Perhydroxylated benzoquinoid covalent triazine framework for ultra-high-density lithium-ion storage

Sustainable organic electrode materials have gained tremendous attention as a major alternative to non-renewable inorganic intercalation electrodes. Structural design flexibility, redox tunability, and low-carbon-footprint processing drive research efforts on highly-stable covalently-assembled organic frameworks. Covalent triazine frameworks (CTFs) with porous non-crystalline features are promising due to easily accessible redox-active sites and bipolar triazine centers capable of undergoing both n-doping and p-doping redox processes, potentially leading to high capacity. However, most of these polymeric materials reported so far still contain a vast portion of electrochemically-inactive units, resulting in a large unit weight per electron uptake, which inevitably lowers the energy density of batteries. Herein, we have utilized the most compact fully-functionalized monomer 1,4-dicyano-2,3,5,6-tetrahydroxybenzene to synthesize a perhydroxylated benzoquinoid CTF, Q-CTF-OH, via trimerization, resulting in a framework structure without any redox-inactive moieties. In situ conversion of the hydroxyl to carbonyl groups during the ionothermal synthesis maximizes storage sites for lithium, allowing Q-CTF-OH to have the highest density of redox centers in all organic framework cathodes known to date. The remaining polar hydroxyl groups are crucial in stabilizing inserted lithium ions and facilitating ionic transport, leading to superior electrochemical performance. Q-CTF-OH delivers a capacity as high as 689 mAh g⁻¹ at 200 mA g⁻¹ and a reversible capacity of 180 mAh g⁻¹ at 20 A g⁻¹ corresponding to 99 % retention after 1000 cycles. This work highlights design strategies for organic-based electrode materials for next-generation sustainable energy storage.