Hierarchical ball-in-ball structured nitrogen-doped carbon microspheres as high performance anode for sodium-ion batteries

Microspheres with ball-in-ball (yolk@void@shell) structure have attracted much attention in energy storage materials. However, current fabrication technologies mainly rely on utilizing silica as templates with HF acid etching or hydrothermal methods to fabricate the ball-in-ball structural microspheres, and an additional step (KOH activation) is adopted to generate porous structure. Apparently, it cannot meet the demand for large-scale industrial production, on account of potential explosive risk, lengthy routes, high costs and utilizing highly toxic chemical reagents. Accordingly, to fully bridge the gap between laboratory scale and actual commercial application, we first report a facile precipitation polymerization approach for synthesis of nitrogen-doped hierarchical ball-in-ball (C@void@C) carbon microspheres both the shell and yolk having hierarchical structure, without employing templates, surfactant, KOH activation or hydrothermal devices. The as-obtained unique carbon microspheres have a large BET specific surface area of 1250.0 m² g⁻¹. In sodium-ion batteries, this anode material presents excellent electrochemical performance, the discharge specific capacity is as high as 472.5 mAh g⁻¹ at 50 mA g⁻¹, and a high reversible specific capacity (104 mAh g⁻¹) is still retained at 5 A g⁻¹ after 1000 cycles.