Organometallic Polymer Constructed by Active Fe-C₁₂N₈ Centers for Boosting Sodium-Ion Storage

Organic-metal coordination materials with rich structural diversity are considered as promising electrode materials for rechargeable sodium-ion batteries. However, the electrochemical performance can be constrained by the limited number of active sites and structural instability under the discharge/charge process. Herein, organometallic polymer microspheres (Fe-PDA-220) with a unique d-π conjugated structure was designed and successfully constructed through a simple synchronous polymerization and coordination reactions. Polymerization of phenylenediamine was initiated by Fe³⁺ and Fe²⁺ ions generated synchronously during the polymerization integrated with poly-aminoquinone chains to form Fe-C₁₂N₈ active centers. Used as electrode materials for sodium-ion batteries, the distinctive Fe-C bond significantly boosts the structural stability, and the π-d conjugation system could facilitate electron transfer. A high reversible capacity of 345 mAh g^-1 was delivered at 0.1 A g^-1 and a capacity of 106 mAh g^-1 was maintained even after discharged/charged at 1.0 A g^-1 for 5000 cycles, outperforming most reported coordination materials. Spectroscopic and electronic analyses revealed that a two-electron reaction occurred per active unit, accompanied by the reversible redox evolution of the C=N groups and Fe ions during the sodiation/desodiation. This work provides a promising and efficient strategy for boosting the electrochemical performance of organic electrode materials by the design of organometallic polymers.