Insights into the coordination enhanced leaching mechanism of spent lithium-ion batteries cathode materials

The ultra-fast leaching of strategic metals from spent lithium-ion batteries cathode materials is achieved by the coordination enhanced leaching with inorganic acids and organic reductants. However, the detailed leaching mechanism that led to the fast leaching kinetics is unclear. This research utilizes the controlled variable method to reveal the coordination enhanced leaching mechanism by a comparison of the thermodynamics and kinetics between sulfate acid coupled with L-ascorbic acid (VC) and hydrochloride acid coupled with VC systems. Thermodynamic calculations, online UV-vis spectroscopy, and in-situ Raman spectroscopy are adopted to interpret the leaching mechanisms in the whole leaching process. The leaching of cobalt is controlled by decomposition of crystal structure and diffusion, respectively, in sulfate acid coupled with VC and hydrochloride acid coupled with VC systems, indicating relatively slow kinetics in the former system. The thermodynamic calculations suggest higher fractions and more stable coordination species, i.e., 26.11% CoCl(H2O)5+ and 8.945% CoCl4(H2O)22- at 363 K, in the latter system, which leads to lower apparent activation energy and might facilitate the rapid leaching kinetics in return. In-situ Raman mapping and depth analysis suggested the formation of coordination species on the crystal structure surfaces, solid-liquid boundaries, and bulk solutions.