Engineering Optimized Defects and Abundant Closed-Pores of Hard Carbons for Enhanced Sodium Storage

Hard carbons (HCs) are widely regarded as promising anode materials for sodium-ion batteries, but their application is confined by low initial coulombic efficiency (ICE) as well as poor specific capacity. Herein, a one-stone-for-two-birds strategy is proposed to optimize defect concentration and engineer closed pores in HCs via the introduction of Zn. The resulting HCs demonstrated a satisfactory ICE (82.1%) and an excellent storage capacity (446.3 mAh g^-1). A fraction of Zn anchored by nitrogen in resin precursor effectively regulated the optimized defect concentration in HCs, thus mitigating side reactions with the electrolyte with high ICE. In the meantime, the volatilization of the remaining Zn generated closed pores during pyrolysis, providing abundant sodium storage sites and significantly increasing the specific capacity. It is revealed that the inorganic-rich solid-electrolyte interfaces are formed onto the modified HCs, which suppressed electrolyte decomposition and further raised the ICE. Additionally, it is found that Zn-single atoms facilitated the diffusion of Na⁺, especially in the low-voltage plateau region, contributing to enhanced plateau capacity. As a result, it is demonstrated in this study that the high sodium storage capacity of the HCs originated from a pore-filling mechanism in the closed pores.