Pyrazine and crown ethers: functional covalent organic polymers for (solar-assisted) high capacity and rate performance lithium-organic battery

Abstract

Multifunctional two-dimensional (2D) covalent organic polymers (COPs) benefiting from unique features display great potential in lithium-organic batteries yet are rarely reported. In this work, we fabricate a novel conjugated polymer (TQ-COP) functioned with flexible crown ethers and pyrazine redox-active sites, used as an organic rechargeable battery cathode. The Fourier transform infrared spectroscopy (FT-IR) and 13C cross-polarization and magic-angle spinning nuclear magnetic resonance (MAS NMR) verify the chemical identity of redox-active units formed during polymerization. In this structure, pyrazine-redox components exhibit a maximum six-electron transfer per active site, resulting in an initial specific capacity of 162 mA h/g(20 mA/g). Because of macrocyclic cavities and preferential coordination with the alkali metal ions of crown ethers, the rate determining step is changed into a charge transfer process, urging TQ-COP to emerge with a superior rate performance (132 mA h/g at 2 C, 81% vs. 0.1 C). Furthermore, the introduction of crown ethers facilitates photo-excited charge-carriers transfer by narrowing the HOMO-LUMO gap, rendering a photo-assisted lithium-organic battery with significant photovoltage responses and ∼10% improvement of battery round-trip efficiency under illumination, demonstrating its promising capability of solar energy utilization. This work proposes new insights into multifunctional COPs for advanced lithium-organic batteries.