Insights into the Na⁺ Storage Mechanism of Phosphorus-Functionalized Hard Carbon as Ultrahigh Capacity Anodes

Hard carbon as a typical anode material for sodium ion batteries has received much attention in terms of its low cost and renewability. Herein, phosphorus-functionalized hard carbon with a specific “honeycomb briquette” shaped morphology is synthesized via electrospinning technology. When applied as an anode material for Na⁺ storage, it exhibits an impressively high reversible capacity of 393.4 mA h g⁻¹ with the capacity retention up to 98.2% after 100 cycles. According to first-principle calculation, the ultrahigh capacity of the as-prepared anode is ascribed to the enhancement of Na-absorption through formation of PO and PC bonds in graphitic layers when doped with phosphorus. Moreover, the increase of electron density around the Fermi level is found to be mainly caused by O atoms instead of P atoms.