Enhancing Sodium-Ion Storage Capacity and Stability in Metal–Organic Coordination Compounds by Bifunctional Coordinated Water Molecule

Abstract

Redox-active organic compounds have received much attention as high-capacity electrodes for rechargeable batteries. However, the high solubility in organic electrolytes during charge and discharge processes hinders the practical exploitation of organic compounds. This study presents a cobalt-based metal–organic coordination compound with bifunctional coordinated water (Co-MOC-H₂O) for sodium-ion storage. The coordinated water enhances interactions between sodium ions and nitrogen atoms in organic ligands through chelation, activating the inert sodium-ion storage sites (C=N). Moreover, the stable hydrogen bonded framework formed by the coordinated water molecules prevents the active organic compounds from dissolving into the electrolyte, thereby enhancing cycling stability. With the bifunctional coordinated water molecules, the Co-MOC-H₂O electrode delivers a high capacity of 403 mAh g⁻¹ at 0.2 A g⁻¹ over 600 cycles and exhibits a capacity retention of 77.9% at 2 A g⁻¹ after 1100 cycles. This work highlights the crucial role of the coordinated water molecules in constructing high capacity and long-life sodium-ion storage materials.