Azobenzene-Phenazine-Based D–A Polymer as a Highly Efficient Bipolar Cathode for Sodium-Ion Batteries

Organic compounds have attracted much attention in the field of rechargeable metal-ion batteries. Advantages such as abundant resources and easily tailored structures make them promising high-energy-density cathode materials. Herein, a new bipolar polymer cathode ABPZ is synthesized, comprising azobenzene and 5,10-dihydrophenazine functional groups. In addition to the phenazine active site, the azo group is also demonstrated to work as an additional storage site. Moreover, the electron-withdrawing azo unit can reduce the highest occupied molecular orbital energy level of the polymer and thus elevate the operating voltage. It is noteworthy that ABPZ as a bipolar cathode material for sodium-ion batteries provides a reversible specific capacity of 283 mAh/g and a high energy density of 792 Wh/kg in a half-cell, superior to the energy density of the inorganic Na3V2(PO4)3 cathode. Moreover, excellent cycling stability is obtained, with 91.7% capacity retention after 300 cycles at a current density of 0.5 C. Considering the bipolar feature of ABPZ, an organic symmetric cell is successfully constructed. This work could provide new insights into the rational design of efficient organic cathodes for SIBs.