Hierarchically Porous Polyacrylonitrile (PAN) 3D Architectures with Anchored Lattice-Expanded λ-MnO₂Nanodots as Freestanding Adsorbents for Superior Lithium Separation

The search for novel lithium-ion sieves (LISs) with more desirable performance seems to be at the cutting edge of lithium separation from seawater and salt-lake brines. In this work, new types of freestanding shape-controllable LISs have been prepared by subtly anchoring λ-MnO2 nanodots on the surface of mesoporous polyacrylonitrile (PAN) nanoskeletons via a scalable sol-gel method. Synergistic effects originate from the hierarchically porous polymer structure, and the high surface ratio of nanodots significantly reduces ion-diffusion resistance, rendering superior kinetics with the adsorption equilibrium being achieved within 3 h for PMO-1. Moreover, PAN not only acts as a binder but also tunes the λ-MnO2 crystal structure by inhibiting the lattice shrinkage of LiMn2O4 during the acid leaching process. The slightly expanded crystal cell enhances the use of active sites, which ultimately results in an excellent saturated adsorption capacity of 49.0 mg g-1 for PMO-1 (based on the mass of λ-MnO2). Flow-through adsorption tests have also been performed by continuously injecting the solution into a PMO-1-packed column to verify its ability for practical applications, which further prove its robust cycling performance and superior lithium separation properties for both seawater and salt-lake brines.