Revisiting the Structure and Electrochemical Performance of Poly(-phenylenediamine) as an Organic Cathode Material.

Poly(-phenylenediamine) (PoPDA) has been recognized as a low-cost electroactive organic material and studied as a cathode for aqueous zinc batteries or as an anode for nonaqueous lithium batteries. However, there remains a lot of confusion about its synthesis, structure, and electrochemical application. Especially, the previously studied PoPDA samples were mostly synthesized at room temperature, which were proved by us to be just a dimer, that is, 2,3-diaminophenazine (DAPZ). By various characterization methods including elemental analysis and mass spectrometry, we verified that the product synthesized at high temperature, PoPDA-H, was a polymer based on DAPZ as the structural repeat unit and with some imperfect substitutes (OH and NHCHCOO). Based on the reversible redox reaction of phenazine units and the stable polymer structure within 1.3-3.8 V vs Li/Li, PoPDA-H was more appropriate to be applied as a cathode rather than as an anode for lithium batteries. It achieved a high energy density of 490 Wh kg (2.12 V × 231 mAh g) at 50 mA g and a high cycling stability (79%@1000th cycle) at 500 mA g, both of which were comparable to previously reported expensive pyrazine- and carbonyl-based polymers. This work clarifies many misunderstandings of PoPDA, which is important to its further development toward practical application in energy-storage devices.