Universal Bulk Pre-Oxidation Strategy for Programming High-Performance Pitch-Derived Hard Carbon Anodes

Pre-oxidation is an essential step to suppress graphitization during the carbonization of pitch-derived hard carbons (HCs) for sodium-ion batteries. However, conventional pre-oxidation typically results in non-uniform, surface-gradient crosslinking within pitch precursors, which impedes oxygen diffusion into the bulk and compromises structural homogeneity. Here, we present a chemical bulk-crosslinking strategy that enables uniform oxygen penetration throughout pitch precursors using ammonium-based small molecules (NH₄H₂PO₄, (NH₄)₂SO₄, (NH₄)₂B₄O₇, or (NH₄)₂HPO₄) as molecular promoters. This approach simultaneously eliminates gradient crosslinking while generating favorable closed porosity and expanded interlayer spacing (0.389 nm). The resulting HC anode achieves a high reversible capacity of 340 mAh g^-1 at 30 mA g^-1, with a dominant plateau capacity (263.9 mAh g^-1, 77.3%), and maintains outstanding rate performance (191.2 mAh g^-1 at 300 mA g^-1) in conventional carbonate electrolytes, surpassing most reported pitch-derived analogues. Through in situ characterization analyses, a sequential "adsorption-intercalation-pore filling" sodium storage mechanism, facilitated by the tailored disorder-pore architecture, is elucidated. This work provides a universal platform for achieving uniform bulk pre-oxidation of carbon precursors, offering fundamental insights and scalable material design principles for high-performance sodium-ion batteries.