Recent advances in hard carbon anodes with high initial Coulombic efficiency for sodium-ion batteries

Initial Coulombic efficiency (ICE) has been widely adopted in battery research as a quantifiable indicator for the lifespan, energy density and rate performance of batteries. Hard carbon materials have been accepted as a promising anode family for sodium-ion batteries (SIBs) owing to their outstanding performance. However, the booming application of hard carbon anodes has been significantly slowed by the low ICE, leading to a reduced energy density at the cell level. This offers a challenge to develop high ICE hard carbon anodes to meet the applications of high-performance SIBs. Here, we discuss the definition and factors of ICE and describe several typical strategies to improve the ICE of hard carbon anodes. The strategies for boosting the ICE of such anodes are also systematically categorized into several aspects including structure design, surface engineering, electrolyte optimization and pre-sodiation. The key challenges and perspectives in the development of high ICE hard carbon anodes are also outlined.