HOMO-Level Alignment as a Design Descriptor for Donor-Acceptor Covalent Organic Frameworks in Aqueous Zinc-Iodine...

Aqueous zinc-iodine batteries (ZIBs) offer high voltage and inherent safety, but face a persistent trade-off among fast kinetics, long cycling stability, and high iodine loading. Donor-acceptor (D-A) covalent organic frameworks (COFs) have emerged as promising iodine hosts, yet the rational design is hindered by an unclear understanding of what truly governs charge-transfer efficiency-with most studies empirically pursuing higher energy density without clear design rules. In this study, we establish highest occupied molecular orbital (HOMO) level alignment relative to iodine's redox potential as the governing descriptor for charge-transfer efficiency. Analyzing a series of units with progressively elevated HOMO levels reveals a clear trend: higher HOMO levels enhance electron donation to iodine and promote charge transfer. The optimally designed NT-COF, which combines a triphenylamine (TPA) donor with favorable HOMO alignment and a naphthalene diimide (NTCDI) acceptor with extended conjugation, exhibits the best electrochemical performance. Even with a high iodine loading (∼50.5 wt%), it delivers stable cycling and reliable low-temperature operation. Spectroscopic and theoretical analyses confirm that the energetic alignment facilitates chemisorptive iodine anchoring, accelerates charge transfer, and suppresses polyiodide dissolution. This synergy confirms that rational COF design must combine energy-level alignment with structural conjugation.