Self-stacked nitrogen-doped carbon nanotubes as long-life air electrode for sodium-air batteries: Elucidating the evolution of discharge product morphology

Abstract

Self-stacked nitrogen-doped carbon nanotubes (NCNTs) and pristine carbon nanotubes (CNTs) on commercial porous polypropylene substrates have been applied as air electrodes for sodium-air batteries (SAB). Both NCNT and CNT air electrodes exhibit highly reversible electrochemical activities and large capacities under low current densities in SABs, while NCNT electrodes show much higher rate performance and extended cycling life under high current densities. The superior electrochemical behavior of NCNT electrodes is attributed to the robust network of aligned NCNTs, which enables rapid oxygen and liquid electrolyte transport while accommodating the volume change originating from discharge product aggregation during cycling. Moreover, uniform coverage of the discharge product has been observed on the NCNT air electrodes, in contrast to the random and discrete dispersion of discharge product on the CNT air electrodes. The unique morphologies and growth mechanism of discharge products on NCNT electrodes are believed to be due to the outstanding catalytic activity of the nitrogen-doped sites in the NCNTs, which play a critical role in the high cycling stability of NCNT air electrodes for SABs.