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

Abstract 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‐Me 4 )14‐crown‐4)‐ ortho ‐carborane ( 14C4 Cb ), 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 ( 14C4 Cb ), whereas strong binding was observed in the cage‐opened reduced nido state ( 14C4 Cb 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 14C4 Cb/ 14C4 Cb 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.