• 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, enhancing sodium storage capacity.
• 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, indicating significant plateau region contribution.
• Mechanistic insights from GITT and in-situ XRD reveal that sodium storage in the low-voltage plateau involves both interlayer insertion and closed pore filling, providing a dual storage mechanism.
• This work offers a low-cost, environmentally friendly route to convert petroleum coke into high-performance anodes for sodium-ion batteries, addressing the need for sustainable energy storage materials.