Efficient separation of Li and Mn from spent LiMn2O4 cathode materials by NH4HSO4 reduction roasting-selective ammonia leaching

With the imminent peak in the retirement of rechargeable lithium-ion batteries worldwide, the development of efficient methods for recycling valuable elements from LIBs has attracted considerable attention. In this work, a novel method of NH 4 HSO 4 reduction roasting-selective ammonia leaching was proposed to thoroughly separate Li and Mn from the spent LiMn 2 O 4 cathode materials . Using the TG-DSC-FTIR, XRD, SEM-EDS and ICP methods, the detailed mechanism of reduction roasting process was revealed, showing that the Mn 4+ and Mn 3+ in the spinel structure of LiMn 2 O 4 were reduced by the NH 4 + from the molten ammonium salt, and transformed into water-soluble MnSO 4 and Li 2 Mn 2 (SO 4 ) 3 . According to the water-leaching performance of the roasting products, the sulfation conversion rate of Mn reaches 98.1 %, while Li is almost completely transformed under the optimal roasting conditions (475 ℃, 60 min). Using the NH 3 ·H 2 O-air leaching-system, complete separation of Li and Mn can be achieved. It was shown that the introduction of air into the ammonia solution at a rate of 0.2 L/min under the optimal leaching condition leads to Mn precipitation in the form of nanoscale Mn 3 O 4 particles with a precipitation rate of over 99.99 %, while Li still remains completely leached. Based on the in situ detection of the oxidation reduction potential and pH during the selective leaching, the key mechanism of efficient Mn precipitation was revealed. that is, the increasing potential accelerates the oxidation of Mn(OH) 2 , and the rapid consumption of Mn(OH) 2 promotes the hydrolysis of Mn 2+ , thereby achieving a significant improvement in the separation efficiency of Li and Mn.