Conjugated polymers of organic carbonyl compounds are promising electrode materials for energy storage devices owing to the renewable development prospects, structural variability, and better insolubility in electrolyte. However, the synthesis methods in solution are cumbersome and complicated in separation and purification, and require the introduction of functional groups and use of expensive catalysts. In this work, a novel conjugated poly(3,4,9,10-perylenetetracarboxylic diimide) (PPI) with superior thermal stability and lower solubility was prepared successfully by a green facile mechanical ball milling strategy. The PPI exhibits enhanced electrochemical dynamics performance, preferable rate capability, higher discharge capacity, and excellent cycling stability of 450 cycles at 0.2 C with higher capacity retention of 85.7% when used as cathode material for sodium-ion battery. Furthermore, the in-situ X-ray diffraction (XRD) and in-situ Raman investigations combined with the Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) were carried out to investigate the sodium storage mechanism. The results indicate that only two sodium ions are bound to two opposite carbonyl groups of PPI monomer to form sodium enolates during normal charging and discharging and to deliver available reversible capacity.

Sodium-ion battery is a potential application system for large-scale energy storage due to the advantage of higher nature abundance and lower production cost of sodium-based materials. However, there exist inevitably the safety problems such as flammability due to the use of the same type of organic liquid electrolyte with lithium-ion battery. Gel polymer electrolytes are being considered as an effective solution to replace conventional organic liquid electrolytes for building safer sodium-ion batteries. In this review paper, the authors present a comprehensive overview of the research progress in electrochemical and physical properties of the gel polymer electrolyte-based sodium batteries. The gel polymer electrolytes based on different polymer hosts namely poly(ethylene oxide), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), poly(vinylidene fluoride-hexafluoro propylene), and other new polymer networks are summarized. The ionic conductivity, ion transference number, electrochemical window, thermal stability, mechanical property, and interfacial issue with electrodes of gel polymer electrolytes, and the corresponding influence factors are described in detail. Furthermore, the ion transport pathway and ion conduction mechanism are analyzed and discussed. In addition, the advanced gel polymer electrolyte systems including flame-retardant polymer electrolytes, composite gel polymer electrolytes, copolymerization, single-ion conducting polymer electrolytes, etc. with more superior and functional performance are classified and summarized. Finally, the application prospects, development opportunities, remaining challenges, and possible solutions are discussed.