Tunable Redox Chemistry and Stability of Radical Intermediates in 2D Covalent Organic Frameworks for High Performance Sodium Ion Batteries

Abstract

Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore, it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability, and long-term cycle life. Herein, we reported the redox and tunable stability of radical intermediates in covalent organic frameworks (COFs) considered as high capacity and stable anode for sodium-ion batteries. The comprehensive characterizations combined with theoretical simulation confirmed that the redox of C−O• and α-C radical intermediates play an important role in the sodiation/desodiation process. Specifically, the stacking behavior could be feasibly tuned by the thickness of 2D COFs, essentially determining the redox reactivity and stability of the α-C radical intermediates and their contributive capacity. The modulation of reversible redox chemistry and stabilization mechanism of radical intermediates in COFs offers a novel entry to design novel high performance organic electrode materials for energy storage and conversion.

A Redox‐Active 2D Metal–Organic Framework for Efficient Lithium Storage with Extraordinary High Capacity

“…The EPR signal exhibits the strongest intensity when the electrode is discharged to 1.2 V, which can be ascribed to the increase of radicals and the transformation from C=O double bond to CÀO single bond due to the acceptance of electrons in the ligands. [16] The intensity of EPR becomes obviously weak when charged to 2.4 V and then significantly decreases with further charge to 3.2 V, suggesting the transformation of C À OC radical to C = O double bond in the first two anodic steps. Interestingly, the EPR signal displays an obvious increase when fully charged to 4.0 V, which could be ascribed to the oxidation of Cu I to Cu II .…”

A Redox‐Active 2D Metal–Organic Framework for Efficient Lithium Storage with Extraordinary High Capacity

“…To gain more insights into the lithium storage mechanism of the 2D Cu‐THQ MOF, ex situ electron paramagnetic resonance (EPR) measurement was conducted upon redox processes (Figure a). The EPR signal exhibits the strongest intensity when the electrode is discharged to 1.2 V, which can be ascribed to the increase of radicals and the transformation from C=O double bond to C−O single bond due to the acceptance of electrons in the ligands . The intensity of EPR becomes obviously weak when charged to 2.4 V and then significantly decreases with further charge to 3.2 V, suggesting the transformation of C−O .…”

Covalent–Organic Frameworks: Advanced Organic Electrode Materials for Rechargeable Batteries

“…Lu and co‐workers proposed a strategy to stabilize the α‐C radical intermediate through the resonance effect of p‐π conjunction and the steric hindrance effect of the rigid geometry. [ 95,96 ] The disappearance of unpaired electrons, triggered by irreversible binding among the intermolecules, would be largely inhibited by electronic resonance effect and steric hindrance effect ( Figure 12 a). [ 96 ] Based on these principles, rigid aromatic rings containing conjugated structure and steric effect are ideal building blocks to control the activity of radical intermediates.…”

“…[ 95,96 ] The disappearance of unpaired electrons, triggered by irreversible binding among the intermolecules, would be largely inhibited by electronic resonance effect and steric hindrance effect ( Figure 12 a). [ 96 ] Based on these principles, rigid aromatic rings containing conjugated structure and steric effect are ideal building blocks to control the activity of radical intermediates. But, how to incorporate these modules economically without the risk of the reduction on the energy density, and meanwhile to improve the stability of radical intermediates become very challenging.…”