Dual-Tuned Bamboo-Derived Hard Carbon via Stepwise Pyrolysis for High-Efficiency Sodium-Ion Storage

Biomass-derived hard carbon, distinguished by its diverse sources, low cost, and good electrochemical properties, has emerged as a highly promising negative electrode material for sodium-ion batteries. However, challenges such as irreversible Na+ loss at the electrolyte interface and the structural properties of materials lead to compromised initial Coulombic efficiency (ICE) and low reversible capacity in conventional hard carbon (HC) materials. Herein, a stepwise pyrolysis strategy to synthesize bamboo-derived hard carbon is proposed. This methodology aims to minimize the specific surface area while simultaneously increasing closed pore density and optimizing interlayer spacing, thereby synergistically enhancing both reversible capacity and ICE. It has been found that the optimized BHC-11–1400 sample, which is prepared through sequential pyrolysis at 1100 and 1400 °C, shows as high as 92.4% of ICE coupled with exceptional cycling stability. Especially, BHC-11–1400 maintains a specific discharge capacity of 265.6 mAh g–1 after 500 cycles at 300 mA g–1, corresponding to an impressive capacity retention of 95.7%.