Hierarchically porous hard carbon with graphite nanocrystals for high-rate sodium ion batteries with improved initial Coulombic efficiency

Practical application of hard carbon materials in sodium-ion batteries is largely limited by their low initial Coulombic efficiency and poor rate performance. An efficient structure design with appropriate porous structure and optimized graphitic degree is highly desired to tackle these problems. Herein, we report a new design of hierarchically porous hard carbon with rich graphite nanocrystals (PHCG). The graphite nanocrystals enable a high-level ordered structure inside hard carbon, leading to an improved initial Coulombic efficiency. And hierarchically porous structure supplies more entrance for Na+ ion diffusion and decreases Na+ ions diffusion distance, leading to a great rate performance. Therefore, the obtained hard carbon as the free-standing anodes exhibit 312 mAh g−1 with an improved initial Coulombic efficiency (77.4%) and excellent cycle performance at high rate (112 mAh g−1 at 5 A g−1 after 3000 cycles).