Nanostructured WSe₂/C composites as anode materials for sodium-ion batteries

Abstract

Carbon-coated WSe₂ nanomaterials are synthesized using solid-state reaction. The 75.57% crystal WSe₂ nanoparticles are uniformly dispersed on a carbon matrix to form WSe₂/C nanomaterials. The WSe₂/C nanomaterials exhibit high discharge capacity and excellent cycling stability, which should be attributed to the buffering effect of the carbon matrix and improved conductivity of the composites.

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Sodium-ion batteries (SIBs) have been brought to the forefront owing to cheap prices, wide distribution, inexhaustible resources, and similar chemical mechanism to lithium-ions.^1,2 Carbonaceous materials,^3,4 titanium-based compounds^5,6 and alloys (Sn, Sb),^7,8 as alternative anode candidates, have been widely investigated. However, lower reversible capacity of carbonaceous materials and titanium-based compounds, and poor high rate and long cycling performances of alloys limit their practical application. It is urgent to exploit Na-host materials with high capacity and excellent rate performance.

Layered transition metal dichalcogenide compounds (TMDCs) MX₂ (M = Mo, Ti, W, Ta, X = S, Se) have been a major concern in various fields such as gas sensors, hydrogen storage, catalysis, energy storage.^9–11 Their crystal structures are composed of hexagonal layers of metal atoms (M) sandwiched between two layers of chalcogen atoms (X). There is strong covalent bonding within the M–X–M layers while only fairly weak van der Waals interactions between neighbouring sandwich layers.^12,13 Thus, Na-ions are easily inserted and extracted from this layered structure, making it as a promising candidate anode for sodium storage.

Among layer-structured metal chalcogenides, WSe₂ has received considerable attention recently owing to its remarkable features of ultralow thermal conductivity,¹⁴ highly hydrophobic sticky surfaces,¹⁵ and efficient p-type field-effect properties.¹⁶ However, little attention has been paid to their energy storage features on account of the difficulties in the synthesis and other related issues. Chen et al.¹⁷ synthesized ordered mesoporous WSe₂ material by a nanocasting method employing mesoporous silica SBA-315 as a hard template for lithium storage. But the preparation process is complex and produces the poisonous gases H₂Se, constraining its practical application. Share et al.¹⁸ conducted pioneering work on use of commercial WSe₂ as a anode material for sodium batteries.

Herein, for the first time, we synthesized WSe₂/C nanomaterials for sodium-ion batteries using solid-state reaction in which W powder, Se powder, and Super P carbon black were annealed in a vacuum glass tube after high energy ball milling. Firstly, this synthetic method enables quantity production of WSe₂/C nanomaterials. Secondly, the annealed WSe₂/C nanomaterials can be directly applied to electrode fabrication without further purification. Thirdly, the carbon black as a conductive matrix can buffer the huge volume change of WSe₂ particles during charge–discharge processes. The obtained WSe₂/C nanomaterials exhibit excellent specific capacity and cycling performance when evaluated as anode for sodium batteries, which make it have good application foreground.

The crystallographic phase of as-prepared samples was identified by X-ray powder diffraction (XRD). Fig. 1a shows the XRD patterns of the WSe₂/C nanomaterials and WSe₂ nanoplates. The obvious diffraction peaks at 13.6°, 31.4°, 37.8°, 47.3°, 55.9° could be assigned to (002), (100), (103), (105), (110) planes of the hexagonal phase of WSe₂ (JCPDS card no. 38-1388). In order to calculate the accurate loading of WSe₂ in the WSe₂/C nanomaterials, the thermogravimetric analysis (TGA) of the WSe₂/C nanomaterials and bare WSe₂ were investigated under air atmosphere. As shown in Fig. 1b, the weight loss of WSe₂/C nanomaterials and raw WSe₂ samples is 48.43% and 31.76% in the temperature range from 200 °C to 800 °C, respectively. Evidently, the weight increases in the thermogravimetric curves of the WSe₂/C nanomaterials and raw WSe₂ samples can be detected in the temperature between 350 °C and 420 °C, which can be attributed to the formation of solid state WO₃ and SeO₂ reacted by WSe₂ and O₂. For the curve of the WSe₂/C nanomaterials, apparent weight loss can be observed after 400 °C, which is related to the sublimation of SeO₂ and oxidation of carbon matrix. The weight loss of the WSe₂/C nanomaterials is equal to the sum of the weight loss of WSe₂ and carbon matrix, which can be described in the follow formula:

A × 48.43% = A × X × 31.76% + A × (1 − X)

A represents the mass of WSe₂/C nanomaterials. X represents the loading of WSe₂ in the WSe₂/C nanomaterials. Therefore, the accurate loading of WSe₂ in the WSe₂/C nanomaterials is calculated to be 75.57%. The high loading of active materials in the nanocomposites is vital to the practical application.

Fig. 1 (a) The XRD patterns and TGA curves (b) of WSe₂/C nanomaterials, WSe₂ nanoplates.

From the scanning electron microscopy (SEM) images (Fig. 2a), it can be observed that WSe₂ bulk has a plate-like morphology and the thickness of each plate is 15–25 nm. As shown in Fig. 2b, the WSe₂/C nanomaterials consist of granular-like agglomerates. Transmission electron microscopy (TEM) images (Fig. 2c and d) clearly demonstrate that the WSe₂ bulk is composed of many polygonous flakes and WSe₂ in WSe₂/C nanomaterials is coated by Super P carbon black. From high-resolution TEM image of WSe₂/C nanomaterials (Fig. 2e), an interlayered spacing of 0.237 nm is found, which is consistent with the d spacing of (103) planes of WSe₂. Well-resolved lattice fingers suggest that the WSe₂ are well crystallized. The corresponding selected area electron diffraction (SAED) patterns (Fig. 2f), indicate the single crystal characteristic of the WSe₂ nanoplates.

Fig. 2 (a) SEM images of WSe₂ nanoplates and (b) WSe₂/C nanomaterials; (c) TEM images of WSe₂ nanoplates and (d and e) WSe₂/C nanomaterials; (f) SEAD pattern of WSe₂/C nanomaterials.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping images in Fig. 3 demonstrate that the W elemental mapping image (Fig. 3c) overlaps with Se elemental mapping image (Fig. 3d) in the mapping region (Fig. 3a), and C elemental also well distributed in the mapping region, revealing the uniform distribution of WSe₂ in the WSe₂/C nanomaterials.

Fig. 3 Elemental mapping images of WSe₂/C nanomaterials.

Na-storage performances of the WSe₂/C nanomaterials and WSe₂ nanoplates probed by cyclic voltammogram (CV), galvanostatic measurements are demonstrated in Fig. 4. The CV profiles of WSe₂/C nanomaterials for the first five cycles at a scan rate of 0.2 mV s⁻¹ are shown in Fig. 4a. The CV shapes are similar with recent reports using carbonate-based electrolyte.¹⁸ One reduction peak at around 0.3 V can be observed in the first cycling according to the insertion of Na⁺. Two oxidation peaks around 1.86 V and 2.14 V can be observed in the initial cycle and are almost the same in the following cycles, which could be attributed to the extraction process of Na⁺. During the subsequent cycles, two reduction peaks at high voltages of 1.4 and 1.8 V were observed, which could be attributed to the insertion process of Na⁺ and the conversion reaction from Na_xWSe₂ to W metal and Na_xSe.¹⁸ A similar phenomenon was also found in Mo-based layered transition metal dichalcogenide compounds (MoS₂, MoSe₂).^19–22 The CV curves of WSe₂ nanoplates (Fig. S1a†) have similar shape with that of WSe₂/C composites. The difference is that two reduction peaks of WSe₂/C composites from second cycle are more apparent than that of WSe₂ nanoplates. The CV curves overlap ratio of WSe₂/C composites is higher than that of WSe₂ nanoplates, demonstrating better reversibility and stability of WSe₂/C composites. The discharge/charge voltage profiles of the WSe₂/C nanomaterials and WSe₂ nanoplates at a current density of 200 mA g⁻¹ are presented in Fig. 4b and S1b.† The plateaus in the discharge/charge voltage profiles of the WSe₂/C nanomaterials and WSe₂ nanoplates are in accord with the distinct peaks in the CV curves. Cycling performances and coulombic efficiencies of the WSe₂/C nanomaterials and WSe₂ nanoplates are exhibited in Fig. 4c. The initial discharge capacity of the WSe₂/C nanomaterials is 467 mA h g⁻¹, and then the capacity is decreased to 294 mA h g⁻¹ in the second cycle. The intercalation of the sodium into WSe₂ interlayers as well as the formation of solid/electrolyte interphase (SEI) may lead to the initial irreversible capacity loss. The WSe₂/C nanomaterials possess a reversible sodium storage capacity of 270 mA h g⁻¹ after 50 discharge/charge cycles without any notable loss. On the contrary, the WSe₂ nanoplates undergo an apparent capacity decline during the 20 cycles. After 50 cycles, the capacity of the WSe₂ nanoplates is decrease to 81 mA h g⁻¹. The initial coulombic efficiency of WSe₂ nanomaterials is 58.5%, it increases to 93.8% in the second cycle and remains higher than 95.1% after 50 cycles. Notably, the coulombic efficiency of WSe₂ nanoplates has a declining trend after 20 cycling, accompanying by the decline of capacity. Fig. 4d shows the capacity of the WSe₂/C nanomaterials and WSe₂ nanoplates under different current densities. Upon cycling under current densities of 100, 200, 500, 800, and 1000 mA g⁻¹, WSe₂/C nanomaterials reversibly delivers the capacities of 294, 260, 240, 230 and 208 mA h g⁻¹, respectively. In comparison, the WSe₂ nanoplates deliver lower capacities of 320, 303, 260, 158 and 53 mA h g⁻¹ under identical current densities. Enhanced electrochemical performance of WSe₂/C nanomaterials anode may contribute to the good distribution of WSe₂ in the WSe₂/C nanomaterials and improved conductivity of the composite. As it can be seen in Fig. S2,† the WSe₂/C nanomaterials still maintain a relatively structural integrity after 50 cycles (Fig. S2c and d†), while the WSe₂ nanoplates have obvious structural failure (Fig. S2a and b†), demonstrating that the nanostructured WSe₂/C composites are robust to withstand the volume change during charge/discharge cycles.

Fig. 4 (a) Cyclic voltammogram profiles of WSe₂/C nanomaterials at a scan rate of 0.2 mV s⁻¹; (b) discharge/charge voltage profiles of WSe₂/C nanomaterials at current density of 200 mA g⁻¹; (c) cycling performance and coulombic efficiency of WSe₂/C nanomaterials and WSe₂ nanoplates at current density of 200 mA g⁻¹ within a voltage range of 0.01–3.0 V; (d) capacity of WSe₂/C nanomaterials and WSe₂ nanoplates under different current density.

In order to investigate the effect of carbon amount on the composites' electrochemical performance, the composites with different carbon amount (Table 1 in ESI†) were fabricated under same condition. As shown in Fig. S3a and b,† sample 1 consist of nanoparticles and the particles is larger than that sample 2 (Fig. 2b) owing to the high carbon loading of sample 1. Compared to sample 1 and sample 2, sample 3 has a special morphology that contains stacks of nanoplates and a large amount of nanoparticles (Fig. S3c and d†), which could contributed to the low carbon loading of sample 3. As shown in Fig. S4a,† the initial discharge capacity of sample 1, sample 2, sample 3 is 425, 460, 340 mA h g⁻¹, respectively. Sample 1 faces a notable capacity loss during cycling and only 207 mA h g⁻¹ can be obtained after 50 cycles, which could be assigned to that lots of WSe₂ nanoplates are not firmly coated by carbon. Sample 2 possesses a reversible sodium storage capacity of 270 mA h g⁻¹ after 50 discharge/charge cycles. Sample 3 exhibits better cycling performance and a reversible sodium storage capacity of 214 mA h g⁻¹ after 50 cycles. Notably, the coulombic efficiencies of sample 3 are higher than sample 1 and sample after 50 cycles. The results demonstrate that sample 2 possesses better cycling stability compared to sample 1 and higher reversible capacity compared to sample 3 within 50 cycles.

In summary, WSe₂/C nanomaterials were synthesized using solid-state reaction in which W powder, Se powder, and Super P carbon black were annealed in a vacuum glass tube after high energy ball milling. The WSe₂/C nanomaterials anode delivers a high specific capacity of 270 mA h g⁻¹ at 200 mA g⁻¹ in sodium-ion batteries. The capacity of 208 mA h g⁻¹ can be obtained even at current density of 1000 mA g⁻¹. The solid-state reaction is robust to withstand the volume change during charge/discharge cycles as evidenced by the good morphology maintenance after cycling in SEM images.

Acknowledgements

The authors acknowledge the financial support of the Teacher Research Fund of Central South University (2013JSJJ027). This project was supported by Grants from the Project of Innovation-driven Plan in Central South University (2015CXS018) and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c5ra25645c

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