Chao Luo

Abstract: Na-ion batteries (NIBs) and K-ion batteries (KIBs) are promising alternatives to Li-ion batteries (LIBs) due to the low cost, abundance, and high sustainability of sodium and potassium resources. However, the high performance of inorganic electrode materials in LIBs does not extend to NIBs and KIBs because of larger ion size of Na⁺/K⁺ than Li⁺ and more complicated electrochemistry. Therefore, it is vital to search for high-performance electrode materials for NIBs/KIBs. To this end, organic electrode materials (OEMs) with the advantages of high structural tunability and abundant structural diversity show great promise in developing high-performance NIBs/KIBs. To achieve advanced OEMs, a fundamental understanding of the structure–performance correlation is desired for rational structure design and performance optimization. Tailoring molecular structures of OEMs can enhance their performance in NIBs/KIBs, however, the substitution rules and the consequent effect on the specific capacity and working potential remain elusive. Herein, we explored the electrochemical performances and reaction mechanisms of various polymer cathode materials based on azo, imine, and carbonyl groups for NIBs/KIBs. The electrochemical performance of polymer cathode materials with an extended conjugated structure such as a naphthalene backbone structure is better than that with benzene and biphenyl structures due to faster kinetics and lower solubility in the electrolyte. It unravels the rational design principle of extending π-conjugation aromatic structures in redox-active polymers to enhance the electrochemical performance. To further optimize the polymer cathodes, nitrogen-doped or single layer graphene is employed to increase the conductivity and mitigate the dissolution of organic materials in the electrolytes. The resulting organic cathodes deliver high specific capacity, long cycle life, and fast-charging capability. Post-cycling characterizations were employed to study the chemical structure and morphology evolution upon cycling, demonstrating that the active centers in the polymer cathode materials can undergo reversible redox reactions with Na⁺/K⁺ for sustainable NIBs/KIBs. Our work provides a valuable guideline for the design principle of high-capacity and stable OEMs for sustainable energy storage.