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

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 discussed 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.