Diffusion-restricted volatile release drives secondary cracking to engineer low-surface-area hard carbon

Low specific surface area (SSA) in hard carbon anodes is crucial for enhancing the initial Coulombic efficiency (ICE) and reversible capacity of sodium-ion batteries (SIBs). Conventional strategies for SSA reduction within high-temperature carbonization processes often rely on additional energy-intensive thermal treatments or complex processes, presenting significant barriers to practical application. This study introduces a simplified and effective approach that operates within a standard high-temperature framework (1300 °C) by solely controlling the packing thickness of coconut shell char (CSC) precursors. Increased packing thickness induces diffusional resistance, hindering volatile release and achieving a volatile release reduction of 7.68 %. This confinement effect promotes in-situ secondary cleavage of volatiles, accelerating carbon deposition and reducing surface defects. Consequently, SSA decreases significantly by 71.52 %, while closed pore formation is enhanced. The optimized CSC-HPT-1300 exhibits dramatically improved electrochemical performance over the low-packing-thickness control, achieving a 15.48 % higher ICE (85 %), a 15.39 % increase in reversible capacity (280 mAh g −1 ), and a 25.90 % enhanced plateau capacity (209 mAh g −1 ). Furthermore, this strategy concurrently increases carbonization mass yield by 4.64 %. This thickness-controlled carbonization presents a straightforward and scalable optimization to standard high-temperature processing for fabricating low-surface-area hard carbons, advancing sustainable SIBs development.