Solvometallurgical recovery of cobalt from lithium-ion battery cathode materials using deep-eutectic solvents

Recycling of cobalt from end-of-life lithium-ion batteries (LIBs) is gaining interest because they are increasingly used in commercial applications such as electrical vehicles. A common LIB cathode material is lithium cobalt oxide (LiCoO2). Besides the cathode, LIBs contain other components, such as metallic aluminium and copper as current collectors, which are often separated at initial hydrometallurgical recycling stages. Leaching of cobalt from LiCoO2 is mainly driven by reducing cobalt(III) in LiCoO2 to cobalt(II) via adding reducing agents. In this work, a green, cheap and safe approach is proposed by using a choline chloride–citric acid deep-eutectic solvent (DES) as lixiviant. Aluminium and copper were evaluated as reducing agents for cobalt(III). After optimisation, lithium and cobalt were quantitatively leached from LiCoO2 in the presence of aluminium and copper. Copper was the most effective reducing agent for cobalt(III), so that no additional reducing agents or a pre-separation step were required. A speciation study of the pregnant leach solution (PLS) confirmed the dominance of chloro complexes. DES leaching was compared with conventional hydrochloric acid leaching, whereby the DES avoided the formation of toxic chlorine gas. Finally, the DES PLS was used as the more polar phase in a non-aqueous solvent extraction process. This process consisted of a copper(I/II) extraction step with the extractant LIX 984, followed by selective extraction of cobalt(II) with the extractant Aliquat 336. Both metals were completely stripped from the loaded organic phases by oxalic acid. The total recovery yield of cobalt was 81%, as a 99.9% pure oxalate precipitate.