Selective sodium(I) capture by olivine FePO4@C electrode in pseudocapacitive deionization: Mechanism and application

Selective extraction of target ions from aqueous solutions by capacitive deionization (CDI) has gradually become a highly efficient separation technology for water desalination. For the first time, a carbon coated olivine phase iron phosphate (O-FePO4@C) composite was developed by electrochemical delithiation of olivine phase LiFePO4@C composite as a pseudocapacitive electrode to explore the selective desalting behavior towards Na+, Mg2+, and Ca2+ ions. Significantly, this work also offered a new strategy for the recycling of waste lithium iron phosphate batteries, which is killing two birds with one stone. Batch experiments reflected both excellent electrosorption capacity (61.3 mg g-1 for Na+) at 1.2 V and good cycling stability of the O-FePO4@C electrode. Cyclic voltammetry (CV) and X-ray diffraction (XRD) combined with density functional theory (DFT) simulation revealed that the electrosorption mechanism of O-FePO4@C is derived from ion intercalation for Na+ and Mg2+, and electrical double layers (EDLs) adsorption for Ca2+, respectively, which exhibited outstandingly selective sodium(I) electrosorption ability. Combining density of state (DOS) of electrons with spectroscopic analysis revealed that the intercalated ions can impact electron configuration and improve the conductivity of the electrode material. The selective desalination performance of O-FePO4@C was also confirmed by treating natural groundwater samples.