Green recycling of spent Li-ion battery cathodes via deep-eutectic solvents

The growth in numbers of electric vehicles (EVs) has meant significant demand for lithium-ion batteries (LIBs), together with a need for recycling of spent LIBs. Current existing pyro- and hydro-metallurgical recovery technologies are not satisfactory due to poor economics and risks to the environment arising from high temperature and strong acid(s) used. However, deep-eutectic solvents (DESs) have attracted research attention for recycling of spent LIBs but are far from maturity. In this Review, we critically assess advances and applications of DESs in LIB cathode recycling, with emphasis on rational design and modification strategies to obviate practical problems. We establish DES development, categorize types, with selected important characteristics and quantitative measurement. We categorize design and recent progress of DESs and establish insight into highly efficient recovery of LIBs, and report new experimental data. We evidence leaching mechanism(s) and metal recovery from leach liquor, and assess limitations in DES stability and reusability, together with process optimization(s). We also provide a comparative flowchart of sustainable DES possibilities and practical challenges, together with a prospective on likely future directions and outcomes. We conclude that DESs permit green-recycling of spent LIB cathodes with recovery of important metal resources. Findings will be of interest and benefit to researchers and manufacturers for development of a circular and environmentally benign battery value-chain.