Deciphering Closed‐Pore Formation and Size‐Dependent Sodium Cluster Filling Behavior for Nanopore‐Confined Derived Hard Carbon Anodes

The low-voltage plateau capacity (LVPC) of hard carbon (HC) anodes for sodium-ion batteries originates from sodium cluster filling within closed pores, and closed-pore engineering of activated carbon (AC) via nanopore-confined pyrolysis is a feasible strategy to produce HC anodes with high LVPC. However, the formation mechanism of closed pores and their size-dependent effects on sodium cluster filling behavior remain poorly understood. Herein, pitch-confined pyrolysis is utilized within the nanopores of AC to transform its open pores into closed pores, with pitch dose regulating closed-pore size. The corresponding closed-pore formation and size evolution mechanisms are meticulously revealed. More importantly, the extensive experimental data combined with DFT calculations establish a universal relationship among electrochemical inflection points, closed-pore sizes, and filling order: i) the inflection points on the discharge curve can serve as a pointer to predict the sizes of the closed pores where sodium cluster filling occurs, and ii) sodium clusters preferentially fill smaller closed pores before larger ones due to the higher filling potential of the former. This work provides new insights into the understanding of closed pore formation and size-dependent effects on sodium cluster filling behavior.