Efficient, Selective Sodium and Lithium Removal by Faradaic Deionization Using Symmetric Sodium Titanium Vanadium Phosphate Intercalation Electrodes

NASICON (sodium superionic conductor) materials are promising host compounds for the reversible capture of Na⁺ ions, finding prior application in batteries as solid-state electrolytes and cathodes/anodes. Given their affinity for Na⁺ ions, these materials can be used in Faradaic deionization (FDI) for the selective removal of sodium over other competing ions. Here, we investigate the selective removal of sodium over other alkali and alkaline-earth metal cations from aqueous electrolytes when using a NASICON-based mixed Ti-V phase as an intercalation electrode, namely, sodium titanium vanadium phosphate (NTVP). Galvanostatic cycling experiments in three-electrode cells with electrolytes containing Na⁺, K⁺, Mg²⁺, Ca²⁺, and Li⁺ reveal that only Na⁺ and Li⁺ can intercalate into the NTVP crystal structure, while other cations show capacitive response, leading to a material-intrinsic selectivity factor of 56 for Na⁺ over K⁺, Mg²⁺, and Ca²⁺. Furthermore, electrochemical titration experiments together with modeling show that an intercalation mechanism with a limited miscibility gap for Na⁺ in NTVP mitigates the state-of-charge gradients to which phase-separating intercalation electrodes are prone when operated under electrolyte flow. NTVP electrodes are then incorporated into an FDI cell with automated fluid recirculation to demonstrate up to 94% removal of sodium in streams with competing alkali/alkaline-earth cations with 10-fold higher concentration, showing process selectivity factors of 3-6 for Na⁺ over cations other than Li⁺. Decreasing the current density can improve selectivity up to 25% and reduce energy consumption by as much as ∼50%, depending on the competing ion. The results also indicate the utility of NTVP for selective lithium recovery.