Encapsulating hollow Fe3O4 in intertwined N-doped carbon nanofibers for high-performance supercapacitors and sodium-ion batteries

As a promising electrode for energy storage, Fe 3 O 4 has many intriguing advantages, such as a high specific capacity, low cost, low toxicity, wide potential window and environmental benignity. However, the multi-phase changes of iron oxide during the charge and discharge process can give rise to a sharp decrease in its capacity. In addition, the low conductivity of Fe 3 O 4 may hinder the charge transfer and ion diffusion during redox process. In order to solve the above issues, this study mainly attempts to design a nanocomposite of Fe 3 O 4 encapsulated in intertwined N-doped carbon nanofibers (CNFs) via using electrospinning and high-temperature calcination. The sealed structure can efficiently relieve the volume effect of Fe 3 O 4 and raise the stability of electrodes. While a 3-dimensional interconnected conductive network composed of CNFs can increase the electroconductibility of electrodes. At the same time, the N-doping increases active sites on the surface of CNFs, providing more space for ions and charges storage. Herein, different amounts of Fe 3 O 4 are encapsulated in N-doped CNFs (Fe 3 O 4 -CNFs). Fe 3 O 4 -CNFs with 40 % content of Fe 3 O 4 (4Fe 3 O 4 -CNFs) deliver splendid electrochemical performances for all-solid-state supercapacitors and sodium-ion batteries. The specific capacitance of 4Fe 3 O 4 -CNFs supercapacitor is 184.5 F g −1 and maintains 86.2 % of initial capacity at 2 A g −1 after 5000 times. Furthermore, 4Fe 3 O 4 -CNFs as the anode for the half cell vs. Na + /Na demonstrate a splendid specific capacity of 628.1 mAh g −1 at 0.02 A g −1 and can maintain 358.1 mA h g −1 after cycling for 200 laps at 500 mA g −1 . Therefore, 4Fe 3 O 4 -CNFs can be widely used in energy storage. Hollow Fe 3 O 4 nanospheres are encapsulated in the N-doped carbon nanofibers (Fe 3 O 4 CNFs) by using electrostatic spinning and high-temperature calcination method. The encapsulated structure can availably ameliorate the volume effect of Fe 3 O 4 and raise the stability of electrodes. While the addition of carbon materials can increase the conductivity of electrodes. At the same time, the N-doping increases active sites on the surface of carbon nanofibers, providing more space for ions and charges storage. The Fe 3 O 4 CNFs and the preparation method have exceptional employments in the field of energy storage. • The Fe 3 O 4 carbon nanofibers with the encapsulated structure are prepared by employing electrospinning method. • The encapsulated structure can availably ameliorate the volume effect of Fe 3 O 4 and raise the stability of electrodes. • The N-doped carbon nanofibers networks can increase the conductivity and against volume expansion.