Alternating Copolymer-based Bipolar Cathode Material for Efficient Symmetric Sodium-Dual-ion Batteries

Abstract

Organic bipolar electrodes are capable of facilitating both n-type and p-type reactions for energy storage, typically offering high theoretical specific capacities. This article presents the design and synthesis of a new bipolar polymer cathode material, PDQZ, which is synthesized by copolymerization of phenanthroline diketone with dihydrophenazine. Dihydrophenazine works to store anions while phenanthroline diketone could store sodium ions. In a sodium-based half-cell, PDQZ demonstrates an energy density of 597 Wh/kg and a reversible specific capacity of 318 mAh/g at a current density of 0.5 C, with a capacity retention rate of 85% after 300 cycles. Notably, even after 4000 cycles at 10 C, the capacity retention rate remains at 78%. Furthermore, symmetrical batteries are successfully assembled based on PDQZ, which shows the "ready-to-charge" characteristic, providing a discharge specific capacity of 112 mAh/g at 1 A/g. More excitingly, stable cycling performance over 2000 cycles are achieved in symmetric cells, with an average decay rate as low as 0.02% per cycle. These results highlight the promising application potential of the bipolar PDQZ in energy storage, and may also shed a light on future design of bipolar polymer electrode materials.