Efficient and sustainable recycling of spent lithium-ion batteries in deep eutectic solvents enhanced by non-solvating diluent

Abstract

Deep eutectic solvents (DESs) have been proposed as a green alternative to traditional acid leaching in recovery of cathode materials for lithium-ion batteries (LIB). However, the high viscosity of DESs leads to slow mass transfer and low solid–liquid separation efficiency, hindering both the leaching rate and recycling efficacy. In this paper, the viscosity and physicochemical properties of DES are regulated by introducing a non-solvating diluent, i.e., propylene carbonate (PC). The modified DES inherits the solvation structure of the original DES with highly concentrated 1-n-Butyl-3-methylimidazolium-chloride ([Bmim]⁺-Cl^–) – Oxalic acid (OA) clusters distributed in the non-solvating PC diluent. This spatial isolation weakens the electrostatic attractions of nearby clusters/ions, leading to decreased viscosity (24.5 mPa·s) and higher Cl^– diffusion coefficient. As a result, DES with PC diluent exhibits markedly improved leaching rate, kinetics and longer cycle life (Co and Mn leaching efficiency above 92 % after 20 cycles). The leaching-precipitating process directly yields the precursor required for cathode fabrication, resulting in a regenerated cathode that demonstrates stable electrochemical cycling performance.