• A precursor transformation strategy using mixed acid treatment and high-temperature carbonization effectively increases interlayer spacing and generates closed pores in petroleum coke-based carbon.
• The optimized carbon anode delivers a high reversible capacity of 356.0 mAh g−1 at 0.02 A g−1, with ~93% of capacity below 1.0 V, suitable for sodium-ion battery anodes.
• GITT and in-situ XRD analyses reveal that sodium storage in the low-voltage plateau region involves both interlayer insertion and closed pore filling mechanisms.
• This work provides a low-cost, environmentally friendly method to convert petroleum coke into high-performance carbon anodes for sodium-ion batteries.