New ‘Blue-Green’ Solution for Recycling World’s Batteries

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The Rice lab of materials scientist Pulickel Ajayan 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.

Image Caption: A solution turns green as it pulls cobalt from a spent lithium-ion cathode. A Rice University laboratory is developing an environmentally friendly method to recover valuable metals from used batteries. Photo by Jeff Fitlow

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

“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,” Ajayan said.

“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,” he 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.”

“This has been attempted before with acids,” said 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 drawbacks, she 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 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.”

A deep eutectic solvent is a mixture of two or more compounds that freezes at temperatures much lower than each of its precursors. In that way, she said, 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,” Tran said. “By selecting the right precursors, inexpensive ‘green’ solvents with interesting properties can be fabricated.”

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.”

Image Caption: 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. They are developing the solvent to extract cobalt and lithium from spent lithium-ion batteries. Photo by Jeff Fitlow

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.

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