A Multifunctional Surface Modifier Capable of Stabilizing and Accelerating Hard Carbon Anode Kinetics via Pore...

Hard carbon (HC), as the most suitable anode material for sodium-ion batteries, still suffers from low Coulombic efficiency and unsatisfactory fast-charge capability, mainly because of its numerous pores that result in severe electrolyte decomposition and sluggish Na⁺ reaction kinetics. Unlike the mainstream solid-electrolyte interphase (SEI) strategy that solely focuses on suppressing the electrolyte decomposition or accelerating the reactions on HC anode surface, this work reveals the pore-sealing effect of an artificial SEI through adopting a multifunctional organic small-molecule salt (methyl blue, MB) as the surface modifier of HC and systematically elucidates its effects on the reaction kinetics of HC anode from the surface to bulk. The MB modification layer effectively seals the surface pores of commercial HC (Type-2) and suppresses electrolyte decomposition. More importantly, theoretical calculations and empirical experiments reveal that MB accelerates the surface-to-bulk Na⁺ reaction kinetics of HC anode, including Na⁺ desolvation, adsorption on HC, transport across the SEI, and subsequent charge transfer and phase transition into Na clusters. Consequently, the MB-modified commercial HC exhibits significantly enhanced Coulombic efficiency, superior rate capability and prolonged cycling life compared with unmodified HC.