Selective separation of Ni from Co and Mn in spent lithium-ion batteries using a novel tetramethylammonium chloride-based deep eutectic solvent

Achieving selective separation of transition metals with close reduction potentials, such as cobalt and nickel, from used lithium-ion batteries (LIBs) remains a key challenge due to their chemical similarity and the complexity of battery waste streams. This study reports a novel deep eutectic solvent, synthesised from maleic acid and tetramethylammonium chloride (M: TMAC), that enables the targeted extraction of critical metals from end-of-life LiNi x Co y Mn 1−x−y O 2 cathodes. Cobalt and manganese are selectively leached as [N(CH 3 ) 4 ] 2 XCl 4 , (TMATC-X, where X = Mn, Co) complexes through a unique solid–liquid phase separation mechanism and are subsequently recovered as a mixture of Co 3 O 4 and MnO. Notably, nickel is selectively electrodeposited in metallic form onto a copper substrate, facilitated by the M: TMAC DES's high electrical conductivity, while lithium is recovered as lithium carbonate. This integrated system provides an efficient pathway for the targeted extraction of valuable metals, aligning with circular economy principles. • A novel TMAC-based DES system was formulated. • Selective and efficient Ni Co removal via solid–liquid phase separation in M: TMAC. • Co and Mn are selectively recovered as a complex with the HBA, TMAC. • Selective electrodeposition of Ni from DES in metallic form on a copper substrate. • Minimises extra purification steps and reagents used for the selective recoveries.