Uncovering the inherent link between pseudo-graphite structure and closed-pore formation in hard carbon

Elucidating the formation mechanism of closed pores is critical for interpreting the sodium storage mechanism and developing high-performance hard carbon anodes for sodium-ion batteries. Although numerous strategies for fabricating closed pores have been developed, the underlying evolutionary principles remain poorly understood. This work identified the inherent relationship between pseudo-graphite structures and closed pores through regulating pseudo-graphite structures in the precursor and analyzing their structural evolution during high-temperature carbonization. Carbonization triggered the conversion of pseudo-graphite structures into graphite-like structures, accompanied by densification of the carbon skeleton and shrinkage of carbon interlayer spacing. These structural charges ultimately induced closed-pore formation. In addition, the sodium storage mechanism of carbon materials at different carbonization stages was investigated. Results revealed that different sodium storage behaviors can occur simultaneously within specific potential ranges, leading to the proposal of a four-stage discharge model for hard carbon anodes: (i) adsorption (>0.2 V), (ii) adsorption-intercalation (0.2-0.1 V), (iii) intercalation-filling (0.1-0.05 V), and (iv) filling (0.05-0.001 V), which can effectively explain the abnormal variation in the Na⁺ diffusion coefficient. This work bridges the gap in understanding the evolution and formation of closed pores, providing insights for the structural design of hard carbon and the interpretation of sodium storage processes.