Robust Covalent Organic Frameworks Comprising Accessible Catalytic Sites Enable Fast-Charging and Long-Cycling Aluminum-Sulfur Batteries.

Abstract

Rechargeable aluminum-sulfur (Al-S) batteries are recognized as a promising option for large-scale energy storage due to high theoretical energy density and cost-effectiveness. However, the sulfur cathodes suffer from sluggish reaction kinetics and severe shuttle effect during cycling. Here we report robust two-dimensional covalent organic frameworks (COFs) as sulfur hosts that features accessible catalytic nitrogen sites and confined microporous channels. The fine regulation of different microporous sizes was achieved by controlling different-length organic ligands of various COFs, finally preparing two kinds of COFs. The sensitive comparation between both COFs demonstrates that smaller microporous channels in COFs possess higher confinement effect for polysulfides due to the stronger capillary forces, facilitating higher Coulombic efficiency and better cycling stability in Al-S batteries. Meanwhile, experimental characterizations and theoretical calculations reveal that accessible catalytic nitrogen sites in COFs promote the multistep conversion kinetics of the sulfur cathode during cycling. Consequently, the small-sized COF confined sulfur cathode exhibits a reversible capacity of 1120 mAh g^-1 at 0.2C and a 93.5% capacity retention after 100 cycles, supporting high capacity and exceptional cycling stability. This work provides a new avenue on rational design of emerging COF materials in Al-S batteries.