Tuning the electrochemical performance of covalent organic framework cathodes for Li- and Mg-based batteries: the influence of electrolyte and binder

Covalent organic frameworks (COFs) are crystalline porous organic polymers that have recently emerged as promising electrode materials for rechargeable batteries. Herein, we present an approach to improve the electrochemical performance of an anthraquinone-based COF (DAAQ-TFP-COF) cathode material in metal anode (Li, Mg) based batteries through proper selection of the electrolyte and binder. Our results show that the combination of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte and poly(tetrafluoroethylene) (PTFE) as binder led to the best electrochemical performance with high utilisation efficiency of the redox sites and specific capacities close to the theoretical value. Using such electrolyte and binder, cyclable symmetric cells consisting of two DAAQ-TFP-COF organic electrodes exemplify 82% capacity retention after 1300 cycles at a high current density of 500 mAh g-1. The high reversibility and stability of the COF electrode material upon cycling were confirmed by ex situ IR spectroscopy. In addition, DAAQ-TFP-COF was explored as cathode in magnesium cells using two different Mg electrolytes; one based on MgCl2 and one containing weakly coordinating anions. Electrochemical characterisation reveals significant differences in the performance of COF in terms of achievable capacities and voltage profiles, pointing towards hindered transport. Our findings demonstrate that the appropriate choice of electrolyte and binder is crucial to maximise the performance of COF-based materials in different post-lithium-ion metal anode batteries.