Pore structure of hard carbon has a fundamen-tal influence on the electrochemical properties in sodium-ion batteries(SIBs).Ultra-micropores(＜0.5 nm)of hard carbon can function as ionic sieves to reduce the diffusion of slo-vated Na+but allow the entrance of naked Na+into the pores,which can reduce the interficial contact between the electro-lyte and the inner pores without sacrificing the fast diffusion kinetics.Herein,a molten diffusion-carbonization method is proposed to transform the micropores(＞1 nm)inside carbon into ultra-micropores(＜0.5 nm).Consequently,the designed carbon anode displays an enhanced capacity of 346 mAh g-1 at 30 mA g-1 with a high ICE value of～80.6％and most of the capacity(～90％)is below 1 V.Moreover,the high-loading elec-trode(～19 mg cm-2)exhibits a good temperature endurance with a high areal capacity of 6.14 mAh cm-2 at 25℃and 5.32 mAh cm-2 at-20℃.Based on the in situ X-ray diffraction and ex situ solid-state nuclear magnetic resonance results,the designed ultra-micropores provide the extra Na+storage sites,which mainly contributes to the enhanced capacity.This proposed strategy shows a good potential for the development of high-performance SIBs.