Redox of anionic and cationic radical intermediates in a bipolar polyimide COF for high-performance dual-ion organic batteries

Bipolar organic materials for dual-ion batteries have attracted immense attention due to the low cost, high theoretical capacity, and potential sustainability. However, the bipolar organic batteries are plagued by low reversible capacity and poor stability. Herein, a novel bipolar polyimide COF with n-type imide units and p-type quaternary nitrogen centers exhibits unique topology structure and is used for dual-ion organic batteries. Detailed analyses reveal that the redox of anionic imide radicals and cationic nitrogen-center radicals was triggered to store the Li+ ions and PF6-, anions, respectively, during the charge/discharge processes. Electrolyte engineering demonstrates that the electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) would inactivate the imide units and result in low capacities and poor reversibility of the COF electrodes. With the unique structure and optimized electrolyte, the bipolar COF exhibits high capacity of 165 mAh/g at 30 mA/g and high capacity retention of 91 % after 4000 cycles at 1 A/g, surpassing most of the bipolar electrodes. This work not only expands the palette to design bipolar materials for rechargeable batteries but also emphasizes the importance of the matching strategy of electrolytes and organic electrodes.

Automated summary

A novel bipolar polyimide COF with unique topology structure demonstrates high capacity and stability for dual-ion organic batteries. The COF stores Li+ ions and PF6- anions through the redox reactions of anionic imide radicals and cationic nitrogen-center radicals respectively. With optimized electrolyte, the COF exhibits high capacity and excellent cyclic performance.