Ultraselective sequestration of Li+ and Mg2+ from brines via a reusable polyoxoniobate-based ion sponge.

Lithium (Li) and magnesium (Mg) are designated as critical mineral materials (CMM) due to their essential roles in clean energy technologies. However, extracting high-purity Li+ from brine remains a formidable challenge owing to the presence of Mg2+, a physicochemical similar ion that often exists in excess. Here, we introduce a polyoxoniobate-based "Mg-PONb sponge" that enables ultraselective and rapid Li+/Mg2+ separation across an exceptionally broad range of Mg/Li ratios (0.02 to 200.63). This framework achieves >99.9% Mg2+ removal with negligible Li+ loss in under 1 min, yielding Li+/Mg2+ selectivity values exceeding 5000. The sponge demonstrates excellent recyclability, maintaining >99% Mg2+ rejection and Li+ permeability across five regeneration cycles without structural degradation. Mechanistic investigations reveal that selective Mg2+ capture originates from strong coordination with terminal oxygens on the PONb cluster, driving rapid formation of porous Mg-PONb frameworks. This work presents a generalizable, scalable strategy for Li+/Mg2+ separation and offers a sustainable path toward enhanced Li and Mg recovery from complex brine sources.