Battery Electrode Materials with Omnivalent Cation Storage for Fast and Charge‐Efficient Ion Removal of Asymmetric Capacitive Deionization

Abstract Capacitive deionization (CDI) that engages porous carbon electrodes constitutes one of the well‐established energy‐efficient desalination methods. However, improvement in desalination performance, including ion removal capacity, ion removal rate, and charge efficiency remains requisite for a wide range of applications. Herein, an ion‐exchange membrane‐free asymmetric CDI is introduced by pairing a metal organic framework (MOF), namely, K 0.03 Cu[Fe(CN) 6 ] 0.65 ·0.43H 2 O and porous carbon. The exclusive intercalation of cations into the MOF prevents the reverse adsorption of co‐ions (anions), thus significantly improving ion removal capacity (23.2 mg g −1 ) and charge efficiency (75.8%). Moreover, by utilizing the advantage of the MOF that diverse mono‐ and divalent cations can be stored in the narrow redox potential range, the asymmetric CDI allows simultaneous capture of mono‐ and divalent cations, thus achieving omnivalent cation removal. Moreover, cations are intercalated in the hydrated forms without a discrete phase transition of the host structure, facilitating rapid desalination by reducing the desolvation energy penalty, which results in a high ion removal rate of 0.24 mg g −1 s −1 . This study offers a new design principle in CDI: the integration of a crystal structure with large ionic channels that enable hydrated intercalation of multivalent ions in a fast and exclusive manner.