π-Conjugated polyimide-based organic cathodes with extremely-long cycling life for rechargeable magnesium batteries

Rechargeable magnesium (Mg) batteries hold great promise for large-scale energy storage applications. However, the high polarity of divalent Mg2+ ions may induce sluggish Mg2+ diffusion kinetics in cathode materials, leading to inferior reversible capacity and rate performance. Herein, we report that aromatic dianhydride-derived polyimides (PIs) with reversible multi-electron redox properties can serve as advanced organic cathode materials for rechargeable Mg batteries. The p-conjugated molecular units of PIs provide abundant redox-active sites for the storage of Mg2+ ions, leading to large open-circuit voltage and high specific capacity. Experimental results and density functional theory (DFT) calculations of two different PI molecules indicate that the relatively narrow HOMO-LUMO energy gap and compact p-p stacking structure can help improve the Mg2+ storage performances. After blended with carbon nanotubes (CNTs) by in-situ polymerization and coupled with ionic liquid-modified non-nucleophilic organic electrolyte, the cathodes based on PI/CNTs composites display high rate performance, as well as impressive long-term cyclability at large current rate of 20C for over 8000 cycles. We expect this work may call forth more efforts to develop advanced organic electrode materials for rechargeable batteries based on multi-electron reactions.