Beyond DESs: Aqueous Redox-Active Amino Chloride Salts for Efficient Hydrometallurgical Recycling of Lithium-Ion Batteries.

Recycling lithium-ion batteries has gained momentum over the past few years owing to the escalating demand for critical materials, with Deep Eutectic Solvents (DES) being noted as alternative green lixiviants for hydrometallurgical pathways. While polarity, pH, and H-bonding contribute to the high leaching efficiencies (LE) of these solvents, the exact role of the eutectic composition remains unclear. Moreover, high viscosities of DESs often lead to high temperature and time requirements, deterring their facile commercialization. Herein, we propose a new class of lixiviants, namely, aqueous amino chloride solutions, which mimic the necessary and sufficient properties of DESs while overcoming the constraints of high viscosity and low thermal conductivity. Among these, Hydroxylammonium chloride or hydroxylamine hydrochloride (HACl:H2O) solution shows remarkable ultrafast leaching performance, with ∼65% LE of all metals within 1 min at room temperature. Closer insights into the leaching pathway reveal that while pH balance and presence of a redox center play a crucial role in this rapid leaching, polarity and eutectic composition are not major contributors. A comparative study of different amino chloride solutions performed here leads to a better understanding of the leaching process and demonstrates that satisfying the necessary properties for leaching in different solvent systems can lead to the future design of smarter, more efficient, and sustainable lixiviant systems.