Edge-Affinity-Directed Selective Pore-Mouth Sealing in Coal-Derived Hard Carbon for Stable and High-Capacity Sodium Storage

Abstract

Sodium-ion batteries are attractive for large-scale energy storage, but the limited capacity of hard-carbon anodes constrains practical energy density. Increasing porosity can boost capacity, but typically enlarges the electrolyte-accessible surface, lowers the initial Coulombic efficiency (ICE) and the low-voltage plateau capacity, especially in low-cost, coal-derived carbons that require aggressive activation. Here, we develop a selective pore-mouth sealing strategy that exploits the strong affinity between oxygen-rich edge sites at pore mouths and a polyacrylonitrile (PAN) precursor to build a high density of closed pores while preserving the internal pore network. PAN preferentially accumulates at pore entrances and undergoes in situ cyclization–carbonization during heat treatment, enabling directional pore-mouth contraction and sealing (pore mouth < 0.35 nm). By tuning the PAN content, the volume and fraction of closed pores are precisely controlled, limiting electrolyte penetration and substantially enhancing low-voltage plateau Na storage. The optimized electrode delivers 338.4 mAh g–1 at 0.05 A g–1 with a plateau capacity of 233 mAh g–1 and an ICE increased by ∼42% compared with activated coal-based hard carbon, outperforming most reported coal-derived hard carbons. Even at 5 A g–1, it retains 102.4 mAh g–1. This pore-mouth engineering offers a practical route to coal-derived hard-carbon anodes with controlled closed-porosity for sodium-ion batteries.