Optimizing the density of electronic donor and acceptor sites in bipolar covalent organic polymers toward high-performance lithium storage

Although donor-acceptor (D-A) strategies have been widely adopted to enhance capacity, the fundamental correlation between acceptor density and electrochemical efficacy remains elusive. In this work, we synthesized three covalent organic polymers (COPs) as model systems, featuring invariant electronic donor (C N) units and a systematically varied density of electron-accepting (C N) moieties. Our findings reveal that increasing the number of acceptor sites from 0 to 6 significantly decreases the electron density on donor sites and induces an uneven charge distribution among acceptor sites. Notably, the bipolar COP PyPz with three acceptor sites, possessing a moderate acceptor density, strikes an optimal balance between electronic environment and site accessibility, delivering a superior reversible capacity of 238.3 mAh g -1 (vs. 179.9 mAh g -1 for PhPz with zero acceptor sites and 141.6 mAh g -1 for DzPz with six acceptor sites) at 1 C after 100 cycles. Additionally, PyPz showed better cycling stability among the three materials, retaining 84.3% of its capacity after 1000 cycles at 5 C. Remarkably, even at a high rate of 10 C, the PyPz cathode retained a capacity of 111.0 mAh g -1 . These results demonstrate that excessive site density paradoxically passivates redox activity, suggesting that strategic density optimization, rather than maximization. This work establishes that optimizing donor-acceptor site density instead of maximizing acceptor content enables high-capacity bipolar organic cathodes and provides an universal guideline for molecular engineering.