Natural Template-Driven Closed-Pore Engineering for Enhanced Sodium-Ion Storage

The formation of closed pores in hard carbon (HC) anodes is crucial for enhancing the low-voltage plateau capacity in sodium-ion batteries. Examples for constructing closed pores exist, but there is a need for a simple, cost-effective approach and a deeper understanding of the relationship between closed-pore formation and open pores in precursors. Here, we present a natural template-driven strategy using bamboo as a sustainable precursor to address these challenges. Removing natural templates during pyrolysis at 800 °C generates open pores (0.5-1.7 nm), particularly those in the 0.5-0.7 nm range, which are critical for closed-pore formation during carbonization. Accordingly, we developed an HC material with a true density (1.67 g cm-3) and a high closed-pore volume of 0.16 cm3 g-1. The optimized HC demonstrates excellent sodium-ion storage performance, achieving a reversible capacity of 369 mA h g-1 and a plateau capacity of 275 mA h g-1 at 30 mA g-1, while retaining 303 mA h g-1 after 300 cycles at 1 A g-1. Mechanistic studies reveal that the high-voltage slope capacity arises from sodium adsorption at defect sites, whereas sodium clustering in closed pores drives the low-voltage plateau. This study offers a sustainable and scalable pathway for advancing HC anodes in sodium-ion batteries.