Resolving the True Sodium Storage Sites in Hard Carbon: The Essential Role of Hydrogen Termination

Abstract

Hard carbon (HC) is regarded as the most promising anode material for sodium-ion batteries (SIBs), yet the intrinsic nature of its sodium (Na) storage sites remains elusive. The crucial but rarely discussed role of hydrogen termination (H-term) at defect sites has long been debated in HC modeling. To resolve this debate, we employ first-principles calculations that explicitly account for H-term to determine the realistic structures of Na storage sites. Through systematic investigation of 16 defect models, we elucidate how H-term regulates Na storage site structures and behaviors at the atomic scale. The results show that H-term alters the configuration and adsorption energetics of Na storage sites by weakening Na-substrate bonding strength and reducing charge transfer. Although H-term passivates most sites, special defect structures (1V_O pyrr , 2V, P_N pyri , 1V_N pyri , and 1V_O pyri ) exhibit optimal adsorption properties and low diffusion barriers (as low as 0.26 eV). Our findings highlight the critical and unavoidable role of H-term in determining true Na storage sites under realistic conditions, thereby providing essential theoretical guidance for the rational design of high-performance HC anodes for SIBs.