Engineering Ultramicroporous Carbon with Abundant C=O as Extended "Slope-Dominated" Sodium Ion Battery Anodes

The overall performance of carbon-based materials for sodium-ion batteries (SIBs) is still restricted by the sluggish transfer kinetics of Na+ and limited rate capability. Specifically, introducing ultramicropores (pores width <0.7 nm) into carbon materials may be effective in improving the anode performance of SIBs. However, due to the difficulty of their synthesis and homogeneity regulation, ultramicroporous carbon as advanced SIB anodes as well as their sodium storage mechanisms have seldom been investigated. Herein, carbon spheres with centralized ultramicropores and abundant C=O is facilely obtained by combining the in situ ion activation and the novel quantitative air-assisted carbonization process for the first time. The quantitative air acts not only as a mild agent to activate the materials, but also as an oxygen dopant for forming oxygen-containing functional groups. As the optimized anode, quantitative air-assisted carbonized spheres with a precursor weight of 0.3 g (QACS-0.3) delivers a high rate capability (265 and 121 mAh g(-1) at 0.05 and 5.0 A g(-1), respectively) and good cycling stability (126 mAh g(-1) after 5000 cycles at 5.0 A g(-1)). Kinetic analysis, theoretical calculation, and ex situ testing technologies reveal that the ultramicropores can facilitate the diffusion of Na+ below 0.2 V; the synergistic effect of defects, C=O, and ultramicropores can extend the discharge curves and effectively improve the adsorption capacity of carbon materials. Additionally, a new discharge model is proposed to explain the sodium storage behaviors in these electrodes.