Light-Driven Lithium Extraction from Mixtures of Alkali Cations Using an Azobipyridine Ligand

Humanity's replacement of fossil-fuel energy with renewable electricity will require extensive deployment of lithium batteries, which necessitates an increase in the rate of production of this vital metal. Current extraction methods from brines and ores are very energy-intensive, as are recycling methods. Here we report a novel Li₅<b>L</b>₂ sandwich structure in which five lithium ions are bound between the azobipyridine groups of two pentagonal ligands. The geometry of these ligands leads to the selectivity for binding Li⁺ in the presence of Na⁺ and K⁺. Upon illumination, the <i>trans</i>-azobipyridine moieties of the ligands isomerize to their <i>cis</i> form, promoting the release of free Li⁺ as the sandwich comes apart. The selective complexation of Li⁺, and its photostimulated release, were used as the basis of a cycle for selectively extracting and releasing Li⁺ ions from a mixture with Na⁺ and K⁺. Solar energy may thus be used directly to purify this metal, which is essential for storing increasingly cheap electricity produced through renewable means.