Cationic Covalent Organic Framework-Modified Polypropylene Separator for High-Performance Lithium Metal Batteries.

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Separator wettability and thermal stability are critical for lithium battery performance.
• Existing commercial separators suffer from low ion selectivity and poor thermal stability, limiting battery efficiency and reliability.
• Cationic covalent organic frameworks (COFs) offer potential for separator modification.

Purpose of the Study:

• To design and synthesize cationic COFs (Br-COF and TFSI-COF) for modifying polypropylene (PP) separators.
• To investigate the effect of these modified separators on lithium-ion transport, anode stability, and overall battery performance.
• To explore the potential of cationic COF-modified separators for next-generation lithium metal batteries.

Main Methods:

• Design and synthesis of two cationic covalent organic frameworks (Br-COF and TFSI-COF) with imidazole cationic groups.
• Modification of commercial polypropylene (PP) separators with the synthesized cationic COFs.
• Molecular dynamics simulations to study anion dissociation and ion transport.
• Electrochemical testing of LiFePO4/TFSI-COF@PP/Li cells.

Main Results:

• Cationic COFs effectively dissociate lithium salt ion clusters, promoting lithium-ion transport.
• Electrostatic interactions anchor solvent anions, reducing side reactions and forming a favorable LiF-rich solid electrolyte interphase (SEI) layer.
• Modified separators exhibit improved wettability and ion transport, inhibiting lithium dendrite growth.
• The LiFePO4/TFSI-COF@PP/Li cell showed a high initial capacity (148.0 mAh g⁻¹ at 0.5 C), excellent Coulombic efficiency (98.0%), and good cycling stability (82.0% retention after 100 cycles).

Conclusions:

• Cationic COF-modified separators significantly enhance lithium battery performance by improving ion transport and anode stability.
• The developed separators demonstrate potential for inhibiting lithium dendrite growth and enabling reliable lithium metal battery operation.
• This study provides a promising strategy for developing advanced separators for high-performance next-generation lithium metal batteries.