Phosphorus-doped coal-derived hard carbon anodes for high-performance sodium-ion batteries: Synergistic regulation of graphitization and sodium adsorption

Abstract

The development of high-performance anode materials is crucial for the commercialization of sodium-ion batteries (SIBs). Coal-derived hard carbon (HC) is a promising candidate due to its abundance and low cost, but its application is hindered by insufficient Na⁺ storage capacity and low initial Coulombic efficiency (ICE) due to uncontrolled graphitization and limited active sites. Here, we introduce a novel phosphoric acid-assisted pre-oxidation strategy to synthesize phosphorus-doped hard carbon (HC-P) with multi-phase structural optimization. Phosphorus atoms are chemically integrated into the carbon matrix via covalent P–C–O bonding during pyrolysis, suppressing graphitization while inducing lamellar distortion. This results in expanded interlayer spacing (0.392 nm), abundant closed pores, and reduced Na⁺ adsorption energy (–1.64 eV). The optimized HC-0.5P achieves a high reversible capacity of 322 mAh g^–1 at 0.1 C (vs. 267 mAh g^–1 for undoped HC) with an ICE of 87.6 %, surpassing most coal-based carbons. In situ Raman mapping and density functional theory calculations reveal a three-stage Na⁺ storage mechanism (adsorption-intercalation-pore filling), enabled by defect-rich architectures and hierarchical porosity. This work provides a cost-effective route to transform low-rank coal into high-performance SIBs anodes, addressing critical challenges in sustainable energy storage.