A facile graft-repair strategy: Subsurface Na2Sx heterostructures in hard carbon anodes for high-capacity and ultrafast sodium-ion storage

Hard carbon (HC) anodes hold great promise for sodium-ion batteries (SIBs), yet they still suffer from insufficient rate capability and low initial coulombic efficiency (ICE). Herein, an innovative approach based on the synergistic graft-repair mechanism is proposed to create the multifunctional Na2Sx heterostructure within the subsurface of HC via a facile ball-milling and calcination method. This structure with specific disulfide bonds enlarges the carbon layer for fast ion transfer, offers extra S active sites to boost capacity, and catalytically reduces electrolytes to form an inorganic-dominated and thinner solid electrolyte interface (∼7.1 nm). The introduced Na+ ions compensate for the irreversible Na uptake at intrinsic defects and oxygen-containing groups. These effects are validated by GITT, EIS, Raman, and depth-profiling XPS measurements. The optimized hard carbon delivers an ultrahigh reversible capacity with superior ICE (514.8 mAh g−1 at 0.05 A g−1 with ICE of 84.1%) and excellent rate capability (87.5 mAh g−1 at 40 A g−1) simultaneously. DFT calculations reveal that the moderate S/Na ratio yields suitable adsorption energy, maintaining the balance of rate performance and ICE. The revealed mechanism of ion transport based on graft-repair effects contributes to boosting the key SIB performance indicators required for practical applications.