Polymers Based on Isolated Pyrazine Units as Ultrahigh-Capacity Cathode Materials for Rechargeable Batteries

Polymer organic cathode materials (OCMs) have emerged as promising candidates for sustainable lithium and postlithium batteries. However, it remains a formidable challenge to synthesize high-performing polymer OCMs in a cost-effective manner. Herein, we proposed a novel material design of utilizing isolated pyrazine units to construct the polymers, potentially elevating the theoretical capacity of OCMs to unprecedented heights. As the pioneering example, poly(pyrazinyl sulfide) (PPZS) featuring thioether linkages has been successfully synthesized through a straightforward one-step polycondensation reaction between 2,5-dibromopyrazine and Li₂S. Its structure has been thoroughly characterized, revealing an average degree of polymerization of 20. As a cathode material for rechargeable lithium batteries, PPZS demonstrates an ultrahigh and stable specific capacity of approximately 450 mA h g^-1 (close to the theoretical value of 487 mA h g^-1) over 100 cycles at a low current rate, accompanied by an average discharge voltage of 1.8 V. In addition, a variety of electrochemical analyses, theoretical calculations, and ex-situ characterizations have been employed to gain insight into the fundamental mechanisms governing the electrochemical redox thermodynamics and kinetics of PPZS. The swelling-reaggregation behavior of polymer OCMs has been emphasized, as a crucial factor for determining the discharge-charge potential profiles and cycling stability.