A biphase coupled cathode enables all-organic rocking-chair lithium ion batteries based on crystalline AB-stacked covalent triazine-based frameworks

Abstract

Organic compounds can solve many problems that lithium ion batteries currently face, such as the unsustainability and limited capacity of inorganic electrodes, due to their abundance, renewability and designability. As a kind of electroactive porous polymer, covalent triazine-based frameworks (CTFs) have shown good potential in energy storage. However, their synthesis usually requires high reaction temperature and a long reaction time or employs toxic organic reagents, resulting in uncontrollable structures, high synthesis cost and negative environmental impact. Herein, an AlCl₃–NaCl–KCl ternary molten salt system with a low eutectic point (∼93 °C) was used for the first time for the synthesis of CTFs, and products with good crystallinity and an AB stacking structure were prepared even at 180 °C, the lowest temperature reported for the synthesis of CTFs in molten salt. Electrochemical tests further indicated that AB-stacked CTFs exhibited better electrochemical performance than the AA-stacked one and could behave as both the cathode and anode of Li-ion batteries. As a consequence, a rocking-chair full cell composed of the CTFs@Li₃PO₄ cathode and a CTF-based anode had been assembled with an initial discharge specific capacity of 101.2 mA h g⁻¹ at 0.2 A g⁻¹. Ex situ FTIR and XPS tests revealed the reversible Li⁺ insertion/extraction at CN of triazine rings and CC of cyclohexadiene rings for the anode and the synergistic lithium storage of triazine rings and Li₃PO₄ based on in situ p-type doping/dedoping of CTFs in the cathode. The concept of a biphase coupled cathode (BPCC) that combines p-type organic molecules and lithium salts for designing a rocking-chair all-organic lithium ion battery will inspire the study of high-energy organic lithium ion batteries beyond dual-ion batteries and open a new avenue for organic energy storage.