In-Plane Exposed Metal Sites in Covalent Organic Frameworks for High-Performance Lithium Sulfur Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Covalent organic frameworks (COFs) offer tunable structures and high porosity.
• Metal-COFs (M-COFs) enhance electrocatalytic performance but suffer from limited exposed metal sites due to stacking modes.
• Existing M-COFs in Li-S batteries have in-plane metal coordination, hindering catalytic activity.

Purpose of the Study:

• To design and synthesize M-COFs with in-plane exposed metal sites for improved Li-S battery performance.
• To investigate the impact of precisely incorporated pyridine N atom pairs on COF structure and catalytic activity.
• To elucidate the mechanism by which exposed metal sites enhance polysulfide conversion.

Main Methods:

• Bottom-up molecular design strategy for M-COF synthesis.
• Incorporation of pyridine N atom pairs into the COF framework.
• Experimental characterizations (e.g., spectroscopy, microscopy) and theoretical calculations (e.g., DFT).

Main Results:

• Successfully constructed M-COFs with in-plane exposed metal sites.
• Demonstrated significantly reduced activation barriers for polysulfide conversion.
• Achieved excellent rate performance (1412 mAh g^-1 at 0.5 C, 842 mAh g^-1 at 5 C).
• Exhibited superior cycle stability with only 0.027% capacity fade per cycle over 2000 cycles at 1 C.

Conclusions:

• The proposed molecular design strategy effectively creates M-COFs with accessible in-plane active sites.
• Exposed metal sites in M-COFs are crucial for efficient polysulfide conversion in Li-S batteries.
• These advanced M-COFs show great promise for next-generation high-performance lithium-sulfur batteries.