Lithium Ion-Conducting Triazacoronene-Based Neutral 2D Covalent Organic Framework as a Solid-State Electrolyte

Solid-state electrolytes (SSEs) are critical for advancing lithium-ion battery (LIB) technology, as traditional liquid electrolytes compromise their safety and lifespan. Polymer electrolytes often lack adequate ionic conductivity, Li⁺ transference number, and the ability to prevent dendrite formation, whereas inorganic electrolytes usually suffer from poor processability and electrode wettability despite high ionic conductivity and mechanical strength, necessitating new SSE materials. Composed of tailor-made organic building blocks that can bind and transport Li⁺ ions, crystalline, porous covalent organic frameworks (COFs) offer the best of both worlds. Here, we present a new, neutral hexagonal 2D COF, HMTAC-COF1, constructed from a heteroatom-rich hexamethoxy-triazacoronene (HMTAC) building block with multiple Li⁺ binding sites, which, upon Li⁺-doping exhibits impressive ionic conductivity (4.95 × 10^-5 S/cm at 20°C and 1.95 × 10^-4 S/cm at 120°C), low activation energy (0.25 eV), and a high transference number (0.57). When employed as an SSE in a Li || Li⁺@HMTAC-COF1 || LiFePO₄ coin cell, it displayed a promising specific capacity (181 mAh/g) with >99% Coulombic efficiency over multiple cycles. These results demonstrate the potential of heteroatom-rich neutral COFs as SSEs for next-generation LIBs.