In Situ Tracking of Radical Evolution in a Conjugated Covalent Organic Framework for Reversible Sodium Storage

Area of Science:

• Materials Science
• Electrochemistry
• Organic Chemistry

Background:

• Covalent organic frameworks (COFs) are attractive for sodium-ion storage due to their structure and redox sites.
• The mechanism of radical intermediates in COF electrochemical processes is not well understood.

Purpose of the Study:

• To investigate the role of radical intermediates in the sodium-ion storage mechanism of COFs.
• To elucidate the sequential sodium-ion storage pathway in a specific COF material.

Main Methods:

• Synthesis of a highly crystalline β-ketoenamine-linked COF (BT-COF-AA).
• In situ electron paramagnetic resonance (EPR) spectroscopy to observe transient radical intermediates.
• Electrochemical testing for rate capability and cycling stability.
• Theoretical calculations to support experimental findings.

Main Results:

• BT-COF-AA demonstrated exceptional rate capability and ultralong cycling stability (>10,000 cycles at 5 A g⁻¹).
• In situ EPR spectroscopy directly observed transient radical intermediates during sodiation.
• A sequential sodium-ion storage mechanism involving nitrogen and carbonyl oxygen sites was elucidated.
• Reversible radical evolution was identified as key to the material's performance.

Conclusions:

• The study reveals a radical-mediated charge storage mechanism in COFs for sodium-ion batteries.
• Direct observation of radical intermediates provides fundamental insights into COF electrochemistry.
• Findings guide the rational design of high-performance organic electrode materials for energy storage.