Accessing p- and n-type Polyimide Covalent Organic Frameworks via Post-Synthetic Linker Exchange for High-Performance Cathodes in Sodium-Ion Batteries

Rechargeable metal-ion batteries have become a viable portable technology to fulfill the growing global energy demand and diminishing fossil fuel supply. Due to their abundance of reserves and wide operating temperature range, sodium-ion batteries (<b>SIBs</b>) have been predicted to be a rapidly emerging field in the post-lithium era. However, the sluggish intercalation kinetics of Na⁺ ions with cathode materials have limited their usefulness.<br/>Recently, researchers have been looking into alternate cathode materials to get around these limitations and enhance the energy density and cycle performance of SIBs. Therefore, we have done this study, which focuses on the synthesis of highly crystalline and porous bipolar redox-active polyimide-linked covalent organic framework (COF), and deployed it as a cathode material for SIBs. For this purpose, we have designed and synthesized a <b>TPDA-NDI-COF</b> by incorporating n-type carbonyl-based reversible redox centers into the p-type triphenylamine-based structure via a post-synthetic linker exchange technique. This approach was successful and led to the formation of a highly crystalline and purposeful polyimide, which was not achievable via direct synthesis. The <b>TPDA-NDI-COF</b> COF exhibits a wide potential window of 1.0 to 3.6 V vs. Na⁺/Na, making it a good candidate as a cathode for SIBs. An advantageous specific capacity of 120 mAh/g at 0.02 A/g can be achieved by doping 50% CNT into the COF. This allows the capacity to be maintained at 92 mAh/g at 1.0 A/g, and even after 5000 cycles, COFs demonstrate the cathode's extraordinarily extended longevity. Our synthetic TPDA-NDI-COF shows an average discharge voltage of 2.1 V, outperforming most recently published COF hosts. We believe this approach will pave the way for boosting the synthesis and design of COFs with more desired properties for energy storage and other various applications.