A 1D Co-MOF with Electronic Gating in Ligands for the High-Voltage Cathode of Sodium-Ion Batteries

Abstract

Sodium-ion battery (SIB) cathodes capable of storing both cations and anions offer a promising route to high capacity and high voltage, yet anion intercalation in metal−organic framework (MOF) cathodes remains rarely observed. Here, we report a one-dimensional Co-terephthalate MOF (CityU-84) synthesized at room temperature, which exhibits a wide voltage window of 1.0−4.3 V when evaluated as a cathode in SIBs due to dual-ion intercalation. Ex situ Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) reveal that the framework enables both PF₆⁻ intercalation at high voltages during the charging process and Na⁺ insertion at low voltages during the discharging process. To elucidate the origin of this dual functionality, we compare CityU-84 using unsubstituted terephthalate with its isostructural halogenated analogues (CityU-85 and CityU-86). Both halogenated MOFs show severely degraded electrochemical performance, demonstrating a ligand electronic gating effect, where electron-withdrawing substituents (e.g., Cl and Br) alter the electronic structure of the coordination framework, weakening both anion and cation interactions. Consequently, CityU-84 delivers a high reversible capacity (178 mAh g⁻¹), together with excellent rate capability and outstanding cycling stability (88% capacity retention after 1200 cycles at 200 mA g⁻¹). This work demonstrates that electronic gating of ligands enables dual-ion storage in pristine MOF cathodes, providing a promising platform for high-voltage organic electrode materials.