With the rapid deployment of sodium (Na)-ion batteries (NIBs), recycling spent batteries is essential for reducing system cost and improving sustainability. Hard carbon (HC) anodes are particularly valuable recycling targets because of their complex synthesis and high production cost. Here, combined spectroscopic and structural analyses reveal that HC failure strongly associated with irreversible Na accumulation, continuous solid electrolyte interphase (SEI) growth/decomposition, and degradation of closed-pore microstructures. Regeneration of spent HC is therefore intrinsically challenging, because it requires not only complete removal of electrochemically inactive Na-containing residues and parasitic species, but also precise reconstruction of the disordered closed-pore carbon framework that governs Na storage, while avoiding further disruption of its microstructure. Guided by these insights, we develop a green recycling strategy integrating oxalic acid treatment with thermal processing. This approach removes surface deposits and parasitic species, while oxalic acid induces oxygen-containing functionalities and C–O–C linkages that promote carbon framework rearrangement and pore restoration. As a result, the regenerated HC delivers a reversible capacity of 302.6 mAh g-1 with markedly restored rate capability. This work provides a practical pathway for the sustainable regeneration of HC anodes for large-scale NIB applications.
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