Engineering microporous structures to boost low-voltage sodium storage in anthracite-derived hard carbon anodes

A bottleneck for commercializing sodium-ion batteries is the inadequate plateau capacity of hard carbon anodes, which stems from a deficiency of closed pores. Informed by the "adsorption-insertion-pore filling" mechanism, this work employs a synergistic phosphoric acid activation and pre-oxidation process to construct hard carbon from anthracite with copious closed pores. Consequently, the reversible capacity of the modified hard carbon anode increases by 66.3% to 334.8 mAh g⁻¹, with its ICE remaining at 90.8%, which is attributed to the critical role of closed pores in facilitating Na⁺ transport and storage. Extensive investigations demonstrate that the enhanced plateau capacity (257.0 mAh g⁻¹) exhibits a strong correlation with the micropore volume of the precursor, validating that the plateau capacity primarily relies on a pore-filling mechanism. The universality of this pore-formation strategy is further demonstrated by extending it to phenolic resin. The modified resin-derived hard carbon achieves a reversible capacity of 415.7 mAh g⁻¹, representing a 20.1% enhancement, along with a plateau capacity of 337.2 mAh g⁻¹. Our work not only underscores the pivotal role of precursor cross-linking but also highlights the broad application potential of this pore-formation strategy.