Immobilizing Quinone-Fused Aza-Phenazine into π-d Conjugated Coordination Polymers with Multiple-Active Sites for Sodium-Ion Batteries

The development of coordination polymers with π-d conjugation (CCPs) provides ide prospects for exploring the next generation of environmental-friendliness energy storage systems. Herein, the synthesis, experimental characterizations, and Na-ion storage mechanism of π-d CCPs with multiple-active sites are reported, which use quinone-fused aza-phenazine (AP) and aza-phenazin (AP) as the organic ligands coordinated with the metal center (Ni²⁺). Among them, NiQAP as the cathode material exhibits impressive electrochemical properties applied in sodium-ion batteries (SIBs), including the high initial/stable discharge specific capacities (180.0/225.6 mAh g⁻¹) at 0.05 A g⁻¹, a long-term cycle stability up to 10,000 cycles at 1.0 A g⁻¹ with a high reversible capacity of 100.1 mAh g⁻¹, and good rate capability of 99.6 mAh g⁻¹ even at 5.0 A g⁻¹. Moreover, the Na-ion storage mechanism of NiQAP is also performed by the density functional theory (DFT) calculation, showing multiple-active sites of C≐O and C≐N (in the quinone and phenazine structure) and NiO₄ (in the coordination unit) for Na-ion storage. These results highlight the importance of organic electrode material with the coordination units and provide a foundation for further studying the CCPs with multiple active sites for energy storage systems.