Steam-activation-induced transformation from coal-derived soft carbon to hard carbon for enhanced sodium-ion storage

Anthracite is an appealing, low-cost precursor for anodes in sodium-ion batteries, but conventional high-temperature carbonization typically yields long-range ordered soft carbon with limited Na⁺ storage capability. Here, we present a steam-activation-assisted strategy to redirect the carbonization pathway from soft-carbon-like to hard-carbon-like structures. Anthracite is first steam-activated at 900 °C to introduce an ultramicropore-dominated framework, and then carbonized at 1100–1500 °C to obtain a reconstructed microcrystalline and porosity. N₂/CO₂ adsorption, X-ray diffraction and Raman spectra jointly reveal that steam activation establishes a sub-nanometer pore network and effectively delays graphitization. Consequently, the optimized material successfully maintains expanded interlayer spacing and short-range ordered microcrystals, in sharp contrast to the highly graphitized material from direct carbonization. Benefiting from this tailored microcrystalline, A900(H₂O)-1300 anode delivers an increased reversible capacity of 295 mAh g⁻¹, improved rate capability, and 90.4% capacity retention after 800 cycles. Kinetics analyses further demonstrate reduced interfacial polarization and an increased Na⁺ diffusion coefficient compared with the directly carbonized sample. This work highlights steam activation followed by optimized carbonization as an effective route to engineer coal-derived hard carbons for high-performance sodium-ion batteries.