An environmentally friendly deep eutectic solvent was used to extract valuable elements from the metal oxides commonly used as cathodes in lithium-ion batteries. 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.
Other processes for battery recycling have drawbacks. 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.
The solvent is 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, one can literally obtain a liquid from a simple combination of solids.
Deep eutectic solvents were tested 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 °C (356 °F), the solvent extracted nearly 90 percent of lithium ions, and up to 99 percent of cobalt ions from the powder when certain conditions were satisfied.
Small prototype batteries were created and cycled 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. 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.