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The Rice University lab of materials scientist Pulickel Ajayan has used an environmentally friendly deep eutectic solvent to extract valuable elements from the metal oxides commonly used as cathodes in lithium-ion batteries. The goal, researchers said, is to curtail the use of harsh processes to recycle batteries and keep them out of landfills.

The solvent, made of commodity products choline chloride and ethylene glycol, extracted more than 90% of cobalt from powdered compounds, and a smaller but still significant amount from used batteries. The results appear in Nature Energy.

Battery recycling schematic. Starting with disassembly of the LIB, cathode waste is inserted into a DES (deep eutectic solvent), which is then heated and stirred. Extraction of cobalt and lithium ions occurs through dissolution, and at this step, aluminium foil, binder and conductive carbon can be recovered separately when the leachate is filtered. Cobalt compounds can then be recovered either through precipitation or electrodeposition, allowing reutilization of these valuable materials. Tran et al.

Rechargeable battery waste, particularly from lithium-ion batteries, will become an increasingly menacing environmental challenge in the future as the demand for these through their usage in electric vehicles and other gadgets increases dramatically. 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. 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.

—Pulickel Ajayan

This has been attempted before with acids. 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.

—Rice graduate student and lead author Kimmai Tran

Other processes also have drawbacks, Tran said. Pyrometallurgy involves crushing and mixing at extreme temperatures, and the harmful fumes require scrubbing. Hydrometallurgy requires caustic chemicals, while other “green” solvents that extract metal ions often require additional agents or high-temperature processes to fully capture them.

The deep eutectic solvent 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.

—Kimmai Tran

A deep eutectic solvent is a mixture of two or more compounds that freezes at temperatures much lower than each of its precursors; one can literally obtain a liquid from a simple combination of solids.

The large depression of freezing and melting points is due to the hydrogen bonds formed between the different chemicals. By selecting the right precursors, inexpensive green solvents with interesting properties can be fabricated.

—Kimmai Tran

When Tran joined, the Rice group was already testing a eutectic solution as an electrolyte in next-generation high-temperature supercapacitors. However, said Rice research scientist and co-corresponding author Babu Ganguli, the solution was dissolving the metal oxide supercapacitors; 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.

—Babu Ganguli

That became Tran’s focus, as she tested deep eutectic solvents on metal oxides at different temperatures and time scales. During tests with lithium cobalt oxide powder, the clear solvent yielded a wide spectrum of blue-green colors that indicated the presence of cobalt dissolved within.

At 180 degrees Celsius (356 degrees Fahrenheit), the solvent extracted nearly 90% of lithium ions, and up to 99% of cobalt ions from the powder when certain conditions were satisfied.

The researchers built small prototype batteries and cycled them 300 times before exposing the electrodes to the same conditions. The solvent proved adept at dissolving the cobalt and lithium while separating the metal oxides from the other compounds present in the electrode.

They found that cobalt could be recovered from the eutectic solution through precipitation or even electroplating to a steel mesh, as this latter method potentially allowed for the deep eutectic solvent itself to be reused.

We focused on cobalt. 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.

—Rice alumnus Marco Rodrigues, now a postdoc at Argonne National Laboratory

The Department of Energy is mounting new efforts to advance battery recycling technologies and recently announced a center for Li-ion battery recycling.

The National Science Foundation supported the research through its Graduate Research Fellowship Program.

Resources

• Mai K. Tran, Marco-Tulio F. Rodrigues, Keiko Kato, Ganguli Babu & Pulickel M. Ajayan (2019) “Deep eutectic solvents for cathode recycling of Li-ion batteries”
  Nature Energy doi: 10.1038/s41560-019-0368-4