A comprehensive review on the recovery of cathode active materials via direct recycling from spent Li-ion batteries

The appropriate disposal of spent Li-ion batteries (LIBs) is critical considering the limited resources of strategic metals and negative impacts on the environment as well as economic profits from LIB recycling. The direct recycling process as one of LIB recycling schemes regenerates spent cathode active materials through a series of liberation and relithiation processes without the destruction of active materials. In this review, we comprehensively cover the overall liberation and regeneration process for all kinds of commercial cathode active materials. Specifically, separation technologies that liberate cathode active materials are analyzed based on categorized methodologies including thermal treatment with single or multiple stages, chemical treatment through solvent- or mechanically-assisted dissolution methods, and the combined processes of chemo-thermal treatment. Next, we summarize the degradation mechanisms of cathode active materials and introduce regeneration methods that are mainly adopted in the direct recycling process such as solid-state, hydrothermal, molten-salt methods. We further compare the electrochemical performance of regenerated cathode active materials according to their regeneration process. The impacts of impurities that could exist during the direct recycling process on the structural and electrochemical properties of regenerated cathode active materials are firstly addressed. Lastly, research issues are suggested for the commercialization of the direct recycling process.