Comparative of malonic acid aqueous solution and malonic acid-based deep eutectic solvent for LiCoO2 cathode materials recovery: Leaching efficiency and mechanism

Abstract

Deep eutectic solvents (DESs) and aqueous organic acids are two leaching systems that have achieved remarkable results in hydrometallurgical recovery of spent lithium-ion batteries (LIBs). However, previous studies have only evaluated a single leaching system at a time and thereby failed to compare the differences between DES and organic acids systems in detail. In this study, our purpose was to clarify the differences between the efficiencies of two leaching systems—a malonic acid (MA) aqueous solution and a DES of choline chloride: malonic acid (ChCl:MA) via a systematic investigation of the conditions and mechanisms associated with the leaching of LiCoO2 cathode materials using these two systems. The effects exerted by various process parameters, such as reaction time, temperature, and liquid-to-solid ratio, on the leaching efficiency of these two systems were investigated. The results showed that the Co and Li leaching efficiencies of the DES were higher than those of the MA aqueous solution under identical experimental conditions. The kinetic differences between the two systems were reflected in the fact that the MA aqueous solution was regulated by diffusion, whereas the DES leaching was regulated by chemical reactions. Spectroscopic analysis and density functional theory calculations indicated that MA acts as both a ligand and a reducing agent in aqueous solutions, whereas MA in the DES only acts as a reducing agent. In addition, Co was present in the MA aqueous and DES leaching solutions as octahedral (Co(H2O)2(C3H2O4)2) and tetrahedral ([CoCl4]2-), respectively.