Rational Synthesis of Freestanding NaxV2O5-rGO Paper as the Stable Cathode for Sodium Ion Batteries

Flexible Na_xV₂O₅/rGO papers were successfully prepared via hydrothermal method followed by vacuum filtration as a high-performance cathode for SIBs. The as-prepared Na_xV₂O₅/rGO combined flexibility and high conductivity that can buffer stress and facilitate the fast transportation of electrons during the charge-discharge process. As a result, the as-prepared Na_xV₂O₅-rGO paper can exhibit a reversible Na-ion storage capacity of ∼197 mA h g⁻¹ at 100 mA g⁻¹ and a good cycling performance with 81% capacity retention for 400 cycles at a high current density of 500 mA g⁻¹, showing great potential in flexible energy storage devices.

Introduction

With the increase in environmental concerns, people are eager to search for and develop safe and green energy storage systems (ESS). (Xu et al., 2017; Li et al., 2019). In recent years lithium-ion batteries (LIBs) have been applied to commercial electric vehicles (EVs) and provided a high energy density for large-scale energy storage systems (Xu et al., 2013; Huang et al., 2020), but the costs of lithium and the uneven global distribution of lithium resources gradually restrict their further applications. (Li et al., 2014). Therefore, safe and new cheap ESS that can partially replace LIBs need to be explored (Li et al., 2021). Among the new EES, sodium ion batteries (SIBs) have attracted increasing attention due to the rich potential sources of raw materials and suitable redox potentials (Jiang et al., 2014; Zhao et al., 2020). However, they have approximately 70% larger Na⁺ than Li⁺, which can bring sluggish diffusion kinetics to the solid state electrode materials, leading to the fast capacity decay and poor rate performance for SIBs (Liang et al., 2018; Huang et al., 2019). Thus, it is necessary to constantly study high-performance cathode materials that can meet the demands of both low cost and good electrochemical performance. (Wang et al., 2020). High-performance electrode materials should have several important traits including good electrical conductivity, abundant electrochemical active sites and desirable structure. (Lu et al., 2016; Pei et al., 2017). The above traits should be considered when designing and preparing high-performance sodium ion batteries.

To date, a large variety of sodium ion conductors such as pyrophosphates (Niu et al., 2019), fluorophosphates (Lim et al., 2014), sulfates (Wu et al., 2014), sodium transition-metal oxides (Liu et al., 2020a), polyanions (Senthilkumar et al., 2020), and the Prussian blue metalates (Liu et al., 2020b; Huang et al., 2021) have been proposed as SIB cathodes. Among these different types of SIB cathode materials, vanadium-based oxides have attracted significant attention due to their high Na-storage capacities, diverse structures, and high electrochemical activity. (Córdoba et al., 2019; Jo et al., 2020). In recent years, a variety of sodium vanadates including V₂O₅ polymorphs (Baddour-Hadjean et al., 2018), Na_xVO₂ (Guignard et al., 2013), NaVO₃ (Venkatesh et al., 2014), NaV₃O₈ (Kang et al., 2015), Na_0.33V₂O₅ (Shang et al., 2019), and γ-Na_0.96V₂O₅ (Emery et al., 2018) have been investigated as the SIB cathode materials. Given their good electrochemical performances, people expect to design and prepare high-performance new electrode materials for sodium ion batteries from the various sodium vanadates. More recently, Jingjie Feng et al. reported that Na_xV₂O₅·nH₂O/KB shows a good electrochemical performance for SIB cathode. A high Na-storage capacity of 239 mAh g⁻¹ was acquired at 20 mA g⁻¹ and a good discharge capacity retention rate of 91% can be maintained after 90 cycles at 200 mA g⁻¹ (Feng et al., 2018). Then, Nicolas Emery et al. reported that the γ-Na_0.96V₂O₅ cathode material shows a reversible Na⁺ insertion/extraction behavior and can deliver a specific capacity of 125 mAh g⁻¹ at 0.2°C (Emery et al., 2018). Lately, Yifan Dong et al. studied the NaV₆O₁₅ microflowers as an SIB cathode material, which shows a reversible Na-storage capacity of 126 mAh g⁻¹at 100 mA g⁻¹. It also demonstrates good cycle stability and 87% capacity retention over 2000 cycles at 5 A g⁻¹ (Dong et al., 2020). These studies inspired us to develop a promising SIBs cathode material by controllable preparation of sodium vanadates with desirable morphology, structure and composition. A one dimensional (1D) structure can reduce ion diffusion path and promote the full use of active sites, which could endow cathode material with enhanced Na-ion storage performance (Zhu et al., 2019). In addition, the high electrochemical performance of Na_xV₂O₅ is restricted by its low electron conductivity and large volume change. Design of a composite consisting of 1D Na_xV₂O₅·nH₂O nanobelts and conductive carbon is a viable approach, which has both high electrochemical performance and good mechanical properties (Osman et al., 2021). Graphene, as a typical two-dimensional carbon, can provide elastic matrix for loading cathode materials and simultaneously acts as conductive network for strengthening the dynamical process both of electrons and ion transportation (Zhou et al., 2019; Qu et al., 2021). If we can controll fabrication of the 1D Na_xV₂O₅ composite with the reduced graphene oxide (rGO), the freestanding Na_xV₂O₅/rGO composite cathode with enhanced electrochemical performance would be acquired.

In this work, a freestanding composite (denoted as Na_xV₂O₅/rGO) consisting of δ-Na_xV₂O₅ nanobelts and rGO was fabricated by a facile hydrothermal method combined with vacuum filtration. The freestanding composite composed of the δ-Na_xV₂O₅ nanobelts and rGO offers several advantages. For instance, the bilayered δ-Na_xV₂O₅ nanobelts with a large interlayer spacing and 1D belt-shaped morphology can reduce diffusion distance and provide a small diffusion for fast Na⁺ ion insertion/extraction reactions (Xu et al., 2020). In addition, the rational introduction of rGO can reduce the aggregation of Na_xV₂O₅ nanobelts and improve the conductivity of the whole composite material (Ma et al., 2021), which can strengthen the mechanical integrity of the composite, thus improving the electrochemical performance of the electrode. Besides, compared with the traditional cathodes, the as-prepared freestanding electrode does not use non-conductive binders that can further improve electron transport (Xu et al., 2020). As a result, the as-prepared Na_xV₂O₅/rGO electrode could exhibit a Na-storage capacity of ∼197 mA h g⁻¹ at 100 mA g⁻¹ and good cycling performance with 91% of capacity retention after 150 cycles at 100 mA g⁻¹, and high rate performance with ∼91 mAh g⁻¹ at 500 mA g⁻¹. Moreover, a full Na-ion battery fabricated from the Na_xV₂O₅/rGO cathode and hard carbon anode can deliver a Na-storage capacity of 101 mAh g⁻¹ at 500 mA g⁻¹ and good cycling stability with a capacity retention of >75% over 100 cycles.

Experimental Details

Synthesis of the Ultra-Long Na_xV₂O₅ Nanobelts

The Na_xV₂O₅ was synthesized by a hydrothermal reaction as follows: 3 mmol of sodiumiodide and 1.5 mmol of vanadium pentoxide were put into the 40 ml of 1 mol/L sodium chloride aqueous solution and constantly stirred for 8 h. The mixture was put into a 40 ml Teflon-lined autoclave and placed into an electric oven at 200°C for 24 h. Finally, the obtained products were washed with deionized water and ethanol serval times and dried at 80°C to obtain Na_xV₂O₅.

Synthesis of the Free-Standing Na_xV₂O₅/rGO Paper

The free-standing Na_xV₂O₅/rGO paper was prepared as follows: 120 mg of the Na_xV₂O₅ was synthesized by hydrothermal method and 60 mg of graphene oxide (GO) was synthesized via the modified Hummer’s method. The mixture of Na_xV₂O₅ and GO was ultrasonicated for 2 h followed by constantly stirring overnight until it became homogeneous. The mixed solution of Na_xV₂O₅ and GO was put into the Teflon-lined autoclave and placed at 180°C for 24 h. After the hydrothermal reaction, the product was re-dispersed in deionized water and subsequent vacuum filtration. The paper-like product via peeling was dried at 80°C for 24 h to obtain the free-standing Na_xV₂O₅/rGO composite. The mass loading of the Na_xV₂O₅/rGO electrodes is ∼3 mg/cm².

Electrochemical Measurements

Fabrication of the Sodium Metal- Na_xV₂O₅/rGO Half Cell

The Na_xV₂O₅/rGO paper was directly cut into a round electrode with a diameter of 1 cm as the cathode and the active mass is ∼1.2 mg. The homemade sodium sheets were used both as counter electrodes and reference electrodes. Whatman glass microfiber filters (GF/D) were used as the separator and 1 M NaClO₄ in ethylene carbonate (EC): propylene carbonate (PC) (1:1 by v/v) with 5.0% fluoroethylene carbonate (FEC) as the electrolyte.

Assembly of the Hard Carbon- Na_xV₂O₅/rGO Full Cell

The Na-ion full battery was fabricated in an argon-filled glove box composed of Na_xV₂O₅/rGO cathode and hard carbon anode. The hard carbon anode was prepared by mixing hard carbon, Super P, and polyvinylidene difluoride (8:1:1 wt%) in N-Methylpyrrolidone, and then the slurry was spread onto Cu foil. We controlled the negative capacity/positive capacity ratio, which was ∼1.1 to guarantee cell balance. The anode was firstly cycled between 0.01 and 1.5 V in a SIB half cell before preparing the full SIB cell.

Results and Discussion

Figure 1 schematically illustrates the synthesis procedure of the Na_xV₂O₅/rGO paper. The mixture of the Na_xV₂O₅ and rGO was prepared by hydrothermal treatment of the Na_xV₂O₅ and GO. Then, the freestanding Na_xV₂O₅/rGO paper was obtained by facile vacuum filtration. The crystal structures of the as-synthesized samples (Na_xV₂O₅ and Na_xV₂O₅/rGO) are characterized by X-Ray diffraction (XRD). As shown in Figure 2, the XRD patterns of the synthesized Na_xV₂O₅ can be matched with a pure bilayer δ-Na_xV₂O₅ phase, which is consistent with previously reported results (Feng et al., 2018). Na_xV₂O₅/rGO exhibits similar diffraction patterns compared to that of the Na_xV₂O₅ sample, indicating Na_xV₂O₅ still exhibits high phase purity and GO has no effect on the formation of Na_xV₂O₅·nH₂O after the GO is added into the hydrothermal process. Then, the sodium to vanadium ratio was determined for the as-synthesized Na_xV₂O₅ sample by use of inductively coupled plasma optical emission spectroscopy (ICP-OES). The Na to V ratios in Na_xV₂O₅ samples were determined to be 0.16. When the vanadium level in each formula is set equal to 2.0, the corresponding sodium level in the Na_xV₂O₅ samples equals 0.32. (Supplementary Table S1). To confirm the morphology, SEM and TEM images were recorded on the as-synthesized Na_xV₂O₅ and Na_xV₂O₅/rGO paper. As shown in Figure 3A, one can see that the as-prepared Na_xV₂O₅ shows a long nanobelt morphology with a smooth surface and tens of micrometers in length and about 100–200 nm in width. The detailed crystal structures of Na_xV₂O₅ nanobelts were further characterized by a transmission electron microscope (TEM) (Figures 3B,C). The low-magnification TEM image (Figure 3B) further confirmed that the as-synthesized Na_xV₂O₅ nanobelts have a nanobelt structure, which is consistent with the observation in the SEM images. The high resolution (HR) TEM image demonstrates that these nanobelts are single crystals with an interplanar spacing of 0.36 nm, which corresponds to the (110) plane of the monoclinic phase Na_xV₂O₅ (Figure 3C). A typical scanning TEM image of a single Na_xV₂O₅ nanobelt and the corresponding element mapping images are recorded to further investigate the elemental distribution of the Na_xV₂O₅ nanobelt (Figure 3D). The elements Na, V, and O are homogeneously dispersed in the Na_xV₂O₅ nanobelt. Figures 3E,F exhibit the typical SEM images of Na_xV₂O₅/rGO paper. The interconnected Na_xV₂O₅ and rGO networks were observed, indicating the uniform intimate contact between Na_xV₂O₅ and rGO, which could increase the mechanical strength of freestanding Na_xV₂O₅/rGO hybrid paper to enhance cycling performance. Figures 3G–I shows optical images of freestanding Na_xV₂O₅/rGO paper, which can exhibit the excellent flexibility of Na_xV₂O₅/rGO paper and no obvious cracks were observed at the different bending states. Interestingly, based on the practical requirement, the as-fabricated Na_xV₂O₅/rGO paper can be easily cut into the desired shapes to be directly used as the electrode of flexible SIBs.

FIGURE 1

FIGURE 1. Schematic illustration of the as-prepared Na_xV₂O₅/rGO film.

FIGURE 2

FIGURE 2. XRD patterns of as-synthesized Na_xV₂O₅ powder and Na_xV₂O₅/rGO film.

FIGURE 3

FIGURE 3. (A) SEM of as-synthesized Na_xV₂O₅ powder;(B) TEM; (C) SAED; (D) EDS spectrum and Na, V, and O concentration in the Na_xV₂O₅ nanobelts. (E,F) SEM images of Na_xV₂O₅/rGO film; (G–I) Optical images of Na_xV₂O₅/rGO film at different bending states.

To study the electrochemical performance of Na_xV₂O₅/rGO paper as cathode material for SIBs, the cyclic voltammetry (CV) test of Na_xV₂O₅/rGO nanocomposite electrode was first assessed during the initial three cycles at 0.1 mV s⁻¹ (Figure 4A). The CV curves of Na_xV₂O₅/rGO are overlapping, suggesting the highly reversible electrochemical behaviors. The slightly different shape between the first CV curve and subsequent CV curves may be caused by the electrode polarization. Furthermore, It exhibits six main anodic current peaks around 1.91, 2.75, 2.82, 3.01, 3.15, and 3.65 V (vs. Na⁺/Na), respectively, and six cathodic peaks around 1.65, 2.75, 2.91, 3.12, and 3.50 V (vs. Na⁺/Na). The multiple redox peaks correspond to the multi-step insertion/extraction of Na⁺ in the different available locations with energy differences. Two oxidation peaks (1.91 and 3.65 V) and reduction peaks (1.65 and 3.50 V) with high peak current demonstrate high Na + diffusion kinetics of the Na_xV₂O₅/rGO paper electrode. The cycling profiles of the Na_xV₂O₅/rGO electrode at 100 mA g⁻¹ in the voltage range of 1.5–4.1 V are depicted in Figure 4B. The Na_xV₂O₅/rGO paper electrode shows analogous profiles corresponding well to the redox peaks of CV curves, indicating the good reversibility of the as-fabricated electrode. The dominated discharge/charge plateaus at about 3.6 V attributed to the reduction of V⁵⁺ to V⁴⁺ indicate a high electrochemical activity of V⁴⁺/V⁵⁺(He et al., 2016; Feng et al., 2018).

FIGURE 4

FIGURE 4. (A) The initial three CV curves of the Na_xV₂O₅/rGO electrode at a scanning rate of 0.1 mV s⁻¹; (B) Initial three charge/discharge curves of the Na_xV₂O₅/rGO electrode at a current density of 100 mA g⁻¹. The electrochemical performance of Na_xV₂O₅/rGO electrode: (C) cycling performance; and (D) rate capability. (E) Cycling performance of Na_xV₂O₅/rGO electrode at a current density of 500 mAg⁻¹.

The cycling performance of Na_xV₂O₅/rGO paper as cathode materials for SIBs is further studied, as shown in Figure 4C. The cyclability of the Na_xV₂O₅/rGO paper electrode was first tested at a low current density of 100 mA g⁻¹ (Figure 4C). It can deliver an initial charge/discharge capacity of 84 mAh g⁻¹/197 mAh g⁻¹. In the second and third cycles, the discharge capacity was almost no decay, demonstrating good Na-storage reversibility. After 150 cycles, a discharge specific capacity of 181 mAh g⁻¹ was retained with a high capacity retention of 91% compared with that in the 1st cycle (Figure 4C). On the contrary, the Na_xV₂O₅ electrode exhibit degraded electrochemical performance. The first discharge capacity of the Na_xV₂O₅ electrode could reach 185 mAh g⁻¹, but the discharge capacity was only retained at 131 mAh g⁻¹ after 150 cycles with a capacity retention of 70%. EIS tests are investigated to study the electrode reaction kinetics. Supplementary Figures S1A,B, the semicircle in high frequency (interfacial impedance, Rf) and middle frequency (charge transfer impedance, Rct) become larger after 10 cycles for Na_xV₂O₅ and Na_xV₂O₅/rGO. An equivalent circuit model was established, as shown in Supplementary Figure S1, where Rs is the internal resistance, Rf is the interfacial impedance, Rct represents the charge transfer resistance. As listed in Supplementary Table S2, the cycled NaxV2O5/rGO shows lower Rf and Rct than those of Na_xV₂O₅, indicating faster reaction kinetics of Na_xV₂O₅/rGO.

From the above results, the Na_xV₂O₅/rGO paper electrochemical showed superior cycling stability and strengthened reaction kinetics compared with the rare Na_xV₂O₅ electrodes. The rate performance of the Na_xV₂O₅/rGO paper at different current densities is shown in Figure 4D. The Na_xV₂O₅/rGO paper cathodes could exhibit average specific discharge capacities of 200, 163, 129, 92, and 65 mAh g⁻¹ at current densities of 100, 200, 500, 1000, and 2000 mA g⁻¹, respectively. When the current density comes back to 100 mA g⁻¹, an average specific discharge capacity of 187 mAh g⁻¹ could be preserved, showing the excellent electrochemical performance of the Na_xV₂O₅/rGO paper electrode at the different current densities. Considering the high reversibility of the as-prepared Na_xV₂O₅/rGO paper electrodes, we further evaluated the Na_xV₂O₅/rGO electrode at 500 mA g⁻¹ for 400 cycles as shown in Figure 4E. A specific discharge capacity of 91 mAh g⁻¹ was retained after 400 cycles with a capacity retention of 81%. Therefore, we believe the as-prepared Na_xV₂O₅/rGO paper electrodes exhibited a high rate performance and good cycling stability, indicating the enhanced electrochemical performance of the freestanding electrodes in the present work. To confirm the benefits of the Na_xV₂O₅/rGO paper, a comparison of the Na-storage performance between the as-prepared Na_xV₂O₅/rGO and other vanadium-based cathode materials is listed in Table 1. The electrochemical performance of our synthesized Na_xV₂O₅/rGO is superior to the vanadium-based cathode materials of SIBs recently reported. The good electrochemical performance of our Na_xV₂O₅/rGO films may be due to the following reasons: first, the bilayered Na_xV₂O₅ nanobelts with a large interlayer spacing and 1D belt-shaped morphology can reduce diffusion distance and provide a small diffusion for fast Na⁺ ion insertion/extraction reactions. The other is that the presence of rGO can reduce the aggregation of Na_xV₂O₅ nanobelts and improve the conductivity of the whole composite material, which can strengthen the mechanical integrity of the composite and thus improve the cycling stability of the electrode.

TABLE 1

TABLE 1. Comparison of the electrochemical performance of the electrode materials between this work and previous reports.

Due to the high chemical activity of Na metal, the metallic Na anode would be highly dangerous in practical application. Na-ion full batteries based on hard carbon and Na_xV₂O₅/rGO paper as the anode and cathode electrodes, respectively, were assembled to further study the electrochemical behavior of full SIBs. Figure 5A depicts the typical voltage profiles of the hard carbon anode and Na_xV₂O₅/rGO paper cathode measured in the Na half-cell. Both the Na_xV₂O₅/rGO and hard carbon could achieve reversible Na-ion transport from their respective skeleton structures. Figure 5B shows the charge–discharge profiles of the full battery at 500 mA g⁻¹ with cut-off voltages of 3.8 and 1.0 V. The initial discharge-specific capacities could attain 109 mAh g⁻¹. Charge-discharge specific capacities could also retain 134/135 mAh g⁻¹ with a high Coulombic efficiency of 99% in the second cycle. As shown in Figure 5C, after 100 cycles, the full Na-ion battery retained a discharge specific capacity of 101 mAh g⁻¹ with a capacity retention of >75%, showing the good cycling performance of SIBs based on Na_xV₂O₅/rGO paper electrode and hard carbon electrode. Importantly, the average Coulombic efficiency was higher than 98% during the 100 cycles, suggesting the highly reversible Na-ion storage behavior for the full Na-ion battery. Besides the high capacity retention, the full Na-ion battery also exhibited a good rate of performance. As shown in Figure 5D, the full Na-ion battery delivered average specific discharge capacities of 218, 197, 172, 137, 113, and 97 mAh g⁻¹ at constant current densities of 50, 100, 200, 500, 1000, and 2000 mA g⁻¹ respectively. When the current density was recovered to 50 mA g⁻¹, the reversible capacity came back to 193 mAh g⁻¹, indicating a good tolerance for the rapid Na-ion insertion/extraction cycles. The current work shows that the Na_xV₂O₅/rGO has great potential as the freestanding cathode material for stable sodium ion batteries.

FIGURE 5

FIGURE 5. Demonstration of SIBs based on Na_xV₂O₅/rGO || hard carbon configuration. (A) Typical charge–discharge curves of Na_xV₂O₅/rGO film || Na and hard carbon || Na in a half-cell configuration; (B) Electrochemical charge–discharge voltage profiles of SIBs, cycled at a current rate of 500 mA g⁻¹; (C) Cycling performance of SIBs at a constant current of 500 mA g⁻¹; (D) Rate capability of SIBs at different current rates, increasing from 50 mA g⁻¹ to 2A g⁻¹.

Conclusion

In summary, a flexible Na_xV₂O₅/rGO paper was successfully prepared via the hydrothermal method followed by vacuum filtration as a high-performance cathode for SIBs. The as-prepared Na_xV₂O₅/rGO possessed flexibility and high conductivity that can buffer stress and facilitate the fast transportation of electrons during the charge-discharge process. As a result, the as-prepared Na_xV₂O₅-rGO paper can deliver a reversible Na-ion storage capacity of ∼197 mA h g⁻¹ at 100 mA g⁻¹ and showed a good cycling performance with 81% capacity retention for 400 cycles at a high current density of 500 mA g⁻¹, showing great potentials in flexible energy storage devices.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

JX conceived the concept and directed the research. QW and XL designed the project. QW and XL carried out material synthesis. YY and XYL performed material characterization. JX and XW wrote the paper. All authors discussed the results and commented on the paper.

Funding

This work was supported by the Students’ Innovation and Entrepreneurship Training Program of Zhengzhou University (No.2020cxcy101).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2022.913804/full#supplementary-material

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