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

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_0.03Cu[Fe(CN)₆]_0.65·0.43H₂O 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.