Highly crystalline covalent triazine frameworks modified separator for lithium metal batteries

Abstract

Covalent organic frameworks (COFs) that selectively enable lithium ions transport by their abundant sub-nano or nanosized pores and polar skeleton are considered as emerging coating materials for separators of lithium metal batteries. However, the COF-coated separators that combine high ionic conductivity with excellent lithium ions transference number ($$ {t_{L i^{+}} } $$ ) are still challenging, as the coating layer may increase the transport resistance of ions through the separator due to the elongated pathway. Different from conventional strategies that always focus on developing COFs with distinct structural motifs, this work proposes a crystallinity engineering tactic to improve the ion transport behaviors and thus battery performance. Amorphous (AM-CTF) and highly crystalline covalent triazine frameworks (HC-CTF) were successfully synthesized, and the effect of crystallinity of CTFs on the electrochemical properties of the separators and the battery performance are fully studied. Compared to amorphous covalent triazine framework, HC-CTF features a more regular structure and higher surface area, which further improves the $$ {t_{L i^{+}} } $$ (0.60) and ionic conductivity (0.67 mS cm-1) of the coated separators. The LiFePO4/Li cells assembled with the HC-CTF-coated separator exhibit an ultralong lifespan and extremely high-capacity retention (45.4% at 1 C for 1,000 cycles). This work opens up a new strategy for designing high-performance separators of lithium batteries.