Ambient-Dried Cellulose Nanofibril/H₂TiO₃ Composite Aerogel for Enhanced Lithium Capture at Seawater-Relevant pH

Efficient seawater lithium harvesting requires adsorbents combining high selectivity with scalable, low-energy fabrication. Although H₂TiO₃ lithium-ion sieves (HTO-LIS) exhibit exceptional Li⁺ selectivity, their deployment is hindered by strict alkaline requirements and reliance on energy-intensive shaping processes such as freeze-drying. Herein, we report a cellulose nanofibril (CNF) composite aerogel addressing both challenges simultaneously. Ammonium phytate (AP), a bio-derived polyphosphate, serves a dual function: as a green crosslinker for CNF and as an in situ alkalinity generator, creating a localized alkaline microenvironment around immobilized HTO nanoparticles that enables efficient topotactic H⁺/Li⁺ exchange at seawater-relevant pH 8 without requiring strongly alkaline bulk conditions. Crucially, the AP-crosslinked CNF skeleton withstands capillary forces during ambient-pressure drying, preserving a hierarchical macroporous network without freeze-drying. The resulting AP-CNF@HTO monolith achieves a Li⁺ capacity of 19.08 mg g^-1 at pH 8, a 128.9% enhancement over pristine HTO, with excellent selectivity against competing ions in real seawater and 88.3% capacity retention after ten cycles. This work demonstrates that rational bio-based crosslinker selection can concurrently solve the alkaline microenvironment and ambient-drying challenges of HTO-LIS composites, offering an ambient-pressure-drying route with reduced reliance on energy-intensive freeze-drying for seawater lithium enrichment.