Electrospun Nb2O5 nanorods/microporous multichannel carbon nanofiber film anode for Na+ ion capacitors with good performance

https://doi.org/10.1016/j.jcis.2020.03.122Get rights and content

Highlights

  • Nb2O5 NRs/NMMCNF anode was prepared by electrospinning and annealing treatment.

  • The multichannel structure makes this anode deliver fast kinetics of Na+-storage.

  • Nb2O5 NRs/NMMCNF anode shows superior rate capability and ultralong cycling life.

  • Nb2O5 NRs/NMMCNF//AC SIC delivers high energy density and long-term cycling life.

Abstract

For the disadvantages of both the slow reaction kinetics and the poor conductivity for Nb2O5 electrode materials as sodium-ion capacitors (SICs), Nb2O5 NRs/NMMCNF film electrode with good flexibility and high electrochemical property has been fabricated by electrospinning PAN/PMMA/H2Nb2O6·H2O homogeneous viscous suspension and followed by an annealing treatment, in which the precursor H2Nb2O6·H2O nanorods are obtained by grinding H2Nb2O6·H2O nanowires, and Nb2O5 nanorods are uniformly embedded in nitrogen doped microporous multichannel carbon nanofiber. Benefiting from the multichannel network structure, Nb2O5 NRs/NMMCNF film electrode delivers the fast kinetics of Na+-storage and the superior Na-ion storage performance, it delivers outstanding rate capability (101 mAh g−1 at 4 A g−1) and ultralong lifespan (91% capacity retention after 10,000 cycles at 2 A g−1). A Nb2O5 NRs/NMMCNF//AC SIC based on the Nb2O5 NRs@NMMCNF fiber film anode and the AC cathode is assembled. The energy density of the as-assembled device is as high as 91 Wh kg−1 and its maximum power density is 7499 W kg−1. This work offers a new structure design strategy toward intercalation-type metal oxide electrodes for application in SICs.

Graphical abstract

Nb2O5 NRs/NMMCNF anode with superior rate capability and ultralong cycling life is prepared by electrospinning followed by an annealing treatment. The assembled SIC comprising a Nb2O5 NRs/NMMCNF anode and an activated carbon cathode exhibits high energy density (91 W kg−1 at 75 W kg−1) and good flexibility.

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Introduction

In recent years, with the rapid development of hybrid electric vehicles, electrical equipment, military and aerospace facilities, the power supply with high power density and an appropriate energy density is required, and it is expected to complete the charge and discharge in a few seconds to a few minutes [1], [2]. Recently, alkali metal-ion (Li+, Na+ et al.) capacitors based on the dual energy storage mechanism of batteries and supercapacitors have sparked increasing attention because of their high energy densities, high power output, and long cycle life [3], [4], [5], [6]. Sodium-ion capacitors (SICs) are more economical than lithium-ion capacitors (LICs) owing to its much higher natural abundance and higher ion conductivity than lithium [7], [8]. However, the larger ionic radius (1.02 Å) of Na+ ions makes it have slower diffusion kinetics in anode materials, resulting in unsatisfied rate capability [9]. Therefore, tremendous efforts have been dedicated to develop sodium anode materials with good kinetic property to make up for the difference in kinetics between the anode and cathode electrodes [10]. To date, various nanostructured anode materials such as carbon-based materials [11], transition-metal sulfides/selenides [12], [13], metal oxides (e.g. V2O5 [14], VO2 [15], [16], Na2Ti9O19 [17], MgTi2O5 [18], LiTiNbO5 [19], Nb2O5 [20], Li4Ti5O12 [21], TiNb2O7 [22], TiO2 [23], [24], [25], and NiCo2O4 [26], etc.), and conductive MXene nanosheets [27], [28], have been widely investigated for SICs. Among them, the intercalation-type metal oxides could be interesting high-rate candidates for SICs anodes due to their relatively safety voltage and slight volume expansion during the charge-discharge process for stationary performance. However, the low electrical conductivity of metal oxides limits the electron transport, making their electrochemical property reduction. Therefore, it is great significant to develop intercalation-type oxide anodes with high electrochemical kinetics and conductivity for the practical application of SICs.

Orthorhombic Nb2O5 has attracted high levels of scientific attention as a promising anode candidate because of its typical pseudocapacitive characteristics, safety-voltages and negligible volume expansion for Na+-storage [29], [30]. However, the sluggish reaction kinetics and intrinsic low conductivity (3 × 10−4 S m−1) make it has low electrochemical utilization at high rates [31]. To address these issues, various strategies, such as designing nanostructures (e.g., hollow nanospheres, nanosheets, nanowires, nanoparticles, and nanorods), introducing a carbon matrix, and doping heteroatom, have been widely applied [32], [33], [34], [35], [36]. Among the numerous nanostructures, one-dimensional (1D) nanorods possess outstanding Na+-storage properties owing to the 1D electron transfer channel and shorter ion diffusion distance [37], and the introduced carbon materials can improve the conductivity and alleviate the nanostructure aggregation [38]. Research results indicate that carbon nanofibers with microporous multichannel nanostructure are particularly attractive because they not only possess good electrical conductivity, excellent flexibility, and large surface area to volume ratio, but also can provide the rapid pathways for ionic and electronic transport [39], [40]. Moreover, it is demonstrated that heteroatom (such as N) doped into carbon fibers could further achieve the improved electronic conductivity and numerous defects and active sites [41]. Electrospinning is a versatile and efficient technique for the synthesis of various 1D carbon-based composite nanostructures for Na/Li-ion batteries because of its excellent designability and practical mass production, and it also allows for direct fabrication of flexible free-standing membranes [42], [43], [44], [45], [46]. Although the Li-ion storage property of electrospun Nb2O5 nanofibers has been intensively investigated in recent years [47], [48], [49], [50], their sodium-ion storage properties are hardly researched. Therefore, it is highly desired to fabricate Nb2O5 nanorods/N-doped microporous multichannel carbon nanofiber film anode for SICs with high performance by electrospinning.

Herein, for the disadvantages of both the slow diffusion kinetics and the poor conductivity for Nb2O5 anode materials as SICs, Nb2O5 NRs/NMMCNF film electrode with good flexibility and good rate performance had been fabricated by electrospinning PAN/PMMA/H2Nb2O6·H2O homogeneous viscous suspension and followed by an annealing treatment, in which the precursor H2Nb2O6·H2O nanorods were obtained by grinding H2Nb2O6·H2O nanowires, and Nb2O5 nanorods were uniformly embedded in nitrogen doped microporous multichannel carbon nanofiber. Nb2O5 NRs/NMMCNF film anode delivered outstanding rate capability (101 mAh g−1 at 4 A g−1) and ultralong life span (91% capacity retention after 10,000 cycles at 2 A g−1). By using Nb2O5NRs@NMMCNF film as anode and AC as cathode, Nb2O5 NRs/NMMCNF//ACSIC device was assembled. The energy density of the as-assembled device was as high as 91 W kg−1, the maximum power density was 7499 W kg−1, and the capacity retention rate was 77% after 10,000 cycles. This work offered a new structure design strategy toward intercalation-type metal oxide electrodes for application in SICs.

Section snippets

Preparation of Nb2O5 NRs/NMMCNF fiber film

The precursor, H2Nb2O6·H2O nanowires were first prepared by a hydrothermal method reported by Liu’s group [10]. In brief, 16 g NaOH was dissolved in 40 mL of ultrapure water with magnetic stirring, and then 0.36 g niobium (Nb) powder was added into NaOH solution under continuously stirring for 1 h. The obtained suspension was transferred to a Teflon-lined steel autoclave, it was heated at 130 °C in an electric oven for 20 h and then cooled to room temperature. The obtained sediment was

Results and discussion

The synthetic process of the free-standing Nb2O5 NRs/NMMCNF film is shown in Fig. 1. At firstly, the precursor, Na2Nb2O6·H2O nanowires are prepared by hydrothermally treating a mixed suspension of Nb powder and NaOH solution (Fig. 1a). The as-prepared Na2Nb2O6·H2O nanowires are then transformed into H2Nb2O6·H2O nanowires via an ion exchange process in 1 M diluted HCl (Fig. 1b). The XRD profile of the prepared Na2Nb2O6·H2O is in agreement with the standard monoclinic Na2Nb2O6·H2O with C2/c space

Conclusion

Nb2O5 NRs/NMMCNF film electrode with good flexibility and high electrochemical property has been fabricated by embedding Nb2O5 nanorods in nitrogen doped microporous multichannel carbon nanofiber networks through a simple electrospinning method. The novel multichannel structure consisted of graphited carbon, Nb2O5 NRs uniformly anchored in 3D network, and nitrogen-doped result not only has fast electron transfer and possesses sufficient micropores for electrolyte infiltration, but also inhibits

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51772182, 51902193), the 111 Project, the Fundamental Research Funds for the Central Universities (GK201801010, GK201903050), the Natural Science Basic Research Plan in Shaanxi Province of China (2019JQ-092), and the China Postdoctoral Science Foundation.

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