Low-temperature ionothermal polymerization of phenazine-based small molecules towards ultrastable and high-capacity anodes of aqueous alkaline sodium-ion batteries

Abstract

Organic electrodes have high capacity, adjustable structures and abundant reserves. However, organic small molecules are commonly soluble in aqueous alkaline electrolytes, largely limiting their application as electrode materials. Polymerization is an effective tactic to improve the stability, but it is commonly difficult and costly to access the polymers of electron-deficient conjugated small molecules. Herein, we proposed a simple and green AlCl₃-based molten salt method for the dehydrogenation polymerization of electron-deficient aromatic compounds, and two typical electroactive polymers, polyphenazine (PPZ) and poly(hexaazatrinaphthalene) (PHATN), were synthesized at 250 °C in a high yield. Compared with their monomers, PPZ and PHATN exhibit better conductivities and solvent resistance. Specifically, in 6 M NaOH electrolyte, PHATN delivers a high capacity (196.5 mA h g⁻¹ at 0.1 A g⁻¹), excellent rate performance (122.4 mA h g⁻¹ at 20 A g⁻¹), and superb cycling stability (96.7% capacity retention after 10 000 cycles@10 A g⁻¹), much better than those of HATN. Moreover, the assembled PHATN//Ni(OH)₂ full cell exhibits ultra-stable cyclability with 95.1% capacity retention after 80 000 cycles. This molten salt method provides a straightforward cost-effective approach for the synthesis of electroactive polymers, and further promotes the specific capacity and cycling stability, which will undoubtedly accelerate the development of organic energy storage.