A Redox-Tunable Carborane Crown: Toward Highly Selective Electrochemical Lithium Capture

Lithium is a critical element with a projected exponential rise in demand due to its widespread use in battery energy storage. New methods to extract Li⁺, such as through membrane adsorption-based direct Li⁺ extraction (DLE) technologies, are at various stages of development and aim to separate Li⁺ from brine and even seawater. In this report, we present a fundamentally new class of highly selective Li⁺-capture agent, the carborane-crown compound, 1,2-((6,6,7,7-Me₄)14-crown-4)-ortho-carborane (^14C4Cb), which is electrochemically activated for strong, selective Li⁺ binding over Na⁺ and K⁺. This newly synthesized extractant features a redox-tunable cavity size, giving rise to tunable binding constants for Li⁺ capture, favorable coulombic interactions between the reduced anionic capture agent and the Li⁺ cations, and boasts the benefit of rapid, electrochemically driven capture kinetics. Weak, negligible binding to Li⁺ was observed in the neutral "closo" carborane state (^14C4Cb), whereas strong binding was observed in the cage-opened reduced nido state (^14C4Cb^2-). Equilibrium (K) binding constants were measured through experimental and simulated voltammetry, yielding the following log K_metal (experimental; simulation) values: log K_Li (6.8 ± 0.6; 8.0), log K_Na (3.7 ± 0.2; 4.9), log K_K (1.7 ± 0.2; 2.2). Rapid mass transport of Li⁺ to the electrode surface resulted in the simulated value (log K_Li = 8.0) representing a lower-limit value for log K_Li as described herein. The observed strong binding to Li⁺ over Na⁺ and K⁺ is attributed to both the favorable redox-tunable crown cavity size of the ^14C4Cb/^14C4Cb^2- couple, combined with strong coulombic interactions in the reduced nido state. This platform offers a potential new, rapid, and highly selective technique for Li⁺ capture in next-generation electrochemical DLE technologies.