A bipolar-type covalent organic framework on carbon nanotubes with enhanced density of redox-active sites for high-performance lithium-ion batteries

Abstract

As a special class of crystalline porous polymers, covalent organic frameworks (COFs) containing electrochemical redox-active centers are attractive electrode materials for Li-ion batteries. However, designing and synthesizing COF-based electrode materials with a high loading density of redox-active groups, high redox potential, and increased conductivity to enhance their specific capacities and energy densities is extremely challenging. Herein, a series of novel bipolar-type composite cathodes (denoted as TAPP-Pz-COF-XCNTs, where X = 10, 20, 30 and 40 wt% of CNTs) were prepared by in situ condensation of a new p-type phenazine (Pz)-based building block (5,10-dimethyl-5,10-dihydrophenazine-2,7-dicarbaldehyde, Pz) with a bipolar-type semiconductor (5,10,15,20-tetra(p-aminophenyl) porphyrin, TAPP) in the presence of carbon nanotubes to address these challenges. TAPP-Pz-COF-40%CNTs possessing exceptional conductivity (7.48 × 10⁻³ S m⁻¹) and a mesoporous channel of 2.1 nm enable stable and rapid ion transport. This, in combination with the plentiful p-, n-, and bipolar-type redox active sites, endows the TAPP-Pz-COF-40%CNT cathode with a specific capacity of up to 314 mA h g⁻¹ at 200 mA g⁻¹, a record energy density of 737.5 W h kg⁻¹, which is the highest energy density among the thus far reported organic polymer and COF cathodes for Li-ion batteries, superior ion transport dynamics (10⁻¹² to 10⁻⁸ cm² s⁻¹), and excellent long-term cycling stability (88% after 10 000 cycles at 10 000 mA g⁻¹). Using a series of ex situ characterization studies and density functional theory (DFT) calculations, the reversible conversion of bipolar-type redox-active centers of TAPP-Pz-COF-40%CNTs was revealed and an overall 6 PF₆⁻/6 Li⁺ redox mechanism per asymmetric unit was rationalized.