Clean and efficient recovery of spent LiCoO₂ cathode material: Water-leaching characteristics and low-temperature ammonium sulfate calcination mechanisms

LiCoO₂ lithium-ion batteries are disposed once their life cycle comes to an end, by which large amounts of extremely harmful heavy metals and organic compounds are released into the environment. Further, the spent LiCoO₂ cathode material from spent lithium-ion batteries contains numerous high-grade metals with a high recovery value, such as Li, Co and Mn. Hence, a clean and efficient process is proposed in this research for recovery of spent LiCoO₂ cathode material via low-temperature (NH₄)₂SO₄ calcination and water-leaching.The calcination temperature range between 445.87 and 677.48 °C is determined from analysis of the TG-DTA curve obtained with n (2Li + Co + Mn): n (NH₄)₂SO₄ of molar ratio = 1:1. The sample calcined under conditions of calcination temperature of 600 °C, calcination time of 45 min and an (NH₄)₂SO₄ excess of 1.2 exhibits optimal water-leaching efficiencies of the Li, Co and Mn elements, which are 99.50%, 98.53%, and 94.41%, respectively. Reaction mechanisms of the low-temperature calcination process are investigated with both SEM and XRD analyses of the differently prepared samples and their residues following water-leaching. Results show that the calcination of spent LiCoO₂ cathode material with (NH₄)₂SO₄ is a liquid-solid reaction with the spent cathode material particles being gradually eroded and consumed from the outer layer. In addition, the presence of free high-energy H⁺ released by the decomposition of NH₄ ⁺ generated by molten (NH₄)₂SO₄ plays a leading role in the whole calcination process, the structure of LiCoO₂ and LiMn₂O₄ are broken down before eventually being converted into soluble sulfate salts.