Tuning microstructures of hard carbon for high capacity and rate sodium storage

Hard carbon materials have been attracting great attention as anode of sodium ion batteries (SIBs) owing to their low cost, abundance and high Na-storage capacity. However, the correlation between microstructures and electrochemical behaviors of hard carbon is still ambiguous, which hinders the rational design and tailor of structures and morphologies of hard carbon for high Na-storage capacities and rates. Herein, a series of hard carbon near spheres (HCNSs) are designed and prepared at different carbonization conditions, and the effects of their microstructural evolution on Na-storage behaviors are comprehensively investigated. The results demonstrate that plateau region in discharge curve of hard carbon is ascribed to Na+ insertion into the long-range ordered carbon structure, while the corresponding slope region is related to the Na+ adsorption on the surface. Improving ordering degree with suitable interlayer distance (>0.364 nm) can both enhance the "plateau capacity" and total Na-storage capacity. Meanwhile, enlarging the surface area is able to effectively enhance the "adsorption capacity" at high potential. The optimized product delivers an excellent reversible capacity of 305 mAh g(-1) including a "plateau capacity" of 170 mAh g(-1) at a current density of 0.02 A g(-1) and keeps 210 mAh g(-1) for 1100 cycles at 1 A g(-1). This work provides a theoretical guidance for rationally designing and controlling the microstructures of hard carbon for high-performance SIBs anode.