Engineering Metal Centers of π-d Conjugated Coordination Polymers for Boosting Sodium Storage

Organic electrode materials hold substantial promise for sodium-ion batteries while suffering from poor electronic conductivity and high solubility in common organic electrolytes. Among various organic electrode materials, π-d conjugated coordination polymers (CCPs) are particularly noteworthy due to their exceptional chemical stability and conductivity, which arise from the hybridization between the d orbitals of metal ions and the π orbitals of organic ligands. Herein, we synthesized a variety of CCPs by utilizing 2,5-dihydroxy-1,4-benzoquinone (DHBQ) as a ligand, with M²⁺ (M = Ni, Co, and Mn) ions serving as metal centers. This approach enabled a thorough investigation into the impact of these metal sites on the electrochemical performance of the CCPs. Theoretical calculations demonstrated that Ni-DHBQ exhibits the smallest bandgap and the highest degree of π-d conjugation compared to its analogs, facilitating the transport of Na⁺ ions. Consequently, Ni-DHBQ delivers the highest capacity (157 mAh g⁻¹ at 0.1 A g⁻¹), enhanced rate ability (153.9 mAh g⁻¹ at 0.2 A g⁻¹), and remarkable cycling stability (capacity retention of 92.9% over 500 cycles at 1 A g⁻¹). Additionally, the reaction mechanism of Ni-DHBQ was comprehensively investigated using in situ x-ray diffraction, complemented by ex situ Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. The results suggest that π-conjugated quinone groups are responsible for the reversible accommodation of Na⁺ ions. This work underscores the significance of metal centers within CCPs, offering critical insights into the molecular-level design of CCPs with enhanced sodium-ion storage capabilities.