Understanding the hard carbon's closed pore formation and Na-ion storage beyond the first charge/discharge cycle

The lack of accurate determination of hard carbon's (HC) open and closed pores, but also the systematic analysis of only the first charge/discharge cycle, in half-cells, impedes understanding of Na-ions storage. Therefore, a combined approach based on the adsorption of multiple gases (N 2 , O 2 , H 2 and CO 2 ) and He density is proposed to understand the HC's open porosity evolution and closed pores formation during pyrolysis. At 900 °C the pores are completely open, they partially close in the range of ∼1100–1300 °C, and completely close starting from 1500 °C. The narrower pores (<0.7 nm) close firstly, with the formation of few larger open pores and plenty of closed pores, which increase with temperature. Examination of property-performance relationships from both 1st and 2nd cycles, revealed that the 1st cycle sloping capacity increases with open porosity, O-functional groups and defects, and is associated not only with reversible Na-storage but also with irreversible reactions (SEI formation). On the contrary, the 2nd cycle is only linked with reversible Na-storage, and an optimal property threshold was established. The plateau capacity was found to increase with the increase in graphitic domains and closed pores, no matter the cycle. These findings suggest an adsorption-insertion-filling mechanism.