An Anionic Covalent Organic Framework as an Electrostatic Molecular Rectifier for Stabilizing Mg Metal Electrodes

Despite its potential as a high-capacity battery electrode, magnesium (Mg) metals are highly susceptible to electrolytes, resulting in the formation of unwanted passivation layers, which hinder charge transfer phenomena. Here, we first report an anionic covalent organic framework (a-COF) as an electrostatic molecular rectifier (that can preferentially trap solvent molecules) to stabilize Mg metal electrodes. Compared to a neutral COF (n-COF) as a control sample, the a-COF enhances Mg2+ transport by facilitating the desolvation of Mg2+-solvent complexes and cationic mobility through its negatively charged one-dimensional columns, thereby achieving an ionic conductivity eight times higher than that of the n-COF. In addition, the anionic porous frameworks in contact with Mg metal electrodes enable a uniform Mg2+ flux and interfacial stability with Mg metal electrodes. Consequently, the a-COF exhibited reversible Mg plating/stripping cyclability on Mg metal electrodes compared to the n-COF, demonstrating the electrochemical viability of the anionic frameworks for Mg metal electrode stabilization.