Theoretical Study of a Two-Dimensional Metal-Free Polyporphyrin Framework as Anode Materials for Sodium-Ion Batteries

Two-dimensional metal-free organic materials are promising anodes for sodium-ion batteries (SIBs) due to their structural tunability and lightweight. Using first-principles calculations, we systematically investigate the Na storage performance of a two-dimensional hydrogenated polyporphyrin monolayer (2H-PP). The results reveal that 2H-PP exhibits strong Na adsorption with an adsorption energy of -2.14 eV at the most favorable h-C₈ site, effectively suppressing Na clustering. Bader charge analysis indicates a significant charge transfer of 0.906 |e| from Na to the substrate, confirming ionic interaction and efficient charge accommodation. Upon Na adsorption, 2H-PP retains its metallic character, ensuring good electronic conductivity. Na-ion migration on the 2H-PP surface shows a low diffusion barrier of 0.32 eV, enabling fast ion transport. It is worth noting that 2H-PP can accommodate 56 Na atoms, delivering an excellent theoretical capacity of 1258.03 mAh/g. Moreover, the diffusion barrier slightly decreases to 0.30 eV under 4% biaxial tensile strain, demonstrating robust diffusion kinetics under mechanical deformation. These findings highlight 2H-PP as a promising anode material for SIBs with strong Na binding, high charge transfer, favorable electronic structure, and excellent rate capability.