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.