A Radical-Cationic Covalent Organic Framework to Accelerate Polysulfide Conversion for Long-Durable Lithium-Sulfur...

Area of Science:

• Materials Science
• Electrochemistry
• Organic Chemistry

Background:

• Covalent organic frameworks (COFs) are explored as metal-free hosts for lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries.
• Challenges exist in developing COFs with robust electrocatalytic activity for efficient LiPS conversion and mitigating the shuttling effect.

Purpose of the Study:

• To synthesize and characterize a radical-cationic COF (R-TTF•+ -COF) with enhanced electrical conductivity and catalytic properties for Li-S batteries.
• To investigate the mechanism of LiPS conversion facilitated by the radical cations within the COF structure.

Main Methods:

• Synthesis of a radical-cationic COF (R-TTF•+ -COF) with high electrical conductivity.
• Electrochemical testing of Li-S batteries utilizing the R-TTF•+ -COF as a sulfur host.
• Characterization using solid-state nuclear magnetic resonance (NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy.
• Theoretical simulations to elucidate the interaction between LiPSs and the radical cation sites.

Main Results:

• The R-TTF•+ -COF exhibits superior electrical conductivity (3.9 S m⁻¹).
• Li-S batteries with R-TTF•+ -COF demonstrate remarkable longevity, achieving 1500 cycles with minimal capacity fading (0.027% per cycle at 0.5 C).
• Significantly improved capacity retention at higher current densities (2.0 C) compared to non-radical COFs.
• Experimental and theoretical studies confirm the crucial role of radical cations in catalyzing LiPS conversion via reversible interactions.

Conclusions:

• Radical-cationic COFs serve as highly efficient, conductive, and stable sulfur hosts for Li-S batteries.
• The radical-assisted mechanism significantly enhances LiPS conversion kinetics and suppresses polysulfide shuttling.
• This work presents a new strategy for designing advanced organic catalysts for practical Li-S battery applications.