With a rapid expansion in electric vehicles, a huge amount of the spent Li-ion batteries (LIBs) could be discharged in near future. And thus, the proper handling of the spent LIBs is essential to sustainable development in the industry of electrical vehicles. Among various approaches such as pyrometallurgy, hydrometallurgy, and direct recycling, the hydrometallurgical manner has gained interest in recycling the spent LIBs due to its high effectiveness in recycling raw materials (e.g., lithium, nickel, cobalt, and manganese). However, the hydrometallurgical process not only requires the use of large amounts of acids and water resources but also produces toxic gases and wastewater leading to environmental and economic problems, considering potential economic and environmental problems. Thus, this review aims to provide an overview of conventional and state-of-the-art hydrometallurgical processes to recover valuable metals from spent LIBs. First, we briefly introduce the basic principle and materials of LIBs. Then, we briefly introduce the operations and pros-and cons- of hydrometallurgical processes. Finally, this review proposes future research directions in hydrometallurgy, and its potential opportunities in the fundamental and practical challenges regarding its deployment going forward.

In this study, an experiment is performed to recover the Li in Li2CO3 phase from the cathode active material NMC (LiNiCoMnO2) in waste lithium ion batteries. Firstly, carbonation is performed to convert the LiNiO, LiCoO, and Li2MnO3 phases within the powder to Li2CO3 and NiO, CoO, and MnO. The carbonation for phase separation proceeds at a temperature range of 600oC~800oC in a CO2 gas (300 cc/min) atmosphere. At 600~700oC, Li2CO3 and NiO, CoO, and MnO are not completely separated, while Li and other metallic compounds remain. At 800 oC, we can confirm that LiNiO, LiCoO, and Li2MnO3 phases are separated into Li2CO3 and NiO, CoO, and MnO phases. After completing the phase separation, by using the solubility difference of Li2CO3 and NiO, CoO, and MnO, we set the ratio of solution (distilled water) to powder after carbonation as 30:1. Subsequently, water leaching is carried out. Then, the Li2CO3 within the solution melts and concentrates, while NiO, MnO, and CoO phases remain after filtering. Thus, Li2CO3 can be recovered.