Sustainable Synthesis of Hard Carbon with Abundant Ultramicropores via Magnesium-Ion Catalysis for Kinetically Accelerated Sodium Storage

Abstract

Hard carbon (HC) is an extremely promising anode material for sodium-ion batteries (SIBs) and holds great commercial potential. However, its practical implementation is still constrained by insufficient low-voltage plateau capacity (<0.1 V), low initial Coulombic efficiency (ICE), and sluggish reaction kinetics. Herein, we propose a green and scalable one-step pyrolysis strategy that employs magnesium-ion (Mg2+) catalysis to achieve full-lifecycle regulation of bamboo powder precursors, affording bamboo-derived HC with both high C═O functional groups content and abundant ultramicropores. Specifically, magnesium acetate selectively hydrolyzes hemicellulose and dissolves lignin during pretreatment, while inducing molecular crosslinking between lignin and cellulose. During the subsequent high-temperature carbonization, the crosslinked precursors carbonize to significantly increase the C═O functional groups; meanwhile, Mg2+ ions exert dual functions as a graphitization catalyst and a pore-forming agent, inducing localized graphitic domains to bend and deform, thereby constructing ultramicropores in situ. Benefiting from this synergistic optimization, the as-prepared HC anode delivers a remarkable reversible capacity (412 mAh g–1 at 0.1 C), outstanding rate capability (262 mAh g–1 at 10 C), and robust cyclic stability (81.5% capacity retention after 1000 cycles at 10 C). Electrochemical tests and in situ characterizations validate an adsorption–intercalation–pore-filling mechanism, underscoring the decisive role of ultramicropores in promoting Na+ ion kinetics. This work provides an economical and industrially scalable route for the rational design of closed pores in biomass-derived HC materials, thereby paving the way for the practical deployment of high-performance SIBs in large-scale energy storage systems.