Realizing a High‐Performance Na‐Storage Cathode by Tailoring Ultrasmall Na(2)FePO(4)F Nanoparticles with Facilitated Reaction Kinetics

In this paper, the synthesis of ultrasmall Na(2)FePO(4)F nanoparticles (≈3.8 nm) delicately embedded in porous N‐doped carbon nanofibers (denoted as Na(2)FePO(4)F@C) by electrospinning is reported. The as‐prepared Na(2)FePO(4)F@C fiber film tightly adherent on aluminum foil features great flexibility and is directly used as binder‐free cathode for sodium‐ion batteries, exhibiting admirable electrochemical performance with high reversible capacity (117.8 mAh g(−1) at 0.1 C), outstanding rate capability (46.4 mAh g(−1) at 20 C), and unprecedentedly high cyclic stability (85% capacity retention after 2000 cycles). The reaction kinetics and mechanism are explored by a combination study of cyclic voltammetry, ex situ structure/valence analyses, and first‐principles computations, revealing the highly reversible phase transformation of Na(2)Fe(II)PO(4)F ↔ NaFe(III)PO(4)F, the facilitated Na(+) diffusion dynamics with low energy barriers, and the desirable pseudocapacitive behavior for fast charge storage. Pouch‐type Na‐ion full batteries are also assembled employing the Na(2)FePO(4)F@C nanofibers cathode and the carbon nanofibers anode, demonstrating a promising energy density of 135.8 Wh kg(−1) and a high capacity retention of 84.5% over 200 cycles. The distinctive network architecture of ultrafine active materials encapsulated into interlinked carbon nanofibers offers an ideal platform for enhancing the electrochemical reactivity, electronic/ionic transmittability, and structural stability of Na‐storage electrodes.