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WS2@graphene nanocomposites as anode materials for Na-ion batteries with enhanced electrochemical performances†
Received 30th January 2014, Accepted 27th February 2014
Dawei Su,ab Shixue Dou*a and Guoxiu Wang*b
DOI: 10.1039/c4cc00840e www.rsc.org/chemcomm
WS2@graphene nanocomposites were synthesized by a hydrothermal approach. When applied as anodes in Na-ion batteries, the WS2@graphene nanocomposite exhibited a high reversible sodium storage capacity of about 590 mA h g1. It also demonstrated excellent high rate performance and cyclability.
Na-ion batteries have been extensively studied recently, because they are considered to be a substitute for Li-ion batteries, owing to the low cost of sodium and the abundance of sodium sources.1–3 Various cathode materials for Na-ion batteries have been developed with stable capacities, such as layered transition metal oxides, bilayer V2O5,4,5 Na0.7MnO2 nanoplates,6 P2-NaxCoO2,7 P2-Na2/3[Fe1/2Mn1/2]O2,8 Na2/3(Ni1/3Fe1/3Mn2/3)O2,9 NaCrO2,10,11 and NaxVO2.12 Furthermore, the tunnel-structured transition metal oxide MnO2,13,14 olivine NaFePO4,15 fluoride-based materials, NaMF3 (M = Fe, Mn, V and Ni),16,17 as well as fluorophosphate18 and fluorosulfate19–21 have also been developed as cathode materials. So far, most studies on anodes have focused on hard carbon materials,22–24 even though hard carbon cannot contribute high capacity (less than 300 mA h g1). Alternative oxide anodes such as Na2Ti3O725 and amorphous TiO2-nanotubes26 have been investigated, but still have a capacity less than 300 mA h g1. Nevertheless, it was found that anodes based on the Na alloying reaction can dramatically improve the sodium storage capacity.27,28 It was reported that the SnSb–C nanocomposite achieved a capacity of 544 mA h g1, as well good rate performance and cyclability for Na-ion storage.27 We successfully developed octahedral SnO2 and SnO2@graphene nanocomposites,29,30 which also exhibit high capacity and great cyclablity. Recently, it has been realized that the larger sodium ion31 with higher ionization potential (compared to the Li+ ion) requires a more
open framework in which to move reversibly with acceptable mobility.31 WS2 has a large interlayer spacing along the c-axis (d(002) = 6.18 Å), which can accommodate the large Na ions. Therefore, it is an ideal candidate as the anode material for Na-ion batteries. Herein, we report the in situ hydrothermal synthesis of the WS2@graphene nanocomposite, in which WS2 nanocrystals are uniformly anchored on graphene nanosheets. The as-prepared WS2@graphene nanocomposite demonstrated a high reversible capacity of about 590 mA h g1 in Na-ion batteries and excellent cyclability. The crystallographic phase of the as-prepared WS2@graphene nanocomposite and bare WS2 was identified by XRD, as shown in Fig. 1a. It can be well indexed as having a hexagonal symmetry unit cell (JCPDS: 87-2417) with the space group P63/mmc. As indicated by the XRD peak in the low d-spacing range (2y = 14.321), it can be concluded that the as-prepared WS2@graphene nanocomposites and bare WS2 have a large intercalation space in the crystal structure. Furthermore, WS2@graphene nanocomposites exhibit stronger diffraction peaks than the bare WS2, suggesting much better crystallinity of the as-prepared WS2@graphene nanocomposite. There is no
a
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia. E-mail: [email protected]; Fax: +61-2-4221-5731; Tel: +61-2-4221-4558 b Centre for Clean Energy Technology, School of Chemistry and Forensic Science, Faculty of Science, University of Technology, Sydney, NSW 2007, Australia. E-mail: [email protected]; Fax: +61-2-9514-1460; Tel: +61-2-9514-1741 † Electronic supplementary information (ESI) available: Detailed experimental procedures; XRD patterns, cyclic voltammograms, FESEM images, Nyquist plots, schematic diagram and charge–discharge profiles. See DOI: 10.1039/c4cc00840e
4192 | Chem. Commun., 2014, 50, 4192--4195
Fig. 1 (a) XRD patterns of bare WS2 and the WS2@graphene nanocomposite. (b) TG/DTA curves of the WS2@graphene nanocomposite. (c) and (d) Low and high magnification FESEM images of the as-prepared WS2@graphene nanocomposite, respectively.
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diffraction peak corresponding to graphene in the WS2@graphene nanocomposite XRD patterns, suggesting that the graphene nanosheets have been well separated by the WS2 nanocrystals. Fig. S1 shows the XRD pattern of bare graphene (ESI†). The weight ratio of the WS2 nanocrystals to the graphene nanosheets was determined to be 60 : 40 by TG and DTA measurements as shown in Fig. 1b. The 40 wt% weight loss occurred mainly from 500 to 600 1C, with the feature of an endothermic peak at 542 1C, corresponding to the oxidation of carbon. The morphology of the WS2@graphene nanocomposites was observed by FESEM, as shown in Fig. 1c and d. The low magnification FESEM image (Fig. 1c) clearly shows the flexible graphene nanosheets. In the high magnification FESEM image (Fig. 1d), WS2 nanocrystals homogeneously distributed on the graphene nanosheets can be readily observed. Individual WS2 nanocrystals are wrapped with graphene nanosheets. The sizes of the WS2 nanocrystals are estimated to be about 80 nm. Furthermore the morphological features and crystal structure of the WS2@graphene nanocomposites were analysed by TEM and HRTEM (Fig. 2). The low magnification TEM image (Fig. 2a) presents a typical piece of a WS2@graphene nanosheet. It can be seen that the WS2 nanocrystals are uniformly anchored on the graphene nanosheets. Its corresponding selected area electron diffraction (SAED) pattern (Fig. 2b) presents ring features, which can be well indexed as different crystal planes ((102), (103), (107), and (203)) of hexagonal WS2, confirming the well-defined crystallinity of the as-prepared WS2@graphene nanocomposite. In the high magnification TEM image (Fig. 2c), all the free standing WS2 nanocrystals show clear fringes, which is ascribed to the large interlayer distance
Fig. 2 (a) Low magnification TEM image of the WS2@graphene nanocomposite. (b) Selected area electron diffraction (SAED) pattern corresponding to (a). (c) High magnification TEM image, in which the WS2 nanocrystals can be readily observed. (d) Lattice resolved HRTEM image. (e) Fast-Fourier-transform (FFT) pattern taken from the square area marked in (d). (f) Crystal structure of WS2 viewed along the b-axis.
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(d(002) = 6.18 Å) between the crystal planes along the c-axis of hexagonal WS2. From the lattice resolved HRTEM image (Fig. 2d), we can directly observe the interlayer gap of the (002) crystal planes, as illustrated by the simulated WS2 crystal structure (Fig. 2f). The fastFourier-transform (FFT) patterns taken from the marked square area in Fig. 2d clearly show the different stacks along the c-axis of the hexagonal WS2 ((002), (003), and (004)). This crystal structure consists of layered stacks, separated by a large interlayer spacing (d-spacing of 6.18 Å), which is consistent with the XRD peak at 2y = 14.321. Obviously, the large interlayer gaps between the (002) crystal planes of WS2 could provide a facile channel for Na ion diffusion and space to accommodate the Na ions. This feature makes the as-prepared WS2@graphene nanocomposite a promising electrode candidate for Na-ion batteries. The bare graphene, bare WS2, and WS2@graphene nanocomposite were applied as anode materials in Na-ion batteries, and their electrochemical performances were studied by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. The electrochemical properties were first evaluated by CV measurement, as shown in Fig. S2 (ESI†), in which both WS2 and the WS2@graphene nanocomposite exhibit two obvious redox couples (at around 1.57 and 0.89 V in the cathodic process, and around 2.16 and 1.57 V in the anodic process, respectively). The first redox couple (B1.57 V in the cathodic process, B2.16 V in the anodic process) should be derived from the insertion–extraction of Na ions into/out of the layered WS2 crystals, while the following redox couple (B0.89 V in the cathodic process, B1.57 V in the anodic process) could be ascribed to the reversible reaction between Na ions and WS2 (Na+ + WS2 + e 2 W + NaS2). Another broad peak in the lower discharge voltage range (B0.44 V) should correspond to the generation of the solid electrolyte interphase (SEI). According to previous research on electrode materials, the SEI, which contains an inorganic layer and an organic layer around the particles, will be formed on both the anode and the cathode materials.32,33 The organic SEI layer can be formed and dissolved reversibly, which could contribute to the reversible capacity during the cycling processes. In contrast, the generation of the inorganic SEI layer is an irreversible process, which will result in the irreversible capacity. From the second cycle, the CV curves overlap very well, and there is no shift in the redox couple positions, which suggests their superior stability regarding the electrochemical reactions. The galvanostatic discharge–charge profiles in the 1st and 2nd cycles of bare graphene, bare WS2, and the WS2@graphene nanocomposite exhibit differences as shown in Fig. 3a (tested at a current density of 20 mA g1). The bare WS2 electrode yielded discharge capacities of 513 and 297 mA h g1 in the first and second cycles, respectively (FESEM images of as-prepared bare WS2 can be seen in Fig. S3, ESI†). For the bare graphene nanosheet electrode, the initial specific discharge capacity can reach up to B1000 mA h g1, while it dramatically drops to B240 mA h g1 in the subsequent cycle. The WS2@graphene nanocomposite electrode demonstrated the highest initial discharge capacity of 1250 mA h g1. Unlike the bare graphene electrode, the WS2@graphene nanocomposite electrode can maintain a high capacity after the first cycle (a discharge capacity of 584 mA h g1 was obtained in the second cycle), which is higher than the theoretical capacity of WS2@GE nanocomposites (The theoretical sodium storage capacities for the bare WS2 and the
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Fig. 3 (a) The 1st and 2nd cycle discharge and charge profiles of bare WS2, bare graphene (GE), and the WS2@graphene nanocomposite at a current density of 20 mA g1, respectively. (b) Comparison of cycling performances of bare graphene, bare WS2, and WS2@graphene nanocomposite electrodes at a current density of 20 mA g1. (c) Cycling performance of the WS2@ graphene nanocomposite at current densities of 20, 40, 80, 160, 320, and 640 mA g1. (d) Rate performance of the WS2@graphene nanocomposite at varied current densities, (b) and (c) are recorded from the 2nd cycle.
WS2@nanocomposite are 405 and 514 mA h g1, respectively34). Compared with previous work on SnO2@graphene,29 the WS2@ graphene nanocomposite exhibited a higher initial Coulombic efficiency (45%, while the previously reported SnO2@graphene nanocomposite showed an initial Coulombic efficiency of 36%), which should be ascribed to the improved initial Coulombic efficiency of bare WS2 (55%) than that of bare SnO2 (22%). MoS2 has the same crystal structure as WS2 (P63/mmc). However, WS2 exhibited higher Coulombic efficiency, which is 55% and 89% in the initial cycle and the second cycle, respectively (52% and 86%, respectively for MoS2, as shown in Fig. S4, ESI†). The irreversible capacity in the first cycle could be derived from the formation of the inorganic SEI layer. Furthermore, sodium intercalation into the acetylene black additives also contributed to the irreversibility in the first cycle.35 It can be seen that the Na storage capacities of the bare graphene and WS2@graphene nanocomposite electrodes have stabilised within the 500 cycles, as shown in Fig. 3b, while the bare WS2 electrode cannot sustain the capacity. After 500 cycles, the WS2@graphene nanocomposite electrode still delivered 329 mA h g1 capacity, while the bare WS2 electrode retained a capacity of only 32 mA h g1. Therefore, it is clear that the graphene has benefits for the sustainability of the WS2@graphene nanocomposite electrode. Fig. 3c shows the cycling performance of the WS2@graphene nanocomposite electrode at different current densities. It can be seen that the WS2@graphene nanocomposite presented excellent capacity retention capability at all tested current densities. After 500 cycles, the discharge capacities were maintained at high values: 283 mA h g1 at 40 mA g1, 218 mA h g1 at 80 mA g1, 170 mA h g1 at 160 mA g1, and 148 mA h g1 at 320 mA g1. Even when cycled at 640 mA g1, a discharge capacity of 94 mA h g1 was still obtained after 500 cycles. We also tested the cycling performance of the WS2@graphene nanocomposite at varied current densities (Fig. 3d). After cycling at high current densities, the cell capacity can recover to the original
4194 | Chem. Commun., 2014, 50, 4192--4195
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values if the current density is reversed back to a low current density. This confirmed that the WS2@graphene nanocomposite is tolerant of high rate cycling. The outstanding performance of the WS2@graphene nanocomposite could be ascribed to the addition of graphene, because the graphene nanosheets can serve as conductive media for electron transfer during the discharge and charge processes, which was evidenced by the A.C. impedance measurements (Fig. S5, ESI†). It can be seen that both the bare WS2 and the WS2@graphene electrodes show typical Nyquist plots, consisting of a single depressed semicircle in the high-medium frequency region and an inclined line at low frequency. The numerical value of the diameter of the semicircle on the Z0 axis gives an approximate indication of the charge transfer resistance (Rct). Apparently, the charge transfer impedance of the WS2@graphene electrode is much lower than that of the bare WS2 electrode. The reduced charge transfer impedance is beneficial for the electron kinetics in the electrode material, and consequently, improves the electrochemical performance of the WS2@graphene electrode for sodium storage. Furthermore, the unique three-dimensional (3D) architecture of the WS2@graphene material, shown in Fig. S6 (ESI†) also promotes the integrity of the electrode. The charge and discharge processes are always accompanied by large volume changes, causing deterioration of the cyclablity, which is the main reason for the quick decline in the capacity of the bare WS2 electrode (Fig. S7, ESI†). The graphene matrix of the WS2@graphene nanocomposite can effectively buffer the volume variation during the Na insertion and extraction processes. As a result, the integrity of the electrode can be maintained, leading to an enhanced performance for Na storage. In conclusion, WS2@graphene nanocomposites were synthesized by a simple hydrothermal approach. The XRD, FESEM, and TEM analyses confirmed that the WS2 nanocrystals were homogeneously distributed on the graphene nanosheets. When applied as anode materials in Na-ion batteries, the WS2@graphene nanocomposite showed its highly reactive nature towards sodium storage. It demonstrated a high reversible specific capacity of about 594 mA h g1, excellent cyclability, and a good high rate performance, which could be ascribed to the highly conductive graphene matrix and the unique 3D architecture of the nanocomposite. The WS2@graphene nanocomposite is a promising high performance anode material for Na-ion batteries. This original research was proudly supported by Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). The authors acknowledge use of facilities in the UOW Electron Microscopy Centre. The authors would like to also thank Dr Tania Silver for critical reading.
Notes and references 1 V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. CarreteroGonzalez and T. Rojo, Energy Environ. Sci., 2012, 5, 5884–5901. 2 T. Matsuda, M. Takachi and Y. Moritomo, Chem. Commun., 2013, 49, 2750–2752. 3 J. B. Goodenough, J. Song, M. Xu and L. Wang, Chem. Commun., 2013, 49, 5280–5282. 4 D. Su and G. Wang, ACS Nano, 2013, 7, 11218–11226. 5 S. Tepavcevic, H. Xiong, V. R. Stamenkovic, X. Zuo, M. Balasubramanian, V. B. Prakapenka, C. S. Johnson and T. Rajh, ACS Nano, 2011, 6, 530–538.
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Communication 6 D. Su, C. Wang, H. j. Ahn and G. Wang, Chem.–Eur. J., 2013, 19, 10884–10889. 7 R. Berthelot, D. Carlier and C. Delmas, Nat. Mater., 2011, 10, 74–U73. 8 N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada and S. Komaba, Nat. Mater., 2012, 11, 512–517. 9 D. Kim, E. Lee, M. Slater, W. Q. Lu, S. Rood and C. S. Johnson, Electrochem. Commun., 2012, 18, 66–69. 10 S. Komaba, C. Takei, T. Nakayama, A. Ogata and N. Yabuuchi, Electrochem. Commun., 2010, 12, 355–358. 11 X. Xia and J. R. Dahn, Electrochem. Solid-State Lett., 2012, 15, A1–A4. 12 D. Hamani, M. Ati, J.-M. Tarascon and P. Rozier, Electrochem. Commun., 2011, 13, 938–941. 13 D. Su, H.-J. Ahn and G. Wang, NPG Asia Mater., 2013, 5, 56. 14 D. Su, H.-J. Ahn and G. Wang, J. Mater. Chem. A, 2013, 1, 4845–4850. 15 K. Zaghib, J. Trottier, P. Hovington, F. Brochu, A. Guerfi, A. Mauger and C. M. Julien, J. Power Sources, 2011, 196, 9612–9617. 16 I. D. Gocheva, M. Nishijima, T. Doi, S. Okada, J. Yamaki and T. Nishida, J. Power Sources, 2009, 187, 247–252. 17 Y. Yamada, T. Doi, I. Tanaka, S. Okada and J. Yamaki, J. Power Sources, 2011, 196, 4837–4841. 18 N. Recham, J. N. Chotard, L. Dupont, K. Djellab, M. Armand and J. M. Tarascon, J. Electrochem. Soc., 2009, 156, A993–A999. 19 M. Ati, L. Dupont, N. Recham, J. N. Chotard, W. T. Walker, C. Davoisne, P. Barpanda, V. Sarou-Kanian, M. Armand and J. M. Tarascon, Chem. Mater., 2010, 22, 4062–4068. 20 P. Barpanda, J. N. Chotard, N. Recham, C. Delacourt, M. Ati, L. Dupont, M. Armand and J. M. Tarascon, Inorg. Chem., 2010, 49, 7401–7413.
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ChemComm 21 M. Reynaud, P. Barpanda, G. Rousse, J. N. Chotard, B. C. Melot, N. Recham and J. M. Tarascon, Solid State Sci., 2012, 14, 15–20. 22 R. Alcantara, J. M. Jimenez-Mateos, P. Lavela and J. L. Tirado, Electrochem. Commun., 2001, 3, 639–642. 23 R. Alcantara, P. Lavela, G. F. Ortiz and J. L. Tirado, Electrochem. Solid-State Lett., 2005, 8, A222–A225. 24 D. A. Stevens and J. R. Dahn, J. Electrochem. Soc., 2000, 147, 1271–1273. 25 P. Senguttuvan, G. Rousse, V. Seznec, J.-M. Tarascon and M. Rosa Palacin, Chem. Mater., 2011, 23, 4109–4111. 26 H. Xiong, M. D. Slater, M. Balasubramanian, C. S. Johnson and T. Rajh, J. Phys. Chem. Lett., 2011, 2, 2560–2565. 27 L. F. Xiao, Y. L. Cao, J. Xiao, W. Wang, L. Kovarik, Z. M. Nie and J. Liu, Chem. Commun., 2012, 48, 3321–3323. 28 Y. Xu, Y. Zhu, Y. Liu and C. Wang, Adv. Energy Mater., 2012, 128–133. 29 D. Su, H.-J. Ahn and G. Wang, Chem. Commun., 2013, 49, 3131–3133. 30 D. Su, C. Wang, H.-J. Ahn and G. Wang, Phys. Chem. Chem. Phys., 2013, 15, 12543–12550. 31 Y. L. Cao, L. F. Xiao, M. L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. M. Nie, L. V. Saraf, Z. G. Yang and J. Liu, Nano Lett., 2012, 12, 3783–3787. 32 P. Poizot, S. Laruelle, S. Grugeon and J.-M. Tarascon, J. Electrochem. Soc., 2002, 149, A1212–A1217. ¨m, T. Gustafsson and J. O. Thomas, Electrochim. Acta, 2004, 33 K. Edstro 50, 397–403. 34 H. Liu, D. Su, G. Wang and S. Z. Qiao, J. Mater. Chem., 2012, 22, 17437–17440. 35 M. Shirpour, J. Cabana and M. Doeff, Energy Environ. Sci., 2013, 6, 2538–2547.
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Cite this: Chem. Commun., 2014, 50, 4192
WS2@graphene nanocomposites as anode materials for Na-ion batteries with enhanced electrochemical performances†
Received 30th January 2014, Accepted 27th February 2014
Dawei Su,ab Shixue Dou*a and Guoxiu Wang*b
DOI: 10.1039/c4cc00840e www.rsc.org/chemcomm
WS2@graphene nanocomposites were synthesized by a hydrothermal approach. When applied as anodes in Na-ion batteries, the WS2@graphene nanocomposite exhibited a high reversible sodium storage capacity of about 590 mA h g1. It also demonstrated excellent high rate performance and cyclability.
Na-ion batteries have been extensively studied recently, because they are considered to be a substitute for Li-ion batteries, owing to the low cost of sodium and the abundance of sodium sources.1–3 Various cathode materials for Na-ion batteries have been developed with stable capacities, such as layered transition metal oxides, bilayer V2O5,4,5 Na0.7MnO2 nanoplates,6 P2-NaxCoO2,7 P2-Na2/3[Fe1/2Mn1/2]O2,8 Na2/3(Ni1/3Fe1/3Mn2/3)O2,9 NaCrO2,10,11 and NaxVO2.12 Furthermore, the tunnel-structured transition metal oxide MnO2,13,14 olivine NaFePO4,15 fluoride-based materials, NaMF3 (M = Fe, Mn, V and Ni),16,17 as well as fluorophosphate18 and fluorosulfate19–21 have also been developed as cathode materials. So far, most studies on anodes have focused on hard carbon materials,22–24 even though hard carbon cannot contribute high capacity (less than 300 mA h g1). Alternative oxide anodes such as Na2Ti3O725 and amorphous TiO2-nanotubes26 have been investigated, but still have a capacity less than 300 mA h g1. Nevertheless, it was found that anodes based on the Na alloying reaction can dramatically improve the sodium storage capacity.27,28 It was reported that the SnSb–C nanocomposite achieved a capacity of 544 mA h g1, as well good rate performance and cyclability for Na-ion storage.27 We successfully developed octahedral SnO2 and SnO2@graphene nanocomposites,29,30 which also exhibit high capacity and great cyclablity. Recently, it has been realized that the larger sodium ion31 with higher ionization potential (compared to the Li+ ion) requires a more
open framework in which to move reversibly with acceptable mobility.31 WS2 has a large interlayer spacing along the c-axis (d(002) = 6.18 Å), which can accommodate the large Na ions. Therefore, it is an ideal candidate as the anode material for Na-ion batteries. Herein, we report the in situ hydrothermal synthesis of the WS2@graphene nanocomposite, in which WS2 nanocrystals are uniformly anchored on graphene nanosheets. The as-prepared WS2@graphene nanocomposite demonstrated a high reversible capacity of about 590 mA h g1 in Na-ion batteries and excellent cyclability. The crystallographic phase of the as-prepared WS2@graphene nanocomposite and bare WS2 was identified by XRD, as shown in Fig. 1a. It can be well indexed as having a hexagonal symmetry unit cell (JCPDS: 87-2417) with the space group P63/mmc. As indicated by the XRD peak in the low d-spacing range (2y = 14.321), it can be concluded that the as-prepared WS2@graphene nanocomposites and bare WS2 have a large intercalation space in the crystal structure. Furthermore, WS2@graphene nanocomposites exhibit stronger diffraction peaks than the bare WS2, suggesting much better crystallinity of the as-prepared WS2@graphene nanocomposite. There is no
a
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia. E-mail: [email protected]; Fax: +61-2-4221-5731; Tel: +61-2-4221-4558 b Centre for Clean Energy Technology, School of Chemistry and Forensic Science, Faculty of Science, University of Technology, Sydney, NSW 2007, Australia. E-mail: [email protected]; Fax: +61-2-9514-1460; Tel: +61-2-9514-1741 † Electronic supplementary information (ESI) available: Detailed experimental procedures; XRD patterns, cyclic voltammograms, FESEM images, Nyquist plots, schematic diagram and charge–discharge profiles. See DOI: 10.1039/c4cc00840e
4192 | Chem. Commun., 2014, 50, 4192--4195
Fig. 1 (a) XRD patterns of bare WS2 and the WS2@graphene nanocomposite. (b) TG/DTA curves of the WS2@graphene nanocomposite. (c) and (d) Low and high magnification FESEM images of the as-prepared WS2@graphene nanocomposite, respectively.
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diffraction peak corresponding to graphene in the WS2@graphene nanocomposite XRD patterns, suggesting that the graphene nanosheets have been well separated by the WS2 nanocrystals. Fig. S1 shows the XRD pattern of bare graphene (ESI†). The weight ratio of the WS2 nanocrystals to the graphene nanosheets was determined to be 60 : 40 by TG and DTA measurements as shown in Fig. 1b. The 40 wt% weight loss occurred mainly from 500 to 600 1C, with the feature of an endothermic peak at 542 1C, corresponding to the oxidation of carbon. The morphology of the WS2@graphene nanocomposites was observed by FESEM, as shown in Fig. 1c and d. The low magnification FESEM image (Fig. 1c) clearly shows the flexible graphene nanosheets. In the high magnification FESEM image (Fig. 1d), WS2 nanocrystals homogeneously distributed on the graphene nanosheets can be readily observed. Individual WS2 nanocrystals are wrapped with graphene nanosheets. The sizes of the WS2 nanocrystals are estimated to be about 80 nm. Furthermore the morphological features and crystal structure of the WS2@graphene nanocomposites were analysed by TEM and HRTEM (Fig. 2). The low magnification TEM image (Fig. 2a) presents a typical piece of a WS2@graphene nanosheet. It can be seen that the WS2 nanocrystals are uniformly anchored on the graphene nanosheets. Its corresponding selected area electron diffraction (SAED) pattern (Fig. 2b) presents ring features, which can be well indexed as different crystal planes ((102), (103), (107), and (203)) of hexagonal WS2, confirming the well-defined crystallinity of the as-prepared WS2@graphene nanocomposite. In the high magnification TEM image (Fig. 2c), all the free standing WS2 nanocrystals show clear fringes, which is ascribed to the large interlayer distance
Fig. 2 (a) Low magnification TEM image of the WS2@graphene nanocomposite. (b) Selected area electron diffraction (SAED) pattern corresponding to (a). (c) High magnification TEM image, in which the WS2 nanocrystals can be readily observed. (d) Lattice resolved HRTEM image. (e) Fast-Fourier-transform (FFT) pattern taken from the square area marked in (d). (f) Crystal structure of WS2 viewed along the b-axis.
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(d(002) = 6.18 Å) between the crystal planes along the c-axis of hexagonal WS2. From the lattice resolved HRTEM image (Fig. 2d), we can directly observe the interlayer gap of the (002) crystal planes, as illustrated by the simulated WS2 crystal structure (Fig. 2f). The fastFourier-transform (FFT) patterns taken from the marked square area in Fig. 2d clearly show the different stacks along the c-axis of the hexagonal WS2 ((002), (003), and (004)). This crystal structure consists of layered stacks, separated by a large interlayer spacing (d-spacing of 6.18 Å), which is consistent with the XRD peak at 2y = 14.321. Obviously, the large interlayer gaps between the (002) crystal planes of WS2 could provide a facile channel for Na ion diffusion and space to accommodate the Na ions. This feature makes the as-prepared WS2@graphene nanocomposite a promising electrode candidate for Na-ion batteries. The bare graphene, bare WS2, and WS2@graphene nanocomposite were applied as anode materials in Na-ion batteries, and their electrochemical performances were studied by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. The electrochemical properties were first evaluated by CV measurement, as shown in Fig. S2 (ESI†), in which both WS2 and the WS2@graphene nanocomposite exhibit two obvious redox couples (at around 1.57 and 0.89 V in the cathodic process, and around 2.16 and 1.57 V in the anodic process, respectively). The first redox couple (B1.57 V in the cathodic process, B2.16 V in the anodic process) should be derived from the insertion–extraction of Na ions into/out of the layered WS2 crystals, while the following redox couple (B0.89 V in the cathodic process, B1.57 V in the anodic process) could be ascribed to the reversible reaction between Na ions and WS2 (Na+ + WS2 + e 2 W + NaS2). Another broad peak in the lower discharge voltage range (B0.44 V) should correspond to the generation of the solid electrolyte interphase (SEI). According to previous research on electrode materials, the SEI, which contains an inorganic layer and an organic layer around the particles, will be formed on both the anode and the cathode materials.32,33 The organic SEI layer can be formed and dissolved reversibly, which could contribute to the reversible capacity during the cycling processes. In contrast, the generation of the inorganic SEI layer is an irreversible process, which will result in the irreversible capacity. From the second cycle, the CV curves overlap very well, and there is no shift in the redox couple positions, which suggests their superior stability regarding the electrochemical reactions. The galvanostatic discharge–charge profiles in the 1st and 2nd cycles of bare graphene, bare WS2, and the WS2@graphene nanocomposite exhibit differences as shown in Fig. 3a (tested at a current density of 20 mA g1). The bare WS2 electrode yielded discharge capacities of 513 and 297 mA h g1 in the first and second cycles, respectively (FESEM images of as-prepared bare WS2 can be seen in Fig. S3, ESI†). For the bare graphene nanosheet electrode, the initial specific discharge capacity can reach up to B1000 mA h g1, while it dramatically drops to B240 mA h g1 in the subsequent cycle. The WS2@graphene nanocomposite electrode demonstrated the highest initial discharge capacity of 1250 mA h g1. Unlike the bare graphene electrode, the WS2@graphene nanocomposite electrode can maintain a high capacity after the first cycle (a discharge capacity of 584 mA h g1 was obtained in the second cycle), which is higher than the theoretical capacity of WS2@GE nanocomposites (The theoretical sodium storage capacities for the bare WS2 and the
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Fig. 3 (a) The 1st and 2nd cycle discharge and charge profiles of bare WS2, bare graphene (GE), and the WS2@graphene nanocomposite at a current density of 20 mA g1, respectively. (b) Comparison of cycling performances of bare graphene, bare WS2, and WS2@graphene nanocomposite electrodes at a current density of 20 mA g1. (c) Cycling performance of the WS2@ graphene nanocomposite at current densities of 20, 40, 80, 160, 320, and 640 mA g1. (d) Rate performance of the WS2@graphene nanocomposite at varied current densities, (b) and (c) are recorded from the 2nd cycle.
WS2@nanocomposite are 405 and 514 mA h g1, respectively34). Compared with previous work on SnO2@graphene,29 the WS2@ graphene nanocomposite exhibited a higher initial Coulombic efficiency (45%, while the previously reported SnO2@graphene nanocomposite showed an initial Coulombic efficiency of 36%), which should be ascribed to the improved initial Coulombic efficiency of bare WS2 (55%) than that of bare SnO2 (22%). MoS2 has the same crystal structure as WS2 (P63/mmc). However, WS2 exhibited higher Coulombic efficiency, which is 55% and 89% in the initial cycle and the second cycle, respectively (52% and 86%, respectively for MoS2, as shown in Fig. S4, ESI†). The irreversible capacity in the first cycle could be derived from the formation of the inorganic SEI layer. Furthermore, sodium intercalation into the acetylene black additives also contributed to the irreversibility in the first cycle.35 It can be seen that the Na storage capacities of the bare graphene and WS2@graphene nanocomposite electrodes have stabilised within the 500 cycles, as shown in Fig. 3b, while the bare WS2 electrode cannot sustain the capacity. After 500 cycles, the WS2@graphene nanocomposite electrode still delivered 329 mA h g1 capacity, while the bare WS2 electrode retained a capacity of only 32 mA h g1. Therefore, it is clear that the graphene has benefits for the sustainability of the WS2@graphene nanocomposite electrode. Fig. 3c shows the cycling performance of the WS2@graphene nanocomposite electrode at different current densities. It can be seen that the WS2@graphene nanocomposite presented excellent capacity retention capability at all tested current densities. After 500 cycles, the discharge capacities were maintained at high values: 283 mA h g1 at 40 mA g1, 218 mA h g1 at 80 mA g1, 170 mA h g1 at 160 mA g1, and 148 mA h g1 at 320 mA g1. Even when cycled at 640 mA g1, a discharge capacity of 94 mA h g1 was still obtained after 500 cycles. We also tested the cycling performance of the WS2@graphene nanocomposite at varied current densities (Fig. 3d). After cycling at high current densities, the cell capacity can recover to the original
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values if the current density is reversed back to a low current density. This confirmed that the WS2@graphene nanocomposite is tolerant of high rate cycling. The outstanding performance of the WS2@graphene nanocomposite could be ascribed to the addition of graphene, because the graphene nanosheets can serve as conductive media for electron transfer during the discharge and charge processes, which was evidenced by the A.C. impedance measurements (Fig. S5, ESI†). It can be seen that both the bare WS2 and the WS2@graphene electrodes show typical Nyquist plots, consisting of a single depressed semicircle in the high-medium frequency region and an inclined line at low frequency. The numerical value of the diameter of the semicircle on the Z0 axis gives an approximate indication of the charge transfer resistance (Rct). Apparently, the charge transfer impedance of the WS2@graphene electrode is much lower than that of the bare WS2 electrode. The reduced charge transfer impedance is beneficial for the electron kinetics in the electrode material, and consequently, improves the electrochemical performance of the WS2@graphene electrode for sodium storage. Furthermore, the unique three-dimensional (3D) architecture of the WS2@graphene material, shown in Fig. S6 (ESI†) also promotes the integrity of the electrode. The charge and discharge processes are always accompanied by large volume changes, causing deterioration of the cyclablity, which is the main reason for the quick decline in the capacity of the bare WS2 electrode (Fig. S7, ESI†). The graphene matrix of the WS2@graphene nanocomposite can effectively buffer the volume variation during the Na insertion and extraction processes. As a result, the integrity of the electrode can be maintained, leading to an enhanced performance for Na storage. In conclusion, WS2@graphene nanocomposites were synthesized by a simple hydrothermal approach. The XRD, FESEM, and TEM analyses confirmed that the WS2 nanocrystals were homogeneously distributed on the graphene nanosheets. When applied as anode materials in Na-ion batteries, the WS2@graphene nanocomposite showed its highly reactive nature towards sodium storage. It demonstrated a high reversible specific capacity of about 594 mA h g1, excellent cyclability, and a good high rate performance, which could be ascribed to the highly conductive graphene matrix and the unique 3D architecture of the nanocomposite. The WS2@graphene nanocomposite is a promising high performance anode material for Na-ion batteries. This original research was proudly supported by Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). The authors acknowledge use of facilities in the UOW Electron Microscopy Centre. The authors would like to also thank Dr Tania Silver for critical reading.
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