Growing curly graphene layer boosts hard carbon with superior sodium-ion storage

Benefiting from the distinctive ordering degree and local microstructure characteristics, hard carbon (HC) is considered as the most promising anode for sodium-ion batteries (SIBs). Unfortunately, the low initial Coulombic efficiency (ICE) and limited reversible capacity severely impede its extensive application. Here, a homogeneous curly graphene (CG) layer with a micropore structure on HC is designed and executed by a simple chemical vapor deposition method (without catalysts). CG not only improves the electronic/ionic conductivity of the hard carbon but also effectively shields its surface defects, enhancing its ICE. In particular, due to the spontaneous curling structural characteristics of CG sheets (CGs), the micropores (≤ 2 nm) formed provide additional active sites, increasing its capacity. When used as a sodium-ion battery anode, the HC-CG composite anode displayed an outstanding reversible capacity of 358 mAh·g−1, superior ICE of 88.6%, remarkable rate performance of 145.8 mAh·g−1 at 5 A·g−1, and long cycling life after 1000 cycles with 88.6% at 1 A·g−1. This work provides a simple defect/microstructure turning strategy for hard carbon anodes and deepens the understanding of Na+ storage behavior in the plateau region, especially on the pore-filling mechanism by forming quasi-metallic clusters.