High-capacity and cycling-stable polypyrrole-coated MWCNT@polyimide core-shell nanowire anode for aqueous rechargeable sodium-ion battery

Aqueous rechargeable sodium-ion batteries (ARSBs) have potential applications in large-scale electric energy storage systems because of the non-flammable and fast charge-discharge performance of the aqueous neutral electrolyte as well as low cost and abundance of sodium resources. Herein, we demonstrate polypyrrole-coated MWCNT@polyimide core-shell nanowire based on pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) as a high-capacity and cycling-stable anode material for ARSBs. The as-synthesized MWCNT@polyimide core-shell nanowire exhibits excellent initial discharge capacity as high as 234.9 mA h g⁻¹, which is 83.6% of the theoretical value, owing to the bicontinuous electron/ion transport pathway. During 100 cycles of charge-discharge, however, the considerable swelling of PMDA-ODA polyimide and structural degradation of core-shell structure result in significant deterioration in the performance from 234.9 to 74.6 mAh g⁻¹. In order to improve the cycling stability, conducting polypyrrole is coated on its surface. After 100 charge-discharge cycles, the polypyrrole-coated MWCNT@polyimide core-shell nanowire retains a specific capacity of 209.3 mA h g⁻¹, corresponding to 77.8% of the initial capacity, without any swelling and structural degradation. In impedance study, the changes in the surface and charge transfer resistances during charge-discharge cycles are significantly reduced. As a result, the polypyrrole layer successfully inhibits the structural degradation of the MWCNT@polyimide core-shell nanowire, and consequently improves the cycling performance.