From Micropores to Ultra-micropores inside Hard Carbon: Toward Enhanced Capacity in Room-/Low-Temperature Sodium-Ion Storage

Pore structure of hard carbon has a fundamental 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 slovated Na+ but allow the entrance of naked Na+ into the pores, which can reduce the interficial contact between the electrolyte 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 (similar to 90%) is below 1 V. Moreover, the high-loading elec trode (similar to 19 mg cm(-2)) exhibits a good temperature endurance with a high areal capacity of 6.14 mAh cm(-2) at 25 degrees C and 5.32 mAh cm(-2) at - 20 degrees C. 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.

Automated summary

The pore structure of hard carbon plays a crucial role in the electrochemical properties of sodium-ion batteries. A new molten diffusion-carbonization method was proposed to transform micropores into ultra-micropores, leading to an enhanced capacity and temperature endurance for the carbon anode. The ultra-micropores provide extra Na+ storage sites, contributing to the improved capacity, showing potential for high-performance SIBs.