Unlocking High-Performance Sodium-Ion Batteries with Fluorine-Doped Kenaf Biomass-Derived Hard Carbon for Enhanced Charge-Transfer Kinetics and Moisture Stability

Hard carbon is a leading anode candidate for sodium-ion batteries (SIBs), yet its performance is strongly governed by precursor chemistry, microstructural evolution, and interfacial stability. Here, kenaf, a low-cost biomass, is converted into hard carbon through acid pretreatment and controlled carbonization, enabling the systematic modulation of graphitic ordering and closed-pore formation. The optimized kenaf-derived hard carbon material exhibits a greater contribution from pore-filling storage and delivers a high initial Coulombic efficiency of 91.2% with superior Na⁺ diffusion kinetics and high-rate capability. To address the intrinsic moisture sensitivity of SIBs, a fluorinated surface layer is introduced through poly(vinylidene fluoride)-derived F doping, forming a NaF-rich interphase that enhances hydrophobicity, suppresses NaPF₆ decomposition, and lowers interfacial resistance. The resulting F-doped hard carbon anode maintains excellent cycling stability in both half-cells and full cells, retaining 87.3% capacity over 500 cycles even under 500 ppm of moisture. This combined structural and interfacial engineering strategy offers a scalable pathway toward high-performance SIB anodes.