 times before exposing the electrodes to the same conditions. In this test scenario, the solvent proved proficient at dissolving the cobalt and lithium while separating the metal oxides from the other compounds in the electrode.
The team also found that cobalt could be recovered from the eutectic solution through precipitation or even electroplating to a steel mesh, because this latter method could allow the solvent itself to be reused.
“We focused on cobalt,” said Rice alumnus Marco Rodrigues, now a postdoctoral researcher at Argonne National Laboratory. “From a resource standpoint, it’s the most critical part. The battery in your phone will surely have lots of it. Lithium is very valuable too, but cobalt in particular is not only environmentally scarce but also, from a social standpoint, hard to get.”
Much Work Needed
The path forward will require continued work to make the solvent that is the best at taking out the metal oxides.
“It’s likely we won’t be able to recycle and replace mining completely,” Tran said. “These technologies are relatively new, and there is a lot of optimization that needs to be done, such as exploring other deep eutectic solvents, but we truly believe in the potential for greener ways to do dirty chemistry. Sustainability is in the heart of the work I do and what I want to do for the rest of my career.”
The National Science Foundation supported this research via the Graduate Research Fellowship Program.