Rigid-flexible heptazine-biguanide frameworks enable fast electron delocalization and low-steric-hindrance ammonium-ion storage
Wenyan Du, Yehui Zhang, Hui Fen Duan, Yaokang Lv, Ziyang Song, LiHua GAN, Mingxian Liu
Abstract
Polymer anodes solve the solubility issue of small molecules while offering structure-function merits compared with inorganics for superior ammonium-ion batteries (AIBs), but current research focuses either on rigid polymers for rapid ion transport or flexible ones for high active-site utilization. Here, we design polymeric heptazine-biguanide frameworks (HBFs) via integrating planar three-electron meleme and rotated four-electron chlorhexidine linkers, which harness the advantages of rigid heptazine and flexible biguanide while alleviating their respective shortcomings. Heptazines afford fast electron delocalization, and biguanide chains reduce steric hindrances, leading to ultrahigh utilization of imine sites (99.6%) and ultralow activation energy (0.15 electron volts) in HBFs. Septuple hydrogen-bonded NH 4 + coordination per heptazine-biguanide module enables a record capacity (314 milliampere hours per gram) and an exceptional rate capability (60 amperes per gram) among reported polymer anodes. The structural merits of HBFs also enable state-of-the-art all-polymer AIBs with unprecedented energy density (100.6 watt-hours per kilogram of cell) and long life (120,000 cycles). This work gives a previously unidentified paradigm for designing rigid-flexible organic materials toward better AIBs.