Coupling hybrid membrane capacitive deionization (HMCDI) with electric-enhanced direct contact membrane distillation (EE-DCMD) for lithium/cobalt separation and concentration Separation and Purification Technology

The increasing usage of electronic devices has added up the generation of spent LiCoO2 (LCO) batteries. Hence there is an urgency to develop high-efficiency and environmentally-friendly approaches for LCO electrode recycling by Li+/Co2+ ion-separation and concentration. In this work, an ion-sieving membrane with a lamellar structure was fabricated by self-assembling MXene nanosheets on a hydrophilic PTFE membrane, and the interlayer spacing between 2D MXene pieces was adjusted by doping a certain amount of polyvinyl alcohol (PVA). Such ion-sieving membrane was inserted between cation exchange membrane (CEM) and activated carbon fiber (ACF) electrode, forming a hybrid membrane capacitive deionization (HMCDI) system to enhance the ion-separating performance. The artificial Li+/Co2+ mixed solution was firstly treated by the HMCDI process with a separating factor over 6. The Li-enriched HMCDI effluent was further concentrated and purified using an electric-enhanced direct contact membrane distillation (EE-DCMD) system, providing an ion-concentrating effect over 40-folds and a Co2+ removal rate over 99 %. On this basis, a membrane-based LCO battery recycling pathway was proposed, showing advantages in energy conservation, zero liquid discharging, and chemical agent saving. Moreover, the mechanism of ion-separating in the HMCDI system was elucidated in terms of energy barrier and Li+ selective adsorption. The mechanism of anti-fouling and ion-separating in EE-DCMD was dis-cussed based on the interfacial electrochemical reactions and transmembrane mass transfer. We believe this study can a provide new insight into ion separation and concentration, which was promising in LCO battery recycling.