(NH4)2HPO4-mediated closed-pore/pseudographite synergy in thin-walled hard carbon for enhanced Na+ storage and kinetics

Abstract

Synchronously designing pseudo-graphitic domains featuring expanded interlayer spacing, rich closed pores and short ion transfer paths is crucial to enhancing sodium-ion (Na⁺) kinetics and storage of hard carbon anodes for fast-charging sodium-ion batteries (SIBs), yet remains challenging due to their inherent trade-off. Herein, we propose a (NH₄)₂HPO₄ (DAP)-assisted oxidative etching strategy to tailor kapok-derived carbon precursors with abundant nanopores and crosslinked functional groups, thus stimulating the development of closed pores and graphitic domains during carbonization. The resulting N/P-doped thin-walled (∼600 nm) hard carbon features extended graphitic domains coupled with expanded interlayer spacings and rich closed pores. The unique thin-walled structure can effectively shorten the ion diffusion pathway in-plane and along the thin-walled skeleton. More importantly, expanded graphitic domains with N/P co-doping and rich closed pores can not only provide sufficient active sites for Na⁺ storage through surface adsorption, intercalation, and pore-filling mechanisms but also rapidly construct Na⁺ diffusion channels during intercalation. Consequently, the as-prepared hard carbon anode exhibits superior capacity and remarkable rate performance (334.5 mAh g⁻¹/0.1C and 196.4 mAh g⁻¹/20C) without sacrificing the initial coulombic efficiency (ICE) (92.1%).