Mechanisms underlying the acid leaching process for LiNi0.6Co0.2Mn0.2O2 with and without H2O2

Abstract

Digesting lithium insertion materials in acid solutions is succeeded by into today's discovery and developments of lithium-ion batteries (LIBs), and has recently received significant attention as a leaching process for recycling spent LIBs. To explore more effective and eco-friendly leaching conditions, we elucidated the reaction mechanisms underlying the dissolution of LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) into an HNO₃ solution with or without H₂O₂. Without H₂O₂, delithiated NCM622 powder remained even after 24 h, whereas, with H₂O₂, NCM622 completely dissolved into the solution within 3 h. X-Ray diffraction patterns, transmission electron microscopy, and X-ray Raman spectroscopy all indicated that the residual NCM622 powder maintained the initial Li composition during the entire leaching process with H₂O₂. However, soft X-ray absorption spectroscopy, which provides information on the electronic and oxidation states of a surface, revealed the presence of a delithiated NCM622 phase. Hence, successive oxidation and dissolution reactions on the surface occur during the leaching process with H₂O₂, enabling the rapid and complete dissolution of NCM622. The results of leaching with an eco-friendly organic acid are also presented, along with comparative leaching experiments with other positive electrode materials. These insights into the dissolution of lithium insertion materials in acid solutions could pave the way to more effective and eco-friendly acid leaching and solvent extraction processes.