A 3D porous interconnected NaVPO₄F/C network: preparation and performance for Na-ion batteries

Abstract

A uniform carbon embedded nano-scale NaVPO₄F precursor of 3–5 nm is fabricated by a hydrothermal method and using vitamin C as the carbon source and reducing agent, followed by a sintering process, the carbon embedded NaVPO₄F precursors transformed into a kind of 3D porous interconnected NaVPO₄F/C network. As a Na-ion battery cathode material, the NaVPO₄F/C network obtained at 750 °C achieves a high discharge capacity of 121 mA h g⁻¹ and the sample obtained at 800 °C shows a slightly lower discharge capacity of 101 mA h g⁻¹, but better rate capability and long cycle life. These results indicate that the 3D porous NaVPO₄F/C network not only improves the electronic conductivity of the active material, but also prevents the aggregation of particles, and its open porous structure allows electrolyte penetration, and reduces the diffusion path of the sodium ions, hence maximizing utilization of the electrochemically active NaVPO₄F particles.

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Introduction

Energy storage and conversion technology is widely used in portable electronic products, and this technology is gaining worldwide attention for its transportation applications, along with other large-scale renewable energy storage equipment.^1–3 Among the candidates, despite the popularity of Li-ion batteries in energy storage applications, their relatively high cost and the geographic constraints associated with lithium resources have motivated a search for cheaper and more sustainable alternatives.^4–6 In comparison, sodium resources are inexhaustible and unlimited around the world. After lithium, the electrochemical equivalent and standard potential of sodium are the most advantageous for aprotic battery applications; therefore, sodium-ion batteries are receiving widespread attention.^7–9 However, only a limited number of successful candidates are found for sodium insertion as cathode materials compared to those for lithium ion batteries.

Recently, some layered compounds and F-containing V-based polyanion compounds have been investigated as possible cathode materials for Na-ion batteries, such as Na_xMO₂ (M = V, Co, Mn, Fe),^10–13 NaVPO₄F,¹⁴ Na₃V₂O₂(PO₄)₂F,¹⁵ Na₃V₂(PO₄)₂F₃,¹⁶ and Li_1.1Na_0.4VPO_4.8F_0.7.¹⁷ Among these candidates, NaVPO₄F has recently attracted extensive attention because of its high theoretical specific capacity of 143 mA h g⁻¹ and its open framework structure.¹⁸ NaVPO₄F with a tetragonal structure was first proposed by J. Barker et al.,¹⁸ and various methods were introduced to prepare NaVPO₄F, including two-step solid state reaction, hydrothermal approach and sol–gel synthesis.^18–20 However, the capacity of the reported NaVPO₄F materials is far below its theoretical specific capacity and they show bad cycling performance.

Carbon coating is an effective approach to increase the electronic conductivity and electrochemical performance of the cathode materials.^21–23 There are many studies on enhancing the overall performance of cathode materials by carbon coating and different synthesis methods. Designing a porous network is another effective strategy for improving the performance of the electrode owing to favorable electrolyte absorption and significant reduction of sodium-ion diffusion paths.²⁴ Herein, we report a novel approach to synthesize NaVPO₄F/C network which consists of nanoparticles and microparticles with open interconnected 3D porous microstructure. As shown in Fig. 1, the NaVPO₄F/C precursor obtained by dropping absolute alcohol to NaVPO₄F precursor solution and following by hydrothermal reaction consisted of uniformed nanoparticles and completely coated with carbon. After calcination, the embedded nanosize NaVPO₄F grew up and aggregated together and outer carbon layer played an important role in forming the 3D porous interconnected NaVPO₄F network. This simple approach could be extended to other battery materials to obtain improved electrochemical performance.

Fig. 1 Schematic of design and fabrication of three-dimensional NaVPO₄F/C network and electron and ion transfer pathway inside NaVPO₄F/C network.

Experimental

Material preparation

2 mmol NH₄VO₃ and 2 mmol NaF were dissolved into 10 mL deionized water under stirring at 80 °C in a 100 mL beaker. 2 mmol NH₄H₂PO₄ and 3 mmol vitamin C were dissolved into 10 mL deionized water, and then slowly dropped into the above solution under continuous stirring. Subsequently a blue solution was obtained. Then 50 mL absolute alcohol was added to this mixed solution and kept continuous stirring for 0.5 h. The as obtained blue suspension was transferred into two separate 50 mL Teflon autoclaves. The autoclaves were sealed, kept at 180 °C for 24 h, and then cooled to room temperature naturally. The as-synthesized precipitate was collected by centrifugation and frozen dried to get a black brown precursor. Finally, the prepared precursors were calcined at 750 °C and 800 °C for 6 h under argon, respectively. The samples sintered at 750 °C and 800 °C were denoted as NaVPO₄F-750 and NaVPO₄F-800, respectively. Similarly, in order to optimize the reaction conditions, the samples were prepared in different times and under different temperatures.

Material characterization

The crystal structure of the products was characterized by X-ray diffraction (XRD-7000). Morphology and microstructure of the synthesized materials was investigated by field emission scanning electron microscopy (JSM-6700F, Japan) and transmission electron microscopy (JEM-2010, Japan). The carbon content was tested by CHNOS elemental analysis (vario EL CUBE, Germany).

Electrode fabrication and electrochemical tests

To evaluate the electrochemical performance of the as-prepared sample, CR2032-type coin cells were fabricated in an argon-filled glove box. This coin cell was composed of sodium anode, cathode, Celgard polypropylene separator, and 1 M NaPF₆ solution in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC/DEC, 1 : 1 v/v) as the electrolyte. The cathode was prepared by spreading the slurry of the active material (80 wt%), carbon black (10 wt%) and the poly(vinylidene fluoride) (10 wt%) binder dissolved in N-methyl pyrrolidine on Al foil. The working electrolytes were dried in vacuum at 120 °C for overnight. The loading of the active material (NaVPO₄F/C) was about 2–3 mg cm⁻². The assembled cells were rested for 24 h prior to further testing in order to ensure full absorption of the electrolyte into the working electrode. Charge–discharge tests were performed at room temperature with an Arbin.

Results and discussion

The XRD patterns of the as-synthesized samples display in Fig. 2. In order to optimize the reaction conditions, some experiments were done in different react times and temperatures (see Fig. S1 and S2†). The pattern of Fig. 2a suggests the precursor obtained in hydrothermal reaction at 180 °C for 24 h has a tetragonal symmetry structure (space group I4/mmm). VO₄F₂ octahedra and PO₄ tetrahedra stack an extended three-dimensional framework. The position of its diffraction peaks agrees with the previous work published by Barker et al.^18,19 However, the intensity of its diffraction peaks embodies the characteristics of amorphous materials. After sintering at 750 °C and 800 °C, the XRD results (Fig. 2c and d) are in good agreement with that reported by Zhou et al.,^14,20,25 suggesting the phase of monoclinic crystal (space group C2/c), two PO₄ tetrahedra share two corner-oxygen atoms with two different VO₄F₂ octahedra. While the product formed at 700 °C is a mixed phase of tetragonal structure and monoclinic crystal (Fig. 2b).

Fig. 2 XRD patterns of NaVPO₄F/C composites (a) hydrothermal method at 180 °C for 24 h, (b) precursor after calcined at 700 °C for 6 h, (c) precursor after calcined at 750 °C for 6 h, (d) precursor after calcined at 800 °C for 6 h.

NaVPO₄F have been conventionally synthesized by hydrothermal method or the two-step solid state. The reaction intermediate phase is VPO₄, prefabricated by the high temperature carbon-thermal reduction reaction.¹⁸ In this work, we first prepare uniformly carbon embedded nano-scaled NaVPO₄F precursors by a feasible and environmentally friendly hydrothermal method utilizing vitamin C as both the carbon sources and the reducing agent. And then further to sinter the obtained precursors at high temperature to form 3D porous interconnected NaVPO₄F/C network. The unique architecture of the NaVPO₄F/C precursor used in our work is displayed in Fig. 3. It is clear that the precursor is very uniform nanoparticles, the NaVPO₄F nanoparticles with diameters of 3–5 nm are embedded in the carbon matrix.

Fig. 3 SEM (a) and TEM (b and c) of carbon embedded NaVPO₄F obtained at hydrothermal process.

The product sintered at 750 °C displays 3D interconnected network structure composed of uniform particles (Fig. 4a). These grown particles are connected together and disordered carbon distribute between these big particles (Fig. 4b and c). NaVPO₄F sintered at 750 °C, which is coated by fine amorphous carbon coating layers, displayed clear crystal planes (Fig. 4d). The thickness of the carbon layer is about ∼5 nm (Fig. 4d). The carbon embedded NaVPO₄F precursor can prevent the aggregation of NaVPO₄F particles, and help to form porous interconnected network structure. For the NaVPO₄F samples fired at 800 °C, the particle size is in the range of 1–3 μm; these microparticles interweave together forming an open porous structure. (Fig. 4e and f). These coating carbon on NaVPO₄F nanoparticles create a carbon network and improve the electronic conductivity, the width of bridge-like carbon is about 10–50 nm (Fig. 4g and h). Such a well-organized 3D interconnected network structure facilitates the electrolyte penetration into the NaVPO₄F particles and provides more interface area between the electrolyte and the electrode material. The carbon content in NaVPO₄F/C precursors, NaVPO₄F/C sintered at 750 °C and 800 °C are estimated by elemental analysis. The results showed that the carbon content is about 9.6 wt%, 2.9 wt% and 1.4 wt% of the samples by hydrothermal method, sintered at 750 °C and 800 °C, separately. In additional, as shown in the Fig. S3–S5,† the energy dispersive spectrometer (EDS) mapping shows no impurity elements. All five elements –Na, V, P, O and F are present uniformly throughout the bulk of the sample.

Fig. 4 SEM and TEM of NaVPO₄F/C composites obtained at different temperatures. (a–d) 750 °C and (e–h) 800 °C.

In order to examine the potential application of NaVPO₄F/C network in sodium ion batteries, the electrochemical properties of NaVPO₄F/C composite were tested in the coin half-cells at room temperature in the voltage range of 2.5–4.2 V. Fig. 5a shows the cyclic voltammograms of the obtained composites at a scan rate of 0.1 mV s⁻¹. It is seen that the NaVPO₄F precursor obtained during hydrothermal process shows unobvious peaks and the redox couple is at around 3.55 V/3.65 V. And one wide redox couple (3.1 V/3.5 V) can be observed for the NaVPO₄F sintered at 750 °C. When the sintering temperature increases to 800 °C, two redox couples at 3.15 V/3.45 V and 3.6 V/3.7 V can be identified. Fig. 5b illustrates the typical first cycle profiles of charge–discharge curves at 0.1 C (corresponds to 14.3 mA g⁻¹). The charge and discharge plateaus of NaVPO₄F materials agree with the CV results. Because of different phases, the redox potential of NaVPO₄F precursor is about 0.5 V higher than that of the samples obtained by sintered at high temperature. The specific capacity of NaVPO₄F/C composites (hydrothermal, 750 °C and 800 °C) are 50 mA h g⁻¹, 121 mA h g⁻¹ and 101 mA h g⁻¹, respectively. Due to the looser surface structure and higher carbon content, the NaVPO₄F/C sintered at 750 °C exhibits a better specific capacity than the NaVPO₄F/C sintered at 800 °C. The rate performances of NaVPO₄F/C composite (hydrothermal, 750 °C and 800 °C) at various rates from 0.1 C to 1 C are illustrated in Fig. 5c. The as-synthesized NaVPO₄F-750 demonstrates a good rate capability at rate 0.1 C, 0.2 C and 0.5 C (i.e., the tenth capacities of NaVPO₄F-750 and NaVPO₄F-800 are 115 and 97 mA h g⁻¹ at 0.1 C, the twentieth capacities of NaVPO₄F-750 and NaVPO₄F-800 are 107 and 90 mA h g⁻¹ at 0.2 C, the thirtieth capacities of NaVPO₄F-750 and NaVPO₄F-800 are 91 and 83 mA h g⁻¹ at 0.5 C), which is far higher than that of the NaVPO₄F/C (97.8 mA h g⁻¹ at 0.1 C and 99.1 mA h g⁻¹ at 0.1 C) fabricated by Y. Lu et al.¹⁴ and Y.-L. Ruan et al.²⁶ It is interesting to find that the capacities of the as-synthesized NaVPO₄F-750 and NaVPO₄F-800 are basically equal (about 78 mA h g⁻¹) at 1 C. Fig. 5d further compared the cycle stability of these samples, and the discharge capacity of hydrothermal NaVPO₄F precursor decreases from 46 mA h g⁻¹ to only 14 mA h g⁻¹ after 65 cycles, while the NaVPO₄F-750 and NaVPO₄F-800 show good capacity retention, their discharge capacities reduce from 121 to 92 mA h g⁻¹ and from 101 to 83 mA h g⁻¹ after 65 cycles.

Fig. 5 The electrochemical properties of NaVPO₄F precursor and NaVPO₄F sintered at 750 °C and 800 °C: (a) cyclic voltammograms at a scan rate of 0.1 mV s⁻¹; (b) the typical first cycle profiles of charge–discharge curves at 0.1 C; (c) performance at various rates from 0.1 C to 1 C; (d) cycle performances at 0.1 C.

Conclusion

In this study, a kind of 3D porous interconnected NaVPO₄F/C network was prepared by using carbon embedded nano-scaled NaVPO₄F as precursor. The carbon embedded NaVPO₄F precursor can prevent the aggregation of NaVPO₄F particles, and help to form porous interconnected network structure. The unique 3D network structure provides interconnected open pores and effectively carbon coating, which allows electrolyte penetration, improves the electronic conductivity and reduces the diffusion path of the sodium ions. The results indicate that NaVPO₄F/C network delivered a high discharge capacity and a good long-term cycling life. This novel method could be extended to obtain various porous materials for use in energy storage and conversion, as well as catalysis.

Acknowledgements

This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under cstc2011pt-sy90001, Start-up grant under SWU111071 from Southwest University and Chongqing Science and Technology Commission under cstc2012gjhz90002. The work is also supported by grants from the National Natural Science Foundation of China (no. 21063014, 21163021), Fundamental Research Funds for the Central Universities (SWU113079, XDJK2014C051) and Research Funds for a postdoctor of Chongqing (xm2014097)

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05161d

‡ Maowen Xu and Chuan-Jun Cheng contributed equally to this work.

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