Performance optimization of naphthalene-diimide-based porous organic polymer cathode for sodium-ion batteries

Redox-active organic electrode materials offer several advantages over traditional inorganic compounds, such as structural and chemical tunability for multielectron reactions, high redox stability, sustainability, and environmental friendliness. Herein, a porous organic polymer (POP), condensed from naphthalene diimide (NDI) derivative monomer and triformylphloroglucinol (TFP), has been prepared and used as a cathode material for sodium-ion batteries (SIBs). The electrochemical performance of the rigid amorphous NDI-TFP polymer has been further optimized by exfoliation. A specific capacity almost as high as the theoretical value can be obtained from the exfoliated compound with rate capability and capacity retention far superior to the nonexfoliated polymer despite the low surface area of the exfoliated material. These results are in contrast to the traditional perception that crystalline frameworks with large uniform pores and high surface areas are required as host materials for large-sized guest ions such as Na⁺. Using the exfoliation technique to reduce the stacking thickness and make the redox-active sites more accessible to Na ions, superior electrochemical properties can be achieved. To further elucidate the redox mechanism of the NDI-TFP polymer, several spectroscopic techniques have been used to reveal the multielectron redox activities of the NDI moieties. To the best of our knowledge, the NDI-TFP polymer is the first redox-active amorphous POP to be exfoliated and used as a cathode material for SIBs. The obtained mechanistic understanding of the redox-active POPs may pave the way for the design of organic-based electrode materials for next-generation high-performance energy storage systems.