Electronic band structure engineering of π-d conjugated metal-organic framework for sodium organic batteries

Two-dimensional conjugated metal organic frameworks (2D c-MOFs) hold significant promise as electrode materials for alkali metal ion batteries while their electrochemical properties still lack reasonable and effective regulation. Here, two representative 2D c-MOFs (M-HHTQ/M-HHTP, M=Cu or Ni) as positive electrodes are used as models to explore the basic/microscopic principles of their complex storage mechanism in sodium ion batteries (SIBs). It is demonstrated that the energy storage mechanism of 2D c-MOFs is determined by the interaction between coordination covalent bonds and organic linkers. Theoretical calculations and experiment results have jointly demonstrated that the redox potential and theoretical capacity can be regulated based on the valence of M-O bond and the utilization of anions and cations, respectively. As a result, Cu-HHTQ achieves a high discharge voltage at 2.55 V (vs. Na⁺/Na), a higher stable specific capacity of 208 mAh g^-1 at 0.05 A g^-1, and long cyclability with the capacity retention rate of 100% at 1 A g^-1 after 2000 cycles.