A Porous Azatruxene Covalent Organic Framework as Positive Electrode Materials in Li- and Mg-based Batteries

Covalent-organic frameworks (COFs) containing defined redox-active groups have become competitive materials for next-generation batteries. Although a high rate performance can be anticipated, to date only few examples of p-type COFs have been reported for charge storage, and even fewer examples regarding the use of COFs in multivalent ion batteries exist. We herein report the synthesis of an azatruxene-based COF and its application as positive electrode material in Li- and Mg-based batteries. By solvothermal condensation a highly porous and crystalline COF was obtained, confirmed by N2 adsorption and intense powder X-ray diffraction (PXRD) reflexes. The electron-rich azatruxene monomer shows up to three reversible one-electron oxidations that make it attractive for anion storage. In Li-based cells using a COF/CNT electrode an average discharge potential of 3.9 V is obtained and discharge capacities of up to 70 mAh g−1 at 2 C rate. In Mg batteries using a tetrakis(hexafluoroisopropyloxy) borate electrolyte cycling proceeds with an average discharge voltage of 2.9 V. Even at a fast current rate of 5C, the capacity retention amounts to 84% over 1000 cycles, and the cells deliver an energy density of 112 Wh kg−1.