Biomass-derived hard carbon anodes: An overview on strategies of improving initial Coulombic efficiency for sodium-ion batteries

Sodium-ion batteries (SIBs) possess great potentials in large-scale energy storage, owing to the high content of sodium in the Earth's crust and the low manufacturing cost than the lithium-ion battery (LIB) counterpart. Given the limited Na storage capacity of graphite, the conventional anode for LIBs, hard carbon especially the biomassderived one is considered to be the most potential anodes for SIBs, benefiting from their high Na storage capacity, low cost and easy preparation. Unfortunately, hard carbon still faces many challenges, such as unclear sodium storage mechanism, unsatisfactory performance and low initial Coulombic efficiency (ICE). In this review, we discuss the current state-of-the-art advances on strategies to improve the ICE of hard carbon anodes. We begin with the effect of biomass composition on the performance of hard carbon anode and then, the key strategies for regulating the pore structure. Subsequently, we summarized the methods of ICE improvement. Finally, future directions and challenging perspectives of hard-carbon anodes for SIBs are proposed from the viewpoints of storage mechanisms and electrode structures.