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Abstract：
In this paper, the synthesis of ultrasmall Na2FePO4F nanoparticles (≈3.8 nm) delicately embedded in porous N-doped carbon nanofibers (denoted as Na2FePO4F@C) by electrospinning is reported. The as-prepared Na2FePO4F@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 Na2FeIIPO4F ↔ NaFeIIIPO4F, 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 Na2FePO4F@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.
摘要：
在本文中,报道了通过静电纺丝精细嵌入多孔N掺杂碳纳米纤维（表示为Na2FePO4F@C）中的超小Na2FePO4F纳米颗粒（≈3.8nm）的合成。制备的Na2FePO4F@C纤维膜紧密粘附在铝箔上，具有很大的柔韧性，可直接用作钠离子电池的无粘合剂阴极，表现出令人钦佩的电化学性能与高可逆容量（117.8mAhg-1在0.1C），出色的倍率性能（20C时为46.4mAhg-1），和前所未有的高循环稳定性(2000次循环后85%的容量保持率)。通过循环伏安法的联合研究，探索了反应动力学和机理，异位结构/价态分析，和第一性原理计算，揭示Na2FeIIPO4F的高度可逆相变↔NaFeIIIPO4F，低能势垒的促进Na+扩散动力学，以及快速电荷存储所需的伪电容行为。袋式Na离子全电池还采用Na2FePO4F@C纳米纤维阴极和碳纳米纤维阳极组装，在200次循环中表现出135.8Whkg-1的有希望的能量密度和84.5%的高容量保留。封装在互连碳纳米纤维中的超细活性材料的独特网络结构为增强电化学反应性提供了理想的平台，电子/离子透射率，和Na存储电极的结构稳定性。
