Achieving High Li+-to-Na+ Selectivity in Electrodialysis via Pulsed Electric Fields in FePO4-Coated Membranes

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Abstract Seawater contains a vast lithium inventory but an extremely low Li+ concentration (∼0.21 ppm) and a large excess of Na+ (Li/Na ≈ 1:50000), making Li+/Na+ separation a key bottleneck for lithium recovery. Here we study electrodialysis (ED) based on a Li+-selective Li0.33La0.56TiO3 (LLTO) membrane and show that Na+ accumulation at the membrane surface under continuous direct current (DC) operation imposes a threshold for Li+/Na+ selectivity. To deplete this Na+-rich layer, we apply pulsed electric fields (PEFs) with asymmetric forward–reverse segments. An optimized waveform periodically drives Na+ away from the interface while maintaining the association of Li+ with the membrane, increasing Li+/Na+ selectivity from ∼500 under DC to ∼1800 without sacrificing Li+ flux. Building on this kinetic control, we introduce an FePO4/PVDF coating on LLTO to provide adsorption sites that have affinity for Li and suppress Na+ retention. Under PEF operation, the membrane with an optimal FePO4/PVDF mass ratio of 1.0 yields 14 ppm Li+ and 182.5 ppm Na+ in the extract, corresponding to a Li+/Na+ selectivity of ∼4510. In a multi-ion seawater analogue, 24 h operation delivers 24.5 ppm Li+ with Li+/Na+ and Li+/K+ selectivities of ∼1400 and ∼2600, respectively, while Ca2+ and Mg2+ remain below detection limits. These results show that combining time-dependent field control with lithiophilic surface coatings is an effective route to high Li+/Na+ selectivity in ED using scalable ceramic membranes.

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