Na Storage Activity or Inertness of P‑Configurations in N, P Dual-Doped Carbon Nanofibers: Bulk vs Surface

Heteroatom doping is an effective method to improve the electrochemical properties of hard carbon anodes for sodium-ion batteries. However, the different roles of surface and bulk heteroatoms in Na storage have not been explored much. Herein, N, P dual-doped carbon nanofibers (NP-CNFs) with high doping contents and low surface area are designed to clarify the above issue. It is confirmed that P plays a more crucial role in Na storage compared with N. In addition, surface and bulk P not only possess different configurations but also show distinct Na storage activity. There are only oxidized PO x groups on the surface, which are inactive for Na storage but promote the stability of electrochemistry interphase, while in the bulk phase, unoxidized P–C bonds also emerge except PO x , which shows preeminently reversible Na storage activity, and the PO x groups are activated simultaneously. Furthermore, P-doping changes the reactivity of N-configurations with Na both on the surface and in the bulk phase, exhibiting interesting synergism. As expected, the surface stability, bulk activity, and synergism enable NP-CNFs to achieve superior performance. It could deliver a prominent capacity of 105.6 mAh g–1 at 10 A g–1 after 3000 cycles in half cells and 164.3 mAh g–1 at 1 A g–1 after 200 cycles in full cells.