Sodium Storage Mechanism of Nongraphitic Carbons: A General Model and the Function of Accessible Closed Pores

Nowadays, the sodium storage mechanism of nongraphitic carbon materials still remains controversial despite their promising application as anode materials in sodium-ion batteries. Herein, a series of hard carbons and soft carbons with different crystallinities and pore structures was prepared from coal tar pitch, and a general mechanism is proposed through the analysis and contrast of their microstructures and electrochemical performances. In particular, it was found that sodium adsorption and insertion take place in the form of sloping capacity for both soft carbons and hard carbons, while sodium closed pore filling only exists below 0.1 V in hard carbons corresponding to the plateau capacity. Moreover, we further propose the concept of accessible closed pores (ACPs) and quantify the surface area of ACPs, which exhibits a high linear correlation with the plateau capacity. This general mechanism model will provide theory instruction for the preparation of anode materials with superior performance.