Flexible and additive-free organic electrodes for aqueous sodium ion batteries

Organic materials with redox activities are promising candidates for aqueous flexible sodium ion storage devices (AFSISDs) due to their mechanical flexibility and low dissolution in aqueous electrolytes. However, the advantages of organic electrodes are not fully exploited due to uncontrollable morphologies with sluggish electrochemical kinetics, and lack of deep understanding about the storage mechanism via in situ technologies. Herein, an interfacial self-assembly strategy is proposed to directly construct PTCDI nanofibers onto flexible ITO substrates by a binder-free approach. Combining in situ Raman spectroscopy and DFT calculations, the Na+ storage mechanism based on an enolization reaction (-C=O <-> -C-O-Na) of two carbonyl groups located in the para position of PTCDI is illuminated. Coupled with an activated carbon cathode, the organic AFSISD delivers an ultrahigh energy density of 83.8 W h kg(-1) and a largely improved power density of 3.4 kW kg(-1). The as-prepared organic nanofibers show a superior volumetric energy and high power density (i.e., 7.1 mW h cm(-3) and 289.7 mW cm(-3)). This work opens up new horizons for a wide variety of flexible electronics based on organic electrode materials in aqueous neutral electrolytes.