Simultaneously enabling superior ICE and high rate/cycling stability with a hollow carbon nanosphere anode for sodium-ion storage

Despite tremendous endeavors to tackle the low initial coulombic efficiency (ICE) of hard carbons by reducing the intrinsic defects and porous nanostructure, an ongoing obstacle is the trade-off between the ICE and rate/cycling stability originated from the distinct microstructure-related charge storage process, greatly hindering practical deployment of sodium-ion batteries. Herein, we propose the elaborate manipulation of the microstructure of hollow carbon nanospheres (HCNs) to surmount the above paradox, thus simultaneously achieving high ICE and rate/cycling stability. By increasing the carbonization temperature, gradual ordering of pseudo-graphitic nanodomains with decreased defects and enriched closed pores is observed, contributing to the elevated ICE and large plateau capacity; whereas the rate and cycling stability are gradually deteriorated, mainly due to the dominance of diffusion-limited intercalation and pore filling with sluggish kinetics and large volume variation. Therefore, the optimized HCN with a suitable carbonization temperature of 1300 °C exhibits ICE up to 84% while maintaining remarkable cycling stability up to 10 000 cycles and excellent rate capacity (175 mA h g⁻¹ at 2 A g⁻¹). The ICE is significantly higher than the majority of reported HCNs and the combination of superior ICE and cycle life is still at a record value among hard carbon materials. This work will offer new inspiration for the rational engineering of carbon anode materials with holistic performance optimization towards realistic applications.